JP2004218518A - Air-fuel ratio control method for engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an air-fuel ratio control method for an engine that enables small torque fluctuation when temporarily generating a rich condition of exhaust gas to recover a NOx occlusion capacity of a NOx occlusion reduction type catalyst arranged on an exhaust path of the engine. <P>SOLUTION: This air-fuel ratio control method is designed for the engine 1 having an exhaust emission control system 10 with the NOx occlusion reduction type catalyst 32 arranged on the exhaust path 2 of the engine 1. When the rich condition of the exhaust gas is temporarily generated to regenerate the NOx occlusion reduction type catalyst 32, the loaded condition of the engine 1 is judged. During at the time of a low load, the rich condition of the exhaust gas is generated by retarding air intake throttling and main injection, and at the time of intermediate and high loads, the rich condition of the exhaust gas is generated by post injection. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an air-fuel ratio control method for an engine provided with an exhaust purification system having an NOx storage reduction catalyst in an exhaust passage of the engine, and more particularly, to an engine that creates rich conditions for regeneration of a NOx storage reduction catalyst. The present invention relates to an air-fuel ratio control method.
[0002]
[Prior art]
In order to reduce NOx in exhaust gas of a diesel engine or the like, application of a NOx storage reduction catalyst that stores nitrogen oxides (NOx) has been considered. As shown in FIGS. 6 and 7, this NOx storage reduction type catalyst has a catalyst 32b such as platinum Pt and a NOx storage such as barium oxide (BaO ₂ ) on a carrier 32a such as γ-alumina (Al ₂ O ₃ ). The NOx storage catalyst 32c is formed to support the material (absorbent) 32c, and purifies NOx according to the mechanism for storing and releasing NOx of the NOx storage catalyst.
[0003]
That is, as shown in FIG. 6, in a lean state in which the amount of air is larger than the stoichiometric air-fuel ratio, oxygen (O ₂ ) adheres to the surface of the Pt 32b, and nitrogen monoxide (NO) becomes NO + O ₂ → NO ₂ Is converted to nitrogen dioxide (NO ₂ ), and this NO ₂ is oxidized on Pt and stored in the NOx storage material 3c as NO ₃ ^{− in} the form of Ba (NO ₃ ) ₂ or the like.
[0004]
Further, as shown in FIG. 7, when the rich state of the near stoichiometric air-fuel ratio is, NOx storage material 32c by a decrease in O ₂ concentration was release NOx adsorbed in the lean state, the NOx hydrocarbon (HC) And the like, and is reduced to nitrogen N ₂ by a reaction similar to the three-way catalyst.
[0005]
Then, as in the normal operation of the diesel engine, the continuously stores NOx in the operating state of the lean condition, NOx-absorbing material is changed, for example, Ba (NO _{3) 2,} since storage capacity will reach saturation, In order to achieve sufficient NOx purification performance, before reaching the saturation, the operating condition of the engine is temporarily changed to the operating condition of the rich condition to generate a rich condition in the exhaust gas to recover the storage capacity. There is a need to. Therefore, during the operation of the engine, the operating condition of the engine is switched between a lean condition and a rich condition.
[0006]
However, when switching from the lean condition to the rich condition, there is a problem that the engine output torque sharply increases and a large shock (torque shock) occurs. For this reason, a technique for suppressing torque fluctuation when shifting from such a lean condition to a rich condition (rich spike control) has been proposed.
[0007]
As one of the techniques, when the air-fuel ratio is switched from the lean side to the rich side, torque fluctuations before and after the switching are directly detected, and based on the detected value, the degree of enrichment is changed, and a means other than the air-fuel ratio is used. There is a technique for suppressing the torque fluctuation by changing the engine output torque (for example, see Patent Document 1).
[0008]
[Patent Document 1]
JP-A-8-319862 (Pages 2 to 3)
[0009]
[Problems to be solved by the invention]
In the related art, the generation of the rich condition is often performed by adding post-injection to the two-stage injection of the normal pilot injection and the main injection. When the post injection is performed, there is a problem that the torque of the engine is increased more than necessary and a torque shock occurs.
[0010]
SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to temporarily enrich exhaust gas in order to restore the NOx storage capacity of a NOx storage reduction catalyst provided in an exhaust passage of an engine. An object of the present invention is to provide an engine air-fuel ratio control method with less torque fluctuation when setting conditions.
[0011]
[Means for Solving the Problems]
An air-fuel ratio control method for an engine according to the present invention for achieving the above object is an air-fuel ratio control method for an engine provided with an exhaust purification system having a NOx storage reduction catalyst in an exhaust passage of the engine, the method comprising: In order to regenerate the reduction catalyst, when the exhaust gas is temporarily set to the rich condition, the load condition of the engine is determined, and when the load is low, the rich condition of the exhaust gas is created by the intake throttle and the retard of the main injection, At the time of medium load and high load, the exhaust gas rich condition is created by post-injection.
[0012]
In other words, a method of creating a rich condition is performed by adding post-injection at medium load and high load depending on the load state of the engine, and at low load by performing intake throttle to narrow the intake throttle and retarding main injection, This is performed by increasing the fuel injection amount without lowering the efficiency of the engine and increasing the torque.
[0013]
With this configuration, the exhaust gas rich condition can be reliably established without increasing the torque regardless of the load of the engine.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an air-fuel ratio control method for an engine according to the present invention will be described with reference to the drawings.
[0015]
FIG. 1 shows an exhaust gas purification system for an engine, FIG. 2 shows a flowchart of air-fuel ratio control, and FIGS. 3 to 5 show injection timings for creating a rich condition under medium load and high load.
[0016]
As shown in FIG. 1, in an exhaust gas purification system 10 for an engine, an intake air amount sensor (air mass sensor) 21, a compressor 31 a of a turbocharger with variable nozzle (VNT) 31 and an intercooler 22 are provided in an intake passage 2 of the engine 1. An intake throttle (intake throttle) 23 is provided, a turbine 31b of a VNT 31 and a NOx storage catalyst 32 are provided in an exhaust passage 3, and an EGR cooler 41 and an EGR valve 42 are provided in an EGR passage (exhaust circulation passage) 4. I have.
[0017]
The fuel injection system includes a fuel pump 51 for supplying fuel F from a fuel tank (not shown) to a combustion chamber 54 of the engine, a common rail 52, and a fuel injection valve (injector) 53. A control device 60 called an ECU (engine control unit) for controlling the engine by inputting the degree Acc, the engine speed Ne, the crank angle CA and the like is provided.
[0018]
In the method of controlling the air-fuel ratio of an engine according to the present invention, the engine operating range is determined based on the engine speed Ne and the fuel injection amount Q calculated from the accelerator angle Acc to generate the exhaust gas rich condition. The fuel injection method is determined.
[0019]
When the load is low, the main injection (main injection) is retarded to lower the combustion efficiency and increase the fuel injection amount to create a rich condition. That is, since the rich condition cannot be achieved only by the retard of the main injection, the injection amount is increased by advancing the pilot injection, and the intake air amount is reduced by narrowing the intake throttle 23 so that the rich condition is easily created.
[0020]
In the case of medium load and high load, a rich condition is created by adding post-injection (post-injection) at a crank angle sufficiently delayed so as not to affect the torque. In this case, the pilot injection and the main injection, the EGR valve 42, the VNT 31, and the intake throttle 23 continue the same control as the lean condition before the transition to the rich condition.
[0021]
This air-fuel ratio control is performed according to a control flow as shown in FIG. This control flow is a flow that is executed in parallel with the control flow of the engine, and is started when the operation of the engine is started, and is stopped when the operation of the engine is completed, that is, with the interruption of the engine key OFF. You.
[0022]
When the control flow starts, in step S11, the engine speed Ne and the fuel injection amount Q are input, and the lean set time tls is calculated from the engine speed Ne and the fuel injection amount Q by referring to a lean set time map. .
[0023]
In step S12, the engine speed Ne and the fuel injection amount Q are input in order to perform the operation control under the normal lean condition, and the base-pilot injection amount map and the base pilot injection amount map are obtained from the engine speed Ne and the fuel injection amount Q. The pilot injection timing Tpb, the pilot injection amount Qpb, and the main injection timing Tmb are calculated with reference to the pilot injection timing map and the base / main injection timing map, and the calculated amounts are output. Inject fuel.
[0024]
In the operating state under the lean condition, the pilot injection and the main injection are performed at timings as illustrated in FIG. 3, the air-fuel ratio (A / F) of the exhaust gas is, for example, about 40, and the purification mechanism as illustrated in FIG. , _{O 2} adheres to the surface of Pt32b, NO by reaction of NO + _{O 2} → NO _2, becomes NO _2, the NO ₂ while being oxidized on Pt in the NOx storage material 32c NO ₃ ^- as Ba ( NO ₃ ) _2, etc., and the exhaust gas G is purified and discharged into the atmosphere as purified exhaust gas Gc.
[0025]
In the next step S13, it is determined whether or not the lean condition control has been completed, that is, whether or not the lean integrated time tl operating under the lean condition has exceeded the lean set time tls. Step S12 is repeatedly executed until the process is completed.
[0026]
If it is determined in step S13 that the lean accumulated time tl has exceeded the lean set time tls and the lean condition control has been completed, in step S14, the engine speed Ne and the fuel injection amount Q are input, and the engine The rich set time trs is calculated from the rotational speed Ne and the fuel injection amount Q with reference to the rich set time map.
[0027]
Next, in step S15, it is determined whether the operating state of the engine is in the low load region. This determination is made by referring to the operation region determination map based on whether the portion corresponding to the engine speed Ne and the fuel injection amount Q is a low load region, a medium load region, or a high load region.
[0028]
If it is determined in step S15 that the operation state of the engine is in the low load range, the engine speed Ne and the fuel injection amount Q are input in step S16, and the engine speed Ne and the fuel injection amount Q are input. From the intake throttle valve opening amount map, the rich pilot injection timing map, the rich pilot injection amount map, and the rich main injection timing map, the opening degree Th of the intake throttle for the rich control operation, and the pilot injection timing. Tpr, pilot injection amount Qpr, and main injection timing Tmr are calculated and output, and the calculated amount is used to adjust the intake air amount and perform fuel injection.
[0029]
In the operation state under the rich condition at the time of the low load, the pilot injection, the main injection, and the intake throttle are performed at timings as illustrated in FIG. 4, and the air-fuel ratio (A / F) of the exhaust gas is, for example, about 14.3. in purifying mechanism as shown in FIG. 7, NOx storage material 32c by a decrease in O ₂ concentration was release NOx adsorbed in the lean state, nitrogen N by this NOx is a reaction similar to the three-way catalyst with a reducing agent such as HC Reduced to ₂ . Then, the exhaust gas G becomes a purified exhaust gas Gc and is discharged into the atmosphere.
[0030]
In the next step S17, it is determined whether or not the rich condition control has been completed, that is, whether or not the rich integrated time tr 1 operated under the rich condition has exceeded the rich set time trs. Until the process is completed, step S16 is repeatedly executed. If it is determined in step S17 that the rich condition control has been completed, the process proceeds to step S11.
[0031]
When it is determined in step S15 that the operating state of the engine is not in the low load range, that is, the load is medium or high, the engine speed Ne and the fuel injection amount Q are input in step S18. From the engine speed Ne and the fuel injection amount Q, the rich control is performed by referring to the base pilot injection timing map, the base pilot injection amount map, the base main injection timing map, the post injection timing map, and the post injection amount map, respectively. The pilot injection timing Tpb, pilot injection amount Qpb, main injection timing Tmb, post-injection timing Tpos, and post-injection amount Qpos for the operation are calculated and output, and fuel is injected with the calculated amount.
[0032]
In the operation state under the rich condition at the time of the medium load and the high load, the pilot injection, the main injection, and the post-injection (post-injection) are performed at timings illustrated in FIG. 5, and the air-fuel ratio (A / F) of the exhaust gas is obtained. in but for example, about 14.3, in purifying mechanism as shown in FIG. 7, _{O 2} NOx occluding material 32c by lowering the concentration releases the NOx adsorbed in the lean state, the NOx ternary by a reducing agent such as HC It is reduced to nitrogen N ₂ by a reaction similar catalysts. Then, the exhaust gas G becomes a purified exhaust gas Gc and is discharged into the atmosphere.
[0033]
In the next step S19, it is determined whether or not the rich condition control has been completed, that is, whether or not the rich integrated time tr 1 operated under the rich condition has exceeded the rich set time trs. Until the process is completed, step S18 is repeatedly executed. If it is determined in step S19 that the rich condition control has been completed, the process proceeds to step S11.
[0034]
Steps S11 to S19 are repeatedly executed. If an interruption due to the engine key OFF in step S20 occurs during this execution, the control is terminated in step S21. When the control flow is started next time, the lean integration time tl and the rich integration time tr are stored so that the initial values of the integration times tl and tr can be set to the values at the time of stop when the control flow starts next time. Stop from.
[0035]
According to this air-fuel ratio control method of the engine, the load state of the engine is determined when the exhaust gas is temporarily set to the rich condition in order to regenerate the NOx storage reduction catalyst 32. Exhaust gas rich conditions can be created by retarding the injection, and at medium load and high load, exhaust gas rich conditions can be created by post-injection.
[0036]
【The invention's effect】
According to the air-fuel ratio control method for an engine of the present invention, when the exhaust gas is temporarily set to a rich condition in order to regenerate the NOx storage reduction catalyst, the load state of the engine is determined. A rich condition of exhaust gas is created by the restriction of the throttle and the main injection, and a rich condition of exhaust gas is created by post-injection at medium load and high load, so the torque is increased regardless of the engine load. Therefore, the rich condition of the exhaust gas can be reliably established, and the torque shock at the time of shifting from the lean condition to the rich condition can be prevented.
[Brief description of the drawings]
FIG. 1 is a diagram showing an engine system including an exhaust purification system having a NOx storage reduction catalyst according to an embodiment of the present invention.
FIG. 2 is a diagram showing a control flow according to the embodiment of the present invention.
FIG. 3 is a diagram illustrating timings and injection amounts of pilot injection and main injection under a lean condition.
FIG. 4 is a diagram illustrating timings of pilot injection, main injection, and post-injection and injection amounts under rich conditions when the operation of the engine is in a low load region.
FIG. 5 is a diagram illustrating timings of pilot injection, main injection, and post-injection and injection amounts under rich conditions when the operation of the engine is in a medium load or high load region.
FIG. 6 is a diagram showing a mechanism of NOx storage of a NOx storage reduction catalyst.
FIG. 7 is a diagram showing a mechanism of NOx release of a NOx storage reduction catalyst.
[Explanation of symbols]
1 engine (internal combustion engine)
2 Exhaust passage 3 Intake passage 4 EGR passage 23 Intake throttle (intake throttle)
32 NOx storage reduction catalyst 42 EGR valve 53 Fuel injection valve (injector)
60 Control unit (ECU)

Claims (1)

An air-fuel ratio control method for an engine provided with an exhaust purification system having a NOx storage reduction catalyst in an exhaust passage of the engine, wherein the exhaust gas is temporarily set to a rich condition in order to regenerate the NOx storage reduction catalyst. Judgment of the load condition of the engine at the time of low load, making the exhaust gas rich condition by the intake throttle and retard of the main injection, and making the exhaust gas rich condition by post injection at the time of medium load and high load An air-fuel ratio control method for an engine, comprising:

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