Premixed compression ignition engine and method for controlling premixed compression ignition engine

Control is performed so as to occur SPCCI combustion in which, after an air-fuel mixture in a first area of a combustion chamber that includes an electrode portion of an ignition device is burned by receiving ignition energy, an air-fuel mixture formed in a second area located on an outer periphery of the first area is self-ignited. Control is also performed such that, in a high load operation region of an SPCCI combustion execution region, an air-fuel ratio in the entire combustion chamber becomes richer than a stoichiometric air-fuel ratio and that an air-fuel ratio of the air-fuel mixture in the first area becomes leaner than an air-fuel ratio of the air-fuel mixture in the second area.

TECHNICAL FIELD

The present invention relates to a premixed compression ignition engine which includes an engine body having a cylinder formed with a combustion chamber and self-ignites a mixture of fuel and air in the combustion chamber and to a method for controlling the premixed compression ignition engine.

BACKGROUND ART

Conventionally, in a gasoline engine or the like, it has been studied to perform so-called premixed compression ignition combustion in which a mixture of fuel and air mixed in advance is self-ignited in a combustion chamber.

In the premixed compression ignition combustion, it is possible to increase thermal efficiency as a compression ratio can be increased or the like. On the other hand, in the premixed compression ignition combustion, the air-fuel mixture starts combustion simultaneously at various places in the combustion chamber, so that a pressure in the combustion chamber, that is, an in-cylinder pressure rises abruptly, and consequently combustion noise tends to deteriorate. Further, in the premixed compression ignition combustion, a start timing of combustion, that is, a combustion start timing is liable to change depending on a temperature inside the combustion chamber or the like, and it is difficult to appropriately control ignition and a combustion timing.

On the other hand, for example, Patent Literature 1 discloses an engine which injects fuel into a combustion chamber by dividing it into a succeeding injection and a preceding injection and ignites an air-fuel mixture between the succeeding injection and the preceding injection.

According to the engine disclosed in Patent Literature 1, a temperature of the air-fuel mixture can be increased by flame propagation combustion caused by the ignition, and the air-fuel mixture can be self-ignited. Therefore, a combustion start timing can be appropriately controlled by adjusting an ignition timing. In addition, combustion of the air-fuel mixture formed by the succeeding injection and the air-fuel mixture formed by the preceding injection can be started at different timings, so that a rapid rise of an in-cylinder pressure can be suppressed and deterioration of combustion noise can be suppressed.

However, a demand for thermal efficiency and combustion noise is still high, and it is required to realize suitable premixed compression ignition combustion to further increase the thermal efficiency while further reducing the combustion noise.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

It is an object of the present invention to provide a premixed compression ignition engine capable of suppressing combustion noise while realizing appropriate premixed compression ignition combustion.

Means for Solving the Problems

In order to solve the above problems, the present invention provides a premixed compression ignition engine that includes an engine body formed with a combustion chamber and self-ignites a mixture of fuel and air in the combustion chamber, the engine including: a fuel injection device that injects fuel into the combustion chamber; an ignition device having an electrode portion that faces an inside of the combustion chamber and ignites an air-fuel mixture in the combustion chamber to apply ignition energy to the air-fuel mixture; and a control device that controls the ignition device and the fuel injection device so as to occur, in at least a part of an operation region of the engine, SPCCI combustion in which an air-fuel mixture formed in a first area of the combustion chamber that includes the electrode portion of the ignition device is burned by receiving the ignition energy applied from the ignition device, and thereafter, an air-fuel mixture formed in a second area of the combustion chamber located on an outer periphery of the first area is self-ignited and burned by a pressure rise in the combustion chamber due to the combustion of the air-fuel mixture in the first area, wherein when the control device determines that the engine is operated in a high load operation region serving as an operation region in which an engine load is higher than a reference load set to a value equal to or more than half of a maximum load of the engine and in which the SPCCI combustion is executed, the fuel injection device is controlled so that an air-fuel ratio in the entire combustion chamber becomes richer than a stoichiometric air-fuel ratio and that an air-fuel ratio of the air-fuel mixture in the first area becomes leaner than an air-fuel ratio of the air-fuel mixture in the second area at an ignition timing of the ignition device.

The present invention also provides a method for controlling a premixed compression ignition engine, the engine including a fuel injection device that injects fuel into a combustion chamber and an ignition device provided with an electrode portion that faces an inside of the combustion chamber and ignites a mixture of fuel and air in the combustion chamber to apply ignition energy to the mixture, where SPCCI combustion in which an air-fuel mixture formed in a first area of the combustion chamber that includes the electrode portion of the ignition device is burned by receiving the ignition energy applied from the ignition device, and thereafter, an air-fuel mixture formed in a second area of the combustion chamber located on an outer periphery of the first area is self-ignited and burned by a pressure rise in the combustion chamber due to the combustion of the air-fuel mixture in the first area is performed in at least a part of an operation region of the engine, the method including: a step of determining whether or not the engine is operated in a high load operation region serving as an operation region in which an engine load is higher than a reference load set to a value equal to or more than half of a maximum load of the engine and in which the SPCCI combustion is performed; and a step of controlling the fuel injection device executed when the engine is operated in the high load operation region, so that an air-fuel ratio in the entire combustion chamber becomes richer than a stoichiometric air-fuel ratio and that an air-fuel ratio of the air-fuel mixture in the first area becomes leaner than an air-fuel ratio of the air-fuel mixture in the second area at an ignition timing of the ignition device.

According to the present invention, it is possible to suppress combustion noise while realizing appropriate premixed compression ignition combustion.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described with reference to the accompanying drawings. It should be noted that the following embodiment is an example which embodies the present invention, and does not limit the technical scope of the present invention.

FIG. 1is a diagram showing a configuration of an engine system to which a premixed compression ignition engine of the present invention is applied. The engine system according to the present embodiment includes a four-stroke engine body1, an intake passage30for introducing combustion air into the engine body1, an exhaust passage40for leading out exhaust gas generated in the engine body1.

The engine body1is, for example, an in-line four-cylinder engine in which four cylinders2are disposed in series in a direction orthogonal to a paper surface ofFIG. 1. This engine system is mounted on a vehicle, and the engine body1is used as a drive source of the vehicle. In the present embodiment, the engine body1is driven by receiving supply of fuel including gasoline. Note that the fuel may be gasoline including bioethanol or the like.

(1) Engine Body

FIG. 2is a schematic sectional view of the engine body1.

The engine body1includes a cylinder block3in which a cylinder2is formed, a cylinder head4provided on an upper surface of the cylinder block3, a piston5fitted to the cylinder2so as to reciprocate (vertically move).

A combustion chamber6is formed above the piston5. The combustion chamber6is a so-called pent roof type. A ceiling surface6aof the combustion chamber6(hereinafter simply referred to as the combustion chamber ceiling surface6a) constituted by a lower surface of the cylinder head4has a triangular roof shape composed of two inclined surfaces on an intake side and an exhaust side.

The combustion chamber ceiling surface6aand a crown surface5aof the piston5face each other. In the crown surface5aof the piston5(hereinafter simply referred to as the piston crown surface5a), a cavity10is formed by recessing an area including a center of the piston crown surface5atoward a side opposite to the cylinder head4(downward). In this case, a space between the piston crown surface5aand the combustion chamber ceiling surface6ain an inner space of the cylinder2is referred to as the combustion chamber6, regardless of a position of the piston5and a combustion state of an air-fuel mixture.

In the present embodiment, a geometric compression ratio of the engine body1, that is, a ratio between a volume of the combustion chamber6when the piston5is at a bottom dead center and a volume of the combustion chamber6when the piston5is at a top dead center is set to 16 or more and 35 or less (for example, about 20).

The cylinder head4is provided with an intake port16for introducing air supplied from the intake passage30into the cylinder2(combustion chamber6) and an exhaust port17for leading out exhaust gas generated in the cylinder2to the exhaust passage40. Two intake ports16and two exhaust ports17are formed for each cylinder2.

The cylinder head4is provided with intake valves18for opening and closing openings of the intake ports16on the cylinder2and exhaust valves19for opening and closing openings of the exhaust ports17on the cylinder2.

The cylinder head4is provided with an injector (fuel injection device)22for injecting fuel. The injector22is mounted such that a tip formed with injection holes is located near a center of the combustion chamber ceiling surface6aand faces a center of the combustion chamber6. The injector22injects fuel from the vicinity of the center of the combustion chamber ceiling surface6atoward the piston crown surface5a. The injector22injects fuel in a cone shape (specifically, a hollow cone shape) with a center axis of the cylinder2as the center. A taper angle (spray angle) of the cone is, for example, 90° to 100°.

In the present embodiment, an injector of an outward opening type is used as the injector22. Incidentally, the injector22is not limited to the outward opening type but may be of any configuration as long as it can inject fuel in a cone shape with the center axis of the cylinder2as the center, as described above. For example, the injector22may be a VCO (Valve Covered Orifice) nozzle type injector, a multi-hole type injector provided with a plurality of injection holes at a tip and injecting fuel at a predetermined spray angle, or a swirl injector that injects fuel in a hollow cone shape.

The cylinder head4is provided with an ignition plug (ignition device)23for igniting an air-fuel mixture in the combustion chamber6. The ignition plug23has an electrode portion23aon which an electrode for discharging a spark and applying ignition energy to the air-fuel mixture is formed. The ignition plug23is disposed such that the electrode portion23ais located near the center of the combustion chamber ceiling surface6aand faces the center of the combustion chamber6.

The cylinder head4is further provided with a water injection device24for injecting water (injection water) into the combustion chamber6. The water injection device24is located such that a tip formed with an injection hole is located near the center of the combustion chamber ceiling surface6aand faces the center of the combustion chamber6. The water injection device24injects water from the vicinity of the center of the combustion chamber ceiling surface6atoward the piston crown surface5a. The water injection device24injects water in a cone shape (specifically, a hollow cone shape) with the center axis of the cylinder2as the center. A taper angle (spray angle) of this cone is, for example, 90° to 100°. The water injection device24has, for example, a structure similar to that of the injector22. Hereinafter, the water sprayed into the combustion chamber6by the water injection device24is referred to as injection water as appropriate.

As shown inFIGS. 2 and 3(FIG. 3is a schematic sectional view of the combustion chamber6), the injector22and the water injection device24are disposed so that their tips are close to each other in the vicinity of the center of the combustion chamber ceiling surface6a. The ignition plug23is disposed such that its electrode portion23ais closer to the tip of the injector22than the tip of the water injection device24.

As shown inFIG. 1, in the intake passage30, an air cleaner31and a throttle valve32for opening and closing the intake passage30are provided in order from an upstream side. In the present embodiment, during operation of the engine, the throttle valve32is basically kept fully open or nearly fully open. The throttle valve32is closed only under limited operating conditions such as when the engine is stopped, thereby shutting off the intake passage30.

In the exhaust passage40, a purification device41for purifying exhaust and a condenser42are provided in order from the upstream side. For example, the purification device41incorporates a three-way catalyst.

The condenser42condenses water (water vapor) in the exhaust gas passing through the exhaust passage40. The condenser42and the water injection device24are connected by a water supply passage61. The condensed water produced by the condenser42is supplied to the water injection device24via the water supply passage61. As described above, in the present embodiment, the water injection device24injects the water generated from the exhaust gas into the combustion chamber6. More specifically, the water supply passage61is provided with a water tank43for storing the condensed water generated by the condenser42and a water pump44for pressure-feeding the water in the water tank43. The condensed water is supplied from the water tank43to the water injection device24by the water pump44.

In the exhaust passage40, an EGR device46for returning a part of the exhaust gas passing through the exhaust passage40as EGR gas to the intake passage30is provided. The EGR device46has an EGR passage47that communicates a portion of the intake passage30on a downstream side of the throttle valve32and a portion of the exhaust passage40on an upstream side of the purification device41and an EGR valve48that opens and closes the EGR passage47. Further, in the present embodiment, an EGR cooler49for cooling the EGR gas passing through the EGR passage47is provided in the EGR passage47. The EGR gas is cooled by the EGR cooler49and then recirculated to the intake passage30.

(2) Control System

(2-1) System Configuration

FIG. 4is a block diagram showing a control system of the engine. As shown inFIG. 4, the engine system of the present embodiment is totally controlled by a PCM (Powertrain Control Module, control unit)100. As is well known, the PCM100is a microprocessor composed of a CPU, a ROM, a RAM, and the like.

Various sensors are provided in the vehicle, and the PCM100is electrically connected to these sensors. For example, the cylinder block3is provided with a crank angle sensor SN1for detecting engine speed. The intake passage30is provided with an air flow sensor SN2that detects an amount of air taken into each cylinder2through the intake passage30. The vehicle is provided with an accelerator opening degree sensor SN3for detecting an opening degree (accelerator opening degree) of an accelerator pedal (not shown) operated by a driver.

The PCM100executes various calculations based on input signals from these sensors SN1to SN3, etc., and controls the engine parts such as the ignition plug23, the injector22, the water injection device24, the throttle valve32, the EGR valve48, and the water pump44.

In the present embodiment, the EGR valve48is opened in an entire operation region, and the EGR gas is recirculated to the intake passage30in the entire operation region.

Further, in order to increase thermal efficiency, an ignition timing of the ignition plug23(a timing at which the ignition plug23ignites an air-fuel mixture) is controlled so that the center of gravity of a heat generation rate (a timing at which combustion of 50% of a total amount (mass) of fuel supplied to the combustion chamber6is completed) is in an expansion stroke in the entire operation region.

Further, in the present embodiment, premixed compression ignition combustion is performed in the entire operation region. However, various controls for realizing the premixed compression ignition combustion differ depending on the operation region.

FIG. 5is a control map in which a horizontal axis represents engine speed and a vertical axis represents an engine load. In the present embodiment, as a control region, a low load operation region A1in which the engine load is equal to or smaller than a first load Tq1set in advance, a middle load operation region A2in which the engine load is larger than the first load Tq1and equal to or smaller than a fourth load (reference load) Tq4, and a high load operation region A3in which the engine load is higher than the fourth load Tq4are set. Further, the middle load operation region A2is divided into a middle load first region A2_1in which the engine load is equal to or smaller than a second load Tq2, a middle load second region A2_2in which the engine load is larger than the second load Tq2and equal to or smaller than a third load Tq3, and a middle load third region A2_3in which the engine load is larger than the third load Tq3. The contents of control in each of the operation regions A1to A3performed by the PCM100will be described below. The fourth load Tq4is set to a value equal to or larger than half of a full load. In other words, the fourth load Tq4is set to a value equal to or larger than half of the engine load corresponding to a maximum torque that the engine body can output.

The PCM100determines which operation region the engine body is currently operated. For example, the PCM100executes a step of determining whether or not the engine is operated in the high load operation region A3and a step of determining whether or not the engine is operated with the engine load being the fourth load Tq4. Based on the determination result, the PCM100executes control (step) according to each operation region described below.

(2-2) Low Load Operation Region A1

FIG. 6schematically shows a fuel injection pattern and a heat generation rate dQ in the low load operation region A1. As shown inFIG. 6, a batch injection F10is performed in the low load operation region A1, and an entire amount of fuel supplied to the combustion chamber6in one combustion cycle is injected from the injector22into the combustion chamber6in a first half of a compression stroke. This injected fuel amount (amount of fuel injected from the injector22) is calculated from an engine load calculated from an accelerator opening degree and the like, engine speed, and the like.

In the low load operation region A1, as described above, the entire amount of fuel is injected into the combustion chamber6in the first half of the compression stroke to mix with air. Then, this mixture of fuel and air is heated and pressurized by compression action of the piston5to be self-ignited near a compression top dead center. Accordingly, the premixed compression ignition combustion is realized.

In the low load operation region A1, water injection by the water injection device24is stopped.

(2-3) Middle Load Operation Region

In the middle load operation region A2, the premixed compression ignition combustion by ignition assist, that is, SPCCI (SPark Controlled Compression Ignition) combustion is performed. In other words, the ignition plug23discharges in the air-fuel mixture formed in the combustion chamber6, and an air-fuel mixture around the ignition plug23is forcibly ignited. Then, flame propagates from the periphery of the ignition plug23to the surroundings, a temperature of a surrounding air-fuel mixture is raised, and the air-fuel mixture can be self-ignited.

In the middle load operation region A2, the fuel is injected from the injector22so that an air-fuel ratio of a first air-fuel mixture G1formed in a central area (first area) R1and an area of the combustion chamber6that includes the electrode portion23aof the ignition plug23(includes an area where the electrode portion23ais disposed) is leaner than a stoichiometric air-fuel ratio (an air-fuel ratio A/F of the air-fuel mixture is larger than the stoichiometric air-fuel ratio, and an excess air ratio λ of the air-fuel mixture is larger than 1) at an ignition timing. In the middle load operation region A2, the fuel is injected from the injector22so that an air-fuel ratio of a first air-fuel mixture G1is equal to or larger than an air-fuel ratio of a second air-fuel mixture G2formed in an outer peripheral area (second area) R2of the combustion chamber6located on an outer periphery of the first area R1at an ignition timing. Then, as shown inFIG. 3, the first air-fuel mixture G1formed in the central area R1is subjected to flame propagation combustion (SI combustion), and the second air-fuel mixture G2formed in the outer peripheral area R2is subjected to compression self-ignition combustion (CI combustion).

In the present embodiment, as shown inFIG. 3, the central area R1is approximately an area in which the cavity10is formed as seen from a direction along the center axis of the cylinder2. The outer peripheral area R2is an area outside the cavity10in a radial direction of the cylinder2.

FIGS. 7, 8 and 9schematically show a fuel injection pattern, an ignition timing, and a heat generation rate in the middle load first region A2_1, the middle load second region A2_2, and the middle load third region A2_3, respectively. In the middle load second region A2_2and the middle load third region A2_3, water injection is performed as described later, and water injection patterns are shown together inFIGS. 8 and 9.

As shown inFIGS. 7, 8, and 9, in the middle load operation region A2, the fuel injection patterns in the regions A2_1to A2_3are substantially the same, and the fuel is injected twice into the combustion chamber6. As a result, air-fuel mixtures having different air-fuel ratios are formed in the central area R1and the outer peripheral area R2.

FIG. 10is a view for explaining a procedure of forming the air-fuel mixture in the middle load operation region A2. Chart (1) to (4) ofFIG. 10are views schematically showing states inside the combustion chamber6at different times. In (1) to (4) ofFIG. 10, time passes in this order. Chart (1) ofFIG. 10shows a state inside the combustion chamber6during an intake stroke. Chart (2) to (4) ofFIG. 10show a state inside the combustion chamber6during a compression stroke.

As shown in Chart (1) ofFIG. 10, the injector22executes first fuel injection F21for diffusing the fuel to an entire area of the combustion chamber6. Then, second fuel injection F22is executed after the end of the first fuel injection F21.

The first fuel injection F21is performed in the intake stroke or in a first half of the compression stroke (within a period from a start timing of the intake stroke to an end timing of the first half of the compression stroke). In the present specification, a first stage, a middle stage, and a latter stage of a certain stroke such as the compression stroke refer to a first stage, a middle stage, and a latter stage when this stroke is trisected, and a first half and a second half of the certain stroke refer to a first half and a second half when this stroke is bisected.

An injection amount of the first fuel injection F21(amount of fuel injected into the combustion chamber6by the first fuel injection F21) is made smaller than a value obtained by dividing air existing in the combustion chamber6by the stoichiometric air-fuel ratio.

As shown in Chart (2) ofFIG. 10, after a while after the execution of the first fuel injection F21, the fuel injected into the combustion chamber6by the first fuel injection F21is diffused to almost the entire area of the combustion chamber6. As a result, an air-fuel mixture leaner than the stoichiometric air-fuel ratio and substantially homogeneous is formed in the combustion chamber6. Here, if the first fuel injection F21is executed in the first half of the intake stroke and at a timing at which a flow rate of the air flowing into the combustion chamber6from the intake port16is relatively high, the fuel injected by the first fuel injection F21is more reliably diffused into the entire combustion chamber6, so that a homogeneous air-fuel mixture can be more reliably formed.

Next, the injector22executes the second fuel injection F22for unevenly locating the fuel in the outer peripheral area R2. Specifically, as shown in Chart (3) ofFIG. 10, the second fuel injection F22is performed at such a timing that the fuel injected from the injector22collides with a peripheral portion10cof the cavity10.

As shown in Chart (3) ofFIG. 10, the fuel injected in this manner reaches the peripheral portion10cof the cavity10and then flows toward the ceiling surface6aof the combustion chamber6along the peripheral portion10cof the cavity10. The fuel is introduced into an outer periphery side of the cavity10, that is, the outer peripheral area R2. For example, the second fuel injection F22is performed in the middle stage of the compression stroke (from BTDC 120° CA to BTDC 60° CA).

In the middle load operation region A2, the mixture in the combustion chamber6is stratified by the second fuel injection F22, as shown in Chart (4) ofFIG. 10. In other words, an air-fuel mixture (first air-fuel mixture G1) that is a mixture of air and the fuel injected into the combustion chamber6by the first fuel injection F21and is leaner than the stoichiometric air-fuel ratio is formed in the central area R1. Then, an air-fuel mixture (second air-fuel mixture G2) which is a mixture of air, the fuel injected into the combustion chamber6by the first fuel injection F21, and the fuel injected into the combustion chamber6by the second fuel injection F22and is richer (smaller in air-fuel ratio) than the air-fuel mixture in the central area R1formed only by the fuel of the first fuel injection F21is formed in the outer peripheral area R2. This stratified state is maintained until the ignition timing. At the ignition timing as well, the air-fuel mixture leaner than the stoichiometric air-fuel ratio exists in the central area R1, and the air-fuel mixture richer than the air-fuel mixture in the central area R1exists in the outer peripheral area R2.

After the second fuel injection F22is performed, ignition of the air-fuel mixture is performed by the ignition plug23. For example, ignition of the air-fuel mixture is performed at timing more on an advance angle side than the compression top dead center.

FIG. 11is a graph showing a relationship between the engine load and the air-fuel ratio (air-fuel ratio at an ignition timing) of the air-fuel mixture in each of the areas R1, R2, in the middle load operation region A2and the high load operation region A3.

As shown inFIG. 11, in the middle load operation region A2, an air-fuel ratio AF_R2of the air-fuel mixture in the outer peripheral area R2at the ignition timing is set to the stoichiometric air-fuel ratio irrespective of the engine load.

In the middle load operation region A2, an air-fuel ratio AF_R1of the air-fuel mixture in the central area R1is controlled to a constant value (for example, about 20) irrespective of the engine load when the engine load does not exceed the second load Tq2. When the engine load exceeds the second load Tq2, the air-fuel ratio AF_R1becomes rich (is reduced) as the engine load increases. For example, when the engine load exceeds the second load Tq2, the air-fuel ratio AF_R1is reduced in proportion to the engine load. In order to realize this, in the middle load operation region A2, when the engine load does not exceeds the second load Tq2, a ratio between the injection amount of the first fuel injection F21and an injection amount of the second fuel injection F22is kept almost constant irrespective of the engine load. On the other hand, when the engine load exceeds the second load Tq2, as the engine load increases, a ratio of the injection amount of the first fuel injection F21(a ratio of the injection amount of the first fuel injection F21to a total amount of the fuel supplied to the combustion chamber6per one combustion cycle) is increased, and a ratio of the injection amount of the second fuel injection F22(a ratio of the injection amount of the second fuel injection F22to the total amount of the fuel supplied to the combustion chamber6per one combustion cycle) is reduced.

As shown inFIG. 11, in the present embodiment, in a boundary between the middle load operation region A2and the high load operation region A3, that is, under the operating condition in which the engine load is the fourth load Tq4, the air-fuel ratio AF_R1of the air-fuel mixture in the central area R1is set to the stoichiometric air-fuel ratio. Accordingly, under the operating condition where the engine load is the fourth load Tq4, both of the air-fuel ratio AF_R1of the air-fuel mixture in the central area R1and the air-fuel ratio AF_R2of the air-fuel mixture in the outer peripheral area R2are the same stoichiometric air-fuel ratio. Note that under the operating condition where the engine load is the fourth load Tq4, the second fuel injection F22is stopped.

The water injection patterns in the regions A2_1to A2_3of the middle load operation region A2are different from each other. Next, the water injection patterns will be described.

In the middle load first region A2_1, water injection by the water injection device24is stopped.

In the middle load second region A2_2, water injection by the water injection device24is performed. In the middle load second region A2_2, the water injection pattern is an injection pattern in which injection water exists only in the outer peripheral area R2at the ignition timing.

Specifically, in the middle load second region A2_2, as the second fuel injection F22shown in Chart (3) ofFIG. 10, water injection W1is performed only once at a timing at which the water injected from the water injection device24reaches the peripheral portion10cof the cavity10(for example, the middle stage of the compression stroke) during one combustion cycle, so that the injection water is introduced only to the outer peripheral area R2.

Also in the middle load third region A2_3, water injection by the water injection device24is performed. However, in the middle load third region A2_3, the water injection pattern is a pattern in which the injection water exists in both the central area R1and the outer peripheral area R2at the ignition timing. At this time, a concentration of water in the outer peripheral area R2is made higher than a concentration of water in the central area R1.

Specifically, first, water injection W11is performed at a timing at which the injection water is diffused throughout the combustion chamber6. Thereafter, similar to the water injection W1in the middle load second region A2_2, additional water injection W12is performed at timing when the injection water reaches the peripheral portion10cof the cavity10and is introduced only to the outer peripheral area R2. For example, as shown inFIG. 9, the first water injection W11is performed at a relatively early timing in the middle stage of the compression stroke, and then the next water injection W12is performed at a relatively later timing of the compression stroke. As a result, an air-fuel mixture having a relatively low concentration of the injection water is formed in the central area R1by the first water injection W11, and the concentration of the injection water in the outer peripheral area R2is increased by the next water injection W12.

FIG. 12is a graph showing a relationship between the engine load and the concentration of the injection water (the concentration of the injection water at the ignition timing) in each of the areas R1and R2. As shown inFIG. 12, in the present embodiment, in the operation region including the middle load second region A2_2and the middle load third region A2_3, as the engine load increases, a concentration Cw_R2of the injection water in the outer peripheral area R2is increased. In the middle load third region A2_3, as the engine load increases, a concentration Cw_R1of the injection water in the central area R1is increased. In the middle load third region A2_3, a rate of increase in the concentration of the injection water with respect to the engine load is larger in the central area R1. Accordingly, in the middle load third region A2_3, as the engine load increases, a difference between the concentration Cw_R2of the injection water in the outer peripheral area R2and the concentration Cw_R1of the injection water in the central area R1is reduced.

InFIGS. 7 to 9, the water injections W1, W11, and W12are executed after the second fuel injection F22, but the water injection timing and the fuel injection timing can be independently set. The water injection W1in the middle load second region A2_2and the water injection W11and W12in the middle load third region A2_3may be performed before the second fuel injection F22. For example, the first water injection W11in the middle load third region A2_3may be performed during the intake stroke. However, the water injection W1in the middle load second region A2_2and the latter water injection W12in the middle load third region A2_3are performed in the middle stage of the compression stroke as described above, so that the injection water can exist only in the outer peripheral area R2more reliably.

(2-4) High Load Operation Region

FIG. 13schematically shows a fuel injection pattern, a water injection pattern, ignition timing, and a heat generation rate in the high load operation region A3.

Even in the high load operation region A3, the premixed compression ignition combustion (SPCCI combustion) by ignition assist is performed in the entire region in the same manner as in the middle load operation region A2. Also in the high load operation region A3, in the entire region, similarly to the middle load operation region A2(except when the engine load is the fourth load Tq4), at the ignition timing, fuel is injected from the injector22so that the air-fuel ratio AF_R1of the air-fuel mixture in the central area R1is leaner than the air-fuel ratio AF_R2of the air-fuel mixture in the outer peripheral area R2(F31, F32).

However, in the high load operation region A3, as shown inFIG. 11, the air-fuel ratio AF_R1of the air-fuel mixture in the central area R1and the air-fuel ratio AF_R2of the air-fuel mixture in the outer peripheral area R2are richer than the stoichiometric air-fuel ratio (less than the stoichiometric air-fuel ratio, an excess air ratio λ<1).

In the present embodiment, as described above, the air-fuel ratios of the air-fuel mixtures in the central area R1and the outer peripheral area R2at the fourth load Tq4are set to substantially the stoichiometric air-fuel ratio. When the engine load exceeds the fourth load Tq4, the air-fuel ratios of the air-fuel mixtures in these areas R1, R2are made richer than the stoichiometric air-fuel ratio. When the engine load exceeds the fourth load Tq4, the air-fuel ratios of the air-fuel mixtures in these areas R1, R2decrease as the engine load increases. For example, when the engine load exceeds the fourth load Tq4, the air-fuel ratios of the air-fuel mixtures in these areas R1and R2are reduced in proportion to the engine load as the engine load increases.

In the high load operation region A3, water injection into the combustion chamber6is performed with the same injection pattern as the water injection pattern in the middle load third region A2_3. In other words, in the high load operation region A3, injection water exists in the central area R1and the outer peripheral area R2at the ignition timing, and the water injection is performed so that the concentration Cw_R2of the injection water in the outer peripheral area R2is higher than the concentration Cw_R1of the injection water in the central area R1(W11, W12).

As shown inFIG. 12, also in the high load operation region A3, as in the middle load third region A2_3, the higher the engine load is, the higher the concentration Cw_R1of the injection water in the central area R1and the concentration Cw_R2of the injection water in the outer peripheral area R2. Also in the high load operation region A3, as the engine load increases, a difference between the concentration Cw_R2of the injection water in the outer peripheral area R2and the concentration Cw_R1of the injection water in the central area R1is reduced. In the present embodiment, when the engine load is the maximum load, that is, in a so-called full load, the concentrations are controlled so that the concentration Cw_R2of the injection water in the outer peripheral area R2and the concentration Cw_R1of the injection water in the central area R1coincide with each other.

As described above, in the present embodiment, the premixed compression ignition combustion is realized in the entire operation regions A1to A3. Therefore, thermal efficiency can be enhanced.

In addition, the ignition assist is performed in the middle load operation region A2and the high load operation region A3. Therefore, by adjusting the ignition timing in these operation regions A2, A3, it is possible to control a combustion start timing at an appropriate timing. In other words, controllability of the combustion start timing can be enhanced.

Here, in the middle load operation region A2and the high load operation region A3, as the total amount of fuel supplied to the combustion chamber6increases and an amount of heat generation increases, combustion noise tends to increase.

On the other hand, in the present embodiment, in the middle load operation region A2, the ignition assist is carried out in a state in which the air-fuel ratio AF_R1of the air-fuel mixture in the central area R1including the electrode portion23aof the ignition plug23is leaner than the air-fuel ratio AF_R2of the air-fuel mixture in the outer peripheral area R2. Therefore, it is possible to slow the flame propagation combustion in the central area R1caused by the application of the ignition energy to the air-fuel mixture and to suppress a rapid temperature rise in the combustion chamber6. In addition, it is possible to prevent the compression self-ignition combustion of the air-fuel mixture in the outer peripheral area R2following the flame propagation combustion from being started excessively early and to make the compression self-ignition combustion slow. Therefore, it is possible to suppress a rapid rise of in-cylinder pressure (pressure in the combustion chamber6) accompanying the combustion and to reduce combustion noise. In particular, in the middle load operation region A2, the air-fuel ratio AF_R1of the air-fuel mixture in the central area R1is basically leaner than the stoichiometric air-fuel ratio (excluding a condition that the engine load is the fourth load Tq4). Therefore, in the middle load operation region A2, the flame propagation combustion in the central area R1and the subsequent compression self-ignition combustion can be made slow more reliably, and an increase in the combustion noise can be suppressed more reliably.

In the high load operation region A3, the total amount of fuel to be supplied to the combustion chamber6increases due to the high engine load, and the amount of heat generation in the combustion chamber6increases. Therefore, in the high load operation region A3, the combustion noise tends to be higher than that in the middle load operation region A2. Therefore, if the same control as in the middle load operation region A2is performed in the high load operation region A3, the combustion noise may not be sufficiently suppressed.

On the other hand, in the present embodiment, in the high load operation region A3, while the air-fuel ratio AF_R1of the air-fuel mixture in the central area R1is made leaner than the air-fuel ratio AF_R2of the air-fuel mixture in the outer peripheral area R2, the air-fuel ratios AF_R1, AF_R2of the air-fuel mixtures in these areas R1, R2are made richer than the stoichiometric air-fuel ratio.

Therefore, it is possible to cool the air-fuel mixtures by latent heat of vaporization of a large amount of fuel and to suppress rapid increases in the combustion temperature and the in-cylinder pressure during combustion. In particular, since the air-fuel ratio AF_R2of the air-fuel mixture in the outer peripheral area R2is made richer than the air-fuel ratio AF_R1of the air-fuel mixture in the central area R1at the ignition timing, temperature of the air-fuel mixture in the outer peripheral area R2can be suppressed low. Therefore, it is possible to effectively slow down the compression self-ignition combustion occurring in the outer peripheral area R2and to reduce the combustion noise.

This will be concretely described with reference toFIGS. 14 and 15.FIG. 14is a view corresponding toFIG. 3, andFIG. 15is a diagram showing a heat generation rate.

If the air-fuel ratio in the combustion chamber6is not made richer than the stoichiometric air-fuel ratio in the high load operation region A3, a cooling effect due to the latent heat of vaporization of the fuel is small, so that rapid flame propagation combustion (SI combustion) around the ignition plug23occurs, and that surrounding temperature rapidly rises. Therefore, in this case, as shown inFIG. 14, the compression self-ignition combustion (CI combustion) also occurs in an outer peripheral portion of the central area R1, and the compression self-ignition combustion (CI combustion) starts excessively early. In addition, in this case, a large amount of air-fuel mixture consisting of an air-fuel mixture existing in the outer peripheral portion of the central area R1and the air-fuel mixture existing in the outer peripheral area R2performs the compression self-ignition combustion (CI combustion) in a short period. Accordingly, in this case, as indicated by a broken line inFIG. 15, immediately after the ignition, the air-fuel mixture in the entire combustion chamber6is burned rapidly, the in-cylinder pressure rapidly increases, and the combustion noise increases.

On the other hand, in the present embodiment, in the high load operation region A3, the air-fuel ratio AF_R1of the air-fuel mixture in the central area R1is richer than the stoichiometric air-fuel ratio. Therefore, temperature of the air-fuel mixture in the central area R1can be kept low by the latent heat of vaporization of the large amount of fuel. Accordingly, it is possible to suppress occurrence of the rapid flame propagation combustion (SI combustion) around the ignition plug23, to prevent premature ignition of the air-fuel mixtures (excessively early self-ignition combustion of the air-fuel mixtures) in the outer peripheral portion of the central area R1and the outer peripheral area R2, and to slow the combustion of the air-fuel mixture in the outer peripheral area R2. Furthermore, in the present embodiment, since the air-fuel ratio AF_R2of the air-fuel mixture in the outer peripheral area R2is made richer than the stoichiometric air-fuel ratio, the temperature of the air-fuel mixture in the outer peripheral area R2can also be kept low. In particular, the air-fuel ratio AF_R2of the air-fuel mixture in the outer peripheral area R2is made richer than the air-fuel ratio AF_R1of the air-fuel mixture of the air-fuel mixture in the central area R1, thereby effectively suppressing the temperature of the air-fuel mixture in the outer peripheral area R2low. Therefore, as shown by a solid line inFIG. 15, it is possible to slow the combustion of the air-fuel mixture, that is, the compression self-ignition combustion (CI combustion) of the air-fuel mixture in the outer peripheral area R2and to effectively reduce the combustion noise.

Furthermore, in the present embodiment, water is injected into the combustion chamber6in the high load operation region A3. Therefore, it is possible to further suppress the rise of the combustion temperature by raising specific heat of the air-fuel mixture.

Particularly, in the present embodiment, in the high load operation region A3, the injection water is supplied to both the outer peripheral area R2and the central area R1. Therefore, the injection water supplied to the central area R1slows the flame propagation combustion in the central area R1to suppress the temperature rise in the surroundings. As a result, it is possible to prevent the compression self-ignition combustion from starting excessively early and to slow down the compression self-ignition combustion. Also, the injection water supplied to the outer peripheral area R2suppresses the temperature rise in the outer peripheral area R2and can further slow down the compression self-ignition combustion. Accordingly, the combustion noise can be more reliably reduced in the high load operation region A3.

This also applies to the middle load third region A2_3. In other words, in the present embodiment, since the injection water is supplied to both the outer peripheral area R2and the central area R1also in the middle load third region A2_3, the combustion noise can be reliably reduced.

In the middle load second region A2_2, and in a region where the engine load is relatively low but not sufficiently low, the injection water is supplied only to the outer peripheral area R2. Therefore, while preventing excessive lowering of the temperature in the central area R1and inappropriate propagation of flame caused by supply of the injection water to the area R1, the compression self-ignition combustion in the outer peripheral area R2can be made slow. Therefore, the combustion noise can be reduced while achieving the compression self-ignition combustion.

In the above embodiment, a case where the injection water is supplied into the combustion chamber6in the middle load second region A2_2, the middle load third region A2_3, and the high load operation region A3has been described, but control for supplying the injection water may be omitted. The water injection device24may be omitted. However, if the injection water is supplied into the combustion chamber6in these regions where the engine load is relatively high, it is possible to further reduce the combustion noise.

Further, in the above-described embodiment, a case where the water is injected such that the injection water exists in both the outer peripheral area R2and the central area R1at the ignition timing in the middle load third region A2_3and the high load operation region A3has been described, but water injection to the central area R1may be omitted. However, if the injection water is also present in the central area R1, the combustion noise can be further reduced.

In the above embodiment, when the injection water is supplied to both the central area R1and the outer peripheral area R2(when operating in the middle load third region A2_3and the high load operation region A3), as the engine load increases, the difference between the concentration Cw_R1of the injection water in the central area R1and the concentration Cw_R2of the injection water in the outer peripheral area R2is decreased. However, the relationship between these concentrations is not limited to this, and a concentration difference may be made constant irrespective of the engine load. However, as described above, if this difference is made smaller as the engine load increases, and the concentration Cw_R2of the injection water in the outer peripheral area R2is increased, the combustion noise can be effectively reduced.

Further, in the above-described embodiment, a case where the injection water is distributed unevenly in the central area R1and the outer peripheral area R2by changing the injection timing of the water injection device24has been described. However, a specific configuration for unevenly distributing the injection water is not limited to this. For example, as the water injection device24, a device capable of injecting water to different areas, for example, one capable of injecting water at different injection angles may be used. By changing this injection area (injection angle), water is unevenly distributed. Likewise, also in the injector22, fuel may be unevenly distributed by changing an injection area or the like.

In the above embodiment, a case where the EGR valve52is opened to recirculate the EGR gas to the intake passage30in the whole operation region has been described, but the EGR gas may be recirculated only in a part of the operation region. Further, the EGR device46may be omitted. However, if the EGR gas is recirculated in the middle load operation region A2and the high load operation region A3, inert gas in the combustion chamber6is increased, so that it is possible to more reliably suppress a rapid rise of the combustion temperature and an increase in the combustion noise accordingly.

Also, the geometric compression ratio of the engine body is not limited to the above. However, in order to realize appropriate flame propagation combustion while setting the air-fuel ratio of the air-fuel mixture in the central area R1to be lean in the middle load operation region A2or the like and to reliably realize the compression self-ignition combustion of the air-fuel mixture, it is preferable that the geometric compression ratio be set as in the above embodiment.

The above-described specific embodiment mainly includes an invention having the following configurations.

The present invention provides a premixed compression ignition engine that includes an engine body formed with a combustion chamber and self-ignites a mixture of fuel and air in the combustion chamber, the engine including: a fuel injection device that injects fuel into the combustion chamber; an ignition device having an electrode portion that faces an inside of the combustion chamber and ignites an air-fuel mixture in the combustion chamber to apply ignition energy to the air-fuel mixture; and a control device that controls the ignition device and the fuel injection device so as to occur, in at least a part of an operation region of the engine, SPCCI combustion in which an air-fuel mixture formed in a first area of the combustion chamber that includes the electrode portion of the ignition device is burned by receiving the ignition energy applied from the ignition device, and thereafter, an air-fuel mixture formed in a second area of the combustion chamber located on an outer periphery of the first area is self-ignited and burned by a pressure rise in the combustion chamber due to the combustion of the air-fuel mixture in the first area, wherein when the control device determines that the engine is operated in a high load operation region serving as an operation region in which an engine load is higher than a reference load set to a value equal to or more than half of a maximum load of the engine and in which the SPCCI combustion is executed, the fuel injection device is controlled so that an air-fuel ratio in the entire combustion chamber becomes richer than a stoichiometric air-fuel ratio and that an air-fuel ratio of the air-fuel mixture in the first area becomes leaner than an air-fuel ratio of the air-fuel mixture in the second area at an ignition timing of the ignition device.

In this configuration, the air-fuel mixture formed around the electrode portion of the ignition device is forcibly burned by ignition to cause flame propagation, and by increasing temperature in the combustion chamber by the flame propagation, the air-fuel mixture in the second area is self-ignited and burned. Therefore, the air-fuel mixture in the second area can be assuredly self-ignited and burned, whereby thermal efficiency can be enhanced. Also, by adjusting the ignition timing, it is possible to control a combustion start timing of the air-fuel mixture at an appropriate timing.

Moreover, in this configuration, the air-fuel ratio in the entire combustion chamber at the ignition timing is made richer than the stoichiometric air-fuel ratio in the high load operation region. Therefore, it is possible to cool the air-fuel mixture by latent heat of vaporization of a large amount of fuel supplied to the combustion chamber and to keep combustion temperature and in-cylinder pressure at the time of combustion low. In particular, since the air-fuel ratio of the air-fuel mixture in the second area is made richer (smaller) than the air-fuel ratio of the air-fuel mixture in the first area at the ignition timing, it is possible to keep temperature of the air-fuel mixture in the second area low. Therefore, it is possible to effectively slow down the compression self-ignition combustion occurring in the second area, to suppress a rapid rise of the in-cylinder pressure, and to suppress combustion noise to a small extent.

In the above configuration, it is preferable that, when the control device determines that the engine is operated in a state in which the engine load is the reference load, the fuel injection device is controlled so that the air-fuel ratio in the entire combustion chamber becomes the stoichiometric air-fuel ratio and that the air-fuel ratio of the air-fuel mixture in the first area and the air-fuel ratio of the air-fuel mixture in the second area both become the stoichiometric air-fuel ratio at the ignition timing.

According to this configuration, a relatively large amount of fuel is supplied to the combustion chamber even in a state in which the engine load is the reference load, so that a rapid rise in the in-cylinder pressure can be suppressed, and the combustion noise can be suppressed small.

In the above configuration, it is preferable that when the control device determines that the engine is operated in the high load operation region, in a case where the air-fuel ratio of the air-fuel mixture in the first area is set to a first air-fuel ratio at the ignition timing and the air-fuel ratio of the air-fuel mixture in the second area is set to a second air-fuel ratio at the ignition timing, the fuel injection device is controlled so that both the first air-fuel ratio and the second air-fuel ratio become richer than the stoichiometric air-fuel ratio, that both the first air-fuel ratio and the second air-fuel ratio decrease as the engine load increases, and that a decreasing rate of the second air-fuel ratio to an increase amount of the engine load is greater than a decreasing rate of the first air-fuel ratio to the increase amount of the engine load.

In this way, in the high load operation region, required engine torque is realized by the large amount of fuel supplied to the combustion chamber. Moreover, temperature rises in the first area and the second area, an increase in the in-cylinder pressure, and an increase in the combustion noise are appropriately suppressed by the large amount of fuel. Further, by setting the decreasing rate of the air-fuel ratio in the second area to be larger than the decreasing rate of the air-fuel ratio in the first area with respect to the increase amount of the engine load, the compression self-ignition combustion in the second area can be effectively slowed down. This effectively suppresses the increase in the combustion noise.

In the above configuration, it is preferable that when the control device determines that the engine is operated in the high load operation region, the fuel injection device is caused to perform a first fuel injection for injecting fuel in an intake stroke or in a first half of a compression stroke and a second fuel injection for injecting fuel after an end of the first fuel injection.

According to this configuration, it is possible to make the air-fuel ratios of the first area and the second area of the combustion chamber different from each other with a simple configuration in which the fuel is injected twice at different timings. In addition, the fuel is injected into the combustion chamber in a second half of the compression stroke or after a second half of the compression stroke, that is, at timing close to a compression top dead center, so that the temperature of the air-fuel mixture immediately before a start of the combustion can be effectively lowered by the latent heat of vaporization of the injected fuel.

In the above configuration, it is preferable that the engine body is provided with a piston that reciprocates in a cylinder defining the combustion chamber, the piston has a crown surface facing a ceiling portion of the combustion chamber, the ignition device is provided at a center of the ceiling portion of the combustion chamber, the fuel injection device is provided at the center of the ceiling portion of the combustion chamber and injects fuel toward the crown surface of the piston, and when the control device determines that the engine is operated in the high load operation region, the fuel injection device is caused to perform the first fuel injection so that an air-fuel mixture is formed in both the first area and the second area, and thereafter, the fuel injection device is caused to perform the second fuel injection so that the fuel is added to the air-fuel mixture formed in the second area.

This can make the air-fuel ratio in the second area richer than the air-fuel ratio in the first area with a simple configuration.

In the above-described configuration, it is preferable that a water injection device that injects water into the combustion chamber and supplies injection water to an air-fuel mixture is further provided, wherein when the control device determines that the engine is operated in the high load operation region, the water injection device is controlled so that the injection water exists in at least the second area at the ignition timing.

In this way, it is possible to further suppress the temperature rise of the air-fuel mixture in the second area in the high load operation region and to further reduce the combustion noise.

In the above configuration, it is preferable that a geometric compression ratio of the engine body is 16 or more and 35 or less.

In this way, the air-fuel mixture can be self-ignited by compression and burned more reliably.

The present invention also provides a method for controlling a premixed compression ignition engine, the engine including a fuel injection device that injects fuel into a combustion chamber and an ignition device provided with an electrode portion that faces an inside of the combustion chamber and ignites a mixture of fuel and air in the combustion chamber to apply ignition energy to the mixture, where SPCCI combustion in which an air-fuel mixture formed in a first area of the combustion chamber that includes the electrode portion of the ignition device is burned by receiving the ignition energy applied from the ignition device, and thereafter, an air-fuel mixture formed in a second area of the combustion chamber located on an outer periphery of the first area is self-ignited and burned by a pressure rise in the combustion chamber due to the combustion of the air-fuel mixture in the first area is performed in at least a part of an operation region of the engine, the method including: a step of determining whether or not the engine is operated in a high load operation region serving as an operation region in which an engine load is higher than a reference load set to a value equal to or more than half of a maximum load of the engine and in which the SPCCI combustion is performed; and a step of controlling the fuel injection device executed when the engine is operated in the high load operation region, so that an air-fuel ratio in the entire combustion chamber becomes richer than a stoichiometric air-fuel ratio and that an air-fuel ratio of the air-fuel mixture in the first area becomes leaner than an air-fuel ratio of the air-fuel mixture in the second area at an ignition timing of the ignition device.

According to this control method, the air-fuel mixture in the second area can be assuredly self-ignited and burned in the high load operation region, and thermal efficiency can be enhanced. Also, by adjusting the ignition timing, it is possible to control a combustion start timing of the air-fuel mixture at an appropriate timing. Furthermore, by controlling the air-fuel ratio in the first area and the air-fuel ratio in the second area as described above, it is possible to suppress combustion noise by suppressing combustion temperature and in-cylinder pressure during combustion to be low.

The above method preferably includes: a step of determining whether or not the engine is operated in a state in which the engine load is the reference load; and a step of controlling the fuel injection device executed when it is determined that the engine is operated in the state in which the engine load is the reference load, so that the air-fuel ratio in the entire combustion chamber becomes the stoichiometric air-fuel ratio and that the air-fuel ratio of the air-fuel mixture in the first area and the air-fuel ratio of the air-fuel mixture in the second area both become the stoichiometric air-fuel ratio at the ignition timing.

According to this method, even in the state in which the engine load is the reference load, it is possible to suppress a rapid rise of the in-cylinder pressure and to suppress the combustion noise at a small value.

In the above method, it is preferable that in the step executed when it is determined that the engine is operated in the high load operation region, in a case where the air-fuel ratio of the mixture in the first area is set to a first air-fuel ratio at the ignition timing and the air-fuel ratio of the air-fuel mixture in the second area is set to a second air-fuel ratio at the ignition timing, the fuel injection device is controlled so that both the first air-fuel ratio and the second air-fuel ratio become richer than the stoichiometric air-fuel ratio, that both the first air-fuel ratio and the second air-fuel ratio decrease as the engine load increases, and that a decreasing rate of the second air-fuel ratio to an increase amount of the engine load is greater than a decreasing rate of the first air-fuel ratio to the increase amount of the engine load.

According to this method, while required engine torque is reliably realized in the high load operation region, an increase in the combustion noise is appropriately suppressed.