JP4441973B2 - Engine exhaust purification system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to an engine exhaust purification device.
[0002]
[Prior art]
  Conventionally, as a catalyst for purifying exhaust gas of an internal combustion engine, a three-way catalyst capable of purifying HC (hydrocarbon), CO, and NOx (nitrogen oxide) in the exhaust gas at about the theoretical air-fuel ratio simultaneously and extremely effectively. In a gasoline engine, it is common to use this three-way catalyst and control the air-fuel ratio to approximately the stoichiometric air-fuel ratio in most of the operation region except the full load region.
[0003]
  However, since the diesel engine is in a state in which the air-fuel ratio is considerably lean (for example, A / F ≧ 18) in all normal operation regions, the three-way catalyst cannot be used, and the air-fuel ratio is in a lean state. Then, since the oxygen concentration in the exhaust gas becomes considerably high, it is difficult to sufficiently reduce and purify NOx in such an atmosphere.
[0004]
  On the other hand, a so-called NOx trap catalyst that absorbs NOx in an oxygen-excess atmosphere in which the oxygen concentration in the exhaust gas is a predetermined value (for example, 4%) or more and releases the absorbed NOx by reducing the oxygen concentration is used. There is technology. Since this NOx trap catalyst has a decrease in absorption performance when the NOx absorption amount increases, it is necessary to perform a so-called refresh to release the absorbed NOx before that happens.
[0005]
  Therefore, for example, as described in Japanese Patent Laid-Open No. 6-212961, in a diesel engine, a small amount of fuel is post-injected from the middle of the expansion stroke after the main fuel injection to the exhaust stroke when the refresh should be performed. In this way, the oxygen concentration in the exhaust gas is reduced.
[0006]
  On the other hand, in Japanese Patent Laid-Open No. 9-347524, in a diesel engine, a first catalyst device suitable for reducing and purifying NOx in a low temperature region is disposed upstream of the exhaust passage, and NOx in a high temperature region is disposed downstream thereof. The second catalyst device suitable for reducing and purifying the fuel is disposed, and main fuel injection for generating engine output is performed near the top dead center of the compression stroke, and the amount of hydrocarbons supplied to the catalyst is increased in the expansion stroke or the exhaust stroke. Therefore, it is described that NOx purification is effectively performed by performing the post fuel injection for the purpose and then controlling the fuel injection amount in accordance with the catalyst temperature.
[0007]
  That is, in view of the fact that the catalyst temperature rises due to the heat of HC oxidation reaction at the catalyst when the amount of HC in the exhaust gas increases due to post fuel injection, the NOx purification activity peaks for each of the first and second catalytic devices. When the temperature is lower than this temperature, the post fuel injection amount is increased so that NOx purification is performed near the peak temperature, and when the temperature is higher than the peak temperature, the post fuel injection amount is decreased to suppress the temperature rise of the catalyst. It is intended to purify NOx.
[0008]
  Japanese Patent Laid-Open No. 8-26152 discloses a NOx reduction catalyst in which the main injection of fuel is performed near the top dead center of the compression stroke of the engine and the fuel is post-injected in the expansion stroke or the exhaust stroke. It is described that the post-injection timing is delayed when the temperature is high compared to when the temperature is low. That is, by controlling the post-injection timing, HC having a small carbon number is increased in the exhaust gas when the catalyst temperature is low, and HC having a large carbon number in the exhaust gas is increased when the catalyst temperature is high. That's it. This is based on the knowledge that HC having a small number of carbons effectively works as a reducing agent for purifying NOx on the low temperature side, and HC having a large number of carbons works effectively as a reducing agent for purifying NOx on the high temperature side.
[0009]
[Problems to be solved by the invention]
  By the way, the NOx trap catalyst as described above is configured by supporting a catalyst component such as Ba that absorbs NOx and a catalyst component such as Pt that reduces and decomposes NOx in zeolite. Generally not very expensive. Therefore, it is often necessary to dispose an oxidation catalyst in the exhaust passage upstream of the NOx trap catalyst and oxidize and purify HC and CO in the exhaust gas with the upstream catalyst.
[0010]
  However, when such an oxidation catalyst is provided, HC and CO in the exhaust gas necessary for NOx reduction by the NOx trap catalyst are oxidized and purified by this oxidation catalyst, and a necessary amount of reducing agent is supplied to the NOx trap catalyst. It becomes difficult to be done.
[0011]
  The present invention solves the above problem when an oxidation catalyst is provided upstream of such a NOx trap catalyst.
[0012]
[Means for Solving the Problems]
  For this purpose, the present invention increases the amount of reducing agent such as HC in the exhaust gas by the above-described post-injection in order to release NOx from the NOx trap catalyst, and the reducing agent is oxidized and decomposed as much as possible by the upstream oxidation catalyst. The post-injection is controlled so as not to be performed.
[0013]
  That is, the present invention includes an injection valve that faces a combustion chamber of an engine and injects fuel into the combustion chamber;
  A NOx trap catalyst that is disposed in an exhaust passage extending from the combustion chamber and absorbs NOx in the exhaust gas when the oxygen concentration in the exhaust gas is high, and releases and reduces NOx due to a decrease in the oxygen concentration;
  In order to release NOx from the NOx trap catalyst, fuel is injected from the injection valve at a predetermined timing in the expansion stroke or exhaust stroke after the main injection in which fuel is injected by the injection valve in the vicinity of the top dead center of the compression stroke of the engine. In an engine exhaust purification device comprising a reducing agent increasing means for increasing a reducing agent amount in exhaust gas by a post injection for a predetermined period,
  An oxidation catalyst having an oxidation catalyst function is disposed in the exhaust passage upstream of the NOx trap catalyst,
  The reducing agent increasing means is:The post injection timing is set to 10 to 50 ° CA after the top dead center of the compression stroke,The post-injection is performed so that the increase amount of the reducing agent increases or decreases every predetermined time.Control so that the post-injection amount increases at intervals of once every 0.2 to 2 seconds,
When the post-injection amount increases, the amount of the main injection executed before the post-injection is reduced for the cylinder in which the post-injection amount is increased.It is characterized by that.
[0014]
  Here, the term “due to a decrease in oxygen concentration” means that the oxygen concentration in the exhaust gas may be, for example, less than 3 to 4% (preferably less than 1 to 2%), and the air-fuel ratio of the exhaust gas. Corresponds to a rich state that is approximately in the vicinity of the stoichiometric air-fuel ratio or smaller than the stoichiometric air-fuel ratio. Note that the air-fuel ratio of the exhaust gas is the ratio of the total amount of air supplied to the exhaust passage to the total amount of fuel. Unless secondary air or fuel is supplied to the exhaust passage, the air in the combustion chamber is empty. This corresponds to the fuel ratio (the average value of the air-fuel ratio of the combustion chamber, and the same shall apply hereinafter). This reduction in oxygen concentration is performed by increasing the amount of reducing agent in the exhaust gas by the post-injection control of fuel by the reducing agent increasing means.
[0015]
  Thus, in the present invention, since the amount of increase of the reducing agent increases or decreases every predetermined time, the reducing agent is easily blown through without being oxidized and decomposed by the upstream oxidation catalyst. The amount of reducing agent supplied to the NOx trap catalyst can be increased. Thereby, NOx is easily reduced and purified by the NOx trap catalyst.
[0016]
  The reason why such blow-by is likely to occur is not clear, but when the reducing agent is constantly supplied to the oxidation catalyst, the reducing agent is oxidatively decomposed at a substantially constant rate according to the temperature and catalytic ability at that time. In other words, although it is in an equilibrium state, if the amount of the reducing agent increases intermittently, such an equilibrium state cannot be obtained, so that the oxidative decomposition rate of the reducing agent is lowered, and a relatively large amount of blowout is generated.The
[0017]
NaHere, as a catalyst having an oxidation catalyst function, a catalyst mainly supporting a noble metal, Cu or the like, and reacts with NOx in a state where not only the oxidation catalyst and the three-way catalyst but also HC etc. are partially oxidized. It may be a NOx purification catalyst that reduces NOx.
[0018]
  In addition, the present inventionBeforeThe injection timing is set to 10-50 ° CA after the top dead center of the compression stroke, and once every 0.2 seconds to 2 seconds so that the increase amount of the reducing agent increases and decreases every predetermined time. Increase the post-injection amountThe
[0019]
  As a result, the amount of increase in the reducing agent increases or decreases every predetermined time, and the reducing agent easily blows through the upstream side oxidation catalyst and is supplied to the downstream side NOx trap catalyst. The reason for setting the post-injection timing after 10 ° CA after the compression stroke top dead center is to enable post-injection after the main injection is completed. The reason why the post-injection is performed before 50 ° CA is that the combustion gas is slightly cooled by this post-injection and the time for exposure to high temperature is shortened, which is advantageous in suppressing the generation of NOx. The interval for increasing the post-injection amount is set as described above. When the time is shorter than 0.2 seconds, the reducing agent blows out less, and when the time is longer than 2 seconds, NOx is released from the NOx trap catalyst. This is because sometimes the reducing agent is not used effectively because it is not in time.
[0020]
  Note that increasing the post-injection amount at intervals of once every 0.2 seconds means that, for example, if the engine speed is 1500 rpm, it will be performed at intervals of once every 10 main injections. Increasing the amount at an interval of once every 2 seconds means that the amount is increased once every 100 main injections.The
[0021]
MaIn the present invention, as described above, the post-injection timing is set to 10 to 50 ° CA after the top dead center of the compression stroke, and the post-injection amount is increased once every 0.2 to 2 seconds. In this case, when the subsequent injection amount increases, the amount of the main injection executed before the post-injection is reduced for the cylinder in which the post-injection amount is increased. When the post-injection amount increases, it works to increase the engine output, thereby reducing the main fuel injection amount, and avoiding the air-fuel ratio becoming rich and rich by reducing the main fuel injection amount. be able to.
[0022]
Further, according to the present invention, when the post-injection amount is increased at intervals of once every 0.2 to 2 seconds as described above, the subsequent injection timing is set to 30 to 50 ° CA after the top dead center of the compression stroke. be able to. This is advantageous for suppressing the generation of NOx and also reduces the amount of smoke generated.
[0023]
  Further, the present invention provides the engine exhaust purification apparatus as described above, wherein the reducing agent increasing means increases the reducing agent increase amount at the beginning of a predetermined period in which the reducing agent amount in the exhaust gas is increased. Controlling post-injectionCan. That is, if the amount of reducing agent in the exhaust gas is increased to reduce the oxygen concentration in the exhaust gas, a large amount of NOx is released from the NOx trap catalyst immediately after the oxygen concentration is reduced. This is to avoid a state in which the reducing agent is insufficient with respect to NOx.
[0024]
  In another aspect of the present invention, an injection valve that faces the combustion chamber of the engine and injects fuel into the combustion chamber;
A NOx trap catalyst that is disposed in an exhaust passage extending from the combustion chamber and absorbs NOx in the exhaust gas when the oxygen concentration in the exhaust gas is high, and releases and reduces NOx due to a decrease in the oxygen concentration;
In order to release NOx from the NOx trap catalyst, fuel is injected from the injection valve at a predetermined timing in the expansion stroke or exhaust stroke after the main injection in which fuel is injected by the injection valve in the vicinity of the top dead center of the compression stroke of the engine. In an engine exhaust purification device comprising a reducing agent increasing means for increasing a reducing agent amount in exhaust gas by a post injection for a predetermined period,
An oxidation catalyst having an oxidation catalyst function is disposed in the exhaust passage upstream of the NOx trap catalyst,
The reducing agent increasing means controls the post-injection so that the post-injection amount increases intermittently so that the increasing amount of the reducing agent increases or decreases every predetermined time;
When the post-injection amount increases, the amount of the main injection executed before the post-injection is reduced for the cylinder in which the post-injection amount is increased.
[0025]
Therefore, since the increase amount of the reducing agent increases or decreases every predetermined time, the reducing agent is easily blown through the upstream side oxidation catalyst without being decomposed by the upstream side oxidation catalyst, and is supplied to the downstream side NOx trap catalyst. The amount of reducing agent can be increased. Thereby, NOx is easily reduced and purified by the NOx trap catalyst. Further, when the post-injection amount increases, it works in the direction of increasing the engine output, so the main fuel injection amount is reduced. By reducing the main fuel injection amount, the air-fuel ratio is prevented from becoming significantly rich. be able to.
[0026]
【The invention's effect】
  As described above, according to the present invention, the oxidation catalyst is disposed in the exhaust passage upstream of the NOx trap catalyst, and the reducing agent in the exhaust gas is discharged by post-injection after the main injection in order to release NOx from the NOx trap catalyst. In the engine exhaust gas purification apparatus in which the amount is increased for a predetermined period, the post-injection is controlled so that the amount of increase of the reducing agent increases or decreases every predetermined time.At the same time, the main injection amount is reduced when the post-injection amount increases due to the changeTherefore, the reducing agent blows through the upstream oxidation catalyst and is easily supplied to the downstream NOx trap catalyst, which is advantageous for NOx reduction purification by the NOx trap catalyst.At the same time, it is advantageous to prevent the engine output from becoming excessively high, and the air-fuel ratio can be avoided from becoming rich and rich.
[0027]
  In addition, the timing of the post-injection is set to 10 to 50 ° CA after the top dead center of the compression stroke, and the post-injection amount is increased once every 0.2 to 2 seconds. For example, the amount of increase of the reducing agent increases or decreases every predetermined time, and the reducing agent easily blows through the upstream side oxidation catalyst and is supplied to the downstream side NOx trap catalyst. Further, after that, the injection timing is set after the top dead center of the compression stroke. According to the one set at 30 to 50 ° CA, it is advantageous in suppressing the generation of NOx and smoke..
[0028]
MaFurthermore, according to what controls the post-injection so that the amount of increase of the reducing agent is increased at the beginning of the predetermined period in which the amount of reducing agent in the exhaust gas is increased, immediately after the oxygen concentration in the exhaust gas has decreased. When a large amount of NOx is released from the NOx trap catalyst, a large amount of reducing agent can be supplied, which is advantageous for NOx reduction purification.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
  Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0030]
  FIG. 1 shows an overall configuration of an exhaust emission control device A for a diesel engine according to an embodiment of the present invention. Reference numeral 1 denotes an engine body of a multi-cylinder diesel engine mounted on a vehicle. The engine body 1 has a plurality of cylinders 2 (only one is shown), and a piston 3 is fitted in each cylinder 2 so as to be reciprocally movable. A combustion chamber 4 is formed. In addition, an injector (fuel injection valve) 5 is disposed at a substantially central portion of the upper surface of the combustion chamber 4 with the injection hole at the tip facing the combustion chamber 4, and the injection hole is provided at a predetermined injection timing for each cylinder. Is opened and closed to inject fuel directly into the combustion chamber 4.
[0031]
  Each injector 5 is connected to a common common rail (pressure accumulating chamber) 6 for storing high-pressure fuel, and a high-pressure supply pump 8 driven by a crankshaft 7 is connected to the common rail 6. The high-pressure supply pump 8 operates so that the fuel pressure in the common rail 6 detected by the pressure sensor 6a is maintained at a predetermined value or more. Further, a crank angle sensor 9 for detecting the rotation angle of the crankshaft 7 is provided. The crank angle sensor 9 is provided with a plate to be detected (not shown) provided at the end of the crankshaft 7 and an outer periphery thereof. The electromagnetic pickup is arranged so as to be opposed to each other, and the electromagnetic pickup outputs a pulse signal corresponding to the passage of protrusions formed at predetermined angles on the entire outer periphery of the plate to be detected. ing.
[0032]
  Reference numeral 10 denotes an intake passage for supplying intake air (air) filtered by an air cleaner (not shown) to the combustion chamber 4 of the engine body 1. A surge tank (not shown) is provided at the downstream end of the intake passage 10. Each passage branched from the surge tank is connected to the combustion chamber 4 of each cylinder 2 by an intake port. Further, the surge tank is provided with an intake pressure sensor 10a for detecting a supercharging pressure supplied to each cylinder 2. In the intake passage 10, in order from the upstream side to the downstream side, a hot film type air flow sensor 11 that detects an intake air flow rate sucked into the engine body 1 and a blower 12 that is driven by a turbine 21 to be described later and compresses the intake air. An intercooler 13 for cooling the intake air compressed by the blower 12 and an intake throttle valve (intake air amount adjusting means) 14 for reducing the cross-sectional area of the intake passage 10 are provided. The intake throttle valve 14 is a butterfly valve provided with a notch so that intake air can flow even in a fully closed state. Like the EGR valve 24 described later, the magnitude of the negative pressure acting on the diaphragm 15 is negative. The opening degree of the valve is controlled by being adjusted by the control electromagnetic valve 16. The intake throttle valve 14 is provided with a sensor (not shown) for detecting the opening degree.
[0033]
  An exhaust passage 20 discharges exhaust gas from the combustion chamber 4 of each cylinder 2, and is connected to the combustion chamber 4 of each cylinder 2 via an exhaust manifold. In this exhaust passage 20, in order from the upstream side to the downstream side, an O 2 sensor 17 that detects the oxygen concentration in the exhaust gas, a turbine 21 that is rotated by the exhaust flow, and HC, CO, and NOx in the exhaust gas. And a catalytic converter 22 capable of purifying the gas. The catalytic converter 22 is provided with a temperature sensor 18 for detecting the temperature of the catalyst.
[0034]
  As shown in FIG. 2, the catalytic converter 22 includes an oxidation catalyst 22a for oxidizing and purifying HC and CO and a NOx trap catalyst 22b arranged in series on the upstream side and the downstream side in the exhaust gas flow direction. It is.
[0035]
  In each of the catalysts 22a and 22b, a catalyst layer is formed on the wall surface of each through hole of a cordierite support having a honeycomb structure having a large number of through holes extending in parallel in the axial direction. The catalyst layer of the oxidation catalyst 22a is formed by supporting catalyst powder obtained by supporting Pt on alumina and ceria on the carrier with a binder. The catalyst layer of the NOx trap catalyst 22b is formed by supporting catalyst powder obtained by supporting Pt and Ba on zeolite on the carrier with a binder. When the oxygen concentration of the exhaust gas is high (for example, when the air / fuel ratio is leaner than the stoichiometric air / fuel ratio (for example, A / F ≧ 18) and the oxygen concentration is 4% or more), the NOx trap catalyst 22b When NOx in the gas is absorbed by Ba and the oxygen concentration is reduced and, for example, near the excess oxygen ratio λ = 1, the absorbed NOx is released and the NOx is reduced and purified by Pt.
[0036]
  An exhaust gas recirculation passage (hereinafter referred to as an EGR passage) 23 that recirculates a part of the exhaust gas to the intake side branches from a portion of the exhaust passage 20 upstream of the turbine 21, and the downstream end of the EGR passage 23 is the intake air. The intake passage 10 is connected to the downstream side of the throttle valve 14. An exhaust gas recirculation amount adjustment valve (exhaust gas recirculation amount adjusting means: hereinafter referred to as an EGR valve) 24 whose opening degree can be adjusted is disposed near the downstream end in the middle of the EGR passage 23, and a part of the exhaust gas in the exhaust passage 20. Is recirculated to the intake passage 10 while the flow rate is adjusted by the EGR valve 24.
[0037]
  The EGR valve 24 is of a negative pressure responsive type, and a negative pressure passage 27 is connected to a negative pressure chamber of the valve box. The negative pressure passage 27 is connected to a vacuum pump (negative pressure source) 29 via a negative pressure control electromagnetic valve 28. The electromagnetic valve 28 is connected to a negative pressure passage by a control signal (current) from an ECU 35 described later. The EGR valve drive negative pressure in the negative pressure chamber is adjusted by communicating / blocking 27, whereby the opening degree of the EGR passage 23 is adjusted linearly.
[0038]
  The turbocharger 25 is a VGT (variable geometry turbo), to which a diaphragm 30 is attached, and a negative pressure acting on the diaphragm 30 is adjusted by an electromagnetic valve 31 for negative pressure control. Thus, the cross-sectional area of the exhaust gas passage is adjusted.
[0039]
  Each injector 5, high pressure supply pump 8, intake throttle valve 14, EGR valve 24, turbocharger 25, etc. are configured to operate in response to control signals from a control unit (Engine Control Unit: hereinafter referred to as ECU) 35. Yes. On the other hand, the ECU 35 receives an output signal from the pressure sensor 6a, an output signal from the crank angle sensor 9, an output signal from the pressure sensor 10a, an output signal from the air flow sensor 11, and an output from the O2 sensor 17. An output signal, an output signal from the temperature sensor 18, an output signal from the lift sensor 26 of the EGR valve 24, and an accelerator opening sensor that detects an operation amount (accelerator opening) of an accelerator pedal (not shown) by the driver of the vehicle. The output signal from 32 is input at least.
[0040]
  The fuel injection amount and fuel injection timing by the injector 5 are controlled according to the operating state of the engine body 1 and the states of the catalysts 22a and 22b, and the common rail pressure by the operation of the high-pressure supply pump 8, that is, the fuel injection pressure. In addition to this, the control of the intake air amount by the operation of the intake throttle valve 14, the control of the exhaust gas recirculation amount by the operation of the EGR valve 24, the operation control (VGT control) of the turbocharger 25, Is to be performed.
[0041]
  (Fuel injection control)
  The ECU 35 is provided with a fuel injection amount map in which an optimum fuel injection amount Qb experimentally determined according to changes in the target torque and the rotational speed of the engine body 1 is electronically stored in a memory. ing. Based on the target torque obtained based on the output signal from the accelerator opening sensor 32 and the engine speed obtained based on the output signal from the crank angle sensor 9, the main injection amount Qb is calculated from the fuel injection amount map. Is read and the excitation time (valve opening time) of each injector 5 is determined based on the main injection amount Qb and the common rail pressure detected by the pressure sensor 6a. By this main fuel injection control, an amount of fuel corresponding to the target torque of the engine body 1 is supplied, and the engine body 1 is operated in a state where the average air-fuel ratio in the combustion chamber 4 is considerably lean (A / F ≧ 18). The
[0042]
  Further, during steady operation (when the change in the accelerator opening is small), when the NOx trap amount of the NOx trap catalyst 22b is equal to or greater than a predetermined value, λ = 1 is set to release NOx from the NOx trap catalyst 22b. In order to reduce and purify this, post-injection control for increasing the amount of reducing agent in the exhaust gas is executed. The post-injection is to inject an appropriate amount of fuel in the expansion stroke or the exhaust stroke after the main injection (main fuel injection).
[0043]
  A feature of the present invention resides in the post-injection control, and the reducing agent increase amount is increased or decreased every predetermined time.changeI try to let them.
[0044]
  The details of the control will be specifically described below based on the control flow shown in FIG. This control is executed at every predetermined crank angle.
[0045]
  First, in step S1 after the start, a crank angle signal, an airflow sensor output, an accelerator opening, a temperature sensor output, and the like are read. In the subsequent step S2, the NOx trap amount Nx of the NOx trap catalyst 22b is estimated. The NOx trap amount Nx can be estimated based on the engine speed or engine history from the previous NOx release control, or simply based on the engine operating time. Alternatively, it may be estimated by arranging a sensor for detecting the NOx concentration in the exhaust gas in the exhaust passage 20 downstream of the NOx trap catalyst 22b.
[0046]
  In step S3, if it is determined that the NOx trap amount Nx exceeds the predetermined value Nxo, the process proceeds to step S4 to increment the timer T, and even if the NOx trap amount Nx does not exceed the predetermined value Nxo, step S5 is performed. When it is determined that the timer T is counting, the process proceeds to step S4 and increments it. The predetermined value Nxo is a NOx trap amount near a limit value at which the NOx trap in the NOx trap catalyst 22b is saturated.
[0047]
  If it is determined in the following step S6 that the timer T has not reached the predetermined value To, the routine proceeds to injection control for setting λ = 1 in step S9 and subsequent steps, and when the timer T has reached the predetermined value To, step S7 is performed. The routine proceeds to step S8 to set the timer T to zero, and then proceeds to step S8 to set the injection amount and the injection timing for operating the engine with the normal air-fuel ratio lean. When it is determined in step S5 that the timer T is not counting, the process proceeds to step S8. The predetermined value To corresponds to a time for performing NOx release control (for example, 2 to 5 seconds).
[0048]
  Step S8 will be described first. Here, the main injection amount Qb and the injection timing Ib are set according to the engine operating state. The main injection amount Qb is read from the fuel injection amount map based on the accelerator opening and the engine speed. The fuel injection amount map records the optimal injection amount Qb experimentally determined according to changes in the accelerator opening and the engine speed, and the main injection amount Qb increases as the accelerator opening increases. The higher the number of revolutions, the greater the setting. The main injection timing Ib is set near the top dead center of the compression stroke. For example, with the BTDC 5 ° CA (crank angle) as a reference, the main injection timing Ib is advanced as the injection amount Qb increases, and conversely as the injection amount Qb decreases. Further, based on the engine water temperature, when the water temperature is low, the main injection timing Ib is retarded by a predetermined amount and the warm-up operation is performed. Further, the post injection amount Qp is set to zero.
[0049]
  Next, the injection control for setting λ = 1 (the oxygen concentration in the exhaust gas is 0.5% or less) will be described. In step S9, the cylinder to which the post injection amount Qpλ is to be increased is determined. The post-injection amount Qpλ is increased for the cylinder in which the post-injection is first performed, and is performed at intervals of 25 post-injections counted from the cylinder. Therefore, the cylinders for which the post-injection amount Qpλ should be increased are the cylinders when the timer T = 1 and the timer T is 21, 41, 61,. Each cylinder when it becomes.
[0050]
  In the subsequent step S10, the main injection amount Qbλ and the injection timing Ibλ for setting λ = 1, the post-injection amount Qpλ and the injection timing Ibλ are set. The post-injection is performed after the main injection for all the cylinders, and the main injection amount Qbλ is the main injection amount Qb during a normal lean operation in order to avoid an excessive increase in engine output due to the effect of the post-injection. The post-injection amount Qpλ is set to, for example, 30 to 50% (for example, 40%) of the main injection amount Qbλ. The main injection timing Ibλ can be made the same as the injection timing Ib during lean operation. The post-injection timing Ibλ is set to 10-50 ° CA after the compression stroke top dead center (ATDC), for example, ATDC 15 ° CA.
[0051]
  In the following step S11, it is determined whether or not the cylinder is to be increased in the post-injection amount. If the cylinder is to be increased, the process proceeds to step S12, and the main injection amount Qbλ is decreased by Δα, and the post-injection amount Qpλ is determined. Increase by Δα. As Δα, 0.5 to 7%, for example, 5% of the main injection amount Qb is given. As a result, the post-injection amount Qpλ is increased once every 25 times. This is because HC as a reducing agent in the exhaust gas blows through the downstream side without being oxidized by the upstream oxidation catalyst 22a. This is to make it easier to reach the NOx trap catalyst 22b on the side. The decrease in the main injection amount Qbλ is to avoid an extra increase in engine output accompanying an increase in the post-injection amount Qpλ.
[0052]
  If it is determined in the subsequent step S13 that the timer T = 1, the routine proceeds to step S14, where the main injection amount Qbλ is further decreased by Δβ, and the post injection amount Qpλ is further increased by Δβ. As Δβ, several percent of the main injection amount Qb, for example, 5% is given. In this way, the initial post-injection amount Qpλ is increased because a large amount of NOx is released from the NOx trap catalyst 22b at the beginning of the shift to λ = 1 operation, so that HC as a reducing agent for reducing this is reduced. This is to avoid a shortage. The decrease in the main injection amount Qbλ is to avoid an extra increase in engine output accompanying an increase in the post-injection amount Qpλ. Thereby, the torque shock at the time of transition from lean to λ = 1 is alleviated.
[0053]
  Further, EGR feedback control is performed based on the output of the air flow sensor 11 so that the EGR amount is increased during the operation of λ = 1 as compared with the lean operation. This is because the fuel efficiency is worse in λ = 1 operation than in lean operation, but the pumping loss is reduced by increasing the EGR, and the fuel is vaporized and atomized by introducing a lot of high-temperature exhaust gas into the combustion chamber. This is to promote.
[0054]
  In the following step S15, Qbλ, Ibλ, Qpλ, and Ibλ set as described above are given as the main injection amount Qb, the main injection timing Ib, the post injection amount Qp, and the post injection timing Ib, and the process proceeds to step S16 and thereafter. When the injection amount and the injection timing for the normal lean operation are set in step S8, the process proceeds to step S16 and subsequent steps. That is, when the main injection timing Ib is reached, the main injection is executed (steps S16 and S17). When the post-injection should be performed for the cylinder that has performed the main injection (step S18), the post-injection timing Ip is reached. The injection is executed and the process returns (steps S19 and S20).
[0055]
  FIG. 4 is a time chart schematically showing the fuel injection timing. FIG. 4A shows the post-injection amount in step S12 when the post-injection amount is not increased while the post-injection amount is set in step S10. (C) in the figure is when the post-injection amount is increased in step S14. For each of the post-injection increases Δα and Δβ, there is no increase form as shown in FIGS. 7B and 7C, and as shown in FIG. It is good also as a form which performs post injection. In this case, the influence of the increase in the post-injection amount due to the second post-injection is less affected by the engine output, so that it is possible to omit the decrease in the main injection amount.
[0056]
  FIG. 5 is a time chart schematically showing the increase / decrease in the post-injection amount. The initial post-injection amount becomes the largest because Δα in step S12 and Δβ in step S14 are added at the beginning of the shift to λ = 1 operation. Thereafter, it increases by the addition of Δα in step S12 every 25 times.
[0057]
  FIG. 6 shows the exhaust gas flowing into the upstream side catalyst 22a when the post-injection is performed “every time” and “every 25 times” with the post-injection timing set to ATDC 90 ° CA in the middle rotation / medium load operation of the engine. 3 is a time chart of the HC concentration of gas (inlet HC) and the HC concentration of exhaust gas flowing out from the catalyst 22a (outlet HC).
[0058]
  Performing the post-injection “every time” means that when the main injection is performed for each cylinder in a predetermined order, the post-injection is performed for each main injection for each cylinder. Performing the post-injection “every 25 times” means that when the main injection is performed for each cylinder in a predetermined order, the post-injection is performed at intervals of once every 25 main injections. For example, when there are four cylinders A, B, C, and D and main injection is performed in this order, when the post-injection is first performed for the A cylinder, the subsequent B, C, D, A. This means that post-injection is not performed for each of the cylinders (thinning), and post-injection is performed for the next B cylinder. In the figure, the post-injection amount of “Every time A” is the same as the post-injection amount of “Every 25 times”, that is, the post-injection amount of “Every 25 times” is one time of “Every time A”. The post-injection amount is 25 times larger than that of “every time B”, and “every time A” has a larger post-injection amount, and “every time C” has a larger post-injection amount.
[0059]
  FIG. 7 shows the data when the injection timing is set to ATDC 45 ° CA only for “every 25 times”, together with the data of “every time A”, “every time B”, and “every time C”.
[0060]
  According to FIG. 6, there is a large difference in the amount of HC in the exhaust gas that is blown through without being oxidized by the upstream catalyst 22a between “every 25 times” and “every time A” where the total amount of the post-injection is the same. Absent. However, as shown in FIG. 7, when the post-injection timing of “every 25 times” is ATDC 45 ° CA, the outlet HC increases to a value close to “every time C” where the post-injection amount is large. From this, it can be seen that if the post-injection is “every 25 times” at ATDC 50 ° CA or less (advance side), HC in the exhaust gas easily blows through the upstream catalyst 22a without being oxidized.
[0061]
  FIG. 8 shows a case where the post-injection is performed “every time”, “every 5 times”, and “every 25 times” in an engine speed of 1500 rpm and medium load operation. The relationship between the injection timing and the HC concentration (pre-catalyst HC) of exhaust gas flowing into the upstream catalyst 22a 10 seconds after the start of post-injection and the HC concentration of exhaust gas flowing out from the catalyst 22a (post-catalyst HC) is shown. . FIG. 9 shows the same relationship 30 seconds after the start of post-injection, and FIG. 10 shows the same relationship 90 seconds after the start of post-injection. The exhaust gas temperature at the inlet of the upstream catalyst 22a is about 270 ° C. Further, the total amount of post-injection “every time”, “every 5 times”, and “every 25 times” is the same.
[0062]
  8 to 10, in the case of “every 25 times” post-injection, when the post-injection timing is less than ATDC 60 ° CA (advance side), the amount of blow-through HC passing through the upstream catalyst 22a without being oxidized. In particular, when the post-injection timing is set to ATDC 30 to 50 ° CA at “every 25 times”, it can be seen that the HC blow-off performance is good.
[0063]
  From the above experimental results of FIGS. 6 to 10, the post-injection is performed every time in the operation of λ = 1 as in the above-described embodiment, but thereafter the injection amount is increased every predetermined time (every 25 times in the above-mentioned embodiment). If the post-injection control is performed, the amount of HC that blows through the upstream catalyst 22a and reaches the downstream NOx trap catalyst 22b increases without increasing the total amount of post-injection. From this NOx trap catalyst 22b, It can be seen that it is advantageous to reduce and purify the released NOx efficiently, that is, without deteriorating the fuel consumption.
[0064]
  FIG. 11 shows the cases where the post-injection is performed “every time”, “every 5 times”, and “after every 25 times” in the medium load operation at the engine speed 1500 rpm. The relationship between the post-injection timing and the NOx concentration of the exhaust gas flowing into the upstream catalyst 22a 10 seconds and 30 seconds after the start of post-injection is shown. The total amount of post-injection was set to about 4% of the total amount of main injection in all cases.
[0065]
  When the post-injection timing is advanced, the NOx concentration is low in each of “every time”, “every 5 times”, and “every 25 times”, but in “every 25 times”, the post-injection time is set to ATDC 30 to When the temperature is around 50 ° CA, the NOx concentration is significantly reduced. Therefore, as in the above-described embodiment, if post-injection control is performed such that the post-injection is performed every time in the operation of λ = 1, but thereafter the injection amount is increased every predetermined time (every 25 times in the above-described embodiment), NOx It can be seen that an effect of reducing the generation amount can be obtained.
[0066]
  FIG. 12 shows the post-injection timing and smoke (煤) amount when the post-injection is performed every time (the post-injection amount is 5% of the main injection amount) in the operation at 2000 rpm of the engine and Pe = 0.57 MPa (no EGR). Shows the relationship. According to FIG. 10, it can be seen that when the post-injection is performed, the smoke amount is reduced as compared with the case where there is no post-injection, and when the post-injection timing is advanced, it is advantageous for reducing the smoke.
[0067]
  In addition, although the said embodiment is related with a direct injection type diesel engine, this invention is applicable also to a direct injection type gasoline engine.
[Brief description of the drawings]
FIG. 1 is a diagram showing an overall configuration of an exhaust emission control device for a diesel engine according to an embodiment of the present invention.
FIG. 2 is a diagram showing a configuration of a catalytic converter.
FIG. 3 is a flowchart of fuel injection control.
FIG. 4 is a time chart schematically showing fuel injection timing.
FIG. 5 is a time chart schematically showing an increase / decrease in the post-injection amount.
FIG. 6 is a time chart of the HC concentration (inlet HC) of exhaust gas flowing into the upstream catalyst and the HC concentration (outlet HC) of exhaust gas flowing out from the catalyst.
FIG. 7 is a time chart of the HC concentration (inlet HC) of exhaust gas flowing into the upstream catalyst and the HC concentration (outlet HC) of exhaust gas flowing out from the catalyst.
FIG. 8 is a graph showing the relationship between the post injection timing and the upstream pre-catalyst HC concentration and the upstream post-catalyst HC concentration 10 seconds after the start of post injection.
FIG. 9 is a graph showing the relationship between the post-injection timing and the upstream pre-catalyst HC concentration and the upstream post-catalyst HC concentration 30 seconds after the start of post-injection.
FIG. 10 is a graph showing the relationship between the post-injection timing and the upstream pre-catalyst HC concentration and the upstream post-catalyst HC concentration 90 seconds after the start of post-injection.
FIG. 11 is a graph showing the relationship between the post-injection timing and the NOx concentration of the exhaust gas flowing into the upstream catalyst 10 seconds and 30 seconds after the start of post-injection.
FIG. 12 is a graph showing the relationship between the post-injection timing and the smoke (soot) amount.
[Explanation of symbols]
  A Exhaust gas purification device
  1 Diesel engine
  2-cylinder
  4 Combustion chamber
  5 Injector (fuel injection valve)
  20 Exhaust passage
  22 Catalytic converter
  22a Upstream oxidation catalyst
  22b NOx trap catalyst downstream
  35 ECU (control unit)

Claims (4)

An injection valve that faces the combustion chamber of the engine and injects fuel into the combustion chamber;
A NOx trap catalyst that is disposed in an exhaust passage extending from the combustion chamber and absorbs NOx in the exhaust gas when the oxygen concentration in the exhaust gas is high, and releases and reduces NOx due to a decrease in the oxygen concentration;
In order to release NOx from the NOx trap catalyst, fuel is injected from the injection valve at a predetermined timing in the expansion stroke or exhaust stroke after the main injection in which fuel is injected by the injection valve in the vicinity of the top dead center of the compression stroke of the engine. In an engine exhaust purification device comprising a reducing agent increasing means for increasing a reducing agent amount in exhaust gas by a post injection for a predetermined period,
An oxidation catalyst having an oxidation catalyst function is disposed in the exhaust passage upstream of the NOx trap catalyst,
The reducing agent increasing means sets the timing of the post-injection to 10 to 50 ° CA after the top dead center of the compression stroke, and sets the post-injection to 0 so that the increasing amount of the reducing agent increases and decreases every predetermined time. .Control to increase the post-injection amount at intervals of once every 2 to 2 seconds,
An exhaust emission control device for an engine, characterized in that when the post-injection amount increases, the amount of the main injection that is executed before the post-injection is reduced for the cylinder in which the post-injection amount is increased . The exhaust emission control device for an engine according to claim 1 ,
An exhaust emission control device for an engine, characterized in that the post-injection timing is 30 to 50 ° CA after the top dead center of the compression stroke. The exhaust emission control device for an engine according to claim 1,
The engine exhaust gas purification apparatus according to claim 1, wherein the reducing agent increasing means controls the post-injection so that an increasing amount of the reducing agent is increased at the beginning of a predetermined period of increasing the reducing agent amount in the exhaust gas. An injection valve that faces the combustion chamber of the engine and injects fuel into the combustion chamber;
  A NOx trap catalyst that is disposed in an exhaust passage extending from the combustion chamber and absorbs NOx in the exhaust gas when the oxygen concentration in the exhaust gas is high, and releases and reduces NOx due to a decrease in the oxygen concentration;
  In order to release NOx from the NOx trap catalyst, fuel is injected from the injection valve at a predetermined timing in the expansion stroke or exhaust stroke after the main injection in which fuel is injected by the injection valve in the vicinity of the top dead center of the compression stroke of the engine. In an engine exhaust purification device comprising a reducing agent increasing means for increasing a reducing agent amount in exhaust gas by a post injection for a predetermined period,
  An oxidation catalyst having an oxidation catalyst function is disposed in the exhaust passage upstream of the NOx trap catalyst,
  The reducing agent increasing means controls the post-injection so that the post-injection amount increases intermittently so that the increasing amount of the reducing agent increases or decreases every predetermined time;
  An exhaust emission control device for an engine, characterized in that, when the post-injection amount increases, the amount of the main injection executed before the post-injection is reduced for the cylinder in which the post-injection amount is increased.

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