JP4428904B2 - Oxygen concentration detection apparatus and oxygen concentration detection method - Google Patents

Oxygen concentration detection apparatus and oxygen concentration detection method Download PDF

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JP4428904B2
JP4428904B2 JP2002015870A JP2002015870A JP4428904B2 JP 4428904 B2 JP4428904 B2 JP 4428904B2 JP 2002015870 A JP2002015870 A JP 2002015870A JP 2002015870 A JP2002015870 A JP 2002015870A JP 4428904 B2 JP4428904 B2 JP 4428904B2
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oxygen concentration
fuel
exhaust
exhaust system
air
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JP2003214245A (en
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栄記 守谷
辰優 杉山
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Toyota Motor Corp
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Priority to EP20030001529 priority patent/EP1333171B1/en
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D41/00—Electrical control of supply of combustible mixture or its constituents
    • F02D41/24—Electrical control of supply of combustible mixture or its constituents characterised by the use of digital means
    • F02D41/2406—Electrical control of supply of combustible mixture or its constituents characterised by the use of digital means using essentially read only memories
    • F02D41/2425—Particular ways of programming the data
    • F02D41/2429—Methods of calibrating or learning
    • F02D41/2441—Methods of calibrating or learning characterised by the learning conditions
    • F02D41/2448—Prohibition of learning
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D41/00—Electrical control of supply of combustible mixture or its constituents
    • F02D41/02—Circuit arrangements for generating control signals
    • F02D41/04—Introducing corrections for particular operating conditions
    • F02D41/12—Introducing corrections for particular operating conditions for deceleration
    • F02D41/123—Introducing corrections for particular operating conditions for deceleration the fuel injection being cut-off
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D41/00—Electrical control of supply of combustible mixture or its constituents
    • F02D41/02—Circuit arrangements for generating control signals
    • F02D41/14—Introducing closed-loop corrections
    • F02D41/1438—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor
    • F02D41/1444—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor characterised by the characteristics of the combustion gases
    • F02D41/1454—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor characterised by the characteristics of the combustion gases the characteristics being an oxygen content or concentration or the air-fuel ratio
    • F02D41/1455—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor characterised by the characteristics of the combustion gases the characteristics being an oxygen content or concentration or the air-fuel ratio with sensor resistivity varying with oxygen concentration
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D41/00—Electrical control of supply of combustible mixture or its constituents
    • F02D41/02—Circuit arrangements for generating control signals
    • F02D41/14—Introducing closed-loop corrections
    • F02D41/1438—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor
    • F02D41/1473—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor characterised by the regulation method
    • F02D41/1475—Regulating the air fuel ratio at a value other than stoichiometry
    • F02D41/1476—Biasing of the sensor
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D41/00—Electrical control of supply of combustible mixture or its constituents
    • F02D41/24—Electrical control of supply of combustible mixture or its constituents characterised by the use of digital means
    • F02D41/2406—Electrical control of supply of combustible mixture or its constituents characterised by the use of digital means using essentially read only memories
    • F02D41/2425—Particular ways of programming the data
    • F02D41/2429—Methods of calibrating or learning
    • F02D41/2438—Active learning methods
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D41/00—Electrical control of supply of combustible mixture or its constituents
    • F02D41/24—Electrical control of supply of combustible mixture or its constituents characterised by the use of digital means
    • F02D41/2406—Electrical control of supply of combustible mixture or its constituents characterised by the use of digital means using essentially read only memories
    • F02D41/2425—Particular ways of programming the data
    • F02D41/2429—Methods of calibrating or learning
    • F02D41/2451—Methods of calibrating or learning characterised by what is learned or calibrated
    • F02D41/2474—Characteristics of sensors
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D41/00—Electrical control of supply of combustible mixture or its constituents
    • F02D41/0002—Controlling intake air
    • F02D2041/0022—Controlling intake air for diesel engines by throttle control
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D2250/00—Engine control related to specific problems or objectives
    • F02D2250/32—Air-fuel ratio control in a diesel engine
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D41/00—Electrical control of supply of combustible mixture or its constituents
    • F02D41/0002—Controlling intake air
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D41/00—Electrical control of supply of combustible mixture or its constituents
    • F02D41/02—Circuit arrangements for generating control signals
    • F02D41/14—Introducing closed-loop corrections
    • F02D41/1438—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor
    • F02D41/1444—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor characterised by the characteristics of the combustion gases
    • F02D41/1454—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor characterised by the characteristics of the combustion gases the characteristics being an oxygen content or concentration or the air-fuel ratio
    • F02D41/1456—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor characterised by the characteristics of the combustion gases the characteristics being an oxygen content or concentration or the air-fuel ratio with sensor output signal being linear or quasi-linear with the concentration of oxygen
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D41/00—Electrical control of supply of combustible mixture or its constituents
    • F02D41/02—Circuit arrangements for generating control signals
    • F02D41/18—Circuit arrangements for generating control signals by measuring intake air flow
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D41/00—Electrical control of supply of combustible mixture or its constituents
    • F02D41/24—Electrical control of supply of combustible mixture or its constituents characterised by the use of digital means
    • F02D41/2406—Electrical control of supply of combustible mixture or its constituents characterised by the use of digital means using essentially read only memories
    • F02D41/2425—Particular ways of programming the data
    • F02D41/2429—Methods of calibrating or learning
    • F02D41/2451—Methods of calibrating or learning characterised by what is learned or calibrated
    • F02D41/2454—Learning of the air-fuel ratio control

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Analytical Chemistry (AREA)
  • Combined Controls Of Internal Combustion Engines (AREA)
  • Electrical Control Of Air Or Fuel Supplied To Internal-Combustion Engine (AREA)
  • Exhaust Gas After Treatment (AREA)

Description

【０００１】
【発明の属する技術分野】
本発明は、ディーゼルエンジンの排気中酸素濃度を検出する酸素濃度検出装置及び検出方法に関する。
【０００２】
【従来の技術】
ディーゼルエンジンのように、広い運転領域において高い空燃比（リーン雰囲気）の混合気を燃焼に供して機関運転を行う内燃機関として、排気特性の向上や機関燃焼状態の安定化を図るべく、排気中の酸素濃度が高い状態（リーンな状態）ではＮＯｘを吸収する吸蔵還元型ＮＯｘ触媒や、排気の一部を吸気系に還流させ燃焼ガスの不活性ガス量を調整する排気還流（ＥＧＲ：ExhaustGasRecirculation）装置等を搭載するものが広く知られるようになった。吸蔵還元型触媒やＥＧＲ装置の機能を十分活用するためには、当該機関の排気系に設けられ排気中の酸素濃度を逐次モニタする酸素濃度センサの役割が極めて重要となる。酸素濃度センサは、排気中の酸素濃度に応じた検出信号を出力するので、その出力値に基づき排気中の正確な酸素濃度を把握するには、正確な酸素濃度がわかっている基準ガスと、酸素濃度センサの出力値との間で周期的に校正を行うことが望ましい。
【０００３】
例えば特開平１０−２１２９９９号公報に記載されたディーゼルエンジンは、燃料カットの実施中にスロットル弁を開くことで、吸気系に導入した空気を、燃料と混合させたり機関燃焼に供したりすることなく、燃焼室を介しそのまま排気系内に取り入れ、その際、排気系に設けられた酸素濃度センサが出力する検出信号を大気（基準ガス）の酸素濃度に対応する出力値として記憶することにより、排気中の酸素濃度検出にかかる検量を行うシステムを備える。燃料カットの実施中に（機関燃焼が行われない条件下において）スロットル弁を開けば、排気系内でガス交換が生じるため、酸素濃度センサの検出素子を吸入空気（大気）に直接晒すことができ、大気（基準ガス）の酸素濃度に対応する酸素濃度センサの出力値を正確に把握することができる。
【０００４】
【発明が解決しようとする課題】
ところが、同公報に記載されたシステムでは、スロットル弁の開放に伴う排気系内の変動や、温度低下が著しく、酸素濃度センサの出力値として、一定の条件下における大気中の酸素濃度と正確対応する情報を取得するのが困難であった。
【０００５】
本発明は、このような実情に鑑みてなされたものであって、その目的とするところは、ディーゼルエンジンの排気系に設けられた酸素濃度センサの出力値に基づき、排気中の酸素濃度として常時信頼性の高い値を取得することのできる酸素濃度検出装置を提供することにある。
【０００６】
また、他の目的は、ディーゼルエンジンの排気系に設けられた酸素濃度センサの出力値に基づき、排気中の酸素濃度として常時信頼性の高い値を取得することのできる酸素濃度の検出方法を提供することにある。
【０００７】
【課題を解決するための手段】
上記目的を達成するために、本発明は、ディーゼルエンジンの排気系に設けられ排気中の酸素濃度に応じた信号を出力する酸素濃度センサと、当該エンジンの燃焼室に吸入される空気の流量を調整する流量調整弁と、当該エンジンの運転中、所定条件下で燃料供給を停止させる制御を行う燃料カット制御手段と、前記燃料供給の停止と同期し前記吸入される空気の流量を増大させる吸入空気増量制御手段と、前記燃焼室に吸入される空気の積算量を認識する積算吸入空気量認識手段と、前記認識される積算値が所定値を上回ったところで前記吸入される空気の流量を減じる吸入空気減量制御手段と、前記吸入される空気の流量が減じられた後、前記酸素濃度センサの信号が所定値に収束したところで、前記燃料供給の停止期間中に前記酸素濃度センサが出力した信号に関する数値情報を、基準となる酸素濃度に対応する数値情報として学習する学習手段と、を備えることを要旨とする。
【０００８】
また、当該エンジンが、その排気系から吸気系に排気の一部を還流させる排気還流通路と、該排気還流通路を通じて還流される排気の量を調整する排気還流量調整手段を備えている場合、前記排気還流量調整手段は、前記燃料供給の停止、若しくは前記吸入される空気の流量の増大と同期して、前記還流される排気の量を低減、好ましくは遮断するのがよい。
【０００９】
当該エンジンの運転中、燃料供給が停止している条件下で前記吸入される空気の流量を増大させると、前記排気系内に残留している排気ガスと、吸気系から燃焼室を通じて導入される新気との間でガス交換が行われる。このようなガス交換が行われた後、前記吸入される空気の流量を速やかに低減すれば、排気系内の圧力変動や温度低下を十分に抑制しつつ、酸素濃度が既知である大気を排気系内に滞留させることができる。すなわち同構成によれば、当該エンジンの運転中、燃料供給が停止した場合、圧力や温度の安定を保証しつつ、酸素濃度が既知である大気を速やかに排気系内に充填し、この充填された大気を基準ガスとして、酸素濃度センサの検出信号と排気中の酸素濃度との対応関係を正確に補正・学習することができる。よって、酸素濃度センサによる排気中酸素濃度の検出精度を高めることができる。またこのとき、上記学習の実施に伴う排気系内の圧力変動や温度低下は極めて微小なものとなる。しかも、前記排気系内への大気導入が速やかに行われることから、上記学習の実施機会も拡大されることになる。
【００１０】
また、当該エンジンの排気系に設けられ、排気中の有害成分を浄化する排気浄化用触媒と、前記燃料供給を停止させる制御が行われる際、前記触媒の温度が所定値を下回っている場合には前記学習を禁止する学習禁止手段と、を備えるのが好ましい。
【００１１】
同構成によれば、例えば、上記学習に伴う排気系内の温度低下が確実に回避されるため、排気浄化用触媒の活性状態を低下させるような懸念が生じなくなる。
【００１２】
また、上記構成を有する酸素濃度検出装置は、前記燃料供給の停止と同期して前記吸気流量が増大している際に、燃料供給が復帰した場合、燃料供給量の徐変処理を行う徐変処理手段を備えるのが好ましい。
【００１３】
前記吸入される空気の流量が増大されている条件下で当該エンジンへの燃料供給が復帰すると機関トルクが急変する懸念があるものの、同構成によれば、そのような機関トルクの急変が抑制され、常時ドライバビリティの安定性が確保される。
【００１４】
また、上記構成を有する酸素濃度検出装置は、前記排気系に還元成分を供給する還元成分供給手段と、前記燃料供給を停止させる制御の開始時若しくはその直前に前記排気系への還元成分の供給が行われた場合、前記学習を禁止する学習禁止手段と、を備えるのが好ましい。
【００１５】
また、上記構成を有する酸素濃度検出装置は、前記排気系に還元成分を供給する還元成分供給手段と、前記燃料供給を停止させる制御の開始時から前記学習を終了時までの期間中には、前記排気系への還元成分の供給を禁止する還元成分供給禁止手段と、を備えるのが好ましい。
【００１６】
同構成によれば、基準ガスとしての大気を排気系内に充填する際、還元成分が混入することがなくなるため、上記学習の精度及び信頼性が一層高まる。
【００１７】
また、他の発明は、ディーゼルエンジンの排気系に設けられ排気中の酸素濃度に応じた信号を出力する酸素濃度センサの出力値に基づいて、当該エンジンの排気中酸素濃度を検出する酸素濃度の検出方法であって、当該エンジンの運転中における燃料供給の停止に伴い、当該エンジンの燃焼室に吸入される空気量を増量し、前記燃料供給の停止中、当該エンジンの燃焼室に吸入される空気量の積算値が所定値を上回ったところで当該エンジンの燃焼室に吸入される空気量を減じ、前記酸素濃度センサの出力が所定値に収束したところで、その出力に関する数値情報を基準となる酸素濃度に対応する数値情報として学習し、前記学習した数値情報と前記基準となる酸素濃度との関係を参照し、前記酸素濃度センサの出力値に基づいて当該エンジンの排気中酸素濃度を検出することを要旨とする。
【００１８】
同構成によれば、当該エンジンの運転中、燃料供給が停止した場合、圧力や温度の安定を保証しつつ、酸素濃度が既知である大気を速やかに排気系内に充填し、この充填された大気を基準ガスとして、酸素濃度センサの検出信号と排気中の酸素濃度との対応関係を正確に補正・学習することができる。よって、酸素濃度センサによる排気中酸素濃度の検出精度を高めることができる。またこのとき、上記学習の実施に伴う排気系内の圧力変動や温度低下は極めて微小なものとなる。しかも、前記排気系内への大気導入が速やかに行われることから、上記学習の実施機会も拡大されることになる。
【００１９】
なお、本発明にかかる上記構成を、ガソリンエンジンやガソリン希薄燃焼エンジンに対して適用することにより、上記構成に準じた作用効果を奏することができる。
【００２０】
【発明の実施の形態】
（第１の実施の形態）
以下、本発明の酸素濃度検出装置及び酸素濃度検出方法を、ディーゼルエンジンシステムに適用した第１の実施の形態について説明する。
【００２１】
〔エンジンシステムの構造及び機能〕
図１において、内燃機関（以下、エンジンという）１は、燃料供給系１０、燃焼室２０、吸気系３０及び排気系４０等を主要部として構成される直列４気筒のディーゼルエンジンシステムである。
【００２２】
先ず、燃料供給系１０は、サプライポンプ１１、コモンレール１２、燃料噴射弁１３、遮断弁１４、調量弁１６、燃料添加弁１７、機関燃料通路Ｐ１及び添加燃料通路Ｐ２等を備えて構成される。
【００２３】
サプライポンプ１１は、燃料タンク（図示略）から汲み上げた燃料を高圧にし、機関燃料通路Ｐ１を介してコモンレール１２に供給する。コモンレール１２は、サプライポンプ１１から供給された高圧燃料を所定圧力に保持（蓄圧）する蓄圧室としての機能を有し、この蓄圧した燃料を各燃料噴射弁１３に分配する。燃料噴射弁１３は、その内部に電磁ソレノイド（図示略）を備えた電磁弁であり、適宜開弁して燃焼室２０内に燃料を噴射供給する。
【００２４】
他方、サプライポンプ１１は、燃料タンクから汲み上げた燃料の一部を添加燃料通路Ｐ２を介して燃料添加弁１７に供給する。添加燃料通路Ｐ２には、サプライポンプ１１から燃料添加弁１７に向かって遮断弁１４及び調量弁１６が順次配設されている。遮断弁１４は、緊急時において添加燃料通路Ｐ２を遮断し、燃料供給を停止する。調量弁１６は、燃料添加弁１７に供給する燃料の圧力（燃圧）ＰＧを制御する。燃料添加弁１７は、燃料噴射弁１３と同じくその内部に電磁ソレノイド（図示略）を備えた電磁弁であり、還元剤として機能する燃料を、適宜の量、適宜のタイミングで排気系４０のＮＯｘ触媒ケーシング４２上流に添加供給する。
【００２５】
吸気系３０は、各燃焼室２０内に供給される吸入空気の通路（吸気通路）を形成する。一方、排気系４０は、各燃焼室２０から排出される排気ガスの通路（排気通路）を形成する。
【００２６】
また、このエンジン１には、周知の過給機（ターボチャージャ）５０が設けられている。ターボチャージャ５０は、シャフト５１を介して連結された回転体５２，５３を備える。一方の回転体（タービンホイール）５２は排気系４０内の排気に晒され、他方の回転体（コンプレッサホイール）５３は、吸気系３０内の吸気に晒される。このような構成を有するターボチャージャ５０は、タービンホイール５２が受ける排気流（排気圧）を利用してコンプレッサホイール５３を回転させ、吸気圧を高めるといったいわゆる過給を行う。
【００２７】
吸気系３０において、ターボチャージャ５０に設けられたインタークーラ３１は、過給によって昇温した吸入空気を強制冷却する。インタークーラ３１よりもさらに下流に設けられたスロットル弁３２は、その開度を無段階に調節することのできる電子制御式の開閉弁であり、所定の条件下において吸入空気の流路面積を変更し、同吸入空気の供給量（流量）を調整する機能を有する。
【００２８】
また、エンジン１には、吸気系３０と排気系４０とを連通する排気還流通路（ＥＧＲ通路）６０が形成されている。このＥＧＲ通路６０は、排気の一部を適宜吸気系３０に戻す機能を有する。ＥＧＲ通路６０には、電子制御によって無段階に開閉され、同通路を流れる排気（ＥＧＲガス）の流量を自在に調整することができるＥＧＲ弁６１と、ＥＧＲ通路６０を通過（還流）する排気を冷却するためのＥＧＲクーラ６２が設けられている。
【００２９】
また、排気系４０において、同排気系４０及びＥＧＲ通路６０の連絡部位の下流には、吸蔵還元型ＮＯｘ触媒及びパティキュレートフィルタを収容したＮＯｘ触媒ケーシング４２が設けられている。また、排気系４０のＮＯｘ触媒ケーシング下流には、酸化触媒を収容した酸化触媒ケーシング４３が設けられている。
【００３０】
また、エンジン１の各部位には、各種センサが取り付けられており、当該部位の環境条件や、エンジン１の運転状態に関する信号を出力する。
【００３１】
すなわち、レール圧センサ７０は、コモンレール１２内に蓄えられている燃料の圧力に応じた検出信号を出力する。燃圧センサ７１は、添加燃料通路Ｐ２内を流通する燃料のうち、調量弁１６を介して燃料添加弁１７に導入される燃料の圧力（燃圧）ＰＧに応じた検出信号を出力する。エアフロメータ７２は、吸気系３０内のコンプレッサホイール５３上流において吸入空気の流量（吸気量）ＧＡに応じた検出信号を出力する。酸素濃度センサ７３は、排気系４０のＮＯｘ触媒ケーシング４２下流（酸化触媒ケーシング４３上流）において排気中の酸素濃度に応じて連続的に変化する検出信号を出力する。酸素濃度センサ７３の検出信号は、エンジン１の機関燃焼に供される混合気中の空燃比Ａ／Ｆを演算するためのパラメータとして活用される。排気温度センサ７４は、排気系４０においてＮＯｘ触媒ケーシング４２内の所定部位（後述するハニカム構造体４２ａとパティキュレートフィルタ４２ｂとの間）に取り付けられ、当該部位における排気温度（フィルタ入りガス温度）に応じた検出信号を出力する。
【００３２】
また、アクセルポジションセンサ７６はアクセルペダル（図示略）に取り付けられ、同ペダルの踏み込み量ＡＣＣに応じた検出信号を出力する。クランク角センサ７７は、エンジン１の出力軸（クランクシャフト）が一定角度回転する毎に検出信号（パルス）を出力する。これら各センサ７０〜７７は、電子制御装置（ＥＣＵ）９０と電気的に接続されている。
【００３３】
ＥＣＵ９０は、中央処理装置（ＣＰＵ）９１、読み出し専用メモリ（ＲＯＭ）９２、ランダムアクセスメモリ（ＲＡＭ）９３及びバックアップＲＡＭ９４、タイマーカウンタ９５等を備え、これら各部９１〜９５と、Ａ／Ｄ変換器を含む外部入力回路９６と、外部出力回路９７とが双方向性バス９８により接続されて構成される論理演算回路を備える。
【００３４】
このように構成されたＥＣＵ９０は、上記各種センサの検出信号の処理、例えば酸素濃度センサ７３の検出信号に基づいて機関燃焼に供される混合気中の空燃比Ａ／Ｆを算出するといった演算処理等を行う他、これら各種センサの検出信号等に基づき、燃料噴射弁１３の開閉弁動作に関する制御や、ＥＧＲ弁６１の開度調整、或いはスロットル弁３２の開度調整等、エンジン１の運転状態に関する各種制御を実施する。
【００３５】
また、上記のように構成されたＥＣＵ９０、酸素濃度センサ７３、スロットル弁３２等は、本実施の形態にかかる酸素濃度検出装置を構成する。
【００３６】
〔ＮＯｘ触媒ケーシングの構造及び機能〕
次に、以上説明したエンジン１の構成要素のうち、排気系４０に設けられたＮＯｘ触媒ケーシング４２について、その構造及び機能を詳しく説明する。
【００３７】
ＮＯｘ触媒ケーシング４２の内部には、アルミナ（Ａｌ2Ｏ3）を主成分とするストレートフロー型のハニカム構造体４２ａと、多孔質材料を主成分とするウォールフロー型のパティキュレートフィルタ（以下、単にフィルタという）４２ｂとが、各々排気浄化用触媒として、所定の間隔をあけて直列に配置されている。
【００３８】
ハニカム構造体４２ａを形成する複数の通路には、例えばアルミナからなる担体の層が形成されており、その担体層の表面にＮＯｘ吸蔵剤として機能する例えばカリウム（Ｋ）、ナトリウム（Ｎａ）、リチウム（Ｌｉ）、セシウム（Ｃｓ）のようなアルカリ金属、バリウムＢａ、カルシウムＣａのようなアルカリ土類、ランタン（Ｌａ）、或いはイットリウム（Ｙ）のような希土類と、酸化触媒（貴金属触媒）として機能する例えば白金（Ｐｔ）のような貴金属とが担持されている。なお、担体（ここではアルミナからなる担体層が形成されたハニカム構造体）４２ａ上に混在するよう担持されたこれらＮＯｘ吸蔵剤及び貴金属触媒は、併せてＮＯｘ触媒（吸蔵還元型ＮＯｘ触媒）を構成する。
【００３９】
ＮＯｘ吸蔵剤は、排気中の酸素濃度が高い状態ではＮＯｘを吸蔵し、排気中の酸素濃度が低い状態（還元成分の濃度が高い状態）ではＮＯｘを放出する特性を有する。また、排気中にＮＯｘが放出されたとき、排気中にＨＣやＣＯ等が存在していれば、貴金属触媒がこれらＨＣやＣＯの酸化反応を促すことで、ＮＯｘを酸化成分、ＨＣやＣＯを還元成分とする酸化還元反応が両者間で起こる。すなわち、ＨＣやＣＯはＣＯ2やＨ2Ｏに酸化され、ＮＯｘはＮ2に還元される。
【００４０】
一方、ＮＯｘ吸蔵剤は排気中の酸素濃度が高い状態にあるときでも所定の限界量のＮＯｘを吸蔵すると、それ以上ＮＯｘを吸蔵しなくなる。エンジン１では、ポスト噴射や燃料添加を通じて排気通路のＮＯｘ触媒ケーシング４２上流に断続的に還元成分が供給され、排気中の還元成分の濃度が高まる。ＮＯｘ触媒（ＮＯｘ吸蔵剤）のＮＯｘ吸蔵量が限界量に達する前に、この還元成分がＮＯｘ触媒に吸蔵されたＮＯｘを周期的に放出および還元浄化することになり、ＮＯｘ吸蔵剤のＮＯｘ吸蔵能力を回復させることになる。
【００４１】
一方、フィルタ４２ｂを形成する多孔質材料は、例えばコージライト等のセラミック材料にアルミナ、チタニア、ジルコニア若しくはゼオライト等のコート材をウォッシュコートしたものであり、排気を透過する性質を有する。また、フィルタ４２ｂは、互いに平行をなして延びる上流端が開放され下流端が閉ざされた排気流入通路と、上流端が閉ざされ下流端が開放された排気流出通路とを備えるいわゆるウォールフロー型である。そして、両排気通路間に位置する隔壁の表面及び内部に形成された細孔内に、表面に上記ＮＯｘ吸蔵剤と貴金属触媒とを担持するアルミナ等のコート層（担体層）が形成されている。
【００４２】
このような構造を有するフィルタ４２ｂは、排気中に含まれる煤等の微粒子やＮＯｘ等の有害成分を、以下のメカニズムに基づいて浄化する。
【００４３】
ＮＯｘ吸蔵剤が、貴金属触媒との協働により、排気中の酸素濃度や還元成分量に応じてＮＯｘの吸蔵、放出及び浄化を繰り返し行うことは上述した通りである。その一方、ＮＯｘ吸蔵剤は、このようなＮＯｘの浄化を行う過程で、副次的に活性酸素を生成する特性を有する。フィルタ４２ｂを排気が透過過する際、その排気中に含まれる煤等の微粒子は構造体（多孔質材料）に捕捉される。ここで、ＮＯｘ吸蔵剤の生成する活性酸素は、酸化剤として極めて高い反応性（活性）を有しているため、捕捉された微粒子のうちＮＯｘ触媒の表面や近傍に堆積した微粒子は、この活性酸素と（輝炎を発することなく）速やかに反応し、浄化されることになる。
【００４４】
また、ＮＯｘ触媒ケーシング４２内の上流側に配置されたハニカム構造体（同構造体に担持されたＮＯｘ触媒）４２ａから発生する反応熱は、下流側に配置されたフィルタ４２ｂを効率的に昇温し、当該フィルタ４２ｂによる微粒子の分解作用を高めることになる。
【００４５】
〔燃料噴射制御の概要〕
ＥＣＵ９０は、各種センサの検出信号から把握されるエンジン１の運転条件に基づき燃料噴射制御を実施する。本実施の形態において燃料噴射制御とは、各燃料噴射弁１３を通じた各燃焼室２０内への燃料噴射の実施に関し、燃料の噴射量Ｑ、噴射タイミング、噴射パターンといったパラメータを設定し、これら設定されたパラメータに基づいて個々の燃料噴射弁１３の開閉弁操作を実行する一連の処理をいう。
【００４６】
ＥＣＵ９０は、このような一連の処理を、エンジン１の運転中所定時間毎に繰り返し行う。燃料の噴射量Ｑ及び噴射タイミングは、基本的にはアクセルペダルの踏み込み量ＡＣＣおよびエンジン回転数ＮＥ（クランク角センサのパルス信号に基づいて演算することができるパラメータ）に基づき、予め設定されたマップ（図示略）を参照して決定する。
【００４７】
また、燃料の噴射パターンの設定に関し、ＥＣＵ９０は、圧縮上死点近傍での燃料噴射を主噴射として各気筒について行うことで機関出力を得る他、主噴射に先立つ燃料噴射（以下、パイロット噴射という）や、主噴射に後続する燃料噴射（以下、ポスト噴射という）を、副噴射として適宜選択された時期、選択された気筒について行う。
【００４８】
〔パイロット噴射〕
ディーゼルエンジンでは一般に、圧縮行程終期において、燃焼室内が燃料の自己着火を誘発する温度に達する。とくにエンジンの運転条件が中高負荷領域にある場合、燃焼に供される燃料が燃焼室内に一括して噴射供給されると、この燃料は騒音を伴い爆発的に燃焼する。パイロット噴射を実行することにより、主噴射に先立って供給された燃料が熱源（或いは種火）となり、その熱源が燃焼室内で徐々に拡大して燃焼に至るようになるため、燃焼室内における燃料の燃焼状態が比較的緩慢となり、しかも着火遅れ時間が短縮されるようになる。このため、機関運転に伴う騒音が軽減され、さらには排気中のＮＯｘ量も低減される。
【００４９】
〔ポスト噴射〕
ポスト噴射によって燃焼室２０内に供給される燃料は、燃焼ガス中で軽質なＨＣに改質され、排気系４０に排出される。すなわち、還元剤として機能する軽質なＨＣが、ポスト噴射を通じて排気系４０に添加され、排気中の還元成分濃度を高めることとなる。排気系４０に添加された還元成分は、ＮＯｘ触媒ケーシング４２内のＮＯｘ触媒を介し、同ＮＯｘ触媒から放出されるＮＯｘや、排気中に含まれるその他の酸化成分と反応する。このとき発生する反応熱は、排気やＮＯｘ触媒の温度を上昇させる。ポスト噴射では、燃焼室内へ直接燃料を噴射供給し、しかもその燃料を機関燃焼に関与させないといった条件が要求される。このため、一度に供給できる燃料量には制約があり、所定の昇温効果を得るためには、通常、各燃料噴射弁１３を通じて複数回連続実施する必要がある。しかし、燃焼ガス中で軽質化された燃料は、反応性も高く、例えばアイドル時のように排気の温度が相当に低い条件下であっても、排気やＮＯｘ触媒に対して高い昇温機能を発揮する。すなわち、その活用機会は広い運転領域に及ぶ。
【００５０】
〔燃料添加〕
燃料添加弁１７を通じ、噴霧状態の燃料（還元剤）を排気系４０に直接添加することによっても、ポスト噴射と同様、排気中の還元成分濃度を高め、結果として排気やＮＯｘ触媒の温度を上昇させることができる。燃料添加弁１７によって添加された燃料は、ポスト噴射によるものに比べ、排気中においてより高分子の状態を保持しつつ不均一に分布する傾向がある。また、燃料添加弁１７による燃料添加では、一度に添加することのできる燃料量や添加タイミングの自由度が、ポスト噴射による場合よりも大きい。しかし、燃料添加を通じて供給される噴霧状態の燃料は、排気が予めある程度暖まっていなければ、排気通路の内壁に付着して効率的な昇温機能を発揮することができない。このため、その活用機会は概ね中高負荷領域に限られる。
【００５１】
〔ＳＯｘ被毒回復制御〕
またエンジン１では、機関運転の継続に伴って徐々にハニカム構造体４２ａやフィルタ４２ｂに堆積するＳＯｘ等を除去するために、これらＳＯｘ等を熱分解することができる程度にまでＮＯｘ触媒を昇温する制御（ＳＯｘ被毒回復制御）を、所定周期で実施する。ＳＯｘ被毒回復制御では、上記ポスト噴射および燃料添加の何れか、或いは両者を、比較的長期に亘り連続的に実施する。
【００５２】
〔燃料カット〕
ＥＣＵ９０は、例えば減速時等、特定の運転条件下において燃料カットを実行する。燃料カットとは、運転制御の一環であり、エンジン回転数ＮＥが、エンジン１の運転状態に応じて予め設定される規定数を上回った場合等に、燃焼室１１への燃料供給（燃料噴射）を一時的に停止することによってエンジン１への負担軽減、ＮＯｘ触媒や酸化触媒の加熱防止、或いは燃費向上等を図る制御をいう。燃料カットの実施期間中、エンジン１は失火し、機関燃焼が停止することになる。
【００５３】
〔排気中酸素濃度の検出方法〕
図２（ａ）には、排気系４０に設けられた酸素濃度センサ７３の検出素子主要部の断面構造を示す。
【００５４】
同図２（ａ）に示すように、酸素濃度センサ７３の検出素子は、ジルコニア（Ｚｒ2Ｏ3）等、酸素イオン伝導性及び耐熱性を有する多孔質絶縁材料（板材）７３ａ，７３ｂ，７３ｃを積層して形成される。これら板材７３ａ，７３ｂ，７３ｃの積層体内部には、大気と連通する大気導入空間Ｓ１が形成されている。また、板材７３ａの両面、すなわち検出素子の外部空間（排気通路内の空間）Ｓ２に臨む板面と、検出素子の内部に形成された大気導入空間Ｓ１に臨む板面とには、電極７３ｄ，７３ｅが取り付けられている。また、板材７３ｃには、図示しない電熱式ヒータが内蔵され、検出素子の温度を所定値に保持する。両電極７３ｄ，７３ｅの間に所定電圧が印加されると、両電極７３ｄ，７３ｅ近傍に存在する酸素分子がイオン化され、矢指α方向に沿って板材７３ａを透過する。このとき両電極７３ｄ，７３ｅ間を流れる電流値は、空間Ｓ１及び空間Ｓ２における酸素濃度の差と、定量的に関連する。また、空間Ｓ１内の酸素濃度は、大気中の酸素濃度（例えば２１％）として既知であることから、両電極７３ｄ，７３ｅ間に所定電圧を印加したときに両電極７３ｄ，７３ｅ間を流れる電流値を観測することにより、空間Ｓ２内の酸素濃度（排気中の酸素濃度）を把握することができる。
【００５５】
例えば図３は、電極７３ｄ，７３ｅ間の印加電圧及び電流の関係を示すグラフである。なお、同グラフ中には、空間Ｓ２内（検出対象ガス中）の酸素濃度（酸素過剰率）が異なる複数の条件（酸素過剰率λ＝ａ，ｂ，ｃ，ｄ：ただし、ａ＜ｂ＜ｃ＜ｄ）に対応する印加電圧及び電流間の関係が示されている。
【００５６】
同図３に示すように、印加電圧が高くなるほど電流値も高くなる傾向があるものの、印加電圧が特定範囲にある場合には、電流値がほとんど変化しないことがわかる。このような特定範囲における電流値Ｉ１，Ｉ２，Ｉ３，Ｉ４を、酸素過剰率λ＝ａ，ｂ，ｃ，ｄに対応する限界電流値という。そこで、電極７３ｄ，７３ｅ間を流れる電流値をこのような限界電流値に保持させるための印加電圧（例えば図中に示す電圧値Ｖｘ）を適宜設定し、限界電流値を測定することにより、検出対象ガス中の酸素濃度（酸素過剰率）を定量的に把握することができる。
【００５７】
図４は、空間Ｓ２内の酸素濃度と限界電流値との対応関係を概略的に示すグラフである。同図４に示すように、空間Ｓ２内の酸素濃度（排気中の酸素濃度）が高くなるほど限界電流値も高くなる。そこで、同図４中に示す点Ａ、点Ｂのように、排気中の酸素濃度と限界電流値との対応関係が明らかな２座標を予め決定しておけば、当該２座標を結ぶ線（特性線）を基に、酸素濃度センサ７３の検出信号（限界電流値）から、排気中の酸素濃度を定量することができる
本実施の形態にかかる酸素濃度検出装置は、排気中の酸素濃度と限界電流値との対応関係を決定づけるための検量線の設定に関し、以下の修正を加えることにより、その精度と信頼性を高めている。
【００５８】
図５は、ＥＣＵ９０によって学習及び記憶される排気中の酸素濃度と限界電流値との対応関係を詳細に示すグラフである。
【００５９】
ＥＣＵ９０は、排気中の酸素濃度と限界電流値との対応関係を示す特性線として、直線Ａ−Ｂを適宜のタイミングで学習・記憶する。ここで、電流値ＩＡは、排気中の酸素濃度が０％である条件下での限界電流値に相当し、電流値ＩＢは、酸素濃度が２１％である条件下での限界電流値ＩＢに相当する。排気中の酸素濃度をモニタするに際しては、２座標Ａ，Ｂを結ぶ直線Ａ−Ｂ（特性線）を参照することになる。
【００６０】
ここで、排気中の酸素濃度が０％である条件下（機関燃焼に供される混合気の空燃比が理論空燃比（ストイキ）である条件下）では、センサ素子の個体差や経年変化等に依らず、限界電流値は極めて微弱になる（理論的には「０」となる）。一方、排気中の酸素濃度が２１％である条件下（センサ素子を大気に晒した状態）では、限界電流値（絶対値）は比較的大きな値を示し、その値はセンサ素子の個体差や経年変化等によって変動する（ばらつく）。そこでＥＣＵ９０は、適宜、酸素濃度センサ７３のセンサ素子を大気に晒した状態（これと同等の条件下）で限界電流値を測定し、旧値（例えば電流値ＩＢ′）から新値（例えば電流値ＩＢ）への補正（以下、大気補正という）を行う。
【００６１】
なお、排気中の酸素濃度が０％である条件下において限界電流値は理論的には「０」となる。しかし実際には、ＥＣＵ９０と酸素濃度センサ７３との間に介在する回路等の存在に依り、微弱な限界電流値が検出される。ＥＣＵ９０は、この微弱な限界電流値を定量するために、酸素濃度センサ７３のセンサ素子自体が全く検出信号を出力しない条件下（例えば、機関始動直後等、酸素濃度センサ７３が不活性な状態（低温状態）にある条件下）において、その限界電流値ＩＡ″を検出し、基準値ＩＡ（「０」アンペア）との偏差（以下、回路オフセットいう）ＯＳを認識しておく。そして、上記大気補正を実行する場合も含め、酸素濃度センサ７３の検出信号に基づき排気中の酸素濃度を定量する際には、常時この回路オフセットＯＳを考量することで、酸素濃度検出の精度をさらに高める。
【００６２】
〔酸素濃度検出及びこれに伴う運転制御の実行タイミング〕
図６（ａ）〜（ｅ）は、酸素濃度センサ７３の大気補正が実施される際、エンジン１の運転状態に関連する各種パラメータがどのように推移するのか、各パラメータの推移を同一時間軸上に示すタイムチャートである。なお、各図中において、時刻ｔ１に燃料カットが開始されるものとする。
【００６３】
図６（ａ）には、スロットル弁３２の開度の推移を示す。同図６（ａ）に示すように、ＥＣＵ９０は、時刻ｔ１における燃料カットの開始に伴いスロットル弁３２の開度を所定開度まで徐変減量する（時刻ｔ２）。次に、所定期間（時刻ｔ２〜ｔ３）スロットル弁３２を略全開状態（所定の吸気量を確保するために十分大きな開度）に保持することにより、排気系４０に新気を導入する。その後、スロットル弁３２を全閉にして、燃料カットの実施が継続する限りこの全閉状態を保持する。
【００６４】
図６（ｂ）には、ＥＣＵ９０によって演算される特定時刻を始点として吸気量ＧＡを積算した値の推移を示す。同図６（ｂ）に示すように、ＥＣＵ９０は、スロットル弁３２が全開状態に移行した時点（ｔ２）から吸気量ＧＡの積算を開始する。そして、吸気量ＧＡの積算量が所定値Ｆに達したところで（時刻ｔ３）、スロットル弁３２の開放によって導入された新気が排気系４０内の残留排気ガスと完全に交換されたものと認識し、スロットル弁３２を全閉状態にする（図６（ａ）を併せ参照）。
【００６５】
図６（ｃ）には、排気系４０内の圧力の推移を示す。同図６（ｃ）に示すように、時刻ｔ１〜ｔ３にかけ、スロットル弁３２の開閉弁動作に伴い排気系４０内の圧力は変動する。しかしながら、時刻ｔ３においてスロットル弁３２が全閉状態に移行した後は、燃料カットの実施が継続する限り、大気圧と略同等の値を安定して保持するようになる。
【００６６】
図６（ｄ）には、排気温度センサ７４の検出信号に基づいて推定されるＮＯｘ触媒床温の推移を示す。同図６（ｄ）に示すように、時刻ｔ１〜ｔ３に亘る期間中、排気系４０内に新気が導入されることにより、ＮＯｘ触媒の床温は徐々に低下することになる。しかしながら、時刻ｔ３においてスロットル弁３２が全閉状態に移行すると排気系４０内への新気の導入が停止するため、時刻ｔ３以降、ＮＯｘ触媒の床温低下は抑制される。なお、時刻ｔ１〜ｔ３におけるスロットル弁３２の開度は、効率的な新気導入のために十分大きければよく、必ずしも全開状態にする必要はない。例えば、スロット弁３２開度の増大に応じて増大する吸気量（新気の導入効率）には上限値が存在するため、所定開度（例えば全開状態の９０％）を上回る開度設定を行っても新気の導入効率が変わらない場合がある。また、新気導入を効率的に行いつつ、機関トルクの安定性を保持し、排気特性も良好に維持する等といった観点から、時刻ｔ１〜ｔ３におけるスロットル弁３２の最大開度を設定してもよい。また、通常の開度から最大開度に移行する過程や、最大開度から通常の開度に移行する過程でスロットル弁３２の開度を急変させないように、その開弁動作や閉弁動作に関し平均化処理（徐変処理）を施してもよい。
【００６７】
図６（ｅ）には、酸素濃度センサ７３の検出信号の推移を示す。同図６（ｅ）に示すように、酸素濃度センサ７３の検出信号（限界電流値）は燃料カットの開始に伴って徐々に高くなるが、スロットル弁３２が全開状態となる期間中（時刻ｔ２〜ｔ３）わずかづつ低下する傾向を示す。時刻ｔ２〜ｔ３に亘る酸素濃度センサ７３の出力の低下傾向は、スロットル弁３２が全開状態となったことにより、排気系内のガス圧が上昇するために発生する。その後、スロットル弁３２が全閉状態になると、吸気系３０から導入された新気が排気系４０内の残留排気ガスと完全に交換され、同排気系４０内において滞留している状態になる。このため、酸素濃度センサ７３は、大気中の酸素濃度（２１％）に対応する検出信号を安定して出力するようになる。本実施の形態では、スロットル弁３２を全閉状態に移行させた後（時刻ｔ４以降に）、燃料カットの実施が終了するまでの間、酸素濃度センサ７３の大気補正を継続実施する。
【００６８】
このように、本実施の形態にかかる酸素濃度検出装置は、燃料カットの実施期間中、先ずスロットル弁３２を全開状態にして吸気系３０内の新気を排気系内の残留排気ガスと交換し、その後スロットル弁３２を閉じて排気系４０内の圧力や温度を安定させた上で、大気補正を実施する。
【００６９】
〔大気補正の具体的な手順〕
以下、酸素濃度センサ７３の大気補正について、その具体的な処理手順を説明する。
【００７０】
図７は、ＥＣＵ９０を通じて実行される「酸素濃度センサの大気補正ルーチン」を示すフローチャートである。本ルーチンは、エンジン１の始動後、所定時間毎に繰り返し実行される。
【００７１】
本ルーチンに処理が移行すると、ＥＣＵ９０は先ずステップＳ１０１において、現時点が燃料カットの開始時に相当するか否かを判断する。そして、その判断が肯定であれば処理をステップＳ１０２に移行し、その判断が否定であれば本ルーチンを一旦抜ける。すなわち、ＥＣＵ９０は、燃料カットの開始時において本ルーチンに処理を移行した場合にのみ、ステップＳ１０２以降の処理（酸素濃度センサ７３の大気補正）を行うことになる。
【００７２】
ステップＳ１０２においては、スロットル弁３２を開弁し、この開弁状態を所定期間保持した上で、同弁３２を閉弁する。ここでＥＣＵ９０は、スロットル弁３２の開弁時以降、吸気量ＧＡを積算することにより、吸気系３０から排気系４０への新気の導入量を逐次推定する。そして、所定量の新気が排気系４０内に導入され、排気系４０内に残留していた排気ガスが完全に新気と交換されたものと判断した時点でスロットル弁３２を閉弁する（図６（ａ），（ｂ）を参照）。また、ＥＣＵ９０は、燃料カットの開始（スロットル弁３２の開弁）に併せてＥＧＲ弁６１を閉弁する。ＥＧＲ弁６１の閉弁状態は燃料カットの終了時（大気補正の終了時）まで保持する。
【００７３】
スロットル弁３２を閉弁した後、ＥＣＵ９０は酸素濃度センサ７３の出力が所定範囲内（概ね一定値）に収束するまで待機する。そして、酸素濃度センサ７３の出力が所定範囲に収束したものと判断した時点で、酸素濃度センサ７３が出力する限界電流値（回路オフセットＯＳを考量した値）を認識し（ステップＳ１０３）、この限界電流値を大気中の酸素濃度（２１％）に対応する検出信号として、学習・記憶する（ステップＳ１０４）。
【００７４】
ステップＳ１０４の処理を経た後、ＥＣＵ９０は本ルーチンを一旦抜ける。
【００７５】
本実施の形態にかかる酸素濃度検出装置は、このような手順に従って酸素濃度センサ７３の検出信号と排気中酸素濃度との対応関係を学習し、その学習結果に基づいて排気中の酸素濃度（機関燃焼に供される混合気中の空燃比）を取得する。
【００７６】
図８には、酸素濃度センサ７３の検出信号に基づいて演算（推定）される空燃比の推定精度と空燃比の真値との関係を示す。なお、同図において、横軸は空燃比Ａ／Ｆの真値（以下、ベース空燃比という）に相当し、縦軸は酸素濃度センサ７３の検出信号に基づいて推定される空燃比（以下、検出空燃比という）とベース空燃比との偏差ΔＡ／Ｆに相当する。
【００７７】
本実施の形態による大気補正を行わない場合、例えば破線Ｌ１或いは破線Ｍ１として示すように、酸素濃度センサ７３を構成する検出素子（ジルコニア素子）の特性として、ベース空燃比がストイキから離間するほど偏差ΔＡ／Ｆの絶対値は大きくなる傾向を示す。言い換えれば、ベース空燃比がストイキから離間するほど、ベース空燃比と検出空燃比とが乖離する傾向がある。これに対し、本実施の形態による大気補正を適宜実施すれば、実線Ｌ２或いは実線Ｍ２として示すように、ベース空燃比が変動しても、偏差ΔＡ／Ｆの絶対値は十分小さな値を保持するようになる。すなわち、広い酸素濃度範囲において検出空燃比の信頼性が高まる。
【００７８】
以上説明したように、本実施の形態によれば、エンジン１の運転中、燃料カットが実施される条件下で吸気量ＧＡを増大させ、排気系４０内に残留している排気ガスと、吸気系３０から燃焼室２０を通じて導入される新気との間で一旦ガス交換を行った後、吸気量ＧＡを速やかに低減する。このような一連の動作により、排気系４０内の圧力変動や温度低下を十分に抑制しつつ、酸素濃度が既知である大気を排気系内に速やかに充填する。そして、排気系４０内の圧力や温度の安定を確保しつつ、この充填された大気を基準ガスとして、酸素濃度センサ７３が出力する信号に関する数値情報（例えば限界電流値）を、基準ガスの酸素濃度に対応する数値情報として記憶することで、酸素濃度センサ７３の検出信号と排気中の酸素濃度との対応関係を正確に学習することが可能となる。
【００７９】
このような学習を繰り返すことにより、酸素濃度センサ７３は、その検出素子や検出回路の個体差（ばらつき）や経時劣化の進行度合いに依ることなく、高い検出精度や信頼性を長期に亘って維持することができる。
【００８０】
またこのとき、上記学習（大気補正）の実施に伴う排気系４０内の温度低下は極めて微小なものとなるため、排気浄化用触媒を活性状態に保持する上で排気系４０内の温度を十分高い値に保持することができる。しかも、燃料カットの開始と同期して吸気量ＧＡを増大させ、排気系４０内への大気導入を促進することとなるため、上記学習の実施機会も拡大される。すなわち、燃料カットの実施期間はエンジン１の運転中において限られた時間にすぎないところ、この限られた時間を有効に活用することができるようになる。
【００８１】
なお、本実施の形態では、燃料カットの開始と同期して、スロットル弁３２の開閉弁動作とＥＧＲ弁６１の閉弁動作とを併せて行うことにした。これに対し、ＥＧＲ弁６１は開弁状態にしたまま、スロットル弁３２の開閉弁動作のみを行っても、本実施の形態に準ずる効果を奏することができる。
【００８２】
また、スロットル弁３２は閉弁状態（或いは所定開度）に保持したまま、燃料カットの開始と同期してＥＧＲ弁６１を閉じ、所定期間が経過した後に大気補正を実施することとしても、本実施の形態に準ずる効果を奏することはできる。
【００８３】
（第２の実施の形態）
次に、本発明の酸素濃度検出装置及び酸素濃度検出方法を、ディーゼルエンジンシステムに適用した第２の実施の形態について、上記第１の実施の形態と異なる点を中心に説明する。なお、当該第２の実施の形態にあって、適用対象とするエンジンシステムや酸素濃度検出装置のハードウエア構成（図１〜図４）は、先の第１の実施の形態と同一である。このため、同一の機能および構造を有する部材やハードウエア構成等については同一の符号を用い、ここでの重複する説明は割愛することとする。
【００８４】
当該第２の実施の形態にかかる酸素濃度検出装置は、エンジン１の始動時等、酸素濃度センサ７３の回路オフセットＯＳを定量的に認識する点、また、燃料カットの実施期間中、先ずスロットル弁３２を全開状態にして吸気系３０内の新気を排気系内の残留排気ガスと交換し、その後スロットル弁３２を閉じて排気系４０内の圧力や温度を安定させた上で大気補正を実施する点においては第１の実施の形態と共通する。しかし、大気補正を実施タイミングと、燃料添加の実施タイミングとが近接しないようにすることで、大気補正の精度や信頼性をさらに高める制御を行う点で、第１の実施の形態とは異なる。
【００８５】
図９（ａ）及び図９（ｂ）は、燃料カットに伴う酸素濃度センサ７３の検出信号の推移を概略的に示すタイムチャートである。なお、両同図９（ａ），（ｂ）中において、時刻ｔ１１は燃料カットの開始時刻に相当する。
【００８６】
先ず図９（ａ）は、燃料添加を一切行わない場合の推移曲線（一点鎖線）と、燃料カットの開始時に燃料添加を行った場合の推移曲線（実線）とを同一時間軸上に示したものである。同図９（ａ）に示すように、燃料添加を一切行わない場合、燃料カットの開始に伴い酸素濃度センサ７３の検出信号は速やかに上昇し（リーン側に移行し）、大気中の酸素濃度に対応する値に達した時点（ｔ１２）で安定する。これに対し、燃料カットの開始に近接するタイミングで燃料添加を行うと、排気系４０内に燃料が滞留するため、排気系４０内における酸素濃度の上昇が遅れる。このため、酸素濃度センサ７３の検出信号が大気中の酸素濃度に対応する値に達する時刻（ｔ１３）も遅延するようになる。
【００８７】
そこで、本実施の形態にかかる酸素濃度検出装置は、燃料カットの開始後、排気系内の酸素濃度が速やかに大気中酸素濃度と同等になるように、燃料カットの開始時から所定期間（例えば図９（ｂ）における時刻ｔ１１〜ｔ１２）内には燃料添加の実施を禁止する制御を行う。また、大気補正の実施中には、燃料添加の実施を禁止するか、或いは燃料添加の実施に伴う酸素濃度センサの検出信号の変化量を推定し、この変化量を減殺する補正を行いつつ（例えば図９（ｂ）において示す仮想線ＭＳＫを逐次作成しつつ）、大気補正を継続する。
【００８８】
〔大気補正の具体的な手順〕
以下、酸素濃度センサ７３の大気補正について、その具体的な処理手順を説明する。
【００８９】
図１０は、ＥＣＵ９０を通じて実行される「酸素濃度センサの大気補正ルーチン」を示すフローチャートである。本ルーチンは、エンジン１の始動後、所定時間毎に繰り返し実行される。
【００９０】
本ルーチンに処理が移行すると、ＥＣＵ９０は先ずステップＳ２０１において、現時点が燃料カットの開始時に相当するか否かを判断する。そして、その判断が肯定であれば処理をステップＳ２０２に移行し、その判断が否定であれば処理をステップＳ２０７にジャンプする。
【００９１】
ステップＳ２０２においては、現在に至るまでの所定期間、燃料添加を実行した履歴があるか否かを判断する。そして、その判断が肯定であれば処理をステップＳ２０３に移行し、その判断が否定であれば処理をステップＳ２０７にジャンプする。すなわち、ＥＣＵ９０は、燃料カットの開始時において本ルーチンに処理を移行し、且つ、燃料カットの開始直前に燃料添加が行われていないといった条件が成立した場合に限り、ステップＳ２０３以降の処理（酸素濃度センサ７３の大気補正）を行うことになる。
【００９２】
ステップＳ２０３においてＥＣＵ９０は、燃料添加の実施を一時的に禁止する。
【００９３】
ステップＳ２０４においては、所定期間、スロットル弁３２を開弁し、この開弁状態を所定期間保持した上で、同弁３２を閉弁する。ここでＥＣＵ９０は、スロットル弁３２の開弁時以降、吸気量ＧＡを積算することにより、吸気系３０から排気系４０への新気の導入量を逐次推定する。そして、所定量の新気が排気系４０内に導入され、排気系４０内に残留していた排気ガスが完全に新気と交換されたものと判断した時点でスロットル弁３２を閉弁する（図６（ａ），（ｂ）を参照）。また、ＥＣＵ９０は、燃料カットの開始（スロットル弁３２の開弁）に併せてＥＧＲ弁６１を閉弁する。ＥＧＲ弁６１の閉弁状態は燃料カットの終了時（大気補正の終了時）まで保持する。
【００９４】
スロットル弁３２を閉弁した後、ＥＣＵ９０は酸素濃度センサ７３の出力が所定範囲内（概ね一定値）に収束するまで待機する。そして、酸素濃度センサ７３の出力が所定範囲に収束したものと判断した時点で、酸素濃度センサ７３が出力する限界電流値（回路オフセットＯＳを考量した値）を認識し（ステップＳ２０５）、この限界電流値を大気中の酸素濃度（２１％）に対応する検出信号として、学習・記憶する（ステップＳ２０６）。
【００９５】
続くステップＳ２０７においては、燃料添加の実施を許可する処理、言い換えれば燃料添加の禁止（ステップＳ２０３）を解除する処理を行う。
【００９６】
ステップＳ２０７の処理を経た後、ＥＣＵ９０は本ルーチンを一旦抜ける。
【００９７】
このような手順に従って酸素濃度センサ７３の大気学習を行う本実施の形態によれば、上記第１の実施の形態による効果に加え、大気補正の実施に伴う排気系４０内の温度低下を確実に回避し、排気浄化用触媒の活性状態を低下させないといった更なる効果を奏することができる。
【００９８】
なお、本実施の形態において、燃料カットの開始時やその直前に燃料添加が実施された場合には大気補正の実施を禁止することとしたように、排気中の酸素濃度に対し一時的な変動を生じせしめる他の制御（例えばパイロット噴射、ポスト噴射）が燃料カットの直前に実施された場合にも、大気補正の実施を禁止するといった制御構造を適用してもよい。
【００９９】
なお、上記各実施の形態において、スロットル弁３２やＥＧＲ弁６１の開閉弁動作に基づく排気系４０内のガス交換の進行速度や、排気系内における圧力変動や温度変動の度合いは、適用対象となる機関のハードウエア構成によっても異なる。このため、各弁３２，６１の最大開度や最小開度（％）、或いは開度変更の徐変（なまし）率等は、当該機関のハードウエア特性やその運転状況に応じて調整するのが好ましい。このとき、運転状態を反映する各パラメータの推移が、例えば図６（ａ）〜（ｅ）に示す各パラメータ間の関係が実現するように、エアフロメータ７２や排気温度センサ７４の検出信号を逐次フィードバックしながらスロットル弁３２やＥＧＲ弁６１の開度調整を行うのが好ましい。
【０１００】
またさらに、大気補正の精度向上と併せて、燃料カットの実施期間中、或いは実施前後に亘るドライバビリティを良好な状態に維持するといった観点から、スロットル弁３２やＥＧＲ弁６１の開閉弁動作や燃料噴射量に適宜修正を加えるのが好ましい。例えば、燃料カットの開始に伴いスロットル弁３２を開弁状態に保持している期間（図６中における期間ｔ２〜ｔ３）中、エンジン１の運転者がアクセルを踏んだ場合、その時点における要求トルクに見合った量の燃料を直ちに噴射供給するのではなく、燃料噴射量を徐々に増量するのが好ましい。また、排気系４０内におけるガス交換が終了しスロットル弁３２を閉弁する際には（図６中における時刻ｔ３）、スロットル弁３２開度の変更（例えば全閉状態への移行）を急激に行うのではなく、同弁３２を徐々に閉弁する制御を行うのが、いわゆるエンジンブレーキの発生を抑制する上で好ましい。
【０１０１】
またとくに、燃料カットを開始する時点で、ＮＯｘ触媒、パティキュレートフィルタ、或いは酸化触媒の温度が所定値を下回っていると推定されるような場合には、今回の燃料カットに伴う大気補正は行わない（禁止する）よう処理するのが、排気系４０に設けられた各種触媒の活性状態を好適に保持する上で好ましい。
【０１０２】
また、上記各実施の形態では、排気中酸素濃度と定量的に対応する限界電流値を出力する酸素濃度センサに本発明を適用することとしたが、これに限らず排気中酸素濃度と定量的に対応する検出信号を出力する他の酸素濃度センサに本発明を適用して上記各実施の形態と同等若しくはこれに準ずる効果を得ることもできる。また、排気中の酸素成分と関連のある他の排気成分を検出するためのセンサ（例えば窒素酸化物を検出するためのＮＯｘセンサ）に対しても、本発明を均等に適用することができる。
【０１０３】
【発明の効果】
以上説明したように、本発明によれば、エンジンの運転中、燃料供給が停止した場合、圧力や温度の安定を保証しつつ、酸素濃度が既知である大気を速やかに排気系内に充填し、この充填された大気を基準ガスとして、酸素濃度センサの検出信号と排気中の酸素濃度との対応関係を正確に学習することができる。よって、酸素濃度センサによる排気中酸素濃度の検出精度を高めることができる。またこのとき、上記学習の実施に伴う排気系内の圧力変動や温度低下は極めて微小なものとなる。しかも、前記排気系内への大気導入が速やかに行われることから、上記学習の実施機会も拡大されることになる。
【図面の簡単な説明】
【図１】 本発明の第１の実施の形態にかかるディーゼルエンジンシステムを示す概略構成図。
【図２】 同実施の形態において適用される酸素濃度センサの検出素子主要部の断面構造を示す略図。
【図３】 同実施の形態において適用される酸素濃度センサの二電極間の印加電圧及び電流の関係を示すグラフ。
【図４】 同実施の形態において適用される酸素濃度センサの限界電流値と酸素濃度との対応関係を概略的に示すグラフ。
【図５】 同実施の形態において、電子制御装置に記憶される排気中の酸素濃度と限界電流値との対応関係を詳細に示すグラフ。
【図６】 同実施の形態において、エンジンの運転状態に関連する各種パラメータについて、大気補正の実施に伴う推移を同一時間軸上に示すタイムチャート。
【図７】 同実施の形態において適用される酸素濃度センサの大気補正手順を示すフローチャート。
【図８】 酸素濃度センサの検出信号に基づいて推定される空燃比の推定精度と空燃比の真値との関係を示すグラフ。
【図９】 燃料カットに伴う酸素濃度センサの検出信号の推移を概略的に示すタイムチャート。
【図１０】 本発明の第２の実施の形態において適用される酸素濃度センサの大気補正手順を示すフローチャート。
【符号の説明】
１ エンジン（内燃機関）
１０ 燃料供給系
１１ サプライポンプ
１２ コモンレール
１３ 燃料噴射弁
１６ 調量弁
１７ 燃料添加弁
２０ 燃焼室
３０ 吸気系
３１ インタークーラ
３２ スロットル弁
４０ 排気系
４２ ＮＯｘ触媒ケーシング
４３ 酸化触媒ケーシング
５０ ターボチャージャ
５１ シャフト
５２ タービンホイール
５３ コンプレッサホイール
６０ ＥＧＲ通路
６１ ＥＧＲ弁
６２ ＥＧＲクーラ
７０ レール圧センサ
７１ 燃圧センサ
７２ エアフロメータ
７３ 酸素濃度センサ
７４ 排気温度センサ
７６ アクセルポジションセンサ
７７ クランク角センサ
９０ 電子制御装置（ＥＣＵ）
Ｐ１ 機関燃料通路
Ｐ２ 添加燃料通路[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an oxygen concentration detection device and a detection method for detecting the oxygen concentration in exhaust gas of a diesel engine.
[0002]
[Prior art]
As an internal combustion engine that operates an engine by using a high air-fuel ratio (lean atmosphere) mixture for combustion in a wide operating range, such as a diesel engine, in order to improve exhaust characteristics and stabilize the engine combustion state In a state where the oxygen concentration of the exhaust gas is high (lean state), the NOx storage reduction catalyst that absorbs NOx, or part of the exhaust gas is recirculated to the intake system for combustion Gas Those equipped with an exhaust gas recirculation (EGR) device for adjusting the amount of inert gas have become widely known. In order to fully utilize the functions of the storage reduction catalyst and the EGR device, the role of an oxygen concentration sensor provided in the exhaust system of the engine for sequentially monitoring the oxygen concentration in the exhaust gas is extremely important. Since the oxygen concentration sensor outputs a detection signal corresponding to the oxygen concentration in the exhaust gas, in order to grasp the accurate oxygen concentration in the exhaust gas based on the output value, a reference gas having a known accurate oxygen concentration, It is desirable to calibrate periodically with the output value of the oxygen concentration sensor.
[0003]
For example, in a diesel engine described in Japanese Patent Application Laid-Open No. 10-212999, the air introduced into the intake system is not mixed with fuel or subjected to engine combustion by opening a throttle valve during fuel cut. The exhaust gas is directly taken into the exhaust system through the combustion chamber, and the detection signal output from the oxygen concentration sensor provided in the exhaust system is stored as an output value corresponding to the oxygen concentration in the atmosphere (reference gas). The system which performs the calibration concerning the oxygen concentration detection in the inside is provided. If the throttle valve is opened during fuel cut (under the condition that engine combustion is not performed), gas exchange occurs in the exhaust system. Therefore, the detection element of the oxygen concentration sensor may be directly exposed to the intake air (atmosphere). And the output value of the oxygen concentration sensor corresponding to the oxygen concentration in the atmosphere (reference gas) can be accurately grasped.
[0004]
[Problems to be solved by the invention]
However, in the system described in the publication, fluctuations in the exhaust system due to opening of the throttle valve and temperature decrease are significant, and the oxygen concentration sensor output value accurately corresponds to the oxygen concentration in the atmosphere under certain conditions. It was difficult to get information.
[0005]
The present invention has been made in view of such circumstances, and the object of the present invention is to constantly calculate the oxygen concentration in the exhaust based on the output value of the oxygen concentration sensor provided in the exhaust system of the diesel engine. An object of the present invention is to provide an oxygen concentration detection device capable of acquiring a highly reliable value.
[0006]
Another object of the present invention is to provide an oxygen concentration detection method that can always obtain a highly reliable value as the oxygen concentration in the exhaust gas based on the output value of the oxygen concentration sensor provided in the exhaust system of the diesel engine. There is to do.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides an oxygen concentration sensor provided in an exhaust system of a diesel engine that outputs a signal corresponding to the oxygen concentration in the exhaust, and a flow rate of air sucked into a combustion chamber of the engine. A flow regulating valve to be adjusted, a fuel cut control means for controlling the fuel supply to be stopped under a predetermined condition during operation of the engine, and an intake for increasing the flow rate of the sucked air in synchronization with the stop of the fuel supply Air increase control means, integrated intake air amount recognition means for recognizing the integrated amount of air sucked into the combustion chamber, and reducing the flow rate of the intake air when the recognized integrated value exceeds a predetermined value. After the flow rate of the intake air reduction control means and the flow rate of the intake air is reduced, when the signal of the oxygen concentration sensor converges to a predetermined value, the oxygen concentration is reduced during the fuel supply stop period. Numerical information about the sensor outputs signals to a learning means for learning as numerical information corresponding to the oxygen concentration as a reference, and subject matter to be provided with.
[0008]
When the engine includes an exhaust gas recirculation passage that recirculates part of the exhaust gas from the exhaust system to the intake system, and an exhaust gas recirculation amount adjusting unit that adjusts the amount of exhaust gas recirculated through the exhaust gas recirculation passage. The exhaust gas recirculation amount adjusting means may reduce, preferably shut off, the amount of exhaust gas recirculated in synchronization with the stop of the fuel supply or the increase in the flow rate of the intake air.
[0009]
When the flow rate of the intake air is increased under the condition that the fuel supply is stopped during the operation of the engine, the exhaust gas remaining in the exhaust system and the intake system are introduced through the combustion chamber. Gas exchange is performed with fresh air. If the flow rate of the sucked air is quickly reduced after such gas exchange is performed, the atmosphere having a known oxygen concentration is exhausted while sufficiently suppressing pressure fluctuations and temperature drop in the exhaust system. It can be retained in the system. That is, according to this configuration, when the fuel supply is stopped during operation of the engine, the exhaust system is quickly filled with the atmosphere having a known oxygen concentration while ensuring the stability of pressure and temperature. Using the atmosphere as a reference gas, the correspondence between the detection signal of the oxygen concentration sensor and the oxygen concentration in the exhaust gas can be corrected and learned accurately. Therefore, the detection accuracy of the oxygen concentration in the exhaust gas by the oxygen concentration sensor can be increased. At this time, the pressure fluctuation and temperature drop in the exhaust system due to the execution of the learning are extremely small. In addition, since the introduction of the atmosphere into the exhaust system is performed promptly, the learning opportunities are expanded.
[0010]
In addition, when the exhaust purification catalyst provided in the exhaust system of the engine for purifying harmful components in the exhaust and the control for stopping the fuel supply are performed, the temperature of the catalyst is below a predetermined value. Preferably includes learning prohibiting means for prohibiting the learning.
[0011]
According to this configuration, for example, since the temperature drop in the exhaust system due to the learning is reliably avoided, there is no concern that the active state of the exhaust purification catalyst will be reduced.
[0012]
The oxygen concentration detection apparatus having the above-described configuration is a gradual change process that performs a gradual change process of the fuel supply amount when the fuel supply returns when the intake flow rate increases in synchronization with the stop of the fuel supply. It is preferable to provide a processing means.
[0013]
Although there is a concern that the engine torque may suddenly change when the fuel supply to the engine is restored under a condition where the flow rate of the sucked air is increased, according to the configuration, such a sudden change in the engine torque is suppressed. The stability of drivability at all times is ensured.
[0014]
The oxygen concentration detection device having the above-described configuration includes a reducing component supply means for supplying a reducing component to the exhaust system, and supply of the reducing component to the exhaust system at the start of or immediately before the control for stopping the fuel supply. It is preferable to include learning prohibiting means for prohibiting the learning when the learning is performed.
[0015]
Further, the oxygen concentration detection device having the above configuration includes a reducing component supply means for supplying a reducing component to the exhaust system, and a period from the start of the control for stopping the fuel supply to the end of the learning. It is preferable that a reducing component supply prohibiting unit that prohibits the supply of the reducing component to the exhaust system.
[0016]
According to this configuration, since the reducing component is not mixed when the exhaust gas is filled with the atmosphere as the reference gas, the accuracy and reliability of the learning are further improved.
[0017]
In another aspect of the invention, an oxygen concentration sensor that detects an oxygen concentration in an exhaust gas of the engine based on an output value of an oxygen concentration sensor that is provided in an exhaust system of the diesel engine and outputs a signal corresponding to the oxygen concentration in the exhaust gas. According to a detection method, the amount of air sucked into the combustion chamber of the engine is increased as the fuel supply is stopped during operation of the engine, and the air is sucked into the combustion chamber of the engine while the fuel supply is stopped. When the integrated value of the air amount exceeds a predetermined value, the amount of air taken into the combustion chamber of the engine is reduced, and when the output of the oxygen concentration sensor converges to the predetermined value, the numerical information regarding the output is used as a reference oxygen Learning as numerical information corresponding to the concentration, referring to the relationship between the learned numerical information and the reference oxygen concentration, and based on the output value of the oxygen concentration sensor, And summarized in that to detect the exhaust oxygen concentration.
[0018]
According to this configuration, when the fuel supply is stopped during operation of the engine, the exhaust system is quickly filled with the atmosphere having a known oxygen concentration while ensuring the stability of pressure and temperature. With the atmosphere as the reference gas, the correspondence between the detection signal of the oxygen concentration sensor and the oxygen concentration in the exhaust gas can be corrected and learned accurately. Therefore, the detection accuracy of the oxygen concentration in the exhaust gas by the oxygen concentration sensor can be increased. At this time, the pressure fluctuation and temperature drop in the exhaust system due to the execution of the learning are extremely small. In addition, since the introduction of the atmosphere into the exhaust system is performed promptly, the learning opportunities are expanded.
[0019]
In addition, the effect according to the said structure can be show | played by applying the said structure concerning this invention with respect to a gasoline engine or a gasoline lean combustion engine.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
Hereinafter, a first embodiment in which the oxygen concentration detection device and the oxygen concentration detection method of the present invention are applied to a diesel engine system will be described.
[0021]
[Engine system structure and function]
In FIG. 1, an internal combustion engine (hereinafter referred to as an engine) 1 is an in-line four-cylinder diesel engine system that includes a fuel supply system 10, a combustion chamber 20, an intake system 30, an exhaust system 40, and the like as main parts.
[0022]
First, the fuel supply system 10 includes a supply pump 11, a common rail 12, a fuel injection valve 13, a shutoff valve 14, a metering valve 16, a fuel addition valve 17, an engine fuel passage P1, an addition fuel passage P2, and the like. .
[0023]
The supply pump 11 makes the fuel pumped up from a fuel tank (not shown) into a high pressure and supplies it to the common rail 12 via the engine fuel passage P1. The common rail 12 has a function as a pressure accumulation chamber that holds (accumulates) the high-pressure fuel supplied from the supply pump 11 at a predetermined pressure, and distributes the accumulated fuel to each fuel injection valve 13. The fuel injection valve 13 is an electromagnetic valve provided with an electromagnetic solenoid (not shown) therein, and is appropriately opened to inject and supply fuel into the combustion chamber 20.
[0024]
On the other hand, the supply pump 11 supplies a part of the fuel pumped from the fuel tank to the fuel addition valve 17 via the addition fuel passage P2. A shutoff valve 14 and a metering valve 16 are sequentially arranged from the supply pump 11 toward the fuel addition valve 17 in the addition fuel passage P2. The shutoff valve 14 shuts off the fuel supply P2 in an emergency and stops the fuel supply. The metering valve 16 controls the pressure (fuel pressure) PG of the fuel supplied to the fuel addition valve 17. The fuel addition valve 17 is an electromagnetic valve provided with an electromagnetic solenoid (not shown) in the same manner as the fuel injection valve 13, and the NOx of the exhaust system 40 is supplied with an appropriate amount of fuel that functions as a reducing agent at an appropriate timing. Addition is supplied upstream of the catalyst casing 42.
[0025]
The intake system 30 forms a passage (intake passage) for intake air supplied into each combustion chamber 20. On the other hand, the exhaust system 40 forms a passage (exhaust passage) for exhaust gas discharged from each combustion chamber 20.
[0026]
The engine 1 is provided with a known supercharger (turbocharger) 50. The turbocharger 50 includes rotating bodies 52 and 53 connected via a shaft 51. One rotating body (turbine wheel) 52 is exposed to exhaust in the exhaust system 40, and the other rotating body (compressor wheel) 53 is exposed to intake air in the intake system 30. The turbocharger 50 having such a configuration performs so-called supercharging in which the compressor wheel 53 is rotated using the exhaust flow (exhaust pressure) received by the turbine wheel 52 to increase the intake pressure.
[0027]
In the intake system 30, an intercooler 31 provided in the turbocharger 50 forcibly cools the intake air whose temperature has been raised by supercharging. The throttle valve 32 provided further downstream than the intercooler 31 is an electronically controlled on-off valve whose opening degree can be adjusted steplessly, and changes the flow area of the intake air under predetermined conditions. And the function of adjusting the supply amount (flow rate) of the intake air.
[0028]
Further, an exhaust gas recirculation passage (EGR passage) 60 that communicates the intake system 30 and the exhaust system 40 is formed in the engine 1. The EGR passage 60 has a function of returning a part of the exhaust to the intake system 30 as appropriate. The EGR passage 60 is opened and closed steplessly by electronic control, and an EGR valve 61 that can freely adjust the flow rate of exhaust gas (EGR gas) flowing through the passage, and exhaust gas that passes (refluxs) the EGR passage 60. An EGR cooler 62 for cooling is provided.
[0029]
Further, in the exhaust system 40, a NOx catalyst casing 42 containing an occlusion reduction type NOx catalyst and a particulate filter is provided downstream of the connection part of the exhaust system 40 and the EGR passage 60. Further, an oxidation catalyst casing 43 containing an oxidation catalyst is provided downstream of the NOx catalyst casing of the exhaust system 40.
[0030]
In addition, various sensors are attached to each part of the engine 1, and signals related to the environmental conditions of the part and the operating state of the engine 1 are output.
[0031]
That is, the rail pressure sensor 70 outputs a detection signal corresponding to the fuel pressure stored in the common rail 12. The fuel pressure sensor 71 outputs a detection signal corresponding to the pressure (fuel pressure) PG of the fuel introduced into the fuel addition valve 17 through the metering valve 16 among the fuel flowing through the addition fuel passage P2. The air flow meter 72 outputs a detection signal corresponding to the flow rate (intake amount) GA of intake air upstream of the compressor wheel 53 in the intake system 30. The oxygen concentration sensor 73 outputs a detection signal that continuously changes in accordance with the oxygen concentration in the exhaust gas downstream of the NOx catalyst casing 42 (upstream of the oxidation catalyst casing 43) of the exhaust system 40. The detection signal of the oxygen concentration sensor 73 is used as a parameter for calculating the air-fuel ratio A / F in the air-fuel mixture used for engine combustion of the engine 1. The exhaust temperature sensor 74 is attached to a predetermined portion (between a honeycomb structure 42a and a particulate filter 42b described later) in the NOx catalyst casing 42 in the exhaust system 40, and is set to an exhaust temperature (filtered gas temperature) at the portion. A corresponding detection signal is output.
[0032]
The accelerator position sensor 76 is attached to an accelerator pedal (not shown) and outputs a detection signal corresponding to the depression amount ACC of the pedal. The crank angle sensor 77 outputs a detection signal (pulse) every time the output shaft (crankshaft) of the engine 1 rotates by a certain angle. Each of these sensors 70 to 77 is electrically connected to an electronic control unit (ECU) 90.
[0033]
The ECU 90 includes a central processing unit (CPU) 91, a read only memory (ROM) 92, a random access memory (RAM) 93, a backup RAM 94, a timer counter 95, and the like. These units 91 to 95 and an A / D converter are provided. The external input circuit 96 and the external output circuit 97 are connected to each other via a bidirectional bus 98, and a logic operation circuit is provided.
[0034]
The ECU 90 configured as described above performs processing of detection signals of the various sensors, for example, calculation processing of calculating the air-fuel ratio A / F in the air-fuel mixture to be used for engine combustion based on the detection signal of the oxygen concentration sensor 73. In addition to the above, the operation state of the engine 1 is controlled based on the detection signals of these various sensors, etc., the control relating to the on / off valve operation of the fuel injection valve 13, the opening adjustment of the EGR valve 61, or the opening adjustment of the throttle valve 32 Implement various controls.
[0035]
In addition, the ECU 90, the oxygen concentration sensor 73, the throttle valve 32, and the like configured as described above constitute the oxygen concentration detection device according to the present embodiment.
[0036]
[Structure and function of NOx catalyst casing]
Next, among the components of the engine 1 described above, the structure and function of the NOx catalyst casing 42 provided in the exhaust system 40 will be described in detail.
[0037]
Inside the NOx catalyst casing 42 is alumina (Al 2 O Three ) And a wall flow type particulate filter (hereinafter simply referred to as a filter) 42b mainly composed of a porous material as predetermined exhaust purification catalysts. Are arranged in series with an interval of.
[0038]
In the plurality of passages forming the honeycomb structure 42a, a carrier layer made of alumina, for example, is formed, and for example, potassium (K), sodium (Na), lithium that functions as a NOx storage agent on the surface of the carrier layer. (Li), alkaline metals such as cesium (Cs), alkaline earths such as barium Ba and calcium Ca, rare earths such as lanthanum (La) or yttrium (Y), and function as an oxidation catalyst (noble metal catalyst) For example, a noble metal such as platinum (Pt) is supported. The NOx storage agent and the noble metal catalyst supported so as to be mixed on the support (in this case, the honeycomb structure on which the support layer made of alumina is formed) 42a together constitute a NOx catalyst (storage reduction type NOx catalyst). To do.
[0039]
The NOx storage agent has a characteristic of storing NOx in a state where the oxygen concentration in the exhaust gas is high and releasing NOx in a state where the oxygen concentration in the exhaust gas is low (a state where the concentration of the reducing component is high). Further, when NOx is released into the exhaust gas, if HC, CO, or the like is present in the exhaust gas, the noble metal catalyst promotes an oxidation reaction of these HC and CO, so that NOx is an oxidizing component, and HC and CO is removed. A redox reaction as a reducing component occurs between the two. That is, HC and CO are CO 2 And H 2 Oxidized to O, NOx is N 2 Reduced to
[0040]
On the other hand, if the NOx storage agent stores a predetermined limit amount of NOx even when the oxygen concentration in the exhaust gas is high, the NOx storage agent does not store NOx any more. In the engine 1, the reducing component is intermittently supplied upstream of the NOx catalyst casing 42 in the exhaust passage through post injection and fuel addition, and the concentration of the reducing component in the exhaust increases. Before the NOx occlusion amount of the NOx catalyst (NOx occlusion agent) reaches the limit amount, this reducing component periodically releases and reduces and purifies NOx occluded in the NOx catalyst, and the NOx occlusion capacity of the NOx occlusion agent. Will be restored.
[0041]
On the other hand, the porous material forming the filter 42b is obtained by washing a ceramic material such as cordierite with a coating material such as alumina, titania, zirconia, or zeolite, and has a property of transmitting exhaust gas. Further, the filter 42b is a so-called wall flow type having an exhaust inflow passage having an upstream end opened in parallel with each other and closed at a downstream end, and an exhaust outflow passage having an upstream end closed and a downstream end opened. is there. A coating layer (carrier layer) made of alumina or the like that supports the NOx storage agent and the noble metal catalyst is formed on the surface of the partition wall between the exhaust passages and in the pores formed in the partition. .
[0042]
The filter 42b having such a structure purifies particulates such as soot contained in the exhaust gas and harmful components such as NOx based on the following mechanism.
[0043]
As described above, the NOx storage agent repeatedly stores, releases, and purifies NOx in accordance with the oxygen concentration in the exhaust gas and the amount of the reducing component in cooperation with the noble metal catalyst. On the other hand, the NOx storage agent has a characteristic of generating active oxygen as a secondary in the process of purifying NOx. When the exhaust gas passes through the filter 42b, particulates such as soot contained in the exhaust gas are captured by the structure (porous material). Here, since the active oxygen produced by the NOx storage agent has an extremely high reactivity (activity) as an oxidant, fine particles deposited on or near the surface of the NOx catalyst among the captured fine particles It reacts quickly with oxygen (without emitting a luminous flame) and is purified.
[0044]
Further, the reaction heat generated from the honeycomb structure (NOx catalyst supported on the structure) 42a disposed on the upstream side in the NOx catalyst casing 42 efficiently raises the temperature of the filter 42b disposed on the downstream side. In addition, the fine particle decomposition action by the filter 42b is enhanced.
[0045]
[Overview of fuel injection control]
The ECU 90 performs fuel injection control based on the operating conditions of the engine 1 grasped from the detection signals of various sensors. In the present embodiment, the fuel injection control is related to the fuel injection into each combustion chamber 20 through each fuel injection valve 13 by setting parameters such as the fuel injection amount Q, the injection timing, and the injection pattern. This is a series of processes for executing the opening / closing operation of the individual fuel injection valves 13 based on the set parameters.
[0046]
The ECU 90 repeats such a series of processes every predetermined time during the operation of the engine 1. The fuel injection amount Q and the injection timing are basically a map set in advance based on the accelerator pedal depression amount ACC and the engine speed NE (a parameter that can be calculated based on the pulse signal of the crank angle sensor). Determined with reference to (not shown).
[0047]
Further, regarding the setting of the fuel injection pattern, the ECU 90 obtains engine output by performing fuel injection in the vicinity of compression top dead center for each cylinder as well as fuel output prior to main injection (hereinafter referred to as pilot injection). ) And fuel injection following the main injection (hereinafter referred to as post-injection) are performed for the selected cylinder at the time appropriately selected as the sub-injection.
[0048]
[Pilot injection]
In a diesel engine, generally, at the end of the compression stroke, the combustion chamber reaches a temperature that induces fuel self-ignition. In particular, when the engine operating condition is in the middle and high load region, when fuel supplied for combustion is injected into the combustion chamber all at once, this fuel burns explosively with noise. By performing the pilot injection, the fuel supplied prior to the main injection becomes a heat source (or a seed fire), and the heat source gradually expands in the combustion chamber and leads to combustion. The combustion state becomes relatively slow, and the ignition delay time is shortened. For this reason, noise associated with engine operation is reduced, and further, the amount of NOx in the exhaust gas is also reduced.
[0049]
[Post injection]
The fuel supplied into the combustion chamber 20 by the post injection is reformed into light HC in the combustion gas and discharged to the exhaust system 40. That is, light HC that functions as a reducing agent is added to the exhaust system 40 through post injection, and the concentration of reducing components in the exhaust is increased. The reducing component added to the exhaust system 40 reacts with NOx released from the NOx catalyst via the NOx catalyst in the NOx catalyst casing 42 and other oxidizing components contained in the exhaust. The reaction heat generated at this time raises the temperature of the exhaust and the NOx catalyst. In the post-injection, a condition is required in which fuel is directly injected into the combustion chamber and the fuel is not involved in engine combustion. For this reason, there is a limit to the amount of fuel that can be supplied at one time, and in order to obtain a predetermined temperature rise effect, it is usually necessary to carry out the operation continuously through each fuel injection valve 13 a plurality of times. However, the fuel lightened in the combustion gas is highly reactive, and has a high temperature raising function for the exhaust gas and the NOx catalyst even under conditions where the temperature of the exhaust gas is considerably low, such as during idling. Demonstrate. In other words, the opportunities for its use extend over a wide operating area.
[0050]
[Fuel addition]
Direct addition of sprayed fuel (reducing agent) to the exhaust system 40 through the fuel addition valve 17 also increases the concentration of reducing components in the exhaust, as in post-injection, resulting in an increase in the temperature of the exhaust and NOx catalyst. Can be made. The fuel added by the fuel addition valve 17 tends to be non-uniformly distributed while maintaining a higher polymer state in the exhaust than in the case of post injection. Further, in the fuel addition by the fuel addition valve 17, the amount of fuel that can be added at once and the degree of freedom of the addition timing are greater than in the case of post injection. However, the sprayed fuel supplied through fuel addition cannot adhere to the inner wall of the exhaust passage and exhibit an efficient temperature raising function unless the exhaust is warmed to some extent in advance. For this reason, the utilization opportunities are generally limited to the medium and high load areas.
[0051]
[SOx poisoning recovery control]
Further, in the engine 1, in order to gradually remove SOx and the like deposited on the honeycomb structure 42a and the filter 42b as the engine operation continues, the temperature of the NOx catalyst is increased to such an extent that the SOx and the like can be thermally decomposed. Control (SOx poisoning recovery control) is performed at a predetermined cycle. In the SOx poisoning recovery control, either or both of the post injection and the fuel addition are continuously performed for a relatively long period.
[0052]
[Fuel Cut]
The ECU 90 performs fuel cut under specific operating conditions such as during deceleration. The fuel cut is a part of the operation control, and when the engine speed NE exceeds a predetermined number set in advance according to the operating state of the engine 1, fuel is supplied to the combustion chamber 11 (fuel injection). Is a control for reducing the burden on the engine 1, preventing the NOx catalyst or the oxidation catalyst from being heated, or improving the fuel consumption. During the period of fuel cut, the engine 1 misfires and engine combustion stops.
[0053]
[Method of detecting oxygen concentration in exhaust]
FIG. 2A shows a cross-sectional structure of the main part of the detection element of the oxygen concentration sensor 73 provided in the exhaust system 40.
[0054]
As shown in FIG. 2A, the detection element of the oxygen concentration sensor 73 is zirconia (Zr 2 O Three ) Etc., and is formed by laminating porous insulating materials (plate materials) 73a, 73b, 73c having oxygen ion conductivity and heat resistance. An atmosphere introduction space S1 communicating with the atmosphere is formed inside the laminate of these plate members 73a, 73b, 73c. Further, electrodes 73d, 73d are formed on both surfaces of the plate material 73a, that is, a plate surface facing the external space (space in the exhaust passage) S2 of the detection element and a plate surface facing the air introduction space S1 formed inside the detection element. 73e is attached. In addition, the plate member 73c incorporates an electric heater (not shown), and maintains the temperature of the detection element at a predetermined value. When a predetermined voltage is applied between both electrodes 73d and 73e, oxygen molecules existing in the vicinity of both electrodes 73d and 73e are ionized and pass through the plate material 73a along the direction of the arrow α. At this time, the current value flowing between the electrodes 73d and 73e is quantitatively related to the difference in oxygen concentration in the space S1 and the space S2. Further, since the oxygen concentration in the space S1 is known as the atmospheric oxygen concentration (for example, 21%), the current flowing between the electrodes 73d and 73e when a predetermined voltage is applied between the electrodes 73d and 73e. By observing the value, it is possible to grasp the oxygen concentration in the space S2 (oxygen concentration in the exhaust gas).
[0055]
For example, FIG. 3 is a graph showing the relationship between the applied voltage and current between the electrodes 73d and 73e. In the graph, a plurality of conditions (oxygen excess rates λ = a, b, c, d: where a <b <is different) in the oxygen concentration (oxygen excess rate) in the space S2 (in the detection target gas). The relationship between applied voltage and current corresponding to c <d) is shown.
[0056]
As shown in FIG. 3, although the current value tends to increase as the applied voltage increases, it can be seen that the current value hardly changes when the applied voltage is in a specific range. The current values I1, I2, I3, and I4 in such a specific range are referred to as limit current values corresponding to the oxygen excess ratio λ = a, b, c, and d. Therefore, an appropriate voltage (for example, a voltage value Vx shown in the figure) for holding the current value flowing between the electrodes 73d and 73e at such a limit current value is appropriately set, and the detection is performed by measuring the limit current value. It is possible to quantitatively grasp the oxygen concentration (oxygen excess rate) in the target gas.
[0057]
FIG. 4 is a graph schematically showing the correspondence between the oxygen concentration in the space S2 and the limit current value. As shown in FIG. 4, the limit current value increases as the oxygen concentration in the space S2 (the oxygen concentration in the exhaust gas) increases. Therefore, if two coordinates that clearly show the correspondence between the oxygen concentration in the exhaust gas and the limit current value are determined in advance as shown by points A and B in FIG. 4, a line connecting the two coordinates ( The oxygen concentration in the exhaust gas can be determined from the detection signal (limit current value) of the oxygen concentration sensor 73 based on the characteristic line.
The oxygen concentration detection apparatus according to the present embodiment improves the accuracy and reliability of the calibration curve setting for determining the correspondence between the oxygen concentration in the exhaust gas and the limit current value by making the following modifications. ing.
[0058]
FIG. 5 is a graph showing in detail the correspondence relationship between the oxygen concentration in the exhaust gas and the limit current value learned and stored by the ECU 90.
[0059]
The ECU 90 learns and stores the straight line A-B at an appropriate timing as a characteristic line indicating a correspondence relationship between the oxygen concentration in the exhaust gas and the limit current value. Here, the current value IA corresponds to the limit current value under the condition that the oxygen concentration in the exhaust gas is 0%, and the current value IB is the limit current value IB under the condition where the oxygen concentration is 21%. Equivalent to. When monitoring the oxygen concentration in the exhaust, a straight line AB (characteristic line) connecting the two coordinates A and B is referred to.
[0060]
Here, under the condition that the oxygen concentration in the exhaust gas is 0% (under the condition that the air-fuel ratio of the air-fuel mixture used for engine combustion is the stoichiometric air-fuel ratio (stoichiometric)), individual differences in sensor elements, secular changes, etc. Regardless of the limit value, the limit current value becomes extremely weak (theoretically becomes “0”). On the other hand, under the condition that the oxygen concentration in the exhaust gas is 21% (in a state where the sensor element is exposed to the atmosphere), the limit current value (absolute value) shows a relatively large value. It fluctuates due to changes over time (varies). Therefore, the ECU 90 appropriately measures the limit current value in a state where the sensor element of the oxygen concentration sensor 73 is exposed to the atmosphere (under the same condition), and changes the old value (for example, the current value IB ′) to the new value (for example, the current). Correction to the value IB) (hereinafter referred to as atmospheric correction).
[0061]
The limit current value is theoretically “0” under the condition that the oxygen concentration in the exhaust gas is 0%. In practice, however, a weak limit current value is detected due to the presence of a circuit or the like interposed between the ECU 90 and the oxygen concentration sensor 73. The ECU 90 determines the weak limit current value under the condition that the sensor element itself of the oxygen concentration sensor 73 does not output a detection signal at all (for example, the state in which the oxygen concentration sensor 73 is inactive (for example, immediately after the engine is started)). The critical current value IA ″ is detected under a condition in a low temperature state), and a deviation (hereinafter referred to as circuit offset) OS from the reference value IA (“0” ampere) is recognized. In addition, when the oxygen concentration in the exhaust gas is quantified based on the detection signal of the oxygen concentration sensor 73 including the case where the atmospheric correction is performed, the circuit offset OS is always taken into account, thereby improving the accuracy of oxygen concentration detection. Increase further.
[0062]
[Oxygen concentration detection and operation control execution timing associated therewith]
6A to 6E show how various parameters related to the operating state of the engine 1 change when the oxygen correction of the oxygen concentration sensor 73 is performed. It is a time chart shown above. In each figure, the fuel cut is started at time t1.
[0063]
FIG. 6A shows the transition of the opening degree of the throttle valve 32. As shown in FIG. 6A, the ECU 90 gradually changes the opening of the throttle valve 32 to a predetermined opening with the start of fuel cut at time t1 (time t2). Next, fresh air is introduced into the exhaust system 40 by maintaining the throttle valve 32 in a substantially fully open state (a sufficiently large opening degree to ensure a predetermined intake air amount) for a predetermined period (time t2 to t3). Thereafter, the throttle valve 32 is fully closed, and this fully closed state is maintained as long as the fuel cut is continued.
[0064]
FIG. 6B shows a transition of a value obtained by integrating the intake air amount GA with a specific time calculated by the ECU 90 as a starting point. As shown in FIG. 6B, the ECU 90 starts to integrate the intake air amount GA from the time (t2) when the throttle valve 32 shifts to the fully open state. When the integrated amount of the intake air amount GA reaches the predetermined value F (time t3), it is recognized that the fresh air introduced by opening the throttle valve 32 is completely replaced with the residual exhaust gas in the exhaust system 40. Then, the throttle valve 32 is fully closed (see also FIG. 6A).
[0065]
FIG. 6C shows the transition of the pressure in the exhaust system 40. As shown in FIG. 6C, the pressure in the exhaust system 40 varies with the opening / closing operation of the throttle valve 32 from time t1 to time t3. However, after the throttle valve 32 shifts to the fully closed state at time t3, as long as the fuel cut is continued, a value substantially equal to the atmospheric pressure is stably maintained.
[0066]
FIG. 6D shows the transition of the NOx catalyst bed temperature estimated based on the detection signal of the exhaust temperature sensor 74. As shown in FIG. 6 (d), the bed temperature of the NOx catalyst gradually decreases by introducing fresh air into the exhaust system 40 during the period from time t1 to time t3. However, since the introduction of fresh air into the exhaust system 40 stops when the throttle valve 32 shifts to the fully closed state at time t3, the decrease in the bed temperature of the NOx catalyst is suppressed after time t3. Note that the opening of the throttle valve 32 at times t1 to t3 only needs to be sufficiently large for efficient introduction of fresh air, and does not necessarily need to be fully opened. For example, since there is an upper limit for the intake air amount (new air introduction efficiency) that increases as the opening of the slot valve 32 increases, an opening setting that exceeds a predetermined opening (for example, 90% of the fully opened state) is performed. However, the efficiency of new air introduction may not change. Moreover, even if the maximum opening degree of the throttle valve 32 at the times t1 to t3 is set from the viewpoint of efficiently introducing new air, maintaining the stability of the engine torque, and maintaining the exhaust characteristics well. Good. Further, the valve opening operation and the valve closing operation are not performed so that the opening degree of the throttle valve 32 is not changed suddenly in the process of shifting from the normal opening degree to the maximum opening degree or in the process of changing from the maximum opening degree to the normal opening degree. An averaging process (gradual change process) may be performed.
[0067]
FIG. 6E shows the transition of the detection signal of the oxygen concentration sensor 73. As shown in FIG. 6 (e), the detection signal (limit current value) of the oxygen concentration sensor 73 gradually increases with the start of fuel cut, but during the period when the throttle valve 32 is fully open (time t2). -T3) It shows a tendency to decrease little by little. The decreasing tendency of the output of the oxygen concentration sensor 73 from the time t2 to the time t3 occurs because the gas pressure in the exhaust system increases due to the throttle valve 32 being fully opened. Thereafter, when the throttle valve 32 is fully closed, the fresh air introduced from the intake system 30 is completely exchanged with the residual exhaust gas in the exhaust system 40 and stays in the exhaust system 40. For this reason, the oxygen concentration sensor 73 stably outputs a detection signal corresponding to the oxygen concentration (21%) in the atmosphere. In the present embodiment, after the throttle valve 32 is shifted to the fully closed state (after time t4), the atmospheric concentration correction of the oxygen concentration sensor 73 is continued until the fuel cut is completed.
[0068]
As described above, in the oxygen concentration detection apparatus according to the present embodiment, during the fuel cut period, the throttle valve 32 is first fully opened to replace the fresh air in the intake system 30 with the residual exhaust gas in the exhaust system. Then, after the throttle valve 32 is closed to stabilize the pressure and temperature in the exhaust system 40, atmospheric correction is performed.
[0069]
[Specific procedure for atmospheric correction]
Hereinafter, a specific processing procedure for atmospheric correction of the oxygen concentration sensor 73 will be described.
[0070]
FIG. 7 is a flowchart showing an “atmospheric correction routine for the oxygen concentration sensor” executed through the ECU 90. This routine is repeatedly executed every predetermined time after the engine 1 is started.
[0071]
When the process shifts to this routine, the ECU 90 first determines in step S101 whether or not the current time corresponds to the start of fuel cut. If the determination is affirmative, the process proceeds to step S102. If the determination is negative, the routine is temporarily exited. That is, the ECU 90 performs the processing after step S102 (atmospheric correction of the oxygen concentration sensor 73) only when the processing is shifted to this routine at the start of fuel cut.
[0072]
In step S102, the throttle valve 32 is opened, and the valve 32 is closed after the valve is kept open for a predetermined period. Here, the ECU 90 sequentially estimates the amount of fresh air introduced from the intake system 30 to the exhaust system 40 by integrating the intake air amount GA after the throttle valve 32 is opened. When a predetermined amount of fresh air is introduced into the exhaust system 40 and it is determined that the exhaust gas remaining in the exhaust system 40 has been completely replaced with fresh air, the throttle valve 32 is closed ( (See FIGS. 6A and 6B). Further, the ECU 90 closes the EGR valve 61 in conjunction with the start of fuel cut (opening of the throttle valve 32). The closed state of the EGR valve 61 is maintained until the end of fuel cut (at the end of atmospheric correction).
[0073]
After the throttle valve 32 is closed, the ECU 90 waits until the output of the oxygen concentration sensor 73 converges within a predetermined range (substantially a constant value). When it is determined that the output of the oxygen concentration sensor 73 has converged to a predetermined range, the limit current value output by the oxygen concentration sensor 73 (a value considering the circuit offset OS) is recognized (step S103). The current value is learned and stored as a detection signal corresponding to the oxygen concentration (21%) in the atmosphere (step S104).
[0074]
After the processing of step S104, the ECU 90 once exits this routine.
[0075]
The oxygen concentration detection apparatus according to the present embodiment learns the correspondence between the detection signal of the oxygen concentration sensor 73 and the oxygen concentration in the exhaust gas according to such a procedure, and based on the learning result, the oxygen concentration in the exhaust gas (engine) The air-fuel ratio in the air-fuel mixture used for combustion is acquired.
[0076]
FIG. 8 shows the relationship between the estimation accuracy of the air-fuel ratio calculated (estimated) based on the detection signal of the oxygen concentration sensor 73 and the true value of the air-fuel ratio. In the figure, the horizontal axis corresponds to the true value of the air-fuel ratio A / F (hereinafter referred to as the base air-fuel ratio), and the vertical axis represents the air-fuel ratio (hereinafter referred to as the air-fuel ratio) estimated based on the detection signal of the oxygen concentration sensor 73. This corresponds to a deviation ΔA / F between the detected air-fuel ratio and the base air-fuel ratio.
[0077]
When the atmospheric correction according to the present embodiment is not performed, for example, as indicated by a broken line L1 or a broken line M1, as the characteristic of the detection element (zirconia element) constituting the oxygen concentration sensor 73, the deviation becomes larger as the base air-fuel ratio becomes farther from the stoichiometry. The absolute value of ΔA / F tends to increase. In other words, the base air-fuel ratio tends to deviate from the detected air-fuel ratio as the base air-fuel ratio is further away from the stoichiometry. On the other hand, if the atmospheric correction according to the present embodiment is appropriately performed, the absolute value of the deviation ΔA / F maintains a sufficiently small value even when the base air-fuel ratio fluctuates as shown by the solid line L2 or the solid line M2. It becomes like this. That is, the reliability of the detected air-fuel ratio is increased in a wide oxygen concentration range.
[0078]
As described above, according to the present embodiment, during operation of the engine 1, the intake air amount GA is increased under the condition that the fuel cut is performed, and the exhaust gas remaining in the exhaust system 40 and the intake air After the gas is once exchanged with the fresh air introduced from the system 30 through the combustion chamber 20, the intake air amount GA is rapidly reduced. By such a series of operations, the exhaust system 40 is quickly filled with the atmosphere having a known oxygen concentration while sufficiently suppressing the pressure fluctuation and temperature drop in the exhaust system 40. Then, while ensuring the stability of the pressure and temperature in the exhaust system 40, numerical information (for example, limit current value) regarding the signal output from the oxygen concentration sensor 73 using the filled atmosphere as a reference gas is used as the reference gas oxygen. By storing the numerical information corresponding to the concentration, it is possible to accurately learn the correspondence between the detection signal of the oxygen concentration sensor 73 and the oxygen concentration in the exhaust gas.
[0079]
By repeating such learning, the oxygen concentration sensor 73 maintains high detection accuracy and reliability over a long period of time without depending on individual differences (variations) in the detection elements and detection circuits and the degree of progress of deterioration over time. can do.
[0080]
At this time, the temperature drop in the exhaust system 40 due to the execution of the learning (atmospheric correction) is extremely small, so that the temperature in the exhaust system 40 is sufficiently high to keep the exhaust purification catalyst in an active state. It can be kept at a high value. Moreover, since the intake air amount GA is increased in synchronization with the start of the fuel cut and the introduction of the atmosphere into the exhaust system 40 is promoted, the opportunity for performing the learning is also expanded. That is, the fuel cut execution period is only a limited time during the operation of the engine 1, but the limited time can be used effectively.
[0081]
In the present embodiment, the opening / closing valve operation of the throttle valve 32 and the closing operation of the EGR valve 61 are performed in synchronization with the start of fuel cut. On the other hand, even if only the opening / closing operation of the throttle valve 32 is performed while the EGR valve 61 is in the open state, the effect equivalent to the present embodiment can be obtained.
[0082]
In addition, the throttle valve 32 may be held in a closed state (or a predetermined opening), the EGR valve 61 is closed in synchronization with the start of the fuel cut, and atmospheric correction is performed after a predetermined period has elapsed. An effect similar to that of the embodiment can be achieved.
[0083]
(Second Embodiment)
Next, a second embodiment in which the oxygen concentration detection device and the oxygen concentration detection method of the present invention are applied to a diesel engine system will be described focusing on differences from the first embodiment. In the second embodiment, the hardware configuration (FIGS. 1 to 4) of the engine system and the oxygen concentration detection device to be applied is the same as that of the first embodiment. For this reason, the same reference numerals are used for members, hardware configurations, and the like having the same functions and structures, and redundant descriptions here are omitted.
[0084]
The oxygen concentration detection apparatus according to the second embodiment recognizes the circuit offset OS of the oxygen concentration sensor 73 quantitatively at the time of starting the engine 1 and the throttle valve first during the fuel cut execution period. 32 is fully opened, fresh air in the intake system 30 is exchanged with residual exhaust gas in the exhaust system, and after that, the throttle valve 32 is closed to stabilize the pressure and temperature in the exhaust system 40 and perform air correction. This is common to the first embodiment. However, it is different from the first embodiment in that control for further improving the accuracy and reliability of the atmospheric correction is performed by preventing the atmospheric correction from being performed close to the execution timing of the fuel addition.
[0085]
FIGS. 9A and 9B are time charts schematically showing the transition of the detection signal of the oxygen concentration sensor 73 accompanying the fuel cut. In FIGS. 9A and 9B, time t11 corresponds to the fuel cut start time.
[0086]
First, FIG. 9A shows a transition curve (dotted line) when no fuel is added and a transition curve (solid line) when fuel is added at the start of fuel cut on the same time axis. Is. As shown in FIG. 9A, when no fuel is added, the detection signal of the oxygen concentration sensor 73 quickly rises (shifts to the lean side) with the start of fuel cut, and the oxygen concentration in the atmosphere When the value corresponding to is reached (t12), it becomes stable. On the other hand, if the fuel addition is performed at a timing close to the start of the fuel cut, the fuel stays in the exhaust system 40, so that the increase of the oxygen concentration in the exhaust system 40 is delayed. For this reason, the time (t13) when the detection signal of the oxygen concentration sensor 73 reaches a value corresponding to the oxygen concentration in the atmosphere is also delayed.
[0087]
Therefore, the oxygen concentration detection apparatus according to the present embodiment has a predetermined period (for example, from the start of the fuel cut) so that the oxygen concentration in the exhaust system quickly becomes equal to the atmospheric oxygen concentration after the start of the fuel cut. Control for prohibiting the addition of fuel is performed during times t11 to t12) in FIG. While the atmospheric correction is being performed, the fuel addition is prohibited, or the amount of change in the detection signal of the oxygen concentration sensor that accompanies the fuel addition is estimated, and the amount of change is corrected to reduce ( For example, the atmospheric correction is continued while the virtual lines MSK shown in FIG.
[0088]
[Specific procedure for atmospheric correction]
Hereinafter, a specific processing procedure for atmospheric correction of the oxygen concentration sensor 73 will be described.
[0089]
FIG. 10 is a flowchart showing an “atmospheric correction routine for the oxygen concentration sensor” executed through the ECU 90. This routine is repeatedly executed every predetermined time after the engine 1 is started.
[0090]
When the processing shifts to this routine, the ECU 90 first determines in step S201 whether or not the current time corresponds to the start of fuel cut. If the determination is affirmative, the process proceeds to step S202. If the determination is negative, the process jumps to step S207.
[0091]
In step S202, it is determined whether there is a history of fuel addition for a predetermined period up to the present time. If the determination is affirmative, the process proceeds to step S203. If the determination is negative, the process jumps to step S207. That is, the ECU 90 shifts the process to this routine at the start of the fuel cut, and performs the process (oxygen process) after step S203 only when the condition that the fuel addition is not performed immediately before the start of the fuel cut is satisfied. Atmospheric correction of the density sensor 73 is performed.
[0092]
In step S203, the ECU 90 temporarily prohibits the fuel addition.
[0093]
In step S204, the throttle valve 32 is opened for a predetermined period, and the valve 32 is closed after maintaining the opened state for a predetermined period. Here, the ECU 90 sequentially estimates the amount of fresh air introduced from the intake system 30 to the exhaust system 40 by integrating the intake air amount GA after the throttle valve 32 is opened. When a predetermined amount of fresh air is introduced into the exhaust system 40 and it is determined that the exhaust gas remaining in the exhaust system 40 has been completely replaced with fresh air, the throttle valve 32 is closed ( (See FIGS. 6A and 6B). Further, the ECU 90 closes the EGR valve 61 in conjunction with the start of fuel cut (opening of the throttle valve 32). The closed state of the EGR valve 61 is maintained until the end of fuel cut (at the end of atmospheric correction).
[0094]
After the throttle valve 32 is closed, the ECU 90 waits until the output of the oxygen concentration sensor 73 converges within a predetermined range (substantially a constant value). When it is determined that the output of the oxygen concentration sensor 73 has converged to a predetermined range, the limit current value output by the oxygen concentration sensor 73 (a value considering the circuit offset OS) is recognized (step S205). The current value is learned and stored as a detection signal corresponding to the oxygen concentration (21%) in the atmosphere (step S206).
[0095]
In the subsequent step S207, processing for permitting implementation of fuel addition, in other words, processing for canceling prohibition of fuel addition (step S203) is performed.
[0096]
After the processing of step S207, the ECU 90 once exits this routine.
[0097]
According to the present embodiment in which atmospheric learning of the oxygen concentration sensor 73 is performed according to such a procedure, in addition to the effects of the first embodiment, the temperature drop in the exhaust system 40 due to the execution of the atmospheric correction is reliably ensured. Thus, it is possible to achieve further effects such as avoiding the deterioration of the active state of the exhaust purification catalyst.
[0098]
In the present embodiment, when fuel addition is performed at the start of fuel cut or immediately before the fuel cut, the atmospheric correction is prohibited so that the oxygen concentration in the exhaust gas is temporarily changed. A control structure that prohibits the execution of atmospheric correction may also be applied when other control (for example, pilot injection, post-injection) that causes the occurrence of this is performed immediately before the fuel cut.
[0099]
In each of the above embodiments, the progress rate of gas exchange in the exhaust system 40 based on the on / off valve operation of the throttle valve 32 and the EGR valve 61, and the degree of pressure fluctuation and temperature fluctuation in the exhaust system are applied. It depends on the hardware configuration of the organization. For this reason, the maximum opening degree and the minimum opening degree (%) of each valve 32, 61, the gradual change (annealing) rate of the opening degree change, and the like are adjusted according to the hardware characteristics of the engine and the operating state thereof. Is preferred. At this time, the detection signals of the air flow meter 72 and the exhaust temperature sensor 74 are sequentially applied so that the transition of each parameter reflecting the operation state realizes the relationship between the parameters shown in FIGS. 6 (a) to 6 (e), for example. It is preferable to adjust the opening degree of the throttle valve 32 and the EGR valve 61 while feeding back.
[0100]
Furthermore, in addition to improving the accuracy of atmospheric correction, the operation of the on / off valves of the throttle valve 32 and the EGR valve 61 and the fuel from the viewpoint of maintaining a good drivability during the fuel cut period or before and after the fuel cut. It is preferable to appropriately modify the injection amount. For example, when the driver of the engine 1 steps on the accelerator during a period (period t2 to t3 in FIG. 6) in which the throttle valve 32 is kept open with the start of fuel cut, the required torque at that time It is preferable to gradually increase the fuel injection amount rather than immediately injecting and supplying the fuel corresponding to the above. Further, when the gas exchange in the exhaust system 40 is finished and the throttle valve 32 is closed (time t3 in FIG. 6), the change of the throttle valve 32 opening (for example, transition to the fully closed state) is suddenly performed. In order to suppress the occurrence of so-called engine braking, it is preferable to perform control to gradually close the valve 32 instead of performing this.
[0101]
In particular, when it is estimated that the temperature of the NOx catalyst, the particulate filter, or the oxidation catalyst is lower than a predetermined value at the time of starting the fuel cut, the atmospheric correction associated with the current fuel cut is performed. It is preferable to perform treatment so as not to be present (prohibited) in order to suitably maintain the active state of various catalysts provided in the exhaust system 40.
[0102]
In each of the above embodiments, the present invention is applied to the oxygen concentration sensor that outputs a limit current value quantitatively corresponding to the exhaust oxygen concentration. By applying the present invention to another oxygen concentration sensor that outputs a detection signal corresponding to the above, an effect equivalent to or equivalent to that of each of the above embodiments can be obtained. Further, the present invention can be equally applied to sensors for detecting other exhaust components related to oxygen components in the exhaust (for example, NOx sensors for detecting nitrogen oxides).
[0103]
【The invention's effect】
As described above, according to the present invention, when the fuel supply is stopped during the operation of the engine, the exhaust system is quickly filled with the atmosphere having a known oxygen concentration while ensuring the stability of the pressure and temperature. The correspondence relationship between the detection signal of the oxygen concentration sensor and the oxygen concentration in the exhaust gas can be accurately learned using the filled atmosphere as a reference gas. Therefore, the detection accuracy of the oxygen concentration in the exhaust gas by the oxygen concentration sensor can be increased. At this time, the pressure fluctuation and temperature drop in the exhaust system due to the execution of the learning are extremely small. In addition, since the introduction of the atmosphere into the exhaust system is performed promptly, the learning opportunities are expanded.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing a diesel engine system according to a first embodiment of the present invention.
FIG. 2 is a schematic diagram showing a cross-sectional structure of a main part of a detection element of an oxygen concentration sensor applied in the embodiment.
FIG. 3 is a graph showing a relationship between applied voltage and current between two electrodes of the oxygen concentration sensor applied in the embodiment.
FIG. 4 is a graph schematically showing a correspondence relationship between a limiting current value and an oxygen concentration of an oxygen concentration sensor applied in the embodiment.
FIG. 5 is a graph showing in detail a correspondence relationship between oxygen concentration in exhaust gas and limit current value stored in the electronic control unit in the embodiment;
FIG. 6 is a time chart showing, on the same time axis, changes accompanying the execution of atmospheric correction for various parameters related to the operating state of the engine in the same embodiment.
FIG. 7 is a flowchart showing an atmospheric correction procedure of the oxygen concentration sensor applied in the embodiment.
FIG. 8 is a graph showing the relationship between the estimation accuracy of the air-fuel ratio estimated based on the detection signal of the oxygen concentration sensor and the true value of the air-fuel ratio.
FIG. 9 is a time chart schematically showing a transition of a detection signal of an oxygen concentration sensor accompanying a fuel cut.
FIG. 10 is a flowchart showing an atmospheric correction procedure of the oxygen concentration sensor applied in the second embodiment of the present invention.
[Explanation of symbols]
1 engine (internal combustion engine)
10 Fuel supply system
11 Supply pump
12 Common rail
13 Fuel injection valve
16 Metering valve
17 Fuel addition valve
20 Combustion chamber
30 Intake system
31 Intercooler
32 Throttle valve
40 Exhaust system
42 NOx catalyst casing
43 Oxidation catalyst casing
50 turbocharger
51 shaft
52 Turbine wheel
53 Compressor wheel
60 EGR passage
61 EGR valve
62 EGR cooler
70 Rail pressure sensor
71 Fuel pressure sensor
72 Air flow meter
73 Oxygen concentration sensor
74 Exhaust temperature sensor
76 Accelerator position sensor
77 Crank angle sensor
90 Electronic control unit (ECU)
P1 Engine fuel passage
P2 added fuel passage

Claims (4)

ディーゼルエンジンの排気系に設けられ排気中の酸素濃度に応じた信号を出力する酸素濃度センサと、
当該エンジンの燃焼室に吸入される空気の流量を調整する流量調整弁と、
当該エンジンの運転中、所定条件下で燃料供給を停止させる制御を行う燃料カット制御手段と、
前記燃料供給の停止と同期し、前記吸入される空気の流量を増大させる吸入空気増量制御手段と、
前記燃焼室に吸入される空気を積算することにより、前記ディーゼルエンジンの吸気系から前記燃焼室を通じて排気系内へ導入される空気量を認識する積算吸入空気量認識手段と、
前記認識される空気量が、前記吸気系から前記排気系内へ導入される空気によって前記排気系に残留している排気ガスがガス交換され得る量を上回ったところで前記エンジンの燃焼室に対する空気の吸入を遮断させる吸入空気減量制御手段と、
前記燃焼室に対する空気の吸入が遮断された後、前記酸素濃度センサの信号が所定値に収束したところで、前記燃料供給の停止期間中に前記酸素濃度センサが出力した信号に関する数値情報を、基準となる酸素濃度に対応する数値情報として学習する学習手段と、
当該エンジンの排気系において前記酸素濃度センサより上流に設けられ、排気中の有害成分を浄化する排気浄化用触媒と、
前記燃料供給を停止させる制御が開始される時点で前記排気浄化用触媒の温度が所定値を下回っている場合には、前記学習及び前記学習に伴う吸入空気増量制御を禁止する学習禁止手段と、
を備えることを特徴とする酸素濃度検出装置。An oxygen concentration sensor that is provided in the exhaust system of the diesel engine and outputs a signal corresponding to the oxygen concentration in the exhaust;
A flow rate adjusting valve for adjusting the flow rate of air sucked into the combustion chamber of the engine;
Fuel cut control means for performing control to stop fuel supply under predetermined conditions during operation of the engine;
Intake air increase control means for increasing the flow rate of the intake air in synchronization with the stop of the fuel supply;
Integrated air intake amount recognition means for recognizing the amount of air introduced from the intake system of the diesel engine into the exhaust system through the combustion chamber by integrating the air taken into the combustion chamber;
When the recognized amount of air exceeds the amount by which the exhaust gas remaining in the exhaust system can be exchanged by the air introduced from the intake system into the exhaust system, the amount of air to the combustion chamber of the engine Intake air reduction control means for blocking inhalation;
After the intake of air into the combustion chamber is shut off, when the signal of the oxygen concentration sensor converges to a predetermined value, numerical information regarding the signal output by the oxygen concentration sensor during the fuel supply stop period is used as a reference. Learning means for learning as numerical information corresponding to the oxygen concentration
Set upstream of the oxygen concentration sensor in an exhaust system of the engine vignetting, an exhaust purifying catalyst for purifying harmful components in the exhaust,
Learning prohibiting means for prohibiting the learning and intake air increase control accompanying the learning when the temperature of the exhaust purification catalyst is below a predetermined value at the time when the control for stopping the fuel supply is started;
An oxygen concentration detection device comprising: 前記燃料供給の停止と同期して前記吸気流量が増大している際に、燃料供給が復帰した場合、燃料供給量の徐変処理を行う徐変処理手段を備えることを特徴とする請求項１記載の酸素濃度検出装置。  2. A gradual change processing means for performing a gradual change process of the fuel supply amount when the fuel supply is restored when the intake flow rate is increasing in synchronization with the stop of the fuel supply. The oxygen concentration detection apparatus described. 前記排気系に還元成分を供給する還元成分供給手段と、
前記燃料供給を停止させる制御の開始時若しくはその直前に前記排気系への還元成分の供給が行われた場合、前記学習を禁止する学習禁止手段と、
を備えることを特徴とする請求項１又は２記載の酸素濃度検出装置。Reducing component supply means for supplying a reducing component to the exhaust system;
A learning prohibiting means for prohibiting the learning when the reducing component is supplied to the exhaust system at the start of the control for stopping the fuel supply or immediately before the start;
The oxygen concentration detection apparatus according to claim 1, further comprising: 前記排気系に還元成分を供給する還元成分供給手段と、
前記燃料供給を停止させる制御の開始時から前記学習を終了時までの期間中には、前記排気系への還元成分の供給を禁止する還元成分供給禁止手段と、
を備えることを特徴とする請求項１〜３の何れかに記載の酸素濃度検出装置。Reducing component supply means for supplying a reducing component to the exhaust system;
Reducing component supply prohibiting means for prohibiting the supply of the reducing component to the exhaust system during a period from the start of the control for stopping the fuel supply to the end of the learning;
The oxygen concentration detection apparatus according to any one of claims 1 to 3, further comprising:
JP2002015870A 2002-01-24 2002-01-24 Oxygen concentration detection apparatus and oxygen concentration detection method Expired - Fee Related JP4428904B2 (en)

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ES03001529T ES2240861T3 (en) 2002-01-24 2003-01-23 DEVICE TO DETECT THE CONCENTRATION OF OXYGEN AND METHOD TO DETECT THE CONCENTRATION OF OXYGEN.
EP20030001529 EP1333171B1 (en) 2002-01-24 2003-01-23 Method and device for detecting oxygen concentration

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