JP2004245175A - Controlling method for engine air-fuel ratio - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a controlling method for air-fuel ratios of an engine in which engine output torque fluctuations can be suppressed and does not make an operator feel torque shock when exhaust gas is rendered temporarily under rich condition, in order to renew the NOx storage capacity of NOx storage-reduction catalyst provided at an exhaust passage of the engine. <P>SOLUTION: Controlling method is disclosed for air-fuel ratios of an engine equipped with an exhaust emission purifier 10 having NOx storage-reduction catalyst 32 at an exhaust passage 2 of an engine 1. The amount of injection Qpos is increased on a continual and step-by-step basis in the post-injection for a predetermined period during the early stage after starting post-injection, when post-injection is performed temporarily to render exhaust gas under rich condition, in order to renew the NOx storage-reduction catalyst 32. <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. 8 and 9, 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. 8, 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 2NO + O ₂ → 2NO ₂ Is converted to nitrogen dioxide (NO ₂ ), and this NO ₂ is oxidized on Pt and stored in the NOx storage material 32c as NO ₃ ^{− in} the form of Ba (NO ₃ ) ₂ or the like.
[0004]
Further, as shown in FIG. 9, in the rich state near the stoichiometric air-fuel ratio, the NOx storage material 32c releases the adsorbed NOx in a lean state due to the decrease in the oxygen concentration, and this NOx contains hydrocarbons (HC) and the like. Is reduced to nitrogen N ₂ by a reaction similar to that of 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. Therefore, there has been proposed an air-fuel ratio control method that suppresses torque fluctuation when shifting from such a lean condition to a rich condition (rich spike control).
[0007]
One of them is to purify the exhaust gas of an internal combustion engine that gradually increases the air-fuel ratio of the mixture by performing air-fuel ratio control that gradually reduces the air-fuel ratio of the mixture from a lean air-fuel ratio to a rich air-fuel ratio. An apparatus has been proposed (for example, see Patent Document 1).
[0008]
In this exhaust gas purifying apparatus for an internal combustion engine, the air-fuel ratio control for generating the rich condition is performed by gradually increasing the fuel injection time of the main injection (main injection). However, when the rich condition is generated by the increase of the main injection amount, the influence on the fluctuation of the engine output torque is large. Therefore, the torque fluctuation gradually changes. There is a problem that the torque greatly changes. In addition, if an attempt is made to reduce the temporal change in the output torque, the time required for switching becomes longer, the time required for regeneration of the NOx storage reduction catalyst becomes longer, and the effect on vehicle operation and exhaust gas purification performance is reduced. There is also a problem that the influence of is increased.
[0009]
Further, in order to solve the problem of torque shock, the switching from the lean air-fuel ratio to the rich air-fuel ratio is gradually changed, and based on the air-fuel ratio changed by the air-fuel ratio changing unit in the expansion stroke of the internal combustion engine by the sub-injection unit. 2. Description of the Related Art An exhaust purification device for an in-cylinder injection type internal combustion engine that additionally supplies a fuel amount into a cylinder has been proposed (for example, see Patent Document 2).
[0010]
In this exhaust gas purification apparatus, control is performed such that the total air-fuel ratio is immediately changed to the target air-fuel ratio by sub-injection in order to reduce the NOx purge time. In other words, assuming that the sub-injection has little effect on the output torque, when gradually changing the switching from the lean air-fuel ratio to the rich air-fuel ratio, the target air-fuel ratio and the predetermined air-fuel ratio Is injected in the sub-injection, and a predetermined air-fuel ratio during the switching is supplemented by the sub-injection so that the target air-fuel ratio as a whole is obtained immediately after the switching.
[0011]
The purpose of this sub-injection is to compensate for the gradually increasing rich air-fuel ratio in order to make it rich and quick. When the rich air-fuel ratio becomes the target air-fuel ratio, it becomes zero, and the sub-injection is stopped.
[0012]
However, although this sub-injection has little effect on the output torque, this sub-injection also has a slight effect on torque fluctuation. In driving and low-load driving, there is a problem that a torque shock occurs and gives a driver discomfort.
[0013]
[Patent Document 1]
JP-A-7-166913 (pages 2 and 6)
[0014]
[Patent Document 2]
JP-A-11-223148 (pages 2 to 5)
[0015]
[Problems to be solved by the invention]
In order to solve the problem of torque shock, post-injection (post-injection), which has little effect on engine output torque, is added to two-stage injection of pilot injection and main injection performed under lean conditions corresponding to normal operation. There is an air-fuel ratio control method that generates a rich condition by the added three-stage injection.
[0016]
Although the amount of torque generated by this post-injection is small, the output torque fluctuates slightly when switching from the two-stage injection to the three-stage injection. Therefore, in an operation region with a small load such as an idle operation or a low-load operation, When the post-injection is performed, there is a problem that the output torque of the engine increases more than necessary and a torque shock occurs.
[0017]
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. It is an object of the present invention to provide a method for controlling an air-fuel ratio of an engine that can suppress engine output torque fluctuations and make a driver not feel a torque shock when the conditions are satisfied.
[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 post-injection is temporarily performed to make the exhaust gas rich conditions, the injection amount in the post-injection is continuously or stepwise increased for a predetermined period at the beginning of the post-injection start. The feature is constituted.
[0019]
Therefore, according to the air-fuel ratio control method for an engine of the present invention, the amount of post-injection is gradually increased when a rich condition is generated, so that torque fluctuation due to post-injection is reduced, and torque shock is not generated. It is possible to shift from the lean condition to the rich condition.
[0020]
Further, in the above-described air-fuel ratio control method for an engine, when returning from the rich condition to the lean condition, the post-injection is terminated by continuously or stepwise decreasing the injection amount in the post-injection for a predetermined period. Is done.
[0021]
According to this method, when returning to the lean condition, the post injection amount is gradually reduced, so that it is possible to shift from the rich condition to the lean condition without generating a torque shock.
[0022]
Further, not only the fuel injection system but also the air system such as EGR control (exhaust gas circulation control) and VNT (turbocharger with a variable nozzle) are simultaneously controlled to reduce torque shock.
[0023]
As a result, a change in the driving force can be suppressed when switching between the lean condition and the rich condition, and the switching without causing a torque shock can be performed.
[0024]
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.
[0025]
As shown in FIG. 1, in an exhaust gas purification system 10 for an engine, an intake air amount sensor (mass air flow sensor) 21 and a compressor 31 a of a turbocharger 31 with a variable nozzle (hereinafter referred to as VNT) 31 A cooler 22 and an intake valve (intake throttle) 23 are provided, a turbine 31b of a VNT 31 and a NOx storage reduction catalyst 32 are provided in an exhaust passage 3, and an EGR cooler 41 and an EGR valve are provided in an EGR passage (exhaust circulation passage) 4. 42 are provided.
[0026]
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.
[0027]
In the method for controlling the air-fuel ratio of an engine according to the present invention, the following fuel injection is performed.
[0028]
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. The control flow is started when the operation of the engine is started. When the operation of the engine is completed, that is, when the engine key is turned off, the control flow is stopped. Is done.
[0029]
When the control flow starts, lean condition control is entered. In step S11, the engine speed Ne and the fuel injection amount Q are input, and a lean set time map is referred from the engine speed Ne and the fuel injection amount Q. Then, the lean set time tls is calculated.
[0030]
In the next step S12, it is determined whether or not the control is the Ramp control (smoothing control). The Ramp control is performed immediately after the start of switching from the rich condition to the lean condition or immediately before the end of the lean condition when switching from the lean condition to the rich condition. This is a control that is gradually reduced or increased.
[0031]
It is determined whether or not the Ramp control is performed by determining whether or not the lean integrated time tl, which is the operating time under the lean condition, is before the predetermined time t1, or whether the lean integration time tl is equal to or less than the predetermined time t2 before the lean set time tls. tls-t2) or not. When the lean condition is started for the first time when the engine is started, this Ramp control is not performed. In addition, the predetermined period t1 of the end of the lean condition and the predetermined period t2 of the start of the rich condition may be the same, but need not be the same period.
[0032]
When it is determined in step S12 that the control is not the Ramp control, in step S13, the lean condition control without post-injection is performed for a predetermined time, and the process proceeds to step S15. The control in step S13 is an operation control under a normal lean condition. The engine speed Ne and the fuel injection amount Q are input, and the base pilot injection timing map, the base pilot injection timing map, and the engine speed Ne and the fuel injection amount Q are input. The pilot injection timing Tpb, the pilot injection amount Qpb, and the main injection timing Tmb are calculated with reference to the pilot injection amount map and the base / main injection timing map, and the calculated amounts are output. Inject.
[0033]
In addition, the control amount Vdb of the VNT (turbocharger) 31, the opening degree Thb of the intake valve 23, and the target air mass amount AMs are also calculated. As illustrated in FIG. 6, the VNT 31 is controlled so that the control amount Vdb and the opening degree Thb become the same. And the intake valve 23 are respectively controlled, and the EGR valve 42 is subjected to F / B control (feedback control) so as to achieve the target air mass amount AMs.
[0034]
In the fuel injection control in step S13, the pilot injection and the main injection are performed at timings illustrated in FIG. 3, the air-fuel ratio (A / F) of the exhaust gas is, for example, about 40, and the purification mechanism illustrated in FIG. in, _{O 2} adheres to the surface of Pt32b, NO by reaction of NO + _{O 2} → NO _2, it becomes NO _2, the NO ₂ while being oxidized on Pt in the NOx storage material 32c NO ₃ ^- as Ba (NO ₃ ) The exhaust gas G is stored in the form of ₂ or the like, and the exhaust gas G is purified and discharged into the atmosphere as a purified exhaust gas Gc.
[0035]
When it is determined in step S12 that the control is the Ramp control, the lean condition Ramp control with post-injection is performed for a predetermined time in step S14, and the process proceeds to step S15. As shown in FIG. 4, the fuel injection control in step S14 is a control in which post injection is added to the fuel injection control under normal lean conditions in step S13, and the post injection is performed based on the engine speed Ne and the fuel injection amount Q. The post-injection timing Tpos and the post-injection amount Qpos are calculated with reference to the injection timing map and the post-injection amount map, and the calculated amounts are output, and fuel injection is performed based on the calculated amounts.
[0036]
Immediately before the start of the lean condition, the post-injection amount Qpos is gradually reduced to zero as the lean integration time tl increases, and immediately before the end of the lean condition, the post-injection amount Qpos is reduced from zero as the lean integration time tl increases. Increase gradually.
[0037]
Then, in step S15, 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. Until the processing is completed, step S13 or step S14 is repeatedly executed.
[0038]
When it is determined in step S15 that the lean accumulated time tl has exceeded the lean set time tls and the lean condition control has been completed, in step S16, the engine speed Ne and the fuel injection amount Q are input. The rich set time trs is calculated from the engine speed Ne and the fuel injection amount Q with reference to the rich set time map.
[0039]
Then, control of the rich condition is started, and in step S17, it is determined whether or not the control is the Ramp control. This Ramp control is performed at the start or end of the operation under the rich condition, and is a control for increasing or decreasing the injection amount of the post injection continuously or stepwise.
[0040]
The determination as to whether or not the Ramp control is performed is based on whether the rich integrated time tr, which is the operation time under the rich condition, is before a predetermined time t3, or whether the rich integrated time tr is a predetermined time t4 before the rich set time trs. (trs-t4) or later. Note that the predetermined period t3 of the end of the rich condition and the predetermined period t4 of the start of the lean condition may be the same, but need not necessarily be the same period. That is, the predetermined times t1, t2, t3, and t4 are independent.
[0041]
If it is determined in step S17 that the control is not the Ramp control, the rich condition control is performed for a predetermined time in step S18, and the process proceeds to step S20.
[0042]
As shown in FIG. 5, the fuel injection control in step S18 is a control in which post-injection is added to the normal lean operation control in step S13, and the engine speed Ne and the fuel injection amount Q are input. From the engine speed Ne and the fuel injection amount Q, the pilot injection timing Tpb, the pilot injection amount Qpb, the main injection timing map, the base injection timing map, the base injection timing map, and the base injection timing map are referred to respectively. The post-injection timing Tmb and the post-injection amount Qposr are calculated by referring to the post-injection timing map and the post-injection amount map from the engine speed Ne and the fuel injection amount Q, respectively. The calculated amount is output, and fuel is injected based on the calculated amount.
[0043]
In addition, the control amount Vdr of the VNT (turbocharger) 31, the opening degree Thr of the intake valve 23, and the opening degree EGr of the EGR valve 42 are also calculated, and as illustrated in FIG. 6, the control amount Vdr, the opening degree Thr, and the opening degree. The VNT 31, the intake valve 23, and the EGR valve 42 are controlled so as to achieve EGr.
[0044]
In the fuel injection control in step S18, the pilot injection, the main injection, and the post injection are performed at timings illustrated in FIG. 5, and the air-fuel ratio (A / F) of the exhaust gas is, for example, about 14.3. in purifying mechanism as shown, NOx storage material 32c by lowering the oxygen concentration to release NOx adsorbed in the lean state, the NOx is reduced to nitrogen N ₂ by a reaction similar to the three-way catalyst with a reducing agent such as HC The exhaust gas G becomes a purified exhaust gas Gc and is discharged into the atmosphere.
[0045]
If it is determined in step S17 that the control is the Ramp control, the rich condition Ramp control with post-injection is performed for a predetermined time in step S19, and the process proceeds to step S20.
[0046]
As illustrated in FIG. 4, the fuel injection control in step S19 is a fuel injection control under a normal rich condition, in which the post injection amount Qpos is continuously or stepwise changed to perform the fuel injection.
[0047]
Immediately before the start of the rich condition, the post injection amount Qpos is gradually increased and decreased according to the increase of the rich integration time tr to Qposr, and immediately before the end of the rich condition, the post injection amount Qpos is changed to Qposr according to the increase of the lean integration time tr. Gradually decrease from. The post-injection amount Qpos of the rich condition Ramp control is set so as to change continuously or stepwise from the preceding and succeeding post-injection amount Qpos of the lean condition Ramp control.
[0048]
In the next step S20, 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 or step S19 is repeatedly executed. If it is determined in step S20 that the rich condition control has been completed, the process proceeds to step S11.
[0049]
Steps S11 to S20 are repeatedly executed, and if an interrupt occurs due to the engine key OFF in step S21 during this execution, the control end work is performed in step S22, for example, during the lean condition control or the rich condition control. When the control is stopped, when the control flow starts 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 the stop. Stop.
[0050]
As shown in FIG. 6 showing a time series example of this control, the post injection amount Qpos is zero in the lean condition control, continuously increases in the lean condition Ramp control and the rich condition Ramp control, and is increased in the rich condition control. Qposr.
[0051]
Further, regarding the control of the EGR valve 42, the feedback (F / B) control based on the target air mass amount AMs is performed at the time of the lean condition control, but at the time of the rich condition control, the movement of the EGR valve 42 is accelerated and the rich time is shortened. Then, the position control is performed at the preset opening degree EGr.
[0052]
Regarding the control of the intake valve 23, at the time of the rich condition control, the intake throttle is closed from the opening Thb at the time of the lean condition control to a preset opening Thr to reduce the intake air amount (air mass) and create a rich condition. Make it easier.
[0053]
Regarding the control of the VNT 31, at the time of the rich condition control, the opening degree Vdb at the time of the lean condition control is changed to a preset opening degree Vdr so that the intake air amount is reduced and the rich condition is easily created.
[0054]
According to the air-fuel ratio control method of the engine according to this control flow, when the post-injection is temporarily performed to make the exhaust gas rich in order to regenerate the NOx storage-reduction catalyst 32, the termination of the lean condition is determined. The injection amount Qpos in post-injection can be increased continuously or stepwise from before the period t1 to after a predetermined period t2 after the start of the rich condition.
[0055]
In other words, when switching from the lean condition to the rich condition, in the fuel injection of the normal operation under the lean condition, the post-injection is performed with the small injection amount Qpos at the same timing as the post-injection timing of performing the post-injection under the rich condition. Is started, and the post-injection injection amount Qpos is gradually increased continuously and stepwise even after switching to the rich condition. It can be increased to Qposr. That is, when increasing the injection amount of post injection, Ramp control (smoothing control) can be performed.
[0056]
Then, by starting post-injection in advance from among the lean conditions, it is possible to reduce a change in cylinder (cylinder) pressure due to a change in the fuel injection amount accompanying the switching between the lean condition and the rich condition.
[0057]
Further, according to the air-fuel ratio control method of the engine according to the control flow, when returning from the rich condition to the lean condition, the post-operation is performed for a predetermined period t3 before the end of the rich condition and after a predetermined period t4 after the start of the lean condition. The injection amount Qpos in the injection can be reduced continuously or stepwise. That is, Ramp control can be performed even when the injection amount Qpos of the post-injection decreases.
[0058]
Therefore, according to the air-fuel ratio control method of the engine, even when switching to the rich condition and when returning to the lean condition, the post injection amount gradually changes. Lean conditions can be switched.
[0059]
FIG. 7 shows an example of a change in driving force (output torque) by the air-fuel ratio control method of the engine. As can be seen from the line indicating the fluctuation of the driving force in FIG. 7, the driving force hardly changes before and after the rich period.
[0060]
【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 subjected to the rich condition using the post injection in order to regenerate the NOx storage reduction catalyst, the torque fluctuation due to the post injection is reduced. Since the post-injection amount is gradually increased in order to reduce the amount, the lean condition and the rich condition can be switched without generating a torque shock.
[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 fuel injection in lean condition control;
FIG. 4 is a diagram illustrating fuel injection in lean condition Ramp control and rich condition Ramp control.
FIG. 5 is a diagram illustrating fuel injection in rich condition control.
FIG. 6 is a diagram illustrating an example of a time series of control.
FIG. 7 is a diagram showing a time series of a driving force change at the time of switching between a lean condition and a rich condition.
FIG. 8 is a view showing a mechanism of NOx storage of a NOx storage reduction catalyst.
FIG. 9 is a view showing a mechanism of NOx release and reduction 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 valve 31 VNT (turbocharger with variable nozzle)
32 NOx storage reduction catalyst 42 EGR valve 53 Fuel injection valve (injector)
60 Control unit (ECU)

Claims (2)

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. An air-fuel ratio control method for an engine, wherein the injection amount in the post-injection is increased continuously or stepwise for a predetermined period at the beginning of the post-injection start when the rich condition is set. 2. The air-fuel ratio control method for an engine according to claim 1, wherein when returning from the rich condition to the lean condition, the injection amount in the post-injection is reduced continuously or stepwise for a predetermined period to end the post-injection.

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