JP6264422B1 - Control device for premixed compression ignition engine - Google Patents

Abstract

In a premixed compression ignition type engine, both suppression of combustion noise and ensuring of combustion stability are achieved. A control device for a premixed compression ignition type engine receives an injector 6 and sensors 51, 52, 53, 54, and 55, and signals from the sensors, and causes the injector to perform fuel injection. And a controller (ECU 100) configured to output the above signal. The injector includes a first fuel injection for forming an air-fuel mixture having a first equivalent ratio of less than 1.0 in the combustion chamber, and an air-fuel mixture having a second equivalent ratio of 1.0 or more in the combustion chamber. And the second fuel injection for forming. The controller performs the first fuel injection before the second fuel injection when the engine load is equal to or lower than the predetermined load, and the second fuel injection is higher than the first fuel injection when the engine load is higher than the predetermined load. Do it first. The air-fuel mixture having the first equivalent ratio is subjected to compression ignition, and after the compression ignition, the air-fuel mixture having the second equivalent ratio starts combustion. [Selection] Figure 6

Description

  The technology disclosed herein relates to a control device for a premixed compression ignition engine.

  Patent Document 1 describes a compression ignition engine. In this engine, when the operating state is in a high-load high-rotation operating region, the injector performs a small amount of fuel injection that makes the mixture around the spark plug a fuel-rich mixture before performing post-injection after pre-injection. In addition, the spark plug performs spark ignition. With this ignition assist, the air-fuel mixture formed by the pre-stage injection and the air-fuel mixture formed by the post-stage injection after the ignition assist are compressed and ignited with a time difference. This engine uses an ignition assist to prevent an increase in combustion noise due to a sudden rise in pressure in the cylinder.

JP 2012-241590 A

  The compression ignition type engine has a disadvantage that combustion noise increases when the engine load increases. If the compression ignition combustion is slowed down by various measures such as leaning of the air-fuel mixture and reduction of the intake air temperature, and if it is attempted to suppress the combustion noise, a new problem arises that the combustion stability is lowered. That is, suppression of combustion noise and ensuring of combustion stability are in a trade-off relationship.

  The technology disclosed herein has been made in view of the above points, and an object thereof is to achieve both suppression of combustion noise and ensuring of combustion stability in a premixed compression ignition type engine. .

  The technology disclosed herein relates to a control device for a premixed compression ignition engine. The control device for the premixed compression ignition type engine detects an operating state of the engine and an injector configured to inject fuel into a combustion chamber defined by the upper surface of the piston and the lower surface of the cylinder head. And a controller configured to receive a signal from the sensor and to output a signal for causing the injector to perform fuel injection.

  The injector receives a signal from the controller, a first fuel injection for forming an air-fuel mixture having a first equivalence ratio of less than 1.0 in the combustion chamber, and a second equivalent of 1.0 or more. A second fuel injection for forming an air-fuel mixture with a ratio in the combustion chamber, and the controller performs the first fuel injection when the engine load is equal to or lower than a predetermined load. The second fuel injection is performed prior to the first fuel injection when a signal is output to the injector and the engine load is higher than the predetermined load so that the second fuel injection is performed before the first fuel injection. Output a signal to the injector.

  In this engine, the air-fuel mixture having the first equivalent ratio is subjected to compression ignition, and after the compression ignition, the air-fuel mixture having the second equivalent ratio starts to combust.

  Here, “premixing” means that the air-fuel mixture formed by the fuel injection is ignited and burned after the injector finishes the fuel injection. The “combustion chamber” here is not limited to the meaning of the space formed when the piston reaches compression top dead center. The term “combustion chamber” is used in a broad sense.

  According to this configuration, when the engine load is equal to or lower than the predetermined load, the injector performs the first fuel injection first. The first fuel injection forms an air-fuel mixture having a first equivalent ratio of less than 1.0 in the combustion chamber. Here, according to the study by the present inventors, the relationship between the equivalence ratio of the air-fuel mixture and the compression ignitability is as follows. That is, when the equivalence ratio is 1.0 or more, the temperature in the combustion chamber locally decreases due to the latent heat and sensible heat of the fuel. For this reason, as the equivalence ratio increases, the amount of fuel increases and the temperature decreases, so the ignitability decreases. On the other hand, as the equivalence ratio is less than 1.0 and the equivalence ratio becomes lower, the air-fuel mixture becomes leaner, so that the ignitability decreases. According to the study by the present inventors, an air-fuel mixture having an equivalence ratio of 0.6 to 0.9 has the highest ignitability.

  The injector performs the first fuel injection at a relatively early timing. For example, the injector may perform the first fuel injection at a later timing in the compression stroke. The latter period in the compression stroke corresponds to the latter period when the compression stroke is divided into three equal parts, the first period, the middle period, and the second period. In the second half of the compression stroke, the pressure in the cylinder increases to some extent. The fuel injected from the injector does not diffuse widely into the combustion chamber. By doing so, an air-fuel mixture zone containing an air-fuel mixture with high ignitability can be partially formed in the combustion chamber.

  When the engine load is equal to or lower than the predetermined load, the injector performs the second fuel injection after the first fuel injection. The second fuel injection forms an air-fuel mixture having a second equivalent ratio of 1.0 or more in the combustion chamber. The injection amount of the second fuel injection is relatively large. As described above, when the amount of fuel injection is large, the temperature in the combustion chamber locally decreases due to the latent heat and sensible heat of the fuel. As a result, the air-fuel mixture formed by the second fuel injection is difficult to ignite with compression.

  The injector performs the second fuel injection at a late timing. For example, the injector may perform the second fuel injection at a timing near the compression top dead center. The pressure in the cylinder near the compression top dead center is high. Moreover, the position of the piston is close to the top dead center. The flight distance of the fuel injected from the injector is shortened, and an air-fuel mixture is locally formed in a region near the injector in the combustion chamber. By so doing, a mixture zone with low ignitability can be formed in a region near the injector.

  As a result, when the engine load is equal to or lower than the predetermined load, the air-fuel mixture zone constituted by the air-fuel mixture having the first equivalent ratio of less than 1.0 and the second equivalent ratio of 1.0 or more are mixed in the combustion chamber. And an air-fuel mixture zone constituted by gas. When the piston reaches compression top dead center and the temperature and pressure in the combustion chamber increase, the mixture of the first equivalence ratio, which is a highly ignitable mixture, is compressed and ignited and burns. Then, under the influence of the temperature and pressure that increase with the combustion of the air-fuel mixture, the air-fuel mixture of the second equivalent ratio, which is an air-fuel mixture having a relatively low ignitability, is compressed and ignited and burned. In other words, this engine spatially divides the combustion chamber into a plurality of zones, and causes the air-fuel mixture in the plurality of zones to undergo compression ignition combustion at different times. As a result, it is possible to realize slow combustion while igniting stably in the vicinity of the compression top dead center. As a result, this engine achieves both suppression of combustion noise and ensuring of combustion stability.

  Although the second fuel injection has a relatively large injection amount, the injection timing is late. For this reason, the time required for the air-fuel mixture having the second equivalent ratio of 1.0 or more to be exposed to the high-temperature and high-pressure atmosphere in the combustion chamber is shortened. It is possible to prevent the air-fuel mixture having the second equivalent ratio from being prematurely ignited. That is, the fuel injection mode performed in the order of the first fuel injection and the second fuel injection has high robustness against abnormal combustion.

  Further, when the injector is arranged along the axis of the cylinder and is configured to inject fuel toward the upper surface of the piston, a relatively equivalent ratio formed by the second fuel injection A high air-fuel mixture is formed at the center in the combustion chamber, and an air-fuel mixture with a low equivalence ratio is formed at the outer periphery in the combustion chamber. The air-fuel mixture having a high equivalence ratio has a high temperature during combustion, but since it is away from the wall surface of the combustion chamber, an increase in cooling loss can be suppressed. In addition, since the outer peripheral portion of the combustion chamber approaches the wall surface of the combustion chamber at the time of combustion, the combustion gas in the outer peripheral portion easily radiates heat to the wall surface of the combustion chamber, and the temperature tends to decrease. Because it is low (because the amount of fuel in the air-fuel mixture is small), unburned loss is reduced. Therefore, the fuel injection mode performed in the order of the first fuel injection and the second fuel injection is advantageous in improving the fuel efficiency.

  On the other hand, when the engine load is higher than the predetermined load, the injection is performed in the order of the second fuel injection and the first fuel injection. In the second fuel injection, for example, the second fuel injection may be performed at a middle timing in the compression stroke. The middle term in the compression stroke corresponds to the middle term when the compression stroke is divided into three equal parts, the first half, the middle and the second half. By performing the second fuel injection at an early timing, the flight distance of the fuel injected from the injector becomes long, the fuel reaches a region away from the injector in the combustion chamber, and the equivalence ratio locally reaches the region. A high air-fuel mixture is formed.

  On the other hand, the first fuel injection may be performed, for example, near the compression top dead center. By performing the first fuel injection at a late timing, the flying distance of the fuel injected from the injector is shortened, and an air-fuel mixture having high ignitability is locally formed in a region near the injector in the combustion chamber. .

  Even when the engine load is higher than a predetermined load, an air-fuel mixture zone constituted by an air-fuel mixture having a first equivalent ratio of less than 1.0 and a gas mixture having a second equivalent ratio of 1.0 or more are present in the combustion chamber. Is formed. When the piston reaches compression top dead center and the temperature and pressure in the combustion chamber increase, the air-fuel mixture of the first equivalence ratio is compressed and ignited and combusts. In response, the air-fuel mixture having the second equivalent ratio is combusted by compression ignition. As a result, even when the engine load is higher than the predetermined load, suppression of combustion noise and ensuring of combustion stability are compatible.

  As the engine load increases, the amount of fuel supplied to the combustion chamber increases. Since the first fuel injection forms an air-fuel mixture with high ignitability, the fuel injection amount cannot be changed greatly. Therefore, when the engine load increases, the injection amount of the second fuel injection increases. Here, if the timing of the second fuel injection is late, the fuel vaporization time and the mixing time are shortened, and smoke may be generated.

  Therefore, when the engine load is higher than the predetermined load, the timing of the second fuel injection is advanced. By doing so, it is possible to ensure a long fuel vaporization time and mixing time, and to prevent the occurrence of smoke.

  Further, if the timing of the second fuel injection is advanced, since the injection amount is large, the penetration is increased, the reach of the fuel spray is increased, the pressure in the combustion chamber is low, and the piston is positioned below. In combination, the fuel spray reaches a region away from the injector. A large amount of air in a region away from the injector can be used for combustion. Therefore, if the timing of the second fuel injection is advanced, it is possible to increase the amount of fuel supplied into the combustion chamber while avoiding the second equivalent ratio from becoming too high.

  The controller outputs a signal to the injector so that the first fuel injection is performed before the second fuel injection when the engine speed is equal to or lower than a predetermined engine speed, and the engine speed When the engine speed is higher than the predetermined rotational speed, a signal may be output to the injector so that the second fuel injection is performed before the first fuel injection.

  As described above, if the timing of the second fuel injection is advanced, the fuel vaporization time and the mixing time can be ensured long, and the occurrence of smoke can be prevented. This is also effective in securing a long fuel vaporization time and mixing time when the engine speed increases and the time lapse for the same crank angle change becomes short.

  Therefore, when the engine speed is equal to or lower than the predetermined speed, if the first fuel injection is performed prior to the second fuel injection, as described above, it is advantageous for improving fuel efficiency. On the other hand, when the engine speed is higher than the predetermined engine speed, the emission performance is improved by performing the second fuel injection before the first fuel injection.

  A control device for a premixed compression ignition engine includes an outside air temperature sensor configured to detect an outside air temperature, and the controller is configured such that the outside air temperature is higher than a predetermined temperature based on a signal from the outside air temperature sensor. Sometimes, the predetermined load may be shifted to the high load side.

  When the outside air temperature is high, the temperature before starting compression in the combustion chamber becomes high, so that the possibility of abnormal combustion such as premature ignition during the compression stroke increases. As described above, when the second fuel injection is performed at a late timing, robustness against abnormal combustion is enhanced. Therefore, when the outside air temperature is higher than the predetermined temperature, the predetermined load is shifted to the high load side. That is, the operation region in which fuel is injected in the order of the first fuel injection and the second fuel injection is expanded to the high load side. By doing so, the premixed compression ignition type engine is more robust against abnormal combustion.

  The controller may shift the predetermined rotational speed to a high rotational speed side when the external air temperature is higher than a predetermined temperature based on a signal from the external air temperature sensor.

  Similarly to the above, when the outside air temperature is higher than the predetermined temperature, the predetermined rotation speed is shifted to the high rotation side. That is, the operation region in which the fuel is injected in the order of the first fuel injection and the second fuel injection is expanded to the high rotation side. By doing so, the premixed compression ignition type engine is more robust against abnormal combustion.

  The controller outputs a signal to the injector so that a third fuel injection is performed prior to the first fuel injection when the engine load is equal to or lower than a predetermined load, and enters the combustion chamber immediately before the compression ignition. Is formed from the outer peripheral portion toward the central portion, the air-fuel mixture zone that is leaner than the first equivalent ratio, the air-fuel mixture zone that is the first equivalent ratio, and the air-fuel mixture zone that is the second equivalent ratio. It is good also as.

  By doing so, the equivalence ratio of the air-fuel mixture gradually increases from the outer peripheral portion in the combustion chamber toward the central portion. In other words, excess oxygen increases from the center to the outer periphery of the combustion chamber, which is advantageous for reducing unburned loss. Therefore, it is advantageous for improving the fuel efficiency when the engine load is equal to or less than a predetermined load.

  The controller outputs a signal to the injector to perform a third fuel injection prior to the second fuel injection when the engine load is higher than the predetermined load, and the combustion chamber immediately before compression ignition is output. An air-fuel mixture zone having the second equivalent ratio, an air-fuel mixture zone leaner than the first equivalent ratio, and an air-fuel mixture zone having the first equivalent ratio are formed from the outer peripheral portion toward the center portion. It is good as well.

  By doing so, as described above, the fuel vaporization time and the mixing time can be ensured long when the air-fuel mixture having the second equivalent ratio is formed, which is advantageous in improving the emission performance.

  As described above, according to the control device for the premixed compression ignition type engine, when the engine load is equal to or lower than the predetermined load, the first fuel injection for forming an air-fuel mixture having an equivalent ratio of less than 1.0 is performed. The fuel efficiency is improved while achieving both suppression of combustion noise and ensuring of combustion stability by performing before the second fuel injection for forming the air-fuel mixture having a second equivalent ratio of 1.0 or more. When the engine load is higher than the predetermined load, the second fuel injection is performed prior to the first fuel injection, thereby reducing emissions while ensuring both combustion noise suppression and combustion stability. Performance is improved.

FIG. 1 is a diagram illustrating a configuration of a premixed compression ignition type engine. FIG. 2 is a cross-sectional view illustrating the configuration of the combustion chamber. FIG. 3 is a block diagram illustrating the configuration of the control device for the premixed compression ignition engine. FIG. 4A is a diagram illustrating an operating region of the engine at a normal outside air temperature. FIG. 4B is a diagram exemplifying an engine operation region at a high outside air temperature. FIG. 5 is a diagram illustrating the relationship between the equivalence ratio of the air-fuel mixture and the ignition timing. FIG. 6 is a diagram illustrating the fuel injection timing in the high load region and conceptually showing the state of the air-fuel mixture formed in the combustion chamber by each injection. FIG. 7 is a cross-sectional view illustrating a change in heat generation rate, a change in dP / dθ, and a change in pressure in a high load region. FIG. 8 is a diagram illustrating the fuel injection timing in the middle load region and a diagram corresponding to FIG. 6 conceptually showing the state of the air-fuel mixture formed in the combustion chamber by each fuel injection. FIG. 9 is a flowchart of fuel injection control executed by the ECU. FIG. 10 is a diagram corresponding to FIG. 2 for explaining the cavity volume with respect to the total volume of the combustion chamber at the compression top dead center. FIG. 11 is a diagram illustrating the relationship between the mass combustion ratio in the ignition zone and the combustion efficiency in all zones. FIG. 12 is a diagram illustrating the relationship between the cavity diameter / bore diameter, the combustion efficiency, and the combustion period. FIG. 13 is a diagram illustrating the relationship between the volume ratio of the cavity with respect to the total volume of the combustion chamber, the combustion efficiency, and the combustion period.

  Hereinafter, an embodiment of a control device for a premixed compression ignition engine will be described in detail with reference to the drawings. The following description is an example of a control device for a premixed compression ignition engine. FIG. 1 is a diagram illustrating a configuration of a premixed compression ignition type engine. FIG. 2 is a cross-sectional view illustrating the configuration of the combustion chamber. FIG. 3 is a block diagram illustrating the configuration of the control device for the premixed compression ignition engine.

(Entire engine configuration)
FIG. 1 shows a configuration of a premixed compression ignition type engine (hereinafter simply referred to as an engine 1) according to an embodiment. The fuel of the engine 1 is gasoline in this embodiment. The fuel may be gasoline containing bioethanol or the like. The fuel of the engine 1 may be any fuel as long as it is a liquid fuel containing at least gasoline.

  The engine 1 includes a cylinder block 12 and a cylinder head 13 placed on the cylinder block 12. A plurality of cylinders 11 are formed inside the cylinder block 12. 1 and 2 show only one cylinder. The engine 1 is a multi-cylinder engine. A piston 3 is slidably inserted in each cylinder 11. The piston 3 is connected to the crankshaft 15 via a connecting rod 14. The piston 3 defines a combustion chamber 17 together with the cylinder 11 and the cylinder head 13. The “combustion chamber” is not limited to the meaning of the space formed when the piston 3 reaches compression top dead center. The term “combustion chamber” may be used in a broad sense. That is, it may mean a space formed by the piston 3, the cylinder 11, and the cylinder head 13 regardless of the position of the piston 3.

  A lower surface 170 of the cylinder head 13 constituting the ceiling portion of the combustion chamber 17 is constituted by an intake side ceiling surface 171 and an exhaust side ceiling surface 172. The intake-side ceiling surface 171 has an upward slope toward the center of the cylinder 11. The intake side ceiling surface 171 is provided with an opening of the intake port 18. The exhaust side ceiling surface 172 also has an upward slope toward the center of the cylinder 11. An opening of the exhaust port 19 is provided on the exhaust-side ceiling surface 172. The intake-side ceiling surface 171 and the exhaust-side ceiling surface 172 are connected at a trough extending in the axial direction of the crankshaft 15. The combustion chamber 17 is a pent roof type combustion chamber. In addition, the position of the valley part of a pent roof can have both the case where it corresponds to the bore | bore center of the cylinder 11, and the case where it does not correspond.

  The upper surface 30 of the piston 3 is raised like a triangular roof by an inclined surface 31 on the intake side and an inclined surface 32 on the exhaust side (see also FIG. 2). The inclined surface 31 is inclined upwardly toward the center of the piston 3 so as to correspond to the intake side ceiling surface 171. The inclined surface 32 is inclined upwardly toward the center of the piston 3 so as to correspond to the exhaust-side ceiling surface 172. The geometric compression ratio of the engine 1 is set to 16 or more and 35 or less. The geometric compression ratio of the engine 1 is high. A concave cavity 34 is formed on the upper surface 30 of the piston 3. The cavity 34 is formed in the center of the piston 3.

  Although only one is shown in FIG. 1, in the engine 1, two intake ports 18 are formed in the cylinder head 13 for each cylinder 11. The intake port 18 communicates with the combustion chamber 17. The intake port 18 is connected to the intake passage 181. The intake passage 181 is provided with a throttle valve 41 (see FIG. 3) for adjusting the intake flow rate.

  Similar to the intake port 18, the engine 1 has two exhaust ports 19 in the cylinder head 13 for each cylinder 11. The exhaust port 19 communicates with the combustion chamber 17. The exhaust port 19 is connected to the exhaust passage 191. Although not shown, an exhaust gas purification system having one or more catalytic converters is disposed in the exhaust passage 191. The catalytic converter includes a three-way catalyst. However, the catalyst is not limited to a three-way catalyst.

  Although not shown, the engine 1 may be a supercharged engine in which a compressor is interposed in the intake passage 181. The supercharger may be either a turbocharger driven by exhaust energy or a mechanical supercharger driven by the engine 1. The engine 1 may be configured as a naturally aspirated engine.

  An intake valve 21 is disposed in the cylinder head 13. The intake valve 21 opens and closes the intake port 18 with respect to the combustion chamber 17. The intake valve 21 reciprocates at a predetermined timing by an intake valve mechanism 23. In this example, the intake valve mechanism 23 has at least a hydraulic or electric variable phase mechanism (Variable Valve Timing: VVT) capable of continuously changing the rotation phase of the intake camshaft within a predetermined angle range. It is configured to include.

  An exhaust valve 22 is disposed in the cylinder head 13. The exhaust valve 22 opens and closes the exhaust port 19 with respect to the combustion chamber 17. The exhaust valve 22 reciprocates at a predetermined timing by an exhaust valve mechanism 24. In this example, the exhaust valve mechanism 24 includes at least a hydraulic or electric VVT.

  An injector 6 that directly injects fuel into the combustion chamber 17 is attached to the cylinder head 13. The injector 6 is disposed in a valley portion of the pent roof where the intake side ceiling surface 171 and the exhaust side ceiling surface 172 intersect. In this configuration example, the injector 6 is disposed such that its injection axis S is along the axis of the cylinder 11 as shown in FIG. There are both cases where the injection axis S coincides with the axis of the cylinder 11 and when the injection axis S deviates from the axis of the cylinder 11. The injector 6 faces the cavity 34 of the piston 3. The injector 6 injects fuel toward the cavity 34.

  The injector 6 is, for example, an external valve opening type injector. The outer valve injector can change the particle size of the fuel spray to be injected by adjusting the lift amount of the outer valve. Utilizing this fact, the engine 1 forms an air-fuel mixture layer in the central portion of the cavity 34 and a heat insulating gas layer around it in some operation regions.

  Further, not only the valve opening type injector, but also a VOC (Valve Covered Orifice) nozzle type injector changes the effective cross-sectional area of the injection port by adjusting the degree of cavitation generated at the nozzle port, and injects. It is possible to change the particle size of the fuel spray. Therefore, similarly to the outer valve-opening injector, it is possible to form an air-fuel mixture layer at the center of the cavity 34 and to form a heat insulating gas layer around the outer periphery thereof.

  In addition, by injecting the fuel heated to a predetermined temperature by the heater into the combustion chamber 17 in a high-pressure atmosphere, the fuel-air mixture layer is formed in the central portion of the cavity 34 by bringing the fuel into a supercritical state. It is possible to form a heat insulating gas layer on the outer periphery. In this technique, the fuel spray injected into the combustion chamber 17 is instantly vaporized, so that the fuel spray penetration is lowered, the fuel spray reach distance is shortened, and an air-fuel mixture layer is formed in the cavity 34 near the injector. To form. The injector is, for example, a multi-hole type injector having a plurality of nozzle holes, and includes a heater for heating the fuel. Further, the injector that brings the fuel into a supercritical state may be an injector other than the above-described configuration.

  Since the configuration of these injectors is well known, detailed description thereof is omitted here.

  The injector 6 is not limited to a configuration in which an air-fuel mixture layer is formed in the central portion of the cavity 34 and a heat insulating gas layer is formed around the mixture layer. The injector 6 may have any configuration.

  As shown in FIG. 3, the engine 1 also includes an EGR system 43 that is configured to reintroduce burnt gas into the cylinder 11. The EGR system 43 includes an external EGR system that re-introduces burned gas into the cylinder 11 via an EGR passage that connects the exhaust passage 191 and the intake passage 181 of the engine 1, and part of the burned gas in the cylinder 11. Including both internal EGR systems that remain substantially within the cylinder 11.

  The engine 1 may include an ignition plug for ignition assistance.

  The control device for the premixed compression ignition engine includes an ECU 100 that controls the operation of the engine 1. The ECU 100 is a controller based on a well-known microcomputer, and includes a central processing unit (CPU) that executes a program, a memory that is configured by, for example, a RAM or a ROM and stores a program and data, and an input of an electric signal. And an input / output (I / O) bus for outputting. The ECU 100 is an example of a controller.

  The ECU 100 includes at least a signal related to the intake air flow from the air flow sensor 51, an accelerator opening signal from the accelerator opening sensor 52 that detects the depression amount of the accelerator pedal, a vehicle speed signal from the vehicle speed sensor 53, and a crank from the crank angle sensor 54. The angular pulse signal, the temperature signal of the coolant of the engine 1 from the water temperature sensor 55, and the temperature signal of the outside air from the outside air temperature sensor 56 are received. Based on these signals, the ECU 100 calculates required torque, predicts the load on the engine 1, and the like. In the engine with a supercharger, the control device further includes a supercharging pressure sensor that detects the supercharging pressure, and the ECU 100 receives a signal related to the supercharging pressure from the supercharging pressure sensor.

  The ECU 100 calculates control parameters of the engine 1 such as a throttle opening signal, a fuel injection pulse, a valve phase angle signal, and the like based on the calculated required torque and the like. The ECU 100 outputs these signals to the throttle valve 41, the injector 6, the EGR system 43, the intake valve mechanism 23, the exhaust valve mechanism 24, and the like.

  As described above, the engine 1 has the geometric compression ratio ε set to 16 or more. The geometric compression ratio may be 40 or less, and particularly preferably 18 or more and 35 or less. Since the expansion ratio increases as the compression ratio increases, the engine 1 is also an engine having a relatively high expansion ratio at the same time as the high compression ratio. The engine 1 is basically configured to burn the fuel injected into the cylinder 11 in the entire operation region by compression ignition. In other words, compression ignition combustion is CAI (Controlled Auto Ignition) combustion. Due to the high geometric compression ratio, compression ignition combustion is stabilized.

  As shown in FIG. 2, the engine 1 is provided on the outer peripheral edge of the piston 3 separately from the flat surface (that is, the inclined surfaces 31 and 32) on the upper surface 30 of the piston 3 in order to increase the geometric compression ratio. When the piston 3 is in the vicinity of the compression top dead center, the surface of the cylinder head 13 is configured to enter the cylinder head 13 side rather than the mating surface of the cylinder head 13 and the cylinder block 12. The combustion chamber 17 of the engine 1 is substantially defined by the upper surface 30 of the piston 3, the lower surface 170 of the cylinder head 13, and the valve head surfaces of the intake valve 21 and the exhaust valve 22. The configuration of the combustion chamber 17 of the engine 1 is not limited to this configuration. In the engine 1, a heat shield layer 173 is provided on a section screen that partitions the combustion chamber 17. The heat shielding layer 173 may be provided on all of these section screens, or may be provided on a part of these section screens.

  The heat shielding layer 173 has a thermal conductivity lower than that of the metal base material constituting the combustion chamber 17. The base material here is, for example, aluminum or an aluminum alloy in the case of the piston 3. The heat shielding layer 173 suppresses the heat of the combustion gas in the combustion chamber 17 being released through the surface that defines the combustion chamber 17. Further, the heat shielding layer 173 preferably has a volumetric specific heat smaller than that of the base material. That is, it is preferable that the heat capacity of the heat shielding layer 173 is reduced, and the temperature of the section screen of the combustion chamber 17 changes following the fluctuation of the gas temperature in the combustion chamber 17. By doing so, the difference between the temperature of the combustion gas and the temperature of the section screen is reduced, so that heat is prevented from being transmitted to the base material through the section screen.

The heat shielding layer 173 may be formed by applying a heat shielding material containing hollow particles (for example, a glass balloon) and a silicon resin as a binder on a section screen and curing the resin by heat treatment. Good. The thermal barrier layer 173 may also be formed by coating a ceramic material such as ZrO ₂ on the section screen by plasma spraying.

(Engine operation control)
FIG. 4A illustrates an operation region of the engine 1. The operating region of the engine 1 is divided into a low load region (A), a medium load region (B), and a high load region (C). The low load region (A) is a region of low rotation and low load. The medium load region (B) includes a region where the load is higher than that of the low load region (A) and a region where the rotational speed is higher than that of the low load region (A). The high load region (C) includes a region having a higher load than the medium load region (B) and a region having a higher load than the medium load region (B).

  As described above, the ECU 100 outputs a signal corresponding to the operating state of the engine 1 to the throttle valve 41, the intake valve mechanism 23, the exhaust valve mechanism 24, and the EGR system 43. Thereby, the gas state in the combustion chamber 17 is adjusted according to the operating state of the engine 1. The EGR system 43 introduces exhaust gas into the combustion chamber 17 over the entire operation region of the engine 1. The EGR rate (that is, the mass ratio of exhaust gas to the total gas in the combustion chamber 17) may be set to 50% or more as described later.

  The ECU 100 also changes the form of fuel injection into the combustion chamber 17 in each of the low load region (A), the medium load region (B), and the high load region (C).

(Fuel injection mode in low load area (A))
The engine 1 forms a heat insulating layer by a gas layer in the combustion chamber 17 during combustion in the low load region (A). By forming a heat insulating layer in addition to the heat shield structure of the combustion chamber 17, the cooling loss of the engine 1 is greatly reduced.

  Specifically, the ECU 100 injects fuel from the injector 6 into the cavity 34 after the compression stroke in the low load region (A). As shown in FIG. 2, stratification is performed in which an air-fuel mixture layer is formed in the center of the cavity 34 in the vicinity of the injector 6 and a gas layer containing fresh air is formed therearound. This gas layer may be only fresh air, and may contain burned gas (that is, EGR gas) in addition to fresh air. There is no problem even if a small amount of fuel is mixed in the gas layer. The gas layer only needs to have a leaner fuel than the gas mixture layer.

  If the gas mixture and the gas mixture layer are formed in the combustion chamber 17 and the gas mixture is subjected to compression ignition combustion, the gas layer between the gas mixture layer and the partition wall of the combustion chamber 17 causes the flame to be cylinder. 11 is suppressed from contacting the wall surface. Further, since the gas layer becomes a heat insulating layer, heat radiation from the partition wall of the combustion chamber 17 is suppressed. As a result, the cooling loss can be greatly reduced.

  It should be noted that the reduction of the cooling loss is converted to the exhaust loss and does not contribute much to the improvement of the thermal efficiency. However, in this engine 1, the cooling loss is reduced by the high expansion ratio. The energy of the corresponding combustion gas is converted into mechanical work. That is, the engine 1 significantly improves thermal efficiency by adopting a configuration that reduces both cooling loss and exhaust loss.

  In order to form such an air-fuel mixture layer and a gas layer in the combustion chamber 17, it is desirable that the gas flow in the combustion chamber 17 is weak at the timing of fuel injection. Therefore, the intake port 18 has a straight shape in which no swirl is generated or hardly generated in the combustion chamber 17, and the tumble flow is configured to be as weak as possible.

(Fuel injection mode in high load region (C))
When the load on the engine 1 increases, the amount of fuel supplied into the combustion chamber 17 increases, resulting in inconvenience that the pressure fluctuation during compression ignition becomes intense and combustion noise increases. For example, when a slow combustion is realized and a combustion noise is suppressed by measures such as reducing the intake air temperature or increasing the EGR rate, a new problem arises that the combustion stability is lowered.

  In view of this, the engine 1 adopts a fuel injection mode different from that of the low load region (A) described above so that combustion noise suppression and combustion stability are maintained in the high load region (C). .

  FIG. 5 shows the relationship between the equivalence ratio of the air-fuel mixture and the ignition timing of the air-fuel mixture. Of the two lines in FIG. 5, one is a line when the EGR rate is 50%, and the other is a line when the EGR rate is 55% higher than that. The difference in EGR rate is the temperature difference in the cylinder 11 before compression, and the EGR rate of 55% is higher than the EGR rate of 50%. Therefore, an EGR rate of 55% is more advantageous for ignitability than an EGR rate of 50%. That is, the ignition timing is advanced when the EGR rate is 55% than when the EGR rate is 50%.

  According to FIG. 5, regardless of the EGR rate, when the equivalence ratio of the air-fuel mixture is 1 or less, the ignition timing is delayed as the equivalence ratio becomes smaller. That is, as the air-fuel mixture becomes leaner and the amount of fuel in the combustion chamber 17 decreases, the air-fuel mixture becomes more difficult to burn.

  When the equivalence ratio of the air-fuel mixture exceeds 1, the ignition timing becomes late as the equivalence ratio increases regardless of the EGR rate. This is because when the amount of fuel injected into the combustion chamber 17 increases, the temperature in the combustion chamber 17 locally decreases due to the latent heat and sensible heat of the fuel. That is, an air-fuel mixture with a high equivalence ratio is less likely to be ignited as the ambient temperature decreases.

  Regardless of the level of the EGR rate, the ignition timing is most advanced when the equivalence ratio of the air-fuel mixture is in the range of 0.6 to 0.9. That is, the ignitability of the air-fuel mixture is maximized.

  When the equivalence ratio of the air-fuel mixture is different, the ignitability (that is, the ignition timing) of the air-fuel mixture is different. In the engine 1, paying attention to this point, by performing split injection in the high load region (C), a plurality of air-fuel mixture zones constituted by air-fuel mixtures having different equivalence ratios are formed in the combustion chamber 17. Then, the ignition timing of the mixture in the plurality of mixture zones is shifted. In other words, the engine 1 is stable in the vicinity of the compression top dead center by dividing the combustion chamber 17 into a plurality of zones spatially and subjecting the mixture in the plurality of zones to compression ignition combustion at different times. Slow combustion is achieved while igniting. As a result, in this engine 1, in the high load region (C), suppression of combustion noise and securing of combustion stability are compatible. In the high load region (C), the EGR rate is set to 50% or more.

  The right diagram of FIG. 6 illustrates the timing when the injector 6 injects fuel into the combustion chamber 17 and the injection amount in the high load region (C). In the right diagram of FIG. 6, the crank angle advances from the top to the bottom (that is, the time advances). The fuel injection amount is indicated by a square area in the right diagram of FIG. 6, and the fuel injection amount increases as the area increases. The left diagram of FIG. 6 conceptually shows the state of the air-fuel mixture formed in the combustion chamber 17 by each fuel injection.

  The injector 6 receives a signal from the ECU 100 and injects fuel into the combustion chamber 17 during the intake stroke. The intake stroke injection 61 may be performed in the latter half of the intake stroke. The latter half of the intake stroke is the latter half when the intake stroke is divided into two equal parts, the first half and the second half. The intake stroke injection 61 may also be performed later in the intake stroke. The latter period of the intake stroke is the latter period when the intake stroke is divided into three equal parts, the first period, the middle period, and the second period. The injection amount of the intake stroke injection 61 is relatively large. By this intake stroke injection 61, a lean, homogeneous or nearly homogeneous air-fuel mixture is formed throughout the combustion chamber 17. The equivalent ratio of the air-fuel mixture is 0.4 to 0.6 (see (1) in FIG. 6).

  Next, the injector 6 receives a signal from the ECU 100 and performs fuel injection in the middle period in the compression stroke. Hereinafter, this fuel injection is referred to as a compression stroke intermediate injection 62. The compression stroke intermediate injection 62 is an example of a second fuel injection. The middle term in the compression stroke corresponds to the middle term when the compression stroke is divided into three equal parts, the first term, the middle term, and the second term. Although the injection amount of the compression stroke intermediate injection 62 is smaller than the injection amount of the intake stroke injection 61, the absolute amount is relatively large. The fuel injected by the compression stroke intermediate injection 62 has a relatively low pressure in the combustion chamber 17 and the piston 3 is positioned relatively downward. Further, since the injection amount is relatively large, penetration is increased, so that the outer periphery of the cavity 34 is reached. The fuel injected by the intake stroke injection 61 and the fuel injected by the compression stroke intermediate injection 62 are combined to form a rich mixture on the outer periphery of the cavity 34. Hereinafter, the region containing the rich air-fuel mixture may be referred to as “over-rich zone”. The equivalent ratio of the air-fuel mixture in the rich zone is set to 1.0 to 1.7 (see (2) in FIG. 6). Since the equivalence ratio is high, the ignitability of the air-fuel mixture in the rich zone is lowered.

  Finally, the injector 6 receives a signal from the ECU 100 and injects fuel into the combustion chamber 17 in the latter half of the compression stroke, more precisely near the compression top dead center. Hereinafter, this fuel injection is referred to as compression top dead center injection 63. The compression top dead center injection 63 is an example of a first fuel injection. The latter period in the compression stroke corresponds to the latter period when the compression stroke is divided into three equal parts, the first period, the middle period, and the second period. The injection amount of the compression top dead center injection 63 is smaller than the injection amount of the compression stroke intermediate injection 62. The fuel injected by the compression top dead center injection 63 remains in the cavity 34 because the injection amount is small, the pressure in the combustion chamber 17 is high, and the piston 3 is positioned above. The air-fuel mixture formed in the cavity 34 is combined with the fuel injected by the intake stroke injection 61 and the fuel injected by the compression top dead center injection 63 to become an air-fuel mixture having a predetermined equivalence ratio. The equivalent ratio of this air-fuel mixture is set to 0.6 to 0.9. That is, as described with reference to FIG. 5, the equivalence ratio has the highest ignitability of the air-fuel mixture. Therefore, after the piston 3 reaches the compression top dead center, the air-fuel mixture in the cavity 34 is first ignited by compression. Hereinafter, the region including the air-fuel mixture in which the equivalence ratio is set to 0.6 to 0.9 may be referred to as “ignition zone”.

  As shown in (3) of FIG. 6, between the ignition zone and the overconcentration zone, an air-fuel mixture having an equivalence ratio lower than those of the ignition zone and the overconcentration zone (that is, an equivalence ratio of 0.4 to 0.6). ) Is formed. This air-fuel mixture is an air-fuel mixture formed by the intake stroke injection 61. Hereinafter, a region including a gas mixture having a low equivalence ratio may be referred to as a “lean zone”. Since the equivalence ratio is low, the ignitability of the air-fuel mixture in the lean zone decreases.

  In this way, three zones of an ignition zone, a lean zone, and a rich zone are formed in the combustion chamber 17 from the center toward the outside. The equivalence ratio of each zone is different from each other. The average equivalence ratio of the air-fuel mixture is 1 for the entire combustion chamber 17. As a result, the exhaust gas can be purified by the three-way catalyst. The ratio of the injection amounts of the intake stroke injection 61, the compression stroke intermediate injection 62, and the compression top dead center injection 63 is set to 0.55: 0.42: 0.03, for example.

  FIG. 7 shows a change 71 in heat generation rate (upper figure), a change 72 in dP / dθ as an index of combustion noise (middle figure) when the air-fuel mixture in the state shown in FIG. The change 73 (lower figure) of the pressure in the cylinder 11 is shown. As described above, when the piston 3 reaches the compression top dead center and the temperature and pressure of the combustion chamber 17 increase, the mixture in the ignition zone having the highest ignitability located in the center of the combustion chamber 17 is obtained. Compression ignition is started and combustion starts. As shown in the upper diagram of FIG. 7, the heat generation rate gradually increases, but the amount of air-fuel mixture in the ignition zone is relatively small, and the amount of fuel contained in the ignition zone is the fuel supplied into the combustion chamber 17. Since it is only a part of the quantity, the pressure in the combustion chamber 17 is prevented from rising suddenly by combustion (see the lower diagram of FIG. 7). As a result, dP / dθ does not exceed the allowable value of the combustion noise indicated by the broken line in the middle diagram of FIG.

  As combustion of the air-fuel mixture in the ignition zone starts, the temperature and pressure in the combustion chamber 17 increase. As a result, the over-concentrated zone having the second highest ignitability located around the outer periphery of the cavity 34 is compressed and ignited, and combustion starts.

  Finally, the lean zone ignites with compression and begins to burn. Since the lean zone has the largest volume of the air-fuel mixture, the heat generation rate increases as shown in the upper diagram of FIG. 7, but the expansion zone proceeds in the lean zone as shown in the lower diagram of FIG. Therefore, the pressure rise accompanying combustion is prevented from becoming steep. As a result, as shown in the upper diagram or middle diagram of FIG. 7, dP / dθ does not exceed the allowable value of combustion noise. The change in the heat generation rate associated with the combustion of the air-fuel mixture in the ignition zone, the rich zone, and the lean zone is indicated by a thick broken line in the upper diagram of FIG. The waveform of the change in the heat generation rate approaches an ideal waveform for avoiding combustion noise.

  Thus, by forming three zones in the combustion chamber 17 with different equivalence ratios of the air-fuel mixture, the ignition zone, the lean zone, and the rich zone, while ensuring the ignitability in the high load region (C). It becomes possible to slow down the combustion. As a result, the engine 1 can achieve both suppression of combustion noise and maintenance of combustion stability in the high load region (C).

  The fuel injection mode shown in FIG. 6 is particularly advantageous when the amount of fuel injected into the combustion chamber 17 is large. That is, by increasing the injection amount of the compression stroke mid-term injection 62, the penetration increases, the fuel spray reach distance increases, and the fuel spray reaches the outer periphery of the cavity 34 away from the injector 6. become. In addition, a large amount of air existing around the cavity 34 can be used for combustion. Therefore, it is possible to increase the amount of fuel supplied into the combustion chamber 17 while avoiding an excessively high equivalent ratio in the rich zone.

  Further, in the fuel injection mode shown in FIG. 6, an air-fuel mixture having an equivalence ratio of 0.6 to 0.9 is formed by the compression top dead center injection 63, so the injection amount is relatively small. Since the injection period of the compression top dead center injection 63 is short, the time from the end of injection to ignition becomes longer. For this reason, the time for the fuel injected into the combustion chamber 17 to be vaporized by the compression top dead center injection 63 and the time for mixing with the air can be sufficiently secured. This prevents problems such as the occurrence of smoke, for example, and prevents the exhaust gas from deteriorating. This is the same even when the rotation speed of the engine 1 increases and the time when the crank angle changes by the same angle becomes relatively short. The fuel injection mode shown in FIG. 6 is also advantageous in avoiding the deterioration of the exhaust gas properties when the engine 1 has a high rotational speed.

(Fuel injection mode in medium load region (B))
As described above, in the high load region (C), the excessively concentrated zone is formed around the outer periphery of the cavity 34. When the excessively concentrated zone near the wall surface of the combustion chamber 17 burns, the cooling loss tends to increase. Also, when the over-concentrated zone burns, there is a risk that air is insufficient and smoke is generated. Furthermore, since the fuel injection for forming the rich zone is performed in the middle of the compression stroke, the mixture in the rich zone is exposed to a high temperature and high pressure atmosphere in the combustion chamber 17 for a long time. This can lead to abnormal combustion (eg, premature ignition).

  In the medium load region (B), the amount of fuel supplied into the combustion chamber 17 is relatively small. Therefore, in the engine 1, a fuel injection mode different from that in the low load region (A) and the high load region (C) is adopted in the medium load region (B) in order to avoid the above-described problem.

  FIG. 8 illustrates the timing when the injector 6 injects fuel into the combustion chamber 17 and the injection amount in the medium load region (B) (the right diagram in FIG. 8). The fuel injection mode shown in FIG. 8 and the fuel injection mode shown in FIG. 6 have the same load on the engine 1. Moreover, the left figure of FIG. 8 has shown notionally the state of the air fuel mixture formed in the combustion chamber 17 by each fuel injection.

  The injector 6 receives a signal from the ECU 100 and injects fuel into the combustion chamber 17 during the intake stroke. The fuel injection may be performed in the latter half of the intake stroke. The fuel injection amount is relatively large. By this intake stroke injection 81, a lean and homogeneous or nearly homogeneous air-fuel mixture is formed throughout the combustion chamber 17. The equivalent ratio of the air-fuel mixture is 0.4 to 0.6 (see (1) in FIG. 8). The intake stroke injection 81 is substantially the same as the intake stroke injection 61 shown in FIG.

  Next, the injector 6 receives a signal from the ECU 100 and injects fuel at a later stage in the compression stroke. Hereinafter, this fuel injection is referred to as a compression stroke late injection 82. The injection amount of the compression stroke late injection 82 is smaller than the intake stroke injection 81 and smaller than the compression top dead center injection 83 described later. The fuel injected by the compression stroke late injection 82 has a high pressure in the combustion chamber 17 because the piston 3 is approaching the compression top dead center, and does not easily reach the outer peripheral portion of the combustion chamber 17. The fuel injected by the intake stroke injection 81 and the fuel injected by the compression stroke late injection 82 are combined, and in the central portion excluding the outer peripheral portion of the combustion chamber 17, an air-fuel mixture having an equivalence ratio of 0.6 to 0.9. An ignition zone including is formed.

  Finally, the injector 6 performs compression top dead center injection 83 in response to a signal from the ECU 100. The amount of compression top dead center injection 83 is less than intake stroke injection 81 and greater than compression stroke late injection 82. The fuel injected by the compression top dead center injection 83 remains in the cavity 34 because the pressure in the combustion chamber 17 is high and the piston 3 is positioned upward. The air-fuel mixture formed in the cavity 34 is a combination of the fuel injected by the intake stroke injection 81, the fuel injected by the compression stroke late injection 82, and the fuel injected by the compression top dead center injection 83. A rich mixture is formed. That is, a rich zone including a rich mixture with an equivalence ratio of 1.0 to 1.7 is formed in the center of the combustion chamber 17.

  And as shown to (3) of FIG. 8, the lean zone containing the air-fuel | gaseous mixture whose equivalence ratio is 0.4-0.6 is formed outside an ignition zone.

  In this way, three zones of the rich zone, the ignition zone, and the lean zone are formed in the combustion chamber 17 from the center toward the outside. The equivalence ratio of each zone is different from each other. The average equivalence ratio of the air-fuel mixture is 1 for the entire combustion chamber 17. The ratio of the injection amounts of the intake stroke injection 81, the compression stroke late injection 82, and the compression top dead center injection 83 is set to 0.55: 0.14: 0.31, for example.

  When the air-fuel mixture shown in FIG. 8 including the ignition zone, the lean zone, and the rich zone undergoes compression ignition combustion, first, the air-fuel mixture in the ignition zone undergoes compression ignition and starts combustion. Thereafter, the air-fuel mixture in the rich zone undergoes compression ignition, and finally, the air-fuel mixture in the lean zone undergoes compression ignition. Even in the medium load region (B), it is possible to slow down combustion while ensuring ignitability. As a result, the engine 1 can achieve both suppression of combustion noise and maintenance of combustion stability in the medium load region (B).

  Since the air-fuel mixture in the state shown in FIG. 8 forms a lean zone in the outer periphery of the combustion chamber 17, the combustion gas when the rich air-fuel mixture combusts compared to the air-fuel mixture in the state shown in FIG. It is possible to suppress contact with the wall surface of the combustion chamber 17. Thereby, the cooling loss is reduced. At the time of combustion, the outer periphery of the combustion chamber 17 is closer to the wall surface of the combustion chamber than the central portion of the combustion chamber 17 (the outer periphery of the combustion chamber 17 is the combustion chamber with respect to the central portion of the combustion chamber 17. Because the lower surface 170 of the cylinder head 13 which is the wall surface and the upper surface 30 of the piston 3 are closer to each other), the temperature of the combustion gas around the combustion chamber 17 is likely to decrease (heat radiation to the wall surface of the combustion chamber) However, the fuel concentration of the air-fuel mixture gradually decreases from the center of the combustion chamber 17 toward the outside (the amount of fuel occupying the air-fuel mixture in the outer periphery of the combustion chamber 17) Less), so there is less unburned fuel. Thus, by reducing the cooling loss and reducing the unburned loss, the air-fuel mixture in the state shown in FIG. 8 is more advantageous in improving the fuel efficiency than the air-fuel mixture in the state shown in FIG.

  Further, in the fuel injection mode shown in FIG. 8, since the fuel injection that forms the rich zone is performed near the compression top dead center, the time during which the rich mixture is exposed to the high temperature and high pressure atmosphere in the combustion chamber 17. Can be shortened. Therefore, the fuel injection mode shown in FIG. 8 is more robust against abnormal combustion than the fuel injection mode shown in FIG. The fuel injection mode shown in FIG. 8 is effective in avoiding abnormal combustion, for example, when abnormal combustion is likely to occur due to a high outside air temperature.

(Fuel injection control)
FIG. 9 is a flowchart relating to fuel injection control executed by the ECU 100. In step S1 after the start, the ECU 100 detects the operating state of the engine 1 based on signals from the sensors. Specifically, the rotational speed of the engine 1, the load of the engine 1, the intake air temperature, the EGR rate, and the like are detected. When the engine 1 is a supercharged engine, the supercharging pressure is also detected.

  In subsequent step S2, ECU 100 determines, based on the operating state of engine 1, whether or not the combustion noise is expected to be below an allowable value. When the combustion noise falls below the allowable value, the flow proceeds to step S3, and the ECU 100 executes the first injection control. The first injection control corresponds to the fuel injection control in the low load region (A) in FIG. 4A.

  If it is determined in step S2 that the combustion noise does not fall below the allowable value, the flow proceeds to step S4. In step S4, the ECU 100 determines whether it is possible to ensure the fuel mixing time. For example, when the rotational speed of the engine 1 is high, since the time per cycle is short, it may be impossible to ensure the fuel mixing time. Further, even when the load on the engine 1 is high and the fuel injection amount increases, the fuel injection period becomes long, so that it is impossible to ensure the fuel mixing time. When the determination in step S4 is YES, the flow proceeds to step S5. On the other hand, when the determination in step S4 is NO, the flow proceeds to step S8.

  In steps S5 to S7, the ECU 100 executes a second fuel injection corresponding to the fuel injection mode shown in FIG. The second injection control corresponds to the fuel injection control in the medium load region (B) in FIG. 4A.

  In step S5, the ECU 100 outputs a signal to the injector 6, and the injector 6 performs the first fuel injection (that is, the intake stroke injection 81) in the intake stroke. In the subsequent step S6, the ECU 100 outputs a signal to the injector 6, and the injector 6 performs the second fuel injection (that is, the compression stroke late injection 82) in the latter half of the compression stroke. In step S7, the ECU 100 outputs a signal to the injector 6, and the injector 6 executes the third fuel injection (that is, the compression top dead center injection 83) in the vicinity of the compression top dead center. By three fuel injections in steps S5 to S7, an air-fuel mixture as shown in FIG. 8 (3) is formed in the combustion chamber 17, and compression ignition combustion is performed after compression top dead center.

  In step S8, the ECU 100 determines whether the pre-ignition risk is equal to or less than a predetermined value. For example, based on the detection value of the outside air temperature sensor 56, when the outside air temperature is equal to or higher than a predetermined temperature, the ECU 100 determines that the pre-ignition risk exceeds a predetermined value. When the determination in step S8 is NO (in other words, when the outside air temperature is high), the flow proceeds to step S5. When the risk of premature ignition is high, the fuel injection mode shown in FIG. 8 is adopted. As described above, the fuel injection mode shown in FIG. 8 is more robust against abnormal combustion than the fuel injection mode shown in FIG. Therefore, by adopting the fuel injection mode shown in FIG. 8, the engine 1 can avoid abnormal combustion when the risk of pre-ignition is high. This means that when the outside air temperature is high, as shown by an arrow in FIG. 4B, the medium load region (B) for performing fuel injection shown in FIG. 8 is expanded to the high load side and the high rotation side. is there. That is, the ECU 100 shifts the boundary between the medium load region (B) and the high load region (C) to the high load side and the high rotation side when the outside air temperature is high.

  On the other hand, when the determination in step S8 is YES (in other words, when the outside air temperature is not high), the flow proceeds to step S9. In steps S9 to S11, ECU 100 executes a third fuel injection corresponding to the fuel injection mode shown in FIG. The third injection control corresponds to the fuel injection control in the high load region (C) in FIG. 4A.

  In step S9, the ECU 100 outputs a signal to the injector 6, and the injector 6 performs the first fuel injection (that is, the intake stroke injection 61) in the intake stroke. In subsequent step S10, the ECU 100 outputs a signal to the injector 6, and the injector 6 performs the second fuel injection (that is, the compression stroke intermediate injection 62) in the middle of the compression stroke. In step S11, the ECU 100 outputs a signal to the injector 6, and the injector 6 performs the third fuel injection (that is, the compression top dead center injection 63) in the vicinity of the compression top dead center. By the three fuel injections in steps S9 to S11, an air-fuel mixture as shown in FIG. 6 (3) is formed in the combustion chamber 17, and compression ignition combustion is performed after compression top dead center.

(Combustion chamber shape)
As described above, an ignition zone is formed in the cavity 34 in the high load region (C). Then, due to the temperature and pressure in the combustion chamber 17 that is increased by the compression ignition combustion of the air-fuel mixture in the ignition zone, the over-concentrated zone and the lean zone are subjected to compression ignition. Here, if the combustion energy of the air-fuel mixture in the ignition zone is too small, the air-fuel mixture in the rich zone and the lean zone cannot be affected, and the gas mixture in the rich zone and the lean zone may not lead to compression ignition. There is. In order to secure sufficient combustion energy of the air-fuel mixture in the ignition zone, the cavity 34 needs to be sized to some extent. On the other hand, if the cavity 34 is too large, the mass combustion ratio of the combustion of the air-fuel mixture in the ignition zone will be too large, the pressure change in the combustion chamber 17 will become steep, and combustion noise will increase. From such a viewpoint, the cavity 34 of the engine 1 is formed in a predetermined size.

  FIG. 10 is a longitudinal sectional view showing the shape of the combustion chamber 17 when the piston 3 is located at the top dead center. The size of the cavity 34 is defined by the ratio of the cavity volume 9 to the total volume of the combustion chamber 17 when the piston 3 is located at the top dead center. Here, the cavity volume 9 is a volume 91 from the projection plane 93 obtained by projecting the opening 341 of the cavity 34 onto the lower surface 170 of the cylinder head 13 to the opening 341 of the cavity 34 when the piston 3 is located at the top dead center. And the volume 92 of the concave portion of the cavity 34 are added together.

  FIG. 11 shows the relationship between the mass combustion ratio when the ignition zone burns and the combustion efficiency of all the zones (that is, the ignition zone, the rich zone, and the lean zone). In FIG. 11, the EGR rate is changed from 50% to 65%. This is the same as changing the temperature in the combustion chamber 17. When the mass combustion ratio in the ignition zone is small, the combustion efficiency is lower when the EGR rate is 50% than when the EGR rate is 55%. If the EGR rate is 55% or more, the relationship between the mass combustion ratio in the ignition zone and the combustion efficiency in all zones is the same or almost the same.

  According to FIG. 11, the relationship between the mass combustion ratio of the ignition zone and the combustion efficiency of all zones increases to the right. When the mass combustion ratio of the ignition zone is small, the combustion efficiency of all zones decreases, and the mass of the ignition zone When the combustion rate is large, the combustion efficiency of all zones increases. As described above, when the combustion energy of the air-fuel mixture in the ignition zone is small, the influence on the air-fuel mixture in the rich zone and the lean zone is reduced, and the combustion efficiency in all zones is lowered.

  Generally, if the combustion efficiency in the combustion chamber 17 is 80% or more, it can be said that combustion stability is ensured. Therefore, based on the combustion efficiency of 80%, the mass combustion ratio in the ignition zone needs to be at least 25%.

  FIG. 12 shows a graph in which the horizontal axis represents the ratio between the cavity diameter and the bore diameter (that is, cavity diameter / bore diameter), and the vertical axis represents the combustion efficiency and the combustion period. The cavity diameter / bore diameter is proportional to the mass combustion rate of the ignition zone. Therefore, the relationship between the cavity diameter / bore diameter and the combustion efficiency of all zones increases to the right. As described above, on the basis of 80% combustion efficiency, the cavity diameter / bore diameter should be 0.4 or more.

  On the other hand, if the cavity diameter is too large, the mass combustion ratio due to combustion of the air-fuel mixture in the ignition zone becomes too large. As a result, the combustion period becomes too short, and the combustion noise exceeds the allowable value. Based on the combustion noise that can be tolerated, the combustion period must be 10 ° or more in crank angle (note that the rotational speed of the engine 1 is 2000 rpm). Then, as shown in FIG. 12, the cavity diameter / bore diameter needs to be 0.6 or less.

  In FIG. 13, the horizontal axis represents the volume ratio of the cavity 34 (that is, the ratio of the cavity volume 9 to the total volume of the combustion chamber 17 when the piston 3 is located at the top dead center, see FIG. 10). A graph in which the vertical axis represents the combustion efficiency and the combustion period is shown. As described above, based on the combustion efficiency of 80%, the volume ratio of the cavity 34 needs to be 25% or more. Further, based on the combustion noise that can be tolerated, the volume ratio of the cavity 34 needs to be 40% or less.

  The technique disclosed here is not limited to being applied to the engine 1 having the above-described configuration.

1 Engine 11 Cylinder 13 Cylinder Head 17 Combustion Chamber 100 ECU (Controller)
3 Piston 34 Cavity 51 Airflow sensor 52 Accelerator opening sensor 53 Vehicle speed sensor 54 Crank angle sensor 55 Water temperature sensor 56 Outside air temperature sensor 6 Injector

Claims (6)

An injector configured to inject fuel into a combustion chamber defined by an upper surface of the piston and a lower surface of the cylinder head;
A sensor configured to detect the operating state of the engine;
A controller configured to receive a signal from the sensor and to output a signal for causing the injector to perform fuel injection; and
The injector receives a signal from the controller, a first fuel injection for forming an air-fuel mixture having a first equivalence ratio of less than 1.0 in the combustion chamber, and a second equivalent of 1.0 or more. A second fuel injection for forming a mixture of the ratio in the combustion chamber;
The controller outputs a signal to the injector so that the first fuel injection is performed before the second fuel injection when the engine load is equal to or less than a predetermined load, and the engine load is the predetermined fuel load. When the load is higher than the load, a signal is output to the injector so that the second fuel injection is performed before the first fuel injection,
A control apparatus for a premixed compression ignition engine in which the air-fuel mixture having the first equivalent ratio is subjected to compression ignition, and the air-fuel mixture having the second equivalent ratio starts combustion after the compression ignition. The control device for a premixed compression ignition engine according to claim 1,
The controller outputs a signal to the injector so that the first fuel injection is performed before the second fuel injection when the engine speed is equal to or lower than a predetermined engine speed, and the engine speed When the engine speed is higher than the predetermined rotational speed, the control device for the premixed compression ignition engine that outputs a signal to the injector so that the second fuel injection is performed before the first fuel injection. In the control device of the premixed compression ignition engine according to claim 1 or 2,
An outside air temperature sensor configured to detect the outside air temperature;
The controller is a control device for a premixed compression ignition engine that shifts the predetermined load to a high load side when the outside air temperature is higher than a predetermined temperature based on a signal from the outside air temperature sensor. The control device for a premixed compression ignition engine according to claim 2,
An outside air temperature sensor configured to detect the outside air temperature;
The controller is a control device for a premixed compression ignition engine that shifts the predetermined rotational speed to a high rotational speed side when the external air temperature is higher than a predetermined temperature based on a signal from the external air temperature sensor. In the control device of the premixed compression ignition type engine according to any one of claims 1 to 4,
The controller outputs a signal to the injector to perform a third fuel injection prior to the first fuel injection when the engine load is equal to or less than a predetermined load,
In the combustion chamber immediately before the compression ignition, from the outer peripheral portion toward the central portion, the air-fuel mixture zone that is leaner than the first equivalent ratio, the air-fuel mixture zone that has the first equivalent ratio, and the second equivalent A control device for a premixed compression ignition engine in which a mixture zone of a specific ratio is formed. The premixed compression ignition type engine according to any one of claims 1 to 5,
The controller outputs a signal to the injector to perform a third fuel injection before the second fuel injection when the engine load is higher than the predetermined load,
In the combustion chamber immediately before compression ignition, an air-fuel mixture zone having the second equivalence ratio, a gas mixture zone leaner than the first equivalence ratio, and the first equivalent from the outer periphery toward the center. A control device for a premixed compression ignition engine in which a mixture zone of a specific ratio is formed.