JP2009091994A - Combustion control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED To reliably prevent knocking at the time of switching from compression ignition combustion to spark ignition combustion.
When switching from compression ignition combustion to spark ignition combustion, EGR is performed and an air-fuel ratio is enriched to avoid knocking. The basic value of the enriched air-fuel ratio is set according to the engine speed and / or load. Further, a correction value for the enriched air-fuel ratio is set in accordance with the deviation amount of the actual EGR rate with respect to the target EGR rate, and correction is performed so that the richer the richer the deviation amount.
[Selection] Figure 7

Description

  The present invention relates to a combustion control device for an internal combustion engine that switches between spark ignition combustion and compression ignition combustion in accordance with engine operating conditions.

In Patent Document 1, in a combustion control device for an internal combustion engine that switches between spark ignition combustion and compression ignition combustion according to engine operating conditions, when switching from compression ignition combustion to spark ignition combustion, the intake throttle or the compression ratio decreases. It is disclosed that the in-cylinder temperature of the engine is lowered, the fuel is enriched during the time delay of the temperature drop, the knock is avoided, the combustibility is maintained well, and the air-fuel ratio is quickly controlled to stoichiometric thereafter. ing.
JP 2003-184606 A

  By the way, in order to avoid knocking, it is not sufficient to enrich the air-fuel ratio. EGR (exhaust gas recirculation) is performed to recirculate part of the exhaust gas to the intake system. It is desirable to avoid knocking.

However, when knocking is avoided by EGR and enrichment of the air-fuel ratio, there is a delay in EGR, so a countermeasure to avoid knocking is required even during the time delay of EGR.
In view of such a situation, the present invention has an object to reliably avoid knocking at the time of switching from compression ignition combustion to spark ignition combustion and improve drivability.

  Therefore, in the present invention, EGR is performed at the time of switching from compression ignition combustion to spark ignition combustion, and the air-fuel ratio is enriched. At this time, the degree of enrichment is corrected according to the actual EGR rate. The configuration.

  According to the present invention, even if the EGR rate is insufficient due to the time delay of EGR, by increasing the degree of air-fuel ratio enrichment, the knock suppression effect can be maintained and knocking can be reliably avoided. , Combustion stability can be improved.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a system diagram of an internal combustion engine (engine) showing an embodiment of the present invention.
A combustion chamber 4 defined by the cylinder head 1, the cylinder block 2, and the piston 3 is connected to an intake port 6 through an intake valve 5 and is connected to an exhaust port 8 through an exhaust valve 7. The opening / closing timing of the intake valve 5 and the exhaust valve 7 can be controlled by the variable valve gears 9 and 10, respectively.

  As each of the variable valve gears 9, 10, for example, a valve timing variable device capable of variably controlling the valve timing of the intake / exhaust valves (center phase of the valve operating angle) by changing the rotation phase between the crankshaft and the camshaft. (VTC device) and a valve operating angle and valve lift variable device (VEL device) capable of continuously variably controlling the valve operating angle (open period) and the valve lift amount are used in combination. In addition, an electromagnetically driven valve device (EMV device) that drives the intake / exhaust valves by an electromagnetic actuator may be used.

A fuel injection valve 11 and a spark plug 12 face the combustion chamber 4. The fuel injection valve 11 may be disposed in the intake port 6. The spark plug 12 is for spark ignition combustion.
An electric throttle valve 14 is provided in the intake passage 13 upstream of the intake port 6. Further, an EGR passage 16 that leads part of the exhaust gas as EGR gas is led out from the exhaust passage 15 downstream of the exhaust port 8, and this EGR passage 16 is connected to the intake passage 13 downstream of the throttle valve 14. In the middle of the EGR passage 16, for example, a step motor type EGR control valve 17 for controlling the EGR amount is provided. An exhaust purification catalyst (three-way catalyst) 18 is provided in the exhaust passage 15.

  The operations of the variable valve gears 9 and 10, the fuel injection valve 11, the spark plug 12, the electric throttle valve 14, and the EGR control valve 17 are controlled by an engine control unit (ECU) 19.

  The ECU 19 includes an engine speed N detected by a crank angle sensor (not shown), an accelerator opening APO detected by an accelerator opening sensor (not shown), and an intake air amount Qa detected by an air flow meter 20. In addition, information from an air-fuel ratio sensor 21 provided in the exhaust passage 15 upstream of the exhaust purification catalyst 18 is input.

  In the engine of this embodiment, as shown in FIG. 2, the closing timing of the intake valve 5 is set near the bottom dead center by the variable valve operating device 9 in a predetermined operation region on the relatively low rotation / low load side. Thus, while increasing the effective compression ratio (or increasing the in-cylinder temperature by increasing the intake air temperature using the intake air heating device), the compression ignition combustion is performed, and at this time, the air-fuel ratio is made lean. , Save fuel consumption.

  On the other hand, in other operating areas such as the high rotation / high load side and the idling operating area, the closing timing of the intake valve 5 is set after the bottom dead center by the variable valve operating device 9 (the retarded side from the time of compression ignition combustion). Then, while reducing the effective compression ratio (or lowering the in-cylinder temperature by stopping the intake air heating device), spark ignition combustion is performed using the spark plug 12 to ensure high output and idle stability. To do.

FIG. 3 is a main flowchart of the combustion switching control.
In S1, engine speed N and load T are detected as engine operating conditions. As the load T, an accelerator opening, a fuel injection amount, an engine torque, and the like are used.

In S2, based on the detected engine speed N and load T, it is determined which combustion region the current operation region corresponds to in the map of FIG.
As a result of the determination, in the case of the compression ignition combustion region, the process proceeds to S3, and compression ignition combustion is performed while increasing the effective compression ratio.

In the case of the spark ignition combustion region, the process proceeds to S4, and spark ignition combustion is performed while reducing the effective compression ratio.
On the other hand, when the operating region changes from the compression ignition combustion region to the spark ignition combustion region, that is, when switching from the compression ignition combustion to the spark ignition combustion, the process proceeds to S5 and the combustion switching according to the present invention is performed to avoid knocking. Control (FIG. 4) is executed.

FIG. 4 is a flowchart of the combustion switching control from the compression ignition combustion to the spark ignition combustion, and is executed in S5 of FIG.
In S11, the electric throttle valve is slightly throttled from the WOT state (fully opened state) prior to switching from compression ignition combustion to spark ignition combustion. Along with this, the air-fuel ratio also becomes slightly richer from the lean state (A / F = about 30).

In S12, the electric throttle valve is throttled stepwise at the same time as switching from compression ignition combustion to spark ignition combustion.
At the same time, the air-fuel ratio is enriched stepwise.

  The basic value of the enriched air-fuel ratio here is set according to the engine speed and / or load. Specifically, the higher the engine speed, the more severe the knock. Therefore, as shown in FIG. 5A, the richer the basic value of the enriched air-fuel ratio is made richer as the engine speed becomes higher. Further, knocking becomes severer as the engine load becomes higher. Therefore, as shown in FIG. 5B, the basic value of the enriched air-fuel ratio is made richer as the engine load becomes higher.

Then, the fuel injection amount is feedback controlled so as to obtain a target enriched air-fuel ratio.
In addition, the external EGR is introduced step by step simultaneously with the enrichment of the air-fuel ratio. Specifically, the EGR control valve is opened so as to obtain a preset target EGR rate to avoid knocking.

  Note that the target EGR rate may be a constant value, but may be set so as to increase in a severe knock condition according to the engine speed and / or load, as in the basic value of the enriched air-fuel ratio. .

  In this way, knocking is avoided by performing EGR (including increase) and enriching the air-fuel ratio. However, since there is a delay in EGR, it is necessary to take measures to reliably avoid knocking even during the EGR time delay. For this reason, control is performed as follows.

In S13, an actual EGR rate (actual EGR rate) is estimated. In S14, a deviation amount of the actual EGR rate with respect to the target EGR rate (= target EGR rate−actual EGR rate) is calculated.
Since the actual EGR rate changes with a time delay with respect to the target EGR rate, the characteristics of the actual EGR rate (or the amount of deviation from the target EGR rate) are modeled in advance using the target EGR rate and the elapsed time from the start of EGR as parameters. In this case, it is easy to estimate the actual EGR rate (or the deviation from the target EGR rate).

  In addition, the actual EGR rate may be estimated from the opening degree (number of steps) of the EGR control valve, the intake air amount detected by the air flow meter and the time, and further, the EGR control valve downstream of the EGR control valve A temperature sensor may be provided to the actual EGR rate in consideration of the temperature change.

  In S15, the air-fuel ratio is further enriched according to the amount of deviation from the target EGR rate. That is, the correction value of the enriched air-fuel ratio is set so as to become richer as the deviation amount is larger in accordance with the deviation amount of the actual EGR rate with respect to the target EGR rate.

  Specifically, as shown in FIG. 6, the larger the deviation amount of the actual EGR rate from the target EGR rate, the more severe the knock. Therefore, the larger the deviation amount, the richer the air-fuel ratio becomes. Increase the correction value to the minus side.

It should be noted that the post-correction rich air-fuel ratio = the rich air-fuel ratio basic value + the correction value, and the larger the correction value is on the negative side, the smaller the post-correction rich air-fuel ratio is (on the rich side).
In S16, the oxygen storage amount of the exhaust purification catalyst is calculated in order to estimate the change in the oxygen storage amount of the exhaust purification catalyst after the air-fuel ratio enrichment.

  Since compression ignition combustion before enrichment of the air-fuel ratio is performed in a lean state, the oxygen storage amount at this time is in a maximum value state. After the air-fuel ratio is rich, the oxygen storage amount gradually decreases in accordance with the rich air-fuel ratio, the exhaust flow rate (= intake air amount), and the engine speed (or elapsed time). Therefore, the oxygen storage amount can be estimated (calculated) based on these.

  In S17, it is determined whether or not the oxygen storage balance in the catalyst has recovered from the lean state by determining whether or not the oxygen storage amount in the catalyst calculated in S16 has become equal to or less than the predetermined value SL.

  If this determination is NO, the processing of S13 to S17 is repeatedly executed, and the degree of air-fuel ratio enrichment is set while setting the correction value of the enriched air-fuel ratio according to the amount of deviation of the actual EGR amount from the target EGR rate. Correct.

Therefore, when the response delay of EGR is eliminated, the enrichment is reduced according to the decrease in the deviation amount, and knocking is avoided by the EGR and the enrichment of the air-fuel ratio.
If the determination in S17 is YES, the process proceeds to S18, the air-fuel ratio is gradually returned from the rich state to stoichiometric (A / F = 14.7), and the control at the time of switching is terminated.

  In the flowchart of FIG. 4, when enriching the air-fuel ratio, the flow is first enriched to the basic value in S4, and then the air-fuel ratio is further enriched in consideration of the actual EGR rate. However, in practice, since the EGR rate is set to 0 in advance from the time when the enrichment is first performed in S4, the air-fuel ratio is enriched by adding the corresponding correction value, and then the actual EGR rate is increased. It is better to reduce the richness of the air-fuel ratio.

FIG. 7 is a time chart of combustion switching control from compression ignition combustion to spark ignition combustion.
To explain this, the throttle valve is slightly throttled from the WOT state at a time point a prior to switching to spark ignition combustion. Along with this, the air-fuel ratio also becomes slightly rich although it is in a lean state.

At the same time as switching to spark ignition combustion, at time point b, the throttle valve is throttled stepwise, the air-fuel ratio is enriched stepwise, and external EGR is performed.
At this time, a time delay (delayed in the drawing) of the external EGR occurs, and this amount is used as a rich air-fuel ratio correction value (shown correction). By adding to the air-fuel ratio, the EGR response delay can be compensated by the air-fuel ratio enrichment, and knocking can be avoided.

When the EGR response delay is eliminated, the correction value for the enriched air-fuel ratio decreases as the deviation amount decreases, and the air-fuel ratio is eventually enriched only with the basic value.
Thereafter, at time point c, when the oxygen storage amount in the catalyst becomes equal to or less than the predetermined value SL and the oxygen storage balance in the catalyst recovers from the lean state, the air-fuel ratio is gradually returned to stoichiometry. That is, after the NOx purification efficiency of the catalyst is improved, it is returned to stoichiometry.

  As described above, according to the present embodiment, when switching from compression ignition combustion to spark ignition combustion, EGR is performed and the air-fuel ratio is enriched, and at this time, the actual estimated by the EGR rate estimating means By correcting the degree of air-fuel ratio enrichment in accordance with the EGR rate of the engine, even if the EGR rate is insufficient due to the time delay of EGR, the amount of air-fuel ratio enrichment is increased by that amount. The suppression effect can be maintained, knocking can be avoided reliably, and combustion stability can be improved.

  Further, according to the present embodiment, the basic value of the enriched air-fuel ratio is set according to the engine speed and / or the load, so that it is possible to appropriately cope with a high speed or a high load where knocking becomes severe. it can.

  Further, according to the present embodiment, the correction value of the enriched air-fuel ratio is set so as to be richer as the deviation amount is larger in accordance with the deviation amount of the actual EGR rate from the target EGR rate. It is possible to cope with the delay of overtime.

  Further, according to the present embodiment, at the time of switching from compression ignition combustion to spark ignition combustion, EGR is performed and the air-fuel ratio is enriched, and the intake air is throttled by the throttle valve, thereby further increasing the knock suppression effect. be able to.

  Further, according to the present embodiment, it has means for estimating a change in the oxygen storage amount of the exhaust purification catalyst after the air-fuel ratio enrichment, and by gradually returning the air-fuel ratio to stoichiometry due to the decrease in the oxygen storage amount, The rich state can be continued until the NOx conversion efficiency of the exhaust purification catalyst is improved, and the recovery of the NOx conversion efficiency of the catalyst can be accelerated.

Engine system diagram showing an embodiment of the present invention Explanatory diagram of compression ignition combustion region and spark ignition combustion region Main flowchart of combustion switching control Flow chart of combustion switching control from compression ignition to spark ignition Illustration of basic value of enriched air-fuel ratio Explanatory diagram of correction value for enriched air-fuel ratio Time chart of combustion switching control from compression ignition to spark ignition

Explanation of symbols

4 Combustion chamber 5 Intake valve 7 Exhaust valve 9, 10 Variable valve device 11 Fuel injection valve 12 Spark plug 13 Intake passage 14 Electric throttle valve 15 Exhaust passage 16 EGR passage 17 EGR control valve 18 Exhaust purification catalyst 19 ECU
20 Air flow meter 21 Air-fuel ratio sensor

Claims (5)

In an internal combustion engine combustion control device that switches between spark ignition combustion and compression ignition combustion according to engine operating conditions,
At the time of switching from compression ignition combustion to spark ignition combustion, EGR for recirculating a part of the exhaust gas to the intake system and enriching the air-fuel ratio,
At this time, the combustion control apparatus for an internal combustion engine, wherein the degree of air-fuel ratio enrichment is corrected according to the actual EGR rate estimated by the EGR rate estimating means.   2. The combustion control apparatus for an internal combustion engine according to claim 1, wherein the basic value of the enriched air-fuel ratio is set according to the engine speed and / or the load.   The correction value of the enriched air-fuel ratio is set so as to be richer as the deviation amount is larger in accordance with the deviation amount of the actual EGR rate from the target EGR rate. Combustion control device for internal combustion engine.   The intake valve is throttled by a throttle valve together with the implementation of EGR and the enrichment of the air-fuel ratio when switching from compression ignition combustion to spark ignition combustion. Combustion control device for internal combustion engine.   5. The apparatus according to claim 1, further comprising means for estimating a change in an oxygen storage amount of the exhaust purification catalyst after enrichment of the air-fuel ratio, wherein the air-fuel ratio is gradually returned to stoichiometric by decreasing the oxygen storage amount. A combustion control apparatus for an internal combustion engine according to any one of the above.

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