JP2002322928A - Combustion control device for compression ignition type engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable combustion by compression ignition without extremely enhancing a compression ratio. SOLUTION: When an operating range is within a compression ignition range, an operating cycle is changed from a four cycle operation to a six cycle operation, in which compression and expansion processes are successively carried out twice. In the first compression process, fuel is injected inside the combustion chamber by the first injection, wherein stratified combustion of fuel-air mixture is made by spark ignition. Thereby, next compression process start temperature is increased, which leads to compression ignition combustion of fuel supplied by the second injection. As a result, even if the compression ration is 12 or less, the gas temperature inside the cylinder is reliably increased up to the compression ignition temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a combustion control system for a compression ignition engine capable of obtaining stable compression ignition combustion without extremely increasing a compression ratio.

[0002]

2. Description of the Related Art As means for improving the thermal efficiency of a four-cycle engine, it is known to increase the specific heat ratio of the working gas to thereby increase the theoretical thermal efficiency by making the air-fuel mixture lean. Further, by making the air-fuel mixture lean, even when the engine is operated with the same torque, more air is sucked into the engine, so that the pumping loss can be reduced.

[0003] However, leaning of the air-fuel mixture is limited due to prolonged combustion period and unstable combustion. To overcome this problem, in-cylinder injection is used to increase the limit by stratified charge combustion, in which the mixture is stratified around the spark plug while maintaining a stratified state, to ensure ignitability. Since the air-fuel mixture is concentrated, there is a problem that the combustion temperature becomes high and NOx tends to increase.

[0004] On the other hand, a diesel engine burns by compression ignition, so that it has high thermal efficiency and can make the air-fuel ratio significantly lean. However, since the air utilization rate under a high load is poor, the output is low and soot emission is low. May occur, which is a problem in exhaust gas measures.

Therefore, as a means for solving such a problem, there has been proposed a compression ignition type engine in which a gasoline mixture is ignited at multiple points by adiabatic compression without using a spark plug. In the compression ignition type engine, rapid combustion with short flame propagation is realized by multipoint ignition without using spark ignition, so that a local high-temperature portion is hardly formed in the combustion chamber, and NOx emission is greatly reduced. be able to.

[0006]

However, if the compression ratio is set to a high value of about 15 to 18 in order to obtain compression ignition combustion, the combustion pressure increases rapidly during high load operation due to an increase in the fuel injection amount. As a result, knocking is likely to occur.

To cope with this, for example, Japanese Patent Application Laid-Open No. 9-28
Japanese Patent No. 7528 proposes a technique for reducing the temperature of air-fuel mixture in a combustion chamber by external EGR or controlling the temperature of intake air by a cooling device provided in an EGR passage.

However, the combustion control based on the external EGR or the intake air temperature has a problem that the response is slow, and it is not possible to obtain a good follow-up property with respect to a torque change during running.

It is also conceivable to employ a so-called Miller cycle in which the actual compression ratio is set to be smaller than the actual expansion ratio in order to avoid the occurrence of knocking.
When the Miller cycle operation is performed with a high engine load, a sufficient amount of air is not supplied to the combustion chamber, and there is a problem that the torque is reduced.

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention provides a compression ignition engine capable of performing compression ignition combustion over a wide range while suppressing the occurrence of knocking and capable of greatly reducing exhaust emissions in all operation regions. It is an object to provide a combustion control device.

[0011]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides an in-cylinder injector for directly injecting fuel into a combustion chamber, a variable valve mechanism capable of varying valve timing of an exhaust valve and an intake valve. In the combustion control device for a compression ignition type engine provided with the above, when the engine operation region is in the compression ignition region, the operation cycle is switched from the four-cycle operation to the six-cycle operation in which the compression stroke and the expansion stroke are continuously performed twice. Means for injecting a first fuel into the combustion chamber during an intake stroke to a subsequent compression stroke, and injecting a second fuel after shifting to an initial expansion stroke; The mixture formed by the second fuel is ignited in the first compression stroke.

In such a configuration, when the operation region is in the compression ignition region, the operation cycle is switched from the four-cycle operation to the six-cycle operation, the compression stroke and the expansion stroke are continuously performed twice, and in the first compression stroke, By burning the fuel from the first injection, the next compression stroke start temperature is raised, and the gas temperature in the combustion chamber is raised to a temperature at which compression ignition combustion is possible by adiabatic compression during the next compression stroke.

In this case, preferably, 1) the air-fuel mixture formed by the first fuel is burned in a suction expansion stroke by spark ignition by a spark plug.

2) When the operation cycle is switched to the six-cycle operation, the operation cycle switching means forms a negative valve overlap period in which both the intake valve and the exhaust valve close before and after the exhaust top dead center. In addition, the temperature of the new gas sucked during the intake stroke is raised by the residual gas trapped in the combustion chamber during the negative valve overlap period.

3) The amount of fuel injected at the first time is reduced as the engine load increases.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows an overall configuration diagram of a compression ignition type engine.

1 is an engine body, 2 is a piston, 3 is a combustion chamber, 4 is an intake port, 5 is an exhaust port, 6
Denotes an intake valve, 7 denotes an exhaust valve, and a throttle valve 9 is interposed in an intake passage 8 communicating with the intake port 4. The throttle valve 9 is connected to an electronic control throttle device (not shown) for electronically controlling the throttle opening.
Further, an in-cylinder injector 11 is provided at the center of the top surface of the combustion chamber 3.
A curved concave piston cavity 2a is formed on the top surface of the piston 2 opposed to the injection direction of the in-cylinder injector 11. Furthermore,
One side of the combustion chamber 3 (a squish area in the present embodiment)
The ignition portion of the ignition plug 12 is exposed. Note that reference numeral 16 denotes a knock sensor, and 17 denotes a water temperature sensor.

Further, an exhaust passage 2 communicating with the exhaust port 5 is provided.
8, an O2 sensor 18 as an example of an air-fuel ratio detecting means is provided.
A three-way catalyst 29 for purifying by oxidizing O and HC and reducing NOx is interposed. The air-fuel ratio detecting means may be a wide-range air-fuel ratio sensor.

The engine employed in this embodiment is:
The combustion mode can be switched between a super-lean compression ignition combustion having an air-fuel ratio and a normal spark ignition combustion in accordance with the operation range, and the compression ignition combustion can be performed at a NOx generation temperature or lower. Since three-way catalyst 29 rarely occurs and the excess air ratio is high, the three-way catalyst 29 functions as an oxidation catalyst that purifies CO and HC in the exhaust gas by an oxidation reaction.

Further, the engine employed in the present embodiment has a compression ratio of 1 so that abnormal combustion such as knocking can be suppressed and high-load operation by ordinary spark ignition can be realized.
It is set to 2 or less.

An intake valve 6 and an exhaust valve 7 are connected to variable valve mechanisms 13a and 13b, respectively. Each of the variable valve mechanisms 13a and 13b is configured by a known electromagnetically driven valve that can control opening and closing of the intake and exhaust valves at an arbitrary valve timing by an electromagnetic force.

By the way, in the engine according to the present embodiment, when the combustion mode is set to the compression ignition combustion, the operation cycle is changed from the normal four-cycle (one four-stroke cycle) operation to the six-cycle (one six-stroke one cycle) operation. Is switched. In the four-cycle operation, as shown in FIG. 5B, the operation is performed with one cycle of an exhaust stroke → an intake stroke → a compression stroke and an expansion stroke.

On the other hand, in the six-cycle operation, as shown in FIG. 5A, a compression stroke and an expansion stroke are added between the intake stroke and the expansion stroke in the normal four-cycle operation, and the intake stroke as a whole is increased. → Compression stroke → expansion stroke → compression stroke → expansion stroke → exhaust stroke, the compression stroke and the expansion stroke are 2
The operation is performed with six consecutive strokes as one cycle. Therefore, the variable valve mechanism 13b opens the exhaust valve 7 before and after the exhaust stroke, while the variable valve mechanism 13a
Opens the intake valve 6 before and after the intake stroke. As a result, the intake valve 6 and the exhaust valve 7 perform a six-cycle operation at a valve timing that opens once every three engine revolutions.

The signals detected by the above-described sensors are input to an electronic control unit (ECU) 20. An electronic control unit (ECU) 20 includes a CPU 21, a ROM 22,
M23, an input port 24, an output port 25, etc., are constituted mainly by a microcomputer, which are mutually connected by a bidirectional bus 26.

The input port 24 is connected to a crank angle sensor 31 for generating a crank pulse for each set crank angle, in addition to the sensors described above, and generates an output voltage proportional to the amount of depression of an accelerator pedal 32. The load sensor 33 is connected via an A / D converter 34. The output port 25 is connected to each of the variable valve mechanisms 13a, 13a, via an intake valve drive circuit 36a and an exhaust valve drive circuit 36b.
13b, is connected to the ignition plug 12 via an ignition drive circuit 36c, and is further connected to the in-cylinder injector 11 via an injector drive circuit 36d.

The electronic control unit (ECU) 20 determines the operating range based on the engine speed Ne calculated based on the signal from the crank angle sensor 31 and the engine load Lo detected based on the signal from the load sensor 33. It is checked whether it is in the compression ignition region or the spark ignition region. If it is in the compression ignition region, the throttle valve 9 is fully opened, and the fuel injection amount that can obtain the optimal compression ignition combustion is calculated. Perform ignition combustion control. When the operation region is in the spark ignition region, normal spark ignition combustion control is executed.

Since the compression ratio of the engine employed in the present embodiment is set to 12 or less, it is difficult to raise the temperature of the gas in the cylinder to the compression ignition temperature by adiabatic compression during the compression stroke. is there. Therefore, in the present embodiment, in the latter half of the compression stroke, the fuel for combustion control is injected (hereinafter, referred to as “first injection”) to form a stratified mixture, and this stratified mixture is burned by spark ignition. By doing so, the compression stroke start temperature is raised. The main fuel for torque generation is injected between the latter half of the expansion stroke and the first half of the compression stroke (hereinafter referred to as “second injection”), and the adiabatic compression during the compression stroke lowers the temperature of the in-cylinder gas to compression ignition. Raise the temperature to the temperature. At this time, combustion by the first injection (hereinafter, referred to as “first combustion”) is made lean in consideration of combustion by the second injection (hereinafter, referred to as “second combustion”). The second combustion burns the remaining oxygen under conditions from lean to stoichiometric. Each specific concentration is appropriately adjusted depending on operating conditions.

The combustion control, such as fuel injection control and ignition timing control, which is performed by the electronic control unit (ECU) 20 is specifically executed according to a combustion control routine shown in FIG.

In this routine, first, in step S1
Then, based on the engine speed Ne and the engine load Lo, referring to the operation region map shown in FIG. 4, it is determined whether the operation region is in the compression ignition region or the spark ignition region.
When it is in the compression ignition region, the process proceeds to step S2, and when it is in the spark ignition region, the process proceeds to step S5. Note that the compression ignition region in the present embodiment is set to a low-medium rotation and low-medium load region as shown in FIG.
It is set in a high rotation region or a high load region, which is the other region.

In step S2, the throttle valve 9 is fully opened, and then in step S3, the intake valve 6 and the exhaust valve 7 are operated for six cycles with respect to the variable valve mechanisms 13a and 13b. Set and output timing drive signals. Then, as shown in FIG. 5 (a), the intake valve 6 opens once in every three revolutions of the engine during the intake stroke, and the exhaust valve 7 also opens once every three revolutions of the engine.
The valve is opened in the exhaust stroke.

Next, the routine proceeds to step S4, in which compression ignition combustion control is executed, and the routine exits. This compression ignition combustion control is executed according to a compression ignition combustion control subroutine shown in FIG.

In this routine, first, at step S11
Then, the total fuel amount injected from the in-cylinder injector 11 is calculated by a map search or calculation based on the engine load Lo and the engine speed Ne. Next, the process proceeds to step S12, in which the fuel amount at the time of the first injection (hereinafter abbreviated as “the fuel amount at the time of the first injection”) and the second fuel amount according to the engine load Lo.
An injection ratio indicating a ratio with the fuel amount at the time of injection (hereinafter abbreviated as "second injection fuel amount") is calculated. The injection ratio is set to, for example, a maximum of 10% at the time of the first injection-time fuel amount, and is reduced until it reaches 0% as the engine load Lo increases. Therefore, the ratio of the second injection fuel amount increases in the range of 90 to 100% as the engine load Lo increases.

With the increase in the engine load Lo, the fuel amount during the first injection is reduced to control the thermal energy by the first combustion. Therefore, the air-fuel ratio is gradually enriched with the increase in the engine load Lo. Even in such a case, stable compression ignition combustion can be obtained while avoiding abnormal combustion such as knocking, and high output can be achieved.

Next, the process proceeds to step S13, and in this step S13 and the next step S14, step S1 is executed.
Based on the total fuel amount calculated in step 1 and the injection ratio calculated in step S12, the first injection fuel amount and the second injection fuel amount are calculated.

Thereafter, the process proceeds to step S15, and waits until the first injection timing is reached. This first injection timing is a compression stroke after the intake valve 6 is closed, and is used to form a rich mixture around the ignition portion of the ignition plug 12 in a stratified state at the time of a stratified spark ignition timing described later. Is a value indicating when the injection should be started, and may be a fixed value or a variable value calculated back from the ignition timing based on the engine speed Ne.

When the first injection timing has been reached, the process proceeds from step S15 to step S16, where a first injection signal having a pulse width corresponding to the first injection fuel amount is output to the injector drive circuit 36d. Then, the first injection fuel amount calculated in step S13 is injected into the combustion chamber 3 from the in-cylinder injector 11 (see FIG. 5A).

Next, the routine proceeds to step S17, and waits until the stratified spark ignition timing is reached. This stratified spark ignition timing is a fixed value, and the in-cylinder pressure during combustion is determined by the compression top dead center (TD).
The timing is set such that the maximum is obtained just after C).

When the stratified spark ignition timing is reached,
The process proceeds from step S17 to step S18 to output an ignition signal to the ignition drive circuit 36c. Then, the stratified air-fuel mixture formed around the ignition portion of the ignition plug 12 is forcibly ignited, and is subjected to flame propagation combustion (first combustion).
Since the first combustion is also a positive work, the thermal efficiency does not decrease.

Thereafter, the process proceeds to step S19, and waits until the second injection timing is reached. This second injection timing is set to an appropriate timing during the expansion stroke. By setting the second injection timing during the expansion stroke, a uniform mixture can be generated before the gas temperature in the combustion chamber 3 reaches the compression ignition enabling temperature in the compression stroke.

When the second injection timing has been reached, the process proceeds from step S19 to step S20, in which a second injection signal having a pulse width corresponding to the second injection fuel amount is output to the injector drive circuit 36d, and the routine exits. . Then, the in-cylinder injector 11 injects the second-injection fuel amount calculated in step S14 into the combustion chamber 3 (FIG. 5).
(a)).

At this time, the combustion chamber 3 is filled with the relatively high-temperature unburned gas burned by the first combustion, and the unburned gas and the fuel from the second injection are mixed while being uniformly mixed. To the compression stroke.

In the compression stroke, the gas temperature at the start of the compression stroke is increased by the heat energy of the unburned gas. Therefore, even if the compression ratio is 12 or less, the combustion chamber 3
The temperature of the gas inside can be reliably increased to the compression ignition temperature. Then, when the compression ignition temperature is reached, the air-fuel mixture in the combustion chamber 3 is ignited all at once and combustion in which the flame does not propagate,
Multipoint ignition combustion (homogeneous compression ignition combustion) in which an infinite number of spark plugs are arranged is realized.

On the other hand, when it is determined in step S1 of the combustion control routine shown in FIG. 2 that the operation region is in the spark ignition region and the process proceeds to step S5, the intake valve 6 and the exhaust valve are provided to the variable valve mechanisms 13a and 13b. A valve timing drive signal for setting the valve 7 and the four-cycle operation to a four-cycle operation is set and output. Then, as shown in FIG. 5 (b), the intake valve 6 opens once in every two revolutions of the engine during the intake stroke, and the exhaust valve 7 similarly opens once every two revolutions of the engine, the exhaust stroke. Is returned to the normal valve timing.

Next, the routine proceeds to step S6, in which combustion control by ordinary spark ignition is executed, and the routine exits. In the normal spark ignition combustion control, the operation of the throttle valve 9 is returned to the operation linked to the accelerator pedal 32, and the fuel injection amount, the fuel injection timing, the ignition timing, and the like are returned to the normal spark ignition combustion control. Since the normal spark ignition combustion control is known,
The description here is omitted.

As described above, according to the present embodiment, in the compression ignition region, the operation is switched to the six-cycle operation in which the compression stroke and the expansion stroke are added between the intake stroke and the compression stroke in the normal four-cycle operation. In the compression stroke, the fresh air is once compressed, fuel for combustion control is injected into the fresh air, burned by stratified spark ignition to generate unburned gas, and the compression stroke is started by the heat energy of the unburned gas. Since the temperature is raised, stable compression ignition combustion can be obtained even if the compression ratio is 12 or less without extremely increasing the compression ratio of the engine. Furthermore, by variably setting the first injection fuel amount in accordance with the engine load Lo, it is possible to ensure ignitability in a low load region and to suppress knocking in a medium load region. Compression ignition combustion can be performed over a wide range. Further, since the combustion is performed twice during six cycles, a smooth engine rotation can be obtained.

In this case, as shown in FIG. 6, before and after the top dead center (TDC), a negative valve overlap period is formed in which both the exhaust valve 7 and the intake valve 6 are closed. ,
By heating and raising the temperature of fresh air to be sucked in the next intake stroke by the thermal energy of the residual gas confined in the combustion chamber 3 during the negative valve overlap period, the first combustion is also made to be compression ignition combustion. You may do it. Further, as the engine load increases during the negative valve overlap period,
By gradually narrowing the heat energy of the residual gas,
The first combustion timing may be controlled.

[0047]

As described above, according to the present invention,
In the compression ignition region, the operation cycle is switched from four-cycle operation to six-cycle operation, the compression stroke and the expansion stroke are continued twice, and fuel is injected and burned once in the first compression-expansion stroke to perform the next compression stroke. Since the stroke start temperature was raised, without increasing the compression ratio extremely,
By the adiabatic compression during the compression stroke, the gas temperature in the combustion chamber can be reliably raised to a temperature at which compression ignition combustion is possible, and stable compression ignition combustion can be obtained. As a result, it is possible to achieve an improvement in fuel efficiency and a significant reduction in exhaust emissions.

In this case, the ignitability in the low load region is ensured and the occurrence of knocking in the medium load region is suppressed by variably setting the amount of fuel injected at the first time according to the operation region. Thus, compression ignition combustion can be performed over a wide range. As a result, in combination with the normal spark ignition combustion, it is possible to significantly reduce the exhaust emission in the entire operation range.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a compression ignition engine.

FIG. 2 is a flowchart showing a combustion control routine.

FIG. 3 is a flowchart showing a compression ignition combustion control subroutine.

FIG. 4 is an explanatory diagram showing an operation area map.

5A and 5B are explanatory diagrams showing changes in in-cylinder pressure for each operation region, wherein FIG. 5A shows six-cycle operation by compression ignition, and FIG. 5B shows four-cycle operation by spark ignition.

FIG. 6 is an explanatory diagram showing a change in in-cylinder pressure during a six-cycle operation by compression ignition according to another embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine main body 3 Combustion chamber 6 Intake valve 7 Exhaust valve 11 In-cylinder injector 12 Spark plug 13a, 13b Variable valve mechanism

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme court ゛ (Reference) F02D 13/02 F02D 13/02 H J41 / 04 380 41/04 380C 385 385C 41/14 330 41/14 330B 41/40 41/40 DF term (reference) 3G023 AA02 AA05 AA06 AB03 AB06 AC05 AD04 AD29 AG00 AG01 AG02 AG05 3G092 AA04 AA06 AA11 AB02 BB02 BB06 DA03 EA11 FA17 GA05 GA06 GA17 GA18 3G301 HA04 HA08 KA24 KA24 KA24 KA24 MA19 NC02 ND03 PA17Z PB05A PE01Z

Claims (4)

[Claims] An in-cylinder injector for directly injecting fuel into a combustion chamber and a variable valve mechanism capable of varying valve timings of an exhaust valve and an intake valve. When the region is in the compression ignition region, the operation cycle is changed from the 4-cycle operation to the compression stroke and the expansion stroke of 2 cycles.
Operation cycle switching means for switching to six-cycle operation for consecutive times, and injecting the first fuel into the combustion chamber between the intake stroke and the subsequent compression stroke, and transferring the second fuel after shifting to the first expansion stroke. An injection timing setting means for injecting, and a combustion control device for a compression ignition type engine, wherein an air-fuel mixture formed by the first fuel is ignited in a first compression stroke. 2. A combustion control apparatus for a compression ignition type engine according to claim 1, wherein the air-fuel mixture formed by said first fuel is burned in a suction expansion stroke by spark ignition by a spark plug. . 3. The operation cycle switching means forms a negative valve overlap period in which the intake valve and the exhaust valve both close before and after the top dead center of the exhaust when the operation cycle is switched to the six cycle operation. And heating the temperature of the new gas sucked during the intake stroke with the residual gas trapped in the combustion chamber during the negative valve overlap period.
A combustion control device for a compression ignition type engine according to the above. 4. The combustion control system for a compression ignition engine according to claim 1, wherein said first fuel amount is decreased as the engine load increases.