JP5910423B2 - Spark ignition engine - Google Patents

Description

  The technology disclosed herein relates to a spark ignition engine.

  For example, as described in Patent Document 1, a combustion mode in which an air-fuel mixture in a cylinder is ignited by compression is known as a technique for achieving both improvement in exhaust emission performance and improvement in thermal efficiency. In an engine that performs such compression ignition combustion, as the engine load increases, the compression ignition combustion becomes combustion with a sharp pressure rise, leading to an increase in combustion noise. Therefore, as described in Patent Document 1, even in an engine that performs compression ignition combustion, spark ignition combustion is performed not by compression ignition combustion but by driving an ignition plug in an operation region on the high load side. Is common.

  In Patent Document 2, as in the engine of Patent Document 1, in an engine that performs compression ignition combustion in a low load and low rotation region, adjustment of the valve opening period of the intake and exhaust valves is performed in the region where compression ignition combustion is performed. In this way, high-temperature burned gas is retained in the cylinder, and the temperature in the cylinder is increased to promote the compression self-ignition combustion. On the other hand, in the region where the compression ignition combustion is performed, the intake valve Is described in which the burned gas in the cylinder is once blown back to the intake port side by advancing the valve opening timing, and then the burned gas is again introduced into the cylinder together with fresh air. By so doing, the temperature of the burned gas is reduced by fresh air, so that a sudden increase in pressure due to compression ignition combustion is suppressed in a relatively high-load high-rotation region where the compression end temperature pressure becomes high.

JP 2007-154859 A JP 2009-197740 A

  By the way, since the spark ignition combustion has relatively low thermal efficiency, the combustion gas temperature becomes high. On the other hand, in compression ignition combustion, high temperature burned gas is introduced into a cylinder as described in the above-mentioned patent document in order to ensure ignitability. Therefore, in an engine that switches the combustion mode as described in the patent document, immediately after switching from spark ignition combustion to compression ignition combustion, the temperature atmosphere in the cylinder is relatively high, and the spark High-temperature burned gas by ignition combustion is introduced into the cylinder, and the temperature in the cylinder becomes too high. This leads to premature ignition such that the air-fuel mixture in the cylinder ignites, for example, during the compression stroke, and the pressure increase rate (dP / dt) in the cylinder becomes steep, resulting in large combustion noise. There is a risk of inviting.

  Note that switching from spark ignition combustion to compression ignition combustion is not limited to when the engine load decreases, for example, and may be switched even if the engine load is equal, and for example, the engine temperature is reduced. It can also be done when rising from warm to warm.

  The technology disclosed herein has been made in view of such a point, and an object thereof is to avoid an increase in combustion noise when switching from spark ignition combustion to compression ignition combustion.

  The technology disclosed herein is introduced into a cylinder immediately after switching to compression ignition combustion by suppressing the combustion gas temperature by the spark ignition fuel immediately before the switching when switching from spark ignition combustion to compression ignition combustion. The exhaust gas temperature is lowered to lower the temperature state in the cylinder, thereby avoiding premature ignition of the air-fuel mixture and avoiding combustion noise.

  Specifically, the technology disclosed herein relates to a spark ignition engine, and is configured to be disposed facing an engine body having a cylinder and the cylinder, and to ignite an air-fuel mixture in the cylinder. A spark plug, an exhaust gas recirculation means configured to introduce exhaust gas into the cylinder, and at least the spark plug and the exhaust gas recirculation means are controlled to operate the engine body. And a controller.

  The controller performs compression ignition combustion in which the air-fuel mixture in the cylinder is combusted by self-ignition and operates the engine body, and ignites the air-fuel mixture in the cylinder by driving the spark plug. A spark ignition mode in which the engine body is operated by performing spark ignition combustion, and at least in the compression ignition mode, the exhaust gas is introduced into the cylinder through control of the exhaust gas recirculation means. Introducing the controller, when switching from the spark ignition mode to the compression ignition mode, performed a transient combustion mode in which the combustion gas temperature is lower than the combustion gas temperature at the time of spark ignition combustion before the mode switching Later, switch to compression ignition mode.

  According to this configuration, when switching from the spark ignition mode to the compression ignition mode, the transient combustion mode in which the combustion gas temperature is lower than the combustion gas temperature during the spark ignition combustion before the mode switching is performed. As a result, if the combustion mode is switched to the compression ignition mode after the transient combustion mode is performed, the exhaust gas having a relatively low temperature due to the transient combustion mode is introduced into the cylinder, and in the cylinder by the transient combustion mode. Temperature conditions can also be lowered. In other words, since the temperature state in the cylinder immediately after switching to the compression ignition mode becomes low, it is avoided that the fuel in the cylinder is ignited prematurely by being exposed to a high temperature atmosphere. Compressive ignition will start. In this way, it is avoided that the pressure rise is steep, and accordingly, it is also possible to avoid an increase in combustion noise.

In the transient combustion mode, the controller sets the working gas fuel ratio G / F or the air fuel ratio A / F to the working gas fuel ratio G / F or the air fuel at the time of spark ignition combustion before the mode switching. When set to leaner than the ratio A / F. By using A / F lean or G / F lean, the amount of gas with respect to fuel increases, so the combustion gas temperature decreases.

Wherein the controller, wherein the transient combustion mode, by increasing the fresh air amount, the working gas fuel ratio G / F, or, to set the air fuel ratio A / F lean.
Increasing the amount of fresh air at a relatively low temperature lowers the temperature in the cylinder, which is advantageous for further reducing the combustion gas temperature in the transient combustion mode. This is advantageous in further reducing the temperature state in the cylinder immediately after switching from the spark ignition mode to the compression ignition mode and avoiding premature ignition.

In the transient combustion mode, the controller prohibits the introduction of the exhaust gas into the cylinder through the control of the exhaust gas recirculation means, and switches to the compression ignition mode, and then controls the exhaust gas recirculation means. The exhaust gas is introduced into the cylinder.

  The exhaust gas recirculation means is configured to be able to introduce the cooled exhaust gas into the cylinder, and the controller increases the amount of cooled exhaust gas introduced into the cylinder during the transient combustion mode, The working gas fuel ratio G / F may be set to lean.

  Similarly to the above, increasing the introduction amount of the cooled exhaust gas having a relatively low temperature lowers the temperature in the cylinder and further lowers the combustion gas temperature in the transient combustion mode. As a result, the temperature state in the cylinder immediately after switching from the spark ignition mode to the compression ignition mode is further lowered, which is advantageous for avoiding premature ignition.

  The controller operates the engine body in the compression ignition mode when the operating state of the engine body is in a predetermined low load region, and the operating state of the engine body is lower than the predetermined low load region. The engine body is operated in the spark ignition mode when the load is in a high load area where the load is high, and the controller also shifts from the high load area to the low load area as the load of the engine body decreases. At this time, the transient combustion mode may be executed.

  In the spark ignition mode when the operating state of the engine body is in the high load region, the fuel injection amount is relatively large, so the combustion gas temperature is even higher. For this reason, when switching from the spark ignition mode to the compression ignition mode as the load of the engine main body decreases and shifts from the high load region to the low load region, immediately after the switching, an excess due to the high temperature atmosphere in the cylinder is caused. Early ignition is more likely to occur. Therefore, when the transition from the high load region to the low load region is performed, the transient combustion mode is executed immediately before switching from the spark ignition mode to the compression ignition mode. This is advantageous in avoiding an increase in combustion noise. That is, the transient combustion mode is particularly effective at the time of switching from the spark ignition mode to the compression ignition mode accompanying the transition from the high load region to the low load region.

  As described above, this spark ignition engine performs a transient combustion mode in which the combustion gas temperature is lower than the combustion gas temperature at the time of spark ignition combustion immediately before switching from the spark ignition mode to the compression ignition mode. Since the temperature state in the cylinder immediately after switching to the compression ignition mode can be lowered, it is possible to avoid premature ignition and avoid an increase in combustion noise.

It is the schematic which shows the structure of a spark ignition direct injection engine. It is a block diagram concerning control of a spark ignition direct injection engine. It is sectional drawing which expands and shows a combustion chamber. It is a figure which illustrates the operating area of an engine. (A) An example of the fuel injection timing when the intake stroke injection is performed in the CI mode, an example of the heat generation rate of the CI combustion associated therewith, (b) an example of the fuel injection timing when the high pressure retarded injection is performed in the CI mode, , An example of the heat generation rate of CI combustion associated therewith, (c) an example of the fuel injection timing and ignition timing when performing high pressure retarded injection in the SI mode, and an example of the heat generation rate of SI combustion associated therewith, (d) SI 4 is an example of fuel injection timing and ignition timing when split injection of intake stroke injection and high pressure retarded injection is performed in the mode, and the heat generation rate of SI combustion associated therewith. It is a figure which compares the state of SI combustion by a high pressure retarded injection, and the state of conventional SI combustion. It is a time chart explaining the control which concerns on the fuel injection at the time of the switch from SI mode to CI mode. It is a time chart explaining the control which concerns on the combustion temperature at the time of switching from SI mode to CI mode. It is a time chart explaining the control which concerns on the EGR rate at the time of switching from SI mode to CI mode.

  Hereinafter, an embodiment of a spark ignition direct injection engine will be described with reference to the drawings. The following description of preferred embodiments is exemplary. 1 and 2 show a schematic configuration of an engine (engine body) 1. The engine 1 is a spark ignition gasoline engine that is mounted on a vehicle and supplied with fuel containing at least gasoline. The engine 1 includes a cylinder block 11 provided with a plurality of cylinders 18 (only one cylinder is shown in FIG. 1, but four cylinders are provided in series, for example), and the cylinder block 11 is arranged on the cylinder block 11. The cylinder head 12 is provided, and an oil pan 13 is provided below the cylinder block 11 and stores lubricating oil. A piston 14 connected to the crankshaft 15 via a connecting rod 142 is fitted in each cylinder 18 so as to be able to reciprocate. A cavity 141 like a reentrant type in a diesel engine is formed on the top surface of the piston 14 as shown in an enlarged view in FIG. The cavity 141 is opposed to an injector 67 described later when the piston 14 is positioned near the compression top dead center. The cylinder head 12, the cylinder 18, and the piston 14 having the cavity 141 define a combustion chamber 19. The shape of the combustion chamber 19 is not limited to the illustrated shape. For example, the shape of the cavity 141, the top surface shape of the piston 14, the shape of the ceiling portion of the combustion chamber 19, and the like can be changed as appropriate.

  The engine 1 is set to a relatively high geometric compression ratio of 15 or more for the purpose of improving the theoretical thermal efficiency and stabilizing the compression ignition combustion described later. In addition, what is necessary is just to set a geometric compression ratio suitably in the range of about 15-20.

  The cylinder head 12 is provided with an intake port 16 and an exhaust port 17 for each cylinder 18. The intake port 16 and the exhaust port 17 have an intake valve 21 and an exhaust for opening and closing the opening on the combustion chamber 19 side. Each valve 22 is disposed.

  Among the valve systems that drive the intake valve 21 and the exhaust valve 22, respectively, on the exhaust side, the operation mode of the exhaust valve 22 is switched between a normal mode and a special mode, for example, a hydraulically operated variable mechanism (see FIG. 2). Hereinafter, a VVL (Variable Valve Lift) 71 is provided. Although the detailed illustration of the configuration of the VVL 71 is omitted, two types of cams having different cam profiles, a first cam having one cam peak and a second cam having two cam peaks, and the first And a lost motion mechanism that selectively transmits an operating state of one of the second cams to the exhaust valve. When the operating state of the first cam is transmitted to the exhaust valve 22, the exhaust valve 22 operates in the normal mode in which the valve is opened only once during the exhaust stroke, whereas the operating state of the second cam is the exhaust valve. When transmitting to the engine 22, the exhaust valve 22 operates in a special mode in which the exhaust valve is opened during the exhaust stroke and is also opened during the intake stroke so that the exhaust is opened twice. The normal mode and the special mode of the VVL 71 are switched according to the operating state of the engine. Specifically, the special mode is used in the control related to the internal EGR. In the following explanation, operating the VVL 71 in the normal mode and not opening the exhaust twice is referred to as “turning off the VVL 71”, and operating the VVL 71 in the special mode and opening the exhaust twice. , “Turn on VVL 71”. In order to enable switching between the normal mode and the special mode, an electromagnetically driven valve system that drives the exhaust valve 22 by an electromagnetic actuator may be employed. Further, the execution of the internal EGR is not realized only by opening the exhaust gas twice. For example, the internal EGR control may be performed by opening the intake valve 21 twice or by opening the intake valve twice, or by providing a negative overlap period in which both the intake valve 21 and the exhaust valve 22 are closed in the exhaust stroke or the intake stroke. Internal EGR control that causes the fuel gas to remain in the cylinder 18 may be performed.

  As shown in FIG. 2, the exhaust side valve system having the VVL 71 has a variable phase mechanism (hereinafter referred to as VVT (hereinafter referred to as VVT)) that can change the rotational phase of the intake camshaft with respect to the crankshaft 15 as shown in FIG. 72) and a lift variable mechanism (hereinafter referred to as CVVL (Continuously Variable Valve Lift)) 73 capable of continuously changing the lift amount of the intake valve 21. . The VVT 72 may employ a hydraulic, electromagnetic, or mechanical structure as appropriate, and illustration of the detailed structure is omitted. The CVVL 73 can also adopt various known structures as appropriate, and the detailed structure is not shown. By the VVT 72 and the CVVL 73, the intake valve 21 can change its valve opening timing, valve closing timing, and lift amount.

  In addition, an injector 67 that directly injects fuel into the cylinder 18 is attached to the cylinder head 12 for each cylinder 18. As shown in an enlarged view in FIG. 3, the injector 67 is disposed so that its nozzle hole faces the inside of the combustion chamber 19 from the central portion of the ceiling surface of the combustion chamber 19. The injector 67 directly injects an amount of fuel into the combustion chamber 19 at an injection timing set according to the operating state of the engine 1 and according to the operating state of the engine 1. In this example, the injector 67 is a multi-hole injector having a plurality of nozzle holes, although detailed illustration is omitted. Thereby, the injector 67 injects the fuel so that the fuel spray spreads radially from the center position of the combustion chamber 19. As indicated by the arrows in FIG. 3, the fuel spray injected radially from the central portion of the combustion chamber 19 at the timing when the piston 14 is located near the compression top dead center is a cavity formed on the top surface of the piston. It flows along the wall surface of 141. It can be paraphrased that the cavity 141 is formed so that the fuel spray injected at the timing when the piston 14 is located near the compression top dead center is contained therein. This combination of the multi-hole injector 67 and the cavity 141 is an advantageous configuration for shortening the mixture formation period and the combustion period after fuel injection. In addition, the injector 67 is not limited to a multi-hole injector, and may be an outside-opening type injector.

  A fuel tank (not shown) and the injector 67 are connected to each other by a fuel supply path. A fuel supply system 62 including a fuel pump 63 and a common rail 64 and capable of supplying fuel to the injector 67 at a relatively high fuel pressure is interposed on the fuel supply path. The fuel pump 63 pumps fuel from the fuel tank to the common rail 64, and the common rail 64 can store the pumped fuel at a relatively high fuel pressure. When the injector 67 is opened, the fuel stored in the common rail 64 is injected from the injection port of the injector 67. Here, although not shown, the fuel pump 63 is a plunger type pump and is driven by the engine 1. The fuel supply system 62 configured to include this engine-driven pump enables the fuel with a high fuel pressure of 30 MPa or more to be supplied to the injector 67. The fuel pressure may be set to about 120 MPa at the maximum. The pressure of the fuel supplied to the injector 67 is changed according to the operating state of the engine 1 as will be described later. The fuel supply system 62 is not limited to this configuration.

  As shown in FIG. 3, a spark plug 25 that ignites the air-fuel mixture in the combustion chamber 19 is attached to the cylinder head 12. In this example, the spark plug 25 is disposed through the cylinder head 12 so as to extend obliquely downward from the exhaust side of the engine 1. As shown in FIG. 3, the tip of the spark plug 25 is disposed facing the cavity 141 of the piston 14 located at the compression top dead center.

  As shown in FIG. 1, an intake passage 30 is connected to one side of the engine 1 so as to communicate with the intake port 16 of each cylinder 18. On the other hand, an exhaust passage 40 for discharging burned gas (exhaust gas) from the combustion chamber 19 of each cylinder 18 is connected to the other side of the engine 1.

  An air cleaner 31 that filters intake air is disposed at the upstream end of the intake passage 30. A surge tank 33 is disposed near the downstream end of the intake passage 30. The intake passage 30 downstream of the surge tank 33 is an independent passage branched for each cylinder 18, and the downstream end of each independent passage is connected to the intake port 16 of each cylinder 18.

  Between the air cleaner 31 and the surge tank 33 in the intake passage 30, a water-cooled intercooler / warmer 34 that cools or heats the air and a throttle valve 36 that adjusts the amount of intake air to each cylinder 18 are arranged. It is installed. An intercooler bypass passage 35 that bypasses the intercooler / warmer 34 is also connected to the intake passage 30. The intercooler bypass passage 35 is connected to an intercooler for adjusting the flow rate of air passing through the passage 35. A bypass valve 351 is provided. Adjusting the temperature of fresh air introduced into the cylinder 18 by adjusting the ratio between the passage flow rate of the intercooler bypass passage 35 and the passage flow rate of the intercooler / warmer 34 through the opening degree adjustment of the intercooler bypass valve 351. Is possible.

  The upstream portion of the exhaust passage 40 is constituted by an exhaust manifold having an independent passage branched for each cylinder 18 and connected to the outer end of the exhaust port 17 and a collecting portion where the independent passages gather. . A direct catalyst 41 and an underfoot catalyst 42 are connected downstream of the exhaust manifold in the exhaust passage 40 as exhaust purification devices for purifying harmful components in the exhaust gas. Each of the direct catalyst 41 and the underfoot catalyst 42 includes a cylindrical case and, for example, a three-way catalyst disposed in a flow path in the case.

  A portion between the surge tank 33 and the throttle valve 36 in the intake passage 30 and a portion upstream of the direct catalyst 41 in the exhaust passage 40 are used for returning a part of the exhaust gas to the intake passage 30. They are connected via a passage 50. The EGR passage 50 includes a main passage 51 in which an EGR cooler 52 for cooling the exhaust gas with engine coolant is disposed, and an EGR cooler bypass passage 53 for bypassing the EGR cooler 52. ing. The main passage 51 is provided with an EGR valve 511 for adjusting the amount of exhaust gas recirculated to the intake passage 30, and the EGR cooler bypass passage 53 has a flow rate of exhaust gas flowing through the EGR cooler bypass passage 53. An EGR cooler bypass valve 531 for adjustment is provided.

  The engine 1 configured as described above is controlled by a powertrain control module (hereinafter referred to as PCM) 10. The PCM 10 includes a microprocessor having a CPU, a memory, a counter timer group, an interface, and a path connecting these units. This PCM 10 constitutes a controller.

As shown in FIGS. 1 and 2, detection signals from various sensors SW <b> 1 to SW <b> 16 are input to the PCM 10. The various sensors include the following sensors. That is, the air flow sensor SW1 that detects the flow rate of fresh air, the intake air temperature sensor SW2 that detects the temperature of fresh air, the downstream side of the intercooler / warmer 34, and the intercooler / warmer 34 downstream of the air cleaner 31. A second intake air temperature sensor SW3 for detecting the temperature of fresh air after passing through the EGR gas temperature sensor SW4, which is disposed in the vicinity of the connection portion of the EGR passage 50 with the intake passage 30 and detects the temperature of the external EGR gas. An intake port temperature sensor SW5 that is attached to the intake port 16 and detects the temperature of the intake air just before flowing into the cylinder 18, and an in-cylinder pressure sensor SW6 that is attached to the cylinder head 12 and detects the pressure in the cylinder 18. The exhaust passage 40 is disposed in the vicinity of the connection portion of the EGR passage 50, and the exhaust temperature and the exhaust respectively. Exhaust temperature sensor SW7 and exhaust pressure sensor SW8 for detecting a force, and is disposed on the upstream side of the direct catalyst 41, the linear O ₂ sensor SW9, direct catalyst 41 and underfoot catalyst 42 for detecting the oxygen concentration in the exhaust gas The lambda O ₂ sensor SW10 that detects the oxygen concentration in the exhaust gas, the water temperature sensor SW11 that detects the temperature of the engine coolant, the crank angle sensor SW12 that detects the rotation angle of the crankshaft 15, and the vehicle An accelerator opening sensor SW13 for detecting an accelerator opening corresponding to an operation amount of an accelerator pedal (not shown), intake-side and exhaust-side cam angle sensors SW14, SW15, and a common rail 64 of the fuel supply system 62 are attached. The fuel pressure sensor SW16 detects the fuel pressure supplied to the injector 67. The

  The PCM 10 performs various calculations based on these detection signals to determine the state of the engine 1 and the vehicle, and accordingly, the injector 67, the spark plug 25, the intake valve side VVT 72 and CVVL 73, and the exhaust valve side Control signals are output to the actuators of the VVL 71, the fuel supply system 62, and various valves (throttle valve 36, intercooler bypass valve 351, EGR valve 511, and EGR cooler bypass valve 531). Thus, the PCM 10 operates the engine 1.

  FIG. 4 shows an example of the operation region when the engine 1 is warm. This engine 1 is a compression ignition combustion in which combustion is performed by compression self-ignition without ignition by the spark plug 25 in a low load region where the engine load is relatively low for the purpose of improving fuel consumption and exhaust emission performance. I do. However, as the load on the engine 1 increases, in the compression ignition combustion, the combustion becomes too steep and causes problems such as combustion noise. Therefore, in the engine 1, in a high load region where the engine load is relatively high, the compression ignition combustion is stopped, and the engine 1 is switched to the spark ignition combustion using the spark plug 25. As described above, the engine 1 has a CI (Compression Ignition) mode in which compression ignition combustion is performed and an SI (Spark Ignition) mode in which spark ignition combustion is performed in accordance with the operation state of the engine 1, in particular, the load of the engine 1. It is configured to switch. However, the boundary line for mode switching is not limited to the illustrated example.

  The CI mode is further divided into three areas according to the engine load. Specifically, in the region (1) where the load is the lowest in the CI mode, hot EGR gas having a relatively high temperature is introduced into the cylinder 18 in order to improve the ignitability and stability of the compression ignition combustion. This is because the VVL 71 is turned on and the exhaust valve 22 is opened twice during the intake stroke. The introduction of hot EGR gas is advantageous in increasing the compression end temperature in the cylinder 18 and improving the ignitability and stability of the compression ignition combustion in the light load region (1). In the region (1), as shown in FIG. 5 (a), the injector 67 injects fuel into the cylinder 18 at least during the period from the intake stroke to the middle of the compression stroke, thereby producing a homogeneous lean air-fuel mixture. Form. The excess air ratio λ of the air-fuel mixture may be set to, for example, 2.4 or more. By doing so, it is possible to suppress the generation of RawNOx and improve the exhaust emission performance. Then, as shown in FIG. 5A, the lean air-fuel mixture undergoes compression self-ignition near the compression top dead center.

  In the region with a high load in the region (1), specifically in the region including the boundary between the region (1) and the region (2), the fuel in the cylinder 18 is at least in the period from the intake stroke to the middle of the compression stroke. However, the air-fuel ratio of the air-fuel mixture is set to the stoichiometric air-fuel ratio (λ≈1). By setting the stoichiometric air-fuel ratio, the three-way catalyst can be used and, as will be described later, the air-fuel ratio of the air-fuel mixture is also set to the stoichiometric air-fuel ratio in the SI mode, so that switching between the SI mode and the CI mode is possible. The control at the time is simplified, and further, the CI mode can be expanded to the high load side.

  In the CI mode, in the region (2) where the load is higher than that in the region (1), as in the high load side of the region (1), at least in the period from the intake stroke to the middle of the compression stroke, fuel is injected into the cylinder 18. The fuel is injected (see FIG. 5A) to form a homogeneous air / fuel ratio (λ≈1).

  In the region (2), the temperature in the cylinder 18 naturally increases as the engine load increases, so the hot EGR gas amount is reduced to avoid pre-ignition. This is due to the adjustment of the amount of internal EGR gas introduced into the cylinder 18. Further, the hot EGR gas amount may be adjusted by adjusting the external EGR gas amount bypassing the EGR cooler 52.

  In the region (2), a cooled EGR gas having a relatively low temperature is further introduced into the cylinder 18. Thus, by introducing the hot hot EGR gas and the cold cooled EGR gas into the cylinder 18 at an appropriate ratio, the compression end temperature in the cylinder 18 is made appropriate, and the rapid ignition while ensuring the ignitability of the compression ignition. Avoid combustion and stabilize compression ignition combustion. The EGR rate as a ratio of the EGR gas introduced into the cylinder 18 including the hot EGR gas and the cooled EGR gas is as high as possible under the condition that the air-fuel ratio of the air-fuel mixture is set to λ≈1. Set to rate. Therefore, in the region (2), the fuel injection amount increases as the engine load increases, so the EGR rate gradually decreases.

  In the region (3) where the load is highest in the CI mode, including the boundary line between the CI mode and the SI mode, the compression end temperature in the cylinder 18 is further increased. Therefore, as in the region (1) and the region (2) In addition, if fuel is injected into the cylinder 18 during the period from the intake stroke to the middle of the compression stroke, abnormal combustion such as pre-ignition occurs. On the other hand, if a large amount of cooled EGR gas having a low temperature is introduced to lower the compression end temperature in the cylinder, the ignitability of the compression ignition is deteriorated. That is, since compression ignition combustion cannot be stably performed only by temperature control in the cylinder 18, in this region (3), in addition to temperature control in the cylinder 18, it is prematurely devised by devising a fuel injection mode. Stabilize compression ignition combustion while avoiding abnormal combustion such as ignition. Specifically, this fuel injection mode has a fuel pressure significantly higher than that in the conventional case, and as shown in FIG. 5 (b), at least a period from the latter stage of the compression stroke to the initial stage of the expansion stroke (hereinafter referred to as this The fuel is injected into the cylinder 18 within a period (referred to as a retard period). This characteristic fuel injection mode is hereinafter referred to as “high pressure retarded injection” or simply “retarded injection”. Such high-pressure retarded injection stabilizes compression ignition combustion while avoiding abnormal combustion in the region (3). Details of the high-pressure retarded injection will be described later.

  In the region (3), similarly to the region (2), high-temperature hot EGR gas and low-temperature cooled EGR gas are introduced into the cylinder 18 at an appropriate ratio. As a result, the compression end temperature in the cylinder 18 is appropriately set to stabilize the compression ignition combustion.

  In contrast to the CI mode divided into three regions according to the engine load, the SI mode is divided into two regions, region (4) and region (5), according to the engine speed. ing. The region (4) corresponds to a low speed region when the operation region of the engine 1 is divided into a low speed and a high speed in the illustrated example, and the region (5) corresponds to a high speed region. The boundary between the region (4) and the region (5) is also inclined in the rotational speed direction with respect to the load level in the operation region shown in FIG. 4, but the boundary between the region (4) and the region (5) Is not limited to the illustrated example.

  In each of the region (4) and the region (5), the air-fuel mixture is set to the stoichiometric air-fuel ratio (λ≈1) as in the regions (2) and (3). Therefore, the air-fuel ratio of the air-fuel mixture is made constant at the theoretical air-fuel ratio (λ≈1) across the boundary between the CI mode and the SI mode. This allows the use of a three-way catalyst. In the region (4) and the region (5), the throttle valve 36 is basically fully opened, while the opening amount of the EGR valve 511 is adjusted and the amount of fresh air and the amount of external EGR gas introduced into the cylinder 18 are adjusted. Adjust. Adjusting the gas ratio introduced into the cylinder 18 in this way reduces pump loss and introduces a large amount of EGR gas into the cylinder 18, thereby suppressing the combustion temperature of spark ignition combustion and reducing cooling loss. Figured. In the region (4) and the region (5), the external EGR gas cooled mainly through the EGR cooler 52 is introduced into the cylinder 18. This is advantageous for avoiding abnormal combustion and also has an advantage of suppressing generation of Raw NOx. In the fully open load range, the external EGR is set to zero by closing the EGR valve 511.

  In the regions (4) and (5), the introduction of EGR gas is not introduced, but the introduction is stopped, while the opening of the throttle valve 36 is controlled according to the amount of fuel injected. The amount of fresh air introduced into the cylinder 18 may be adjusted so that the stoichiometric air-fuel ratio (λ≈1).

  As described above, the geometric compression ratio of the engine 1 is set to 15 or more (for example, 18). Since the high compression ratio increases the compression end temperature and the compression end pressure, it is advantageous for stabilizing the compression ignition combustion in the CI mode, particularly in a low load region (for example, the region (1)). On the other hand, the high compression ratio engine 1 has a problem that abnormal combustion such as pre-ignition and knocking is likely to occur in the SI mode which is a high load region.

  Therefore, in the engine 1, in the region (4) and the region (5) of the SI mode, abnormal combustion is avoided by performing the above-described high-pressure retarded injection. More specifically, in the region (4), fuel is injected into the cylinder 18 with a high fuel pressure of 30 MPa or more and within the retard period from the late compression stroke to the early expansion stroke, as shown in FIG. 5 (c). Only high-pressure retarded injection is performed. On the other hand, in the region (5), as shown in FIG. 5 (d), a part of the fuel to be injected is injected into the cylinder 18 within the intake stroke period in which the intake valve 21 is open. The remaining fuel is injected into the cylinder 18 within the retard period. That is, in the region (5), fuel split injection is performed. Here, the intake stroke period during which the intake valve 21 is open is not a period defined based on the piston position, but a period defined based on opening / closing of the intake valve, and the intake stroke referred to here is CVVL73. Depending on the closing timing of the intake valve 21 changed by the VVT 72, the piston may deviate from the time when the piston reaches the bottom dead center of the intake.

  Next, the high pressure retarded injection in the SI mode will be described with reference to FIG. FIG. 6 shows the heat generation rate (upper diagram) and the progress of the unburned mixture reaction in the SI combustion (solid line) by the high pressure retarded injection described above and the conventional SI combustion (broken line) in which fuel injection is performed during the intake stroke. It is a figure which compares the difference in a degree (lower figure). The horizontal axis in FIG. 6 is the crank angle. As a precondition for this comparison, the operating state of the engine 1 is both a high-load low-speed region (that is, region (4)), and the amount of fuel to be injected is the case of SI combustion by high-pressure retarded injection and conventional SI combustion. They are the same as each other.

  First, in the conventional SI combustion, a predetermined amount of fuel is injected into the cylinder 18 during the intake stroke (broken line in the upper diagram). In the cylinder 18, a relatively homogeneous air-fuel mixture is formed after the fuel injection until the piston 14 reaches the compression top dead center. In this example, ignition is executed at a predetermined timing indicated by a white circle after the compression top dead center, thereby starting combustion. After the start of combustion, as shown by the broken line in the upper diagram of FIG. 6, the combustion ends through a peak of the heat generation rate. The time from the start of fuel injection to the end of combustion corresponds to the reaction possible time of the unburned mixture (hereinafter sometimes simply referred to as reaction possible time). As shown by the broken line in the lower diagram of FIG. The reaction of the fuel mixture gradually proceeds. The dotted line in the figure shows the ignition threshold, which is the reactivity with which the unburned mixture reaches ignition, and the conventional SI combustion has a very low reaction time in combination with the low speed range. In the meantime, the reaction of the unburned mixture continues to progress during that time, so the reactivity of the unburned mixture exceeds the ignition threshold before and after ignition, and abnormal combustion such as premature ignition or knocking occurs. cause.

  On the other hand, the high pressure retarded injection aims to shorten the reaction possible time, thereby avoiding abnormal combustion. That is, as shown in FIG. 6, the possible reaction time is a period during which the injector 67 injects fuel ((1) injection period), and after the injection is completed, a combustible mixture is formed around the spark plug 25. (2) The mixture formation period) and the period until the combustion started by ignition is completed ((3) combustion period), that is, (1) + (2 ) + (3). The high-pressure retarded injection shortens the injection period, the mixture formation period, and the combustion period, thereby shortening the reaction time. This will be described in order.

  First, the high fuel pressure relatively increases the amount of fuel injected from the injector 67 per unit time. For this reason, when the fuel injection amount is constant, the relationship between the fuel pressure and the fuel injection period is generally such that the lower the fuel pressure, the longer the injection period, and the higher the fuel pressure, the shorter the injection period. Therefore, the high pressure retarded injection in which the fuel pressure is set to be significantly higher than the conventional one shortens the injection period.

  Further, the high fuel pressure is advantageous for atomization of the fuel spray injected into the cylinder 18 and makes the flight distance of the fuel spray longer. For this reason, the relationship between the fuel pressure and the fuel evaporation time is generally longer as the fuel pressure is lower, and the fuel evaporation time is longer as the fuel pressure is higher. Further, the time until the fuel spray reaches the fuel pressure and the spark plug 25 is generally longer as the fuel pressure is lower, and the time until the fuel spray is higher as the fuel pressure is higher. The air-fuel mixture formation period is a time obtained by adding the fuel evaporation time and the fuel spray arrival time around the spark plug 25. Therefore, the higher the fuel pressure, the shorter the air-fuel mixture formation period. Therefore, the high pressure retarded injection in which the fuel pressure is set to be significantly higher than the conventional case shortens the mixture formation period as a result of the fuel evaporation time and the fuel spray arrival time around the spark plug 25 being reduced. On the other hand, as shown by white circles in the figure, the conventional intake stroke injection at a low fuel pressure significantly increases the mixture formation period. In the SI mode, the combination of the multi-injector type injector 67 and the cavity 141 shortens the time until fuel spray reaches around the spark plug 25 after fuel injection. Effective for shortening.

  Thus, shortening the injection period and the mixture formation period makes it possible to set the fuel injection timing, more precisely, the injection start timing to a relatively late timing. Therefore, in the high pressure retarded injection, as shown in the upper diagram of FIG. 6, fuel injection is performed within the retard period from the latter stage of the compression stroke to the early stage of the expansion stroke. As the fuel is injected into the cylinder 18 at a high fuel pressure, the turbulence in the cylinder becomes stronger and the turbulence energy in the cylinder 18 increases. This high turbulence energy is a timing at which the fuel injection timing is relatively late. Therefore, it is advantageous for shortening the combustion period.

  That is, when the fuel injection is performed within the retard period, the relationship between the fuel pressure and the turbulent energy in the combustion period is generally lower as the fuel pressure is lower and the turbulent energy is lower as the fuel pressure is higher. Becomes higher. Here, even if fuel is injected into the cylinder 18 at a high fuel pressure, if the injection timing is in the intake stroke, the time until the ignition timing is long, or the cylinder 18 is in the compression stroke after the intake stroke. Due to the compression of the inside, the disturbance in the cylinder 18 is attenuated. As a result, when fuel is injected during the intake stroke, the turbulent energy during the combustion period becomes relatively low regardless of the fuel pressure level.

  In general, the relationship between the turbulent energy and the combustion period in the combustion period is such that the lower the turbulent energy, the longer the combustion period, and the higher the turbulent energy, the shorter the combustion period. Therefore, the relationship between the fuel pressure and the combustion period is such that the lower the fuel pressure, the longer the combustion period, and the higher the fuel pressure, the shorter the combustion period. That is, the high pressure retarded injection shortens the combustion period. In contrast, the conventional intake stroke injection at a low fuel pressure has a longer combustion period. The multi-injector type injector 67 is advantageous for improving the turbulent energy in the cylinder 18 and is effective for shortening the combustion period. In addition, the combination of the multi-injector type injector 67 and the cavity 141 provides fuel. Putting the spray in the cavity 141 is also effective for shortening the combustion period.

  As described above, the high pressure retarded injection shortens the injection period, the mixture formation period, and the combustion period, and as a result, as shown in FIG. 6, the fuel injection start timing SOI to the combustion end timing θend are not changed. It is possible to significantly shorten the reaction time of the fuel mixture as compared with the case of fuel injection during the conventional intake stroke. As a result of shortening this reaction possible time, as shown in the upper diagram of FIG. 6, in the conventional intake stroke injection at a low fuel pressure, as shown by a white circle, the reaction progress of the unburned mixture at the end of combustion is shown. However, when the ignition threshold is exceeded and abnormal combustion occurs, the high-pressure retarded injection suppresses the progress of the reaction of the unburned mixture at the end of combustion, as shown by the black circle, to prevent abnormal combustion. It can be avoided. It should be noted that the ignition timing is set to the same timing in the white circle and the black circle in the upper diagram of FIG.

  By setting the fuel pressure to, for example, 30 MPa or more, the combustion period can be effectively shortened. Moreover, the fuel pressure of 30 MPa or more can effectively shorten the injection period and the mixture formation period, respectively. The fuel pressure is preferably set as appropriate according to the properties of the fuel used, which contains at least gasoline. The upper limit may be 120 MPa as an example.

  The high pressure retarded injection avoids the occurrence of abnormal combustion in the SI mode by devising the form of fuel injection into the cylinder 18. Unlike this, it is conventionally known that the ignition timing is retarded for the purpose of avoiding abnormal combustion. The retarding of the ignition timing suppresses the progress of the reaction by suppressing the increase in the temperature and pressure of the unburned mixture. However, retarding the ignition timing leads to a decrease in thermal efficiency and torque, whereas when performing high-pressure retarded injection, the ignition timing can be advanced by an amount that avoids abnormal combustion by devising the form of fuel injection. Since it is possible, thermal efficiency and torque are improved. That is, the high pressure retarded injection not only avoids abnormal combustion, but also makes it possible to advance the ignition timing by the amount that can be avoided, which is advantageous in improving fuel consumption.

  As described above, the high pressure retarded injection in the SI mode can shorten the injection period, the mixture formation period, and the combustion period, but the high pressure retarded injection performed in the CI mode region (3) It is possible to shorten the injection period and the mixture formation period. In other words, the turbulence in the cylinder 18 is increased by injecting the fuel into the cylinder 18 at a high fuel pressure, so that the mixing performance of the atomized fuel is increased and the fuel is injected at a late timing near the compression top dead center. However, a relatively homogeneous air-fuel mixture can be quickly formed.

  In the high pressure retarded injection in the CI mode, fuel is injected at a late timing near the compression top dead center in a relatively high load region, for example, while preventing premature ignition during the compression stroke period, as described above. Since a substantially homogeneous air-fuel mixture is quickly formed, it is possible to reliably perform compression ignition after the compression top dead center. Thus, in the expansion stroke period in which the pressure in the cylinder 18 gradually decreases due to motoring, the compression ignition combustion is performed, so that the combustion becomes slow, and the pressure increase in the cylinder 18 due to the compression ignition combustion (dP / It is avoided that dt) becomes steep. Thus, as a result of eliminating the NVH restriction, the CI mode region is expanded to the high load side.

  Returning to the description of the SI mode, as described above, the high pressure retarded injection in the SI mode shortens the reaction time of the unburned mixture by performing the fuel injection within the retard period. In the low speed range where the engine 1 has a relatively low rotational speed, the actual time for the crank angle change is long, which is effective. On the other hand, in the high speed range where the engine 1 has a relatively high rotational speed, the actual Not very effective due to short time. On the contrary, in the retard injection, since the fuel injection timing is set near the compression top dead center, in-cylinder gas not containing fuel, in other words, air having a high specific heat ratio is compressed in the compression stroke. As a result, in the high speed region, the compression end temperature in the cylinder 18 becomes high, and this high compression end temperature causes knocking. Therefore, when only the retard injection is performed in the region (5), it may occur that the ignition timing must be retarded to avoid knocking.

  Therefore, as shown in FIG. 4, in the region (5) where the rotational speed is relatively high in the SI mode, as shown in FIG. 5 (d), a part of the fuel to be injected is cylinder 18 within the intake stroke period. And the remaining fuel is injected into the cylinder 18 within the retard period. In the intake stroke injection, it is possible to lower the specific heat ratio of the in-cylinder gas (that is, the air-fuel mixture containing fuel) during the compression stroke, thereby keeping the compression end temperature low. Thus, since the compression end temperature is lowered, knocking can be suppressed, so that the ignition timing can be advanced.

  Further, by performing the high pressure retarded injection, as described above, the turbulence becomes strong in the cylinder 18 (combustion chamber 19) near the compression top dead center, and the combustion period is shortened. This is also advantageous in suppressing knocking, and the ignition timing can be further advanced. Thus, in the region (5), by performing split injection of intake stroke injection and high pressure retarded injection, it is possible to improve thermal efficiency while avoiding abnormal combustion.

  In order to shorten the combustion period in the region (5), a multi-point ignition configuration may be adopted instead of performing high pressure retarded injection. That is, a plurality of ignition plugs are arranged facing the combustion chamber, and in the region (5), the intake stroke injection is executed and each of the plurality of ignition plugs is driven to perform multipoint ignition. By doing so, since the flame spreads from each of the plurality of fire types in the combustion chamber 19, the flame spreads quickly and the combustion period is shortened. As a result, the combustion period is shortened similarly to the case where high pressure retarded injection is employed, which is advantageous for improving the thermal efficiency.

(Control when switching from SI mode to CI mode)
Since spark ignition combustion has lower thermal efficiency than compression ignition combustion, the combustion gas temperature is relatively high.

  On the other hand, in the CI mode in which compression ignition combustion is performed, as described above, the internal EGR gas or the external EGR gas is introduced into the cylinder 18 in order to ensure the ignitability of the compression ignition. The state is raised.

  Immediately after switching from the SI mode where the combustion gas temperature is relatively high to the CI mode, the inside of the cylinder 18 is in a high temperature atmosphere, and exhaust gas having a high temperature generated by the spark ignition combustion is introduced into the cylinder 18. Thus, compression ignition combustion is performed in a state where the temperature state in the cylinder 18 is high. In this case, for example, if fuel is injected into the cylinder 18 at a relatively early time such as during the intake stroke, pre-ignition occurs during the compression stroke period, and the rate of increase in pressure in the cylinder 18 (dP / dt) ) May become steep and large combustion noise may occur.

  Therefore, in the engine 1, various transient controls are performed to avoid premature ignition immediately after switching from the SI mode to the CI mode and to avoid an increase in combustion noise.

  Here, the switching from the SI mode to the CI mode is performed, for example, in the warm operation map shown in FIG. 4 from the high load region in which the load of the engine 1 is in the SI mode to the low load region in which the CI mode is set. This can correspond to a transition. That is, the SI mode is switched to the CI mode as the load on the engine 1 is reduced. In the vicinity of the boundary between the SI mode and the CI mode, the engine 1 may be switched from the SI mode to the CI mode when the engine 1 is at an equal load.

  Also, when the temperature of the engine 1 is cold or semi-warm when the temperature is lower than a predetermined temperature, the compression ignition combustion is not stabilized. Therefore, although not shown, the CI mode is not executed, and the engine 1 is operated in all operating regions. SI mode is executed. On the other hand, as shown in FIG. 4, when the temperature of the engine 1 is warm above a predetermined temperature, the CI mode is executed. Therefore, as the temperature of the engine 1 rises and becomes a warm state, the engine load may be equal and the mode may be switched from the SI mode to the CI mode.

(Initial compression ignition mode using high pressure retard)
FIG. 7 shows a control example in which the pre-ignition immediately after switching of the combustion mode is avoided by executing the compression ignition initial mode using high-pressure retarded injection immediately after switching from the SI mode to the CI mode. ing. Specifically, FIG. 7 shows changes in fuel injection timing and spark ignition timing, changes in in-cylinder pressure, changes in the open state of the intake and exhaust valves, throttle valve opening when switching from the SI mode to the CI mode. An example of the change and the change of the gas state in the cylinder is shown. In FIG. 7, the crank angle (that is, time) advances from the left to the right of the page. In FIG. 7, for easy understanding, it is assumed that the engine 1 is switched from the SI mode to the CI mode at an equal load. In the SI mode, the opening degree of the throttle valve 36 is adjusted without introducing EGR gas. I am doing so.

  First, in the leftmost first cycle in FIG. 7, the operation is performed in the SI mode. Here, fuel injection is performed during the intake stroke period, and spark ignition is performed in the vicinity of the compression top dead center. The air-fuel ratio of the air-fuel mixture is set to the stoichiometric air-fuel ratio (λ≈1), and the throttle valve is throttled so that the amount of fresh air is commensurate with the fuel injection amount (the gas state in the cylinder shown in the lowest stage of FIG. 7) See also). In the first cycle in the SI mode, the VVL 71 of the exhaust valve 22 is off (that is, the internal EGR gas is not introduced).

  The subsequent second cycle corresponds to the cycle immediately before switching from the SI mode to the CI mode. Here, in preparation for switching to the CI mode in which the throttle opening is set to fully open, the throttle valve is opened in the fully open direction to increase the gas amount (fresh air amount) in the cylinder 18. The stoichiometric air-fuel ratio is maintained by injecting a fuel amount corresponding to the increased fresh air amount. The fuel injection here is the above-described high-pressure retarded injection, in which fuel injection is performed near the compression top dead center, and spark ignition is executed after the compression top dead center. Thus, the equal load (equal torque) is maintained by delaying the spark ignition timing. Further, the valve opening timing of the intake valve 21 is delayed from the first cycle in preparation for switching to the CI mode in which the VVL 71 is turned on.

  The third cycle corresponds to a cycle immediately after switching from the SI mode to the CI mode, and corresponds to the compression ignition initial mode described above. In the CI mode (more precisely, the compression ignition initial mode), the exhaust gas is opened twice by turning on the VVL 71, so that a part of the burned gas generated by the spark ignition combustion in the second cycle is Introduced in. Since this burned gas is caused by spark ignition combustion, its temperature becomes relatively high. Thus, as a result of the introduction of the hot burned gas, the temperature state in the cylinder 18 becomes relatively high.

  The high pressure retarded injection is used also in the compression ignition initial mode of the third cycle. That is, fuel injection is performed within the period from the late stage of the compression stroke to the early stage of the expansion stroke. By delaying the fuel injection timing in this way, pre-ignition during the compression stroke period is avoided even if the temperature state in the cylinder 18 is relatively high.

  As described above, the high-pressure retarded injection forms a relatively homogeneous mixture at an early stage, and the temperature in the cylinder 18 is relatively high. After the compression top dead center, it is surely ignited by compression and burns stably. In this way, it is avoided that combustion noise increases immediately after switching from the SI mode to the CI mode.

  The combustion gas temperature of compression ignition combustion becomes relatively low. In the CI mode, the air / fuel ratio A / F of the air-fuel mixture is set to the stoichiometric air / fuel ratio, while the EGR gas is introduced, so the working gas / fuel ratio G / F becomes lean. Therefore, the combustion gas temperature is further lowered.

  The fourth cycle corresponds to the end of the compression ignition initial mode, and as in the third cycle, the VVL 71 is turned on and a part of the burned gas generated by the compression ignition combustion in the third cycle is introduced into the cylinder 18. However, as described above, since the burned gas has a relatively low temperature, the temperature state in the cylinder 18 becomes low. Thereby, since premature ignition is avoided, the fuel injection timing is set during the intake stroke period. Thereby, a strong intake air flow and a long air-fuel mixture formation period combine to form a homogeneous air-fuel mixture, and the homogeneous air-fuel mixture is compressed and ignited in the vicinity of compression top dead center.

  FIG. 7 illustrates an example of control when the engine 1 is switched from the SI mode to the CI mode at an equal load. However, in the compression ignition initial mode described above, the engine 1 is operated from a high load to a low load as described above. This is particularly effective when the mode is switched from the SI mode to the CI mode as the load decreases. This is because when the load of the engine 1 decreases from a high load to a low load, the combustion gas temperature can be further increased because the fuel injection amount is relatively large in the high load region in the SI mode. In this case, immediately after switching from the SI mode to the CI mode, the temperature state in the cylinder 18 is further increased, and premature ignition is likely to occur. However, as described above, in the compression ignition initial mode, it is possible to reliably avoid pre-ignition during the compression stroke period by the high pressure retarded injection, so that the combustion noise increases when the mode is switched due to a decrease in engine load. This is definitely avoided.

  As a specific example of combustion mode switching accompanying a decrease in engine load, in the warm-time operation map shown in FIG. 4, from the region (4) or region (5) in the SI mode to the region (2 in the CI mode) ) Or transition to region (1). That is, the compression ignition initial mode may be executed immediately after shifting to the region (2) or the region (1). After completion of the compression ignition initial mode, the fuel injection timing is advanced so that the fuel injection timing set in the region (2) or the region (1) (that is, within the period from the intake stroke to the middle of the compression stroke) is reached. Just horn.

  Here, since the region (3) in the CI mode is a region where the retarded injection is performed as described above, the compression ignition is performed when the region (4) or the region (5) is shifted to the region (3). Execution of the initial mode may be omitted. Further, immediately after the transition from the region (4) or the region (5) to the region (3), the fuel injection timing set in the region (3) (that is, the injection timing within the retard period) Further, a compression ignition initial mode in which fuel is injected at a retarded timing may be executed. The injection timing at this time may be within the retard period, or may be a period further retarded than the retard period. Then, after the compression ignition initial mode ends, the injection timing may be advanced so that the fuel injection timing is within the retard period set in the region (3).

  Note that, as described above, FIG. 7 also shows a case where the engine load is switched from the SI mode to the CI mode at an equal load as the temperature of the engine 1 rises and changes from the cold state to the warm state. Such a compression ignition initial mode is effective in avoiding an increase in combustion noise.

  In the control example of FIG. 7, the compression ignition initial mode is performed for only one cycle, but the compression ignition initial mode may be performed for several cycles. In the control example of FIG. 7, in the SI mode, the opening degree of the throttle valve 36 is adjusted without introducing the EGR gas. However, EGR gas may be introduced.

(Transient combustion mode that lowers the combustion gas temperature immediately before switching the combustion mode)
FIG. 8 shows the switching of the combustion mode by executing the transient combustion mode in which the combustion gas temperature of the spark ignition combustion is lowered by leaning the fuel air-fuel ratio A / F immediately before switching from the SI mode to the CI mode. The control example which avoided the premature ignition immediately after is shown.

  Specifically, FIG. 8 shows a change in the fuel injection timing and the spark ignition timing, a change in the in-cylinder pressure, a change in the open state of the intake and exhaust valves, at the time of switching from the SI mode to the CI mode, as in FIG. FIG. 8 shows an example of a change in the opening of the throttle valve and a change in the gas state in the cylinder. In FIG. 8, for easy understanding, the engine 1 is switched from the SI mode to the CI mode at an equal load. In the SI mode, the opening degree of the throttle valve 36 is adjusted without introducing EGR gas.

  The leftmost first cycle in FIG. 8 is the same as the first cycle in FIG. That is, the operation is performed in the SI mode. Here, fuel injection is performed during the intake stroke period, and spark ignition is performed near the compression top dead center. The air-fuel ratio of the air-fuel mixture is set to the stoichiometric air-fuel ratio (λ≈1), and the throttle valve is throttled so that the new air amount is commensurate with the fuel injection amount. In the first cycle in the SI mode, the VVL 71 of the exhaust valve 22 is off (that is, the internal EGR gas is not introduced).

  The subsequent second cycle corresponds to the cycle immediately before switching from the SI mode to the CI mode. Here, with the fuel amount kept the same as in the first cycle, the throttle valve is opened in the fully open direction to increase the gas amount (fresh air amount) in the cylinder 18. As a result, the air-fuel ratio A / F in the air-fuel mixture in the cylinder 18 becomes lean, and at the same time, the working gas fuel ratio G / F also becomes lean (see also the gas state in the cylinder shown in the lowermost stage in FIG. 8). The fuel injection is the above-described high-pressure retarded injection, in which fuel is injected near the compression top dead center and spark ignition is executed. Thus, by setting A / F (and G / F) to lean, the amount of gas with respect to the amount of fuel increases, so the combustion gas temperature due to spark ignition combustion decreases. Increasing the amount of fresh air introduced at a relatively low temperature is particularly effective in lowering the temperature in the cylinder 18 and lowering the combustion gas temperature. This second cycle corresponds to the transient combustion mode.

  The third cycle corresponds to a cycle immediately after switching from the SI mode to the CI mode, and the exhaust is opened twice by turning on the VVL 71. As a result, a part of the exhaust gas generated by the spark ignition combustion in the second cycle is introduced into the cylinder 18, and the working gas fuel ratio G / F becomes lean. The air fuel ratio A / F is the stoichiometric air fuel ratio (λ≈1). In the third cycle, the internal EGR gas is introduced into the cylinder 18, but as described above, the temperature of the exhaust gas in the second cycle is suppressed to a relatively low temperature, so the temperature state in the cylinder 18 is compared. Lower.

  Thus, in the third cycle, pre-ignition is avoided even if fuel injection is performed during the intake stroke period as previously set in the CI mode, and the homogeneous mixture is compressed and ignited near the compression top dead center. To burn. The fourth cycle is the same as the third cycle.

  In the example of FIG. 8, the NOx purification rate by the three-way catalyst may be reduced by performing spark ignition combustion with the air fuel ratio A / F set to lean in the transient combustion mode. Accordingly, a NOx catalyst (for example, LNT) may be provided.

  Further, during the transient combustion mode corresponding to the second cycle of the control example of FIG. 8, by scavenging the high-temperature residual gas (see the gas state in the cylinder shown in the lowest stage of FIG. 8) in the cylinder 18, While reducing the temperature in the cylinder, the amount of fresh air introduced into the cylinder 18 may be increased by scavenging the residual gas. For the scavenging of the cylinder 18, for example, a dynamic pressure scavenging system that makes the exhaust port have a negative pressure by an ejector effect associated with a high exhaust flow velocity may be employed. Further, a so-called 4-2-1 type exhaust system that applies a negative pressure wave to the exhaust port by exhaust pulsation may be employed.

  Further, in the transient combustion mode corresponding to the second cycle in FIG. 8, the working gas fuel ratio G / F may be set lean by introducing EGR gas into the cylinder 18. The EGR gas introduced in the transient combustion mode is preferably a cooled EGR gas cooled by passing through the EGR cooler 52. This makes the temperature in the cylinder 18 lower, which is advantageous in reducing the combustion gas temperature. Become.

  The transient combustion mode as shown in FIG. 8 is not only applied when the engine 1 is switched from the SI mode to the CI mode at an equal load as shown in FIG. 8, but the engine 1 is changed from a high load to a low load. This is also applicable when switching from the SI mode to the CI mode as the load decreases. It is also effective when the engine 1 is switched from the SI mode to the CI mode as the temperature rises from the cold state to the warm state.

  In the control example of FIG. 8, the transient combustion mode is performed for only one cycle, but the transient combustion mode may be performed for several cycles. Further, in the control example of FIG. 8, EGR gas is not introduced in the SI mode, but EGR gas may be introduced.

  Further, the control example of FIG. 8 and the control example of FIG. 7 can be combined. That is, the compression combustion initial mode using high-pressure retarded injection may be executed in the third cycle in the control example of FIG. 8, that is, immediately after switching to the CI mode. The combination of the transient combustion mode and the compression combustion initial mode may be effective for more reliably avoiding an increase in combustion noise.

(Transient combustion mode that reduces the EGR rate immediately after switching the combustion mode)
FIG. 9 shows that the temperature state in the cylinder 18 immediately after the switching of the combustion mode is lowered by executing the transient combustion mode that temporarily decreases the EGR rate immediately after switching from the SI mode to the CI mode. A control example is shown in which ignition is avoided.

  Specifically, FIG. 9 similarly to FIG. 7, changes in the fuel injection timing and spark ignition timing, changes in the in-cylinder pressure, changes in the open state of the intake and exhaust valves, when switching from the SI mode to the CI mode, FIG. 9 shows an example of a change in the opening of the throttle valve and a change in the gas state in the cylinder. Also in FIG. 9, for the sake of easy understanding, the engine 1 is switched from the SI mode to the CI mode at an equal load. In the SI mode, the opening degree of the throttle valve 36 is adjusted without introducing EGR gas.

  The leftmost first cycle in FIG. 9 is the same as the first cycle in FIG. That is, the operation is performed in the SI mode. Here, fuel injection is performed during the intake stroke period, and spark ignition is performed near the compression top dead center. The air-fuel ratio of the air-fuel mixture is set to the stoichiometric air-fuel ratio (λ≈1), and the throttle valve is throttled so that the new air amount is commensurate with the fuel injection amount. In the first cycle in the SI mode, the VVL 71 of the exhaust valve 22 is off (that is, the internal EGR gas is not introduced).

  The subsequent second cycle corresponds to the cycle immediately before switching from the SI mode to the CI mode. This cycle is also the same as the second cycle in FIG. That is, the throttle valve is opened in the fully open direction, and the gas amount (fresh air amount) in the cylinder 18 is increased. The stoichiometric air-fuel ratio is maintained by injecting a fuel amount corresponding to the increased fresh air amount. The fuel injection here is the high-pressure retarded injection described above, and spark ignition is performed after compression top dead center. By thus delaying the ignition timing, the equal load (equal torque) is maintained. Note that, by delaying the combustion period, the temperature of the exhaust gas (burned gas) increases. Further, the opening timing of the intake valve 21 is retarded from the first cycle.

  The third cycle corresponds to the cycle immediately after switching from the SI mode to the CI mode, and corresponds to the transient combustion mode. Specifically, in spite of switching to the CI mode in which the VVL 71 is turned on, in the transient combustion mode, the VVL 71 is kept off and the exhaust is not opened twice. As a result, the mode switching becomes smooth and the high-temperature burned gas generated by the spark ignition combustion in the second cycle is not introduced into the cylinder 18. As a result, the temperature state in the cylinder 18 becomes relatively low. Note that the internal EGR gas amount may be reduced from the internal EGR gas amount set in the CI mode while the VVL 71 is turned on.

  In the third cycle, fuel injection is performed during the intake stroke period as set in advance in the CI mode. However, since the temperature in the cylinder 18 is relatively low, pre-ignition is avoided, and the homogeneous mixture is Compression ignition occurs near the compression top dead center and combustion occurs. As described above, the temperature of the burned gas in the second cycle increases, and as a result, the temperature of the residual gas in the cylinder 18 in the third cycle increases. As a result, the exhaust gas is not opened twice, but the temperature in the cylinder 18 increases. Too low is avoided. Therefore, it is possible to reliably ignite the homogeneous air-fuel mixture in the vicinity of the compression top dead center and stably burn it. The combustion in the third cycle is compression ignition combustion and the working gas fuel ratio G / F is lean, so the combustion gas temperature becomes low.

  The fourth cycle corresponds to after the end of the transient combustion mode, which is the same as the fourth cycle in FIG. That is, as set in the normal CI mode, the VVL 71 is turned on and a part of the burned gas generated by the compression ignition combustion in the third cycle is introduced into the cylinder 18. Since the temperature is relatively low, the temperature state in the cylinder 18 is low. Thereby, the fuel injection timing is set during the intake stroke period, but premature ignition is avoided. The homogeneous air-fuel mixture is ignited by compression near the compression top dead center.

  The transient combustion mode is not only applied when the engine 1 is switched from the SI mode to the CI mode at an equal load as shown in FIG. 8, but the load of the engine 1 is reduced from a high load to a low load. Accordingly, it is effective even when the SI mode is switched to the CI mode. It is also effective when the engine 1 is switched from the SI mode to the CI mode as the temperature rises from the cold state to the warm state.

  In the control example of FIG. 9, the transient combustion mode is performed for only one cycle, but the transient combustion mode may be performed for several cycles. Further, in the control example of FIG. 9, in the SI mode, the opening degree of the throttle valve 36 is adjusted without introducing the EGR gas, but EGR gas may be introduced.

  Furthermore, the control example of FIG. 9 and the control example of FIG. 7 can be combined. That is, the compression combustion initial mode using the high pressure retarded injection may be executed together with the transient combustion mode immediately after the third cycle in the control example of FIG. 9, that is, immediately after switching to the CI mode. The combination of the transient combustion mode and the compression combustion initial mode may be effective for more reliably avoiding an increase in combustion noise.

  The technique disclosed here is not limited to application to the engine configuration described above. For example, fuel may be injected into the intake port 16 through a port injector provided separately in the intake port 16 instead of the injector 67 provided in the cylinder 18 during the intake stroke period.

  The engine 1 is not limited to an in-line 4-cylinder engine, and may be applied to an in-line 3-cylinder, in-line 2-cylinder, in-line 6-cylinder engine, or the like. Further, the present invention can be applied to various engines such as a V type 6 cylinder, a V type 8 cylinder, and a horizontally opposed 4 cylinder.

  Further, in the above description, the air-fuel ratio of the air-fuel mixture is set to the stoichiometric air-fuel ratio (λ≈1) in the predetermined operation region, but the air-fuel ratio of the air-fuel mixture may be set to lean. However, setting the air-fuel ratio to the stoichiometric air-fuel ratio has the advantage that a three-way catalyst can be used.

  The operation region shown in FIG. 4 is an example, and various operation regions other than this can be provided.

  Further, the high-pressure retarded injection may be divided injection as necessary, and similarly, the intake stroke injection may also be divided injection as necessary. In these divided injections, fuel may be injected in each of the intake stroke and the compression stroke.

1 Engine (Engine body)
10 PCM (controller)
18 cylinder 25 spark plug 50 EGR passage (exhaust gas recirculation means)
51 Main passage (exhaust gas recirculation means)
511 EGR valve (exhaust gas recirculation means)
52 EGR cooler (exhaust gas recirculation means)
53 EGR cooler bypass passage (exhaust gas recirculation means)
531 EGR cooler bypass valve (exhaust gas recirculation means)
62 Fuel supply system (fuel pressure setting mechanism)
67 Injector (fuel injection valve)
71 VVL (exhaust gas recirculation means)

Claims (3)

An engine body having a cylinder;
A spark plug disposed facing the cylinder and configured to ignite an air-fuel mixture in the cylinder;
Exhaust gas recirculation means configured to introduce exhaust gas into the cylinder;
A controller configured to operate the engine body by controlling at least the spark plug and the exhaust gas recirculation means, and
The controller performs compression ignition combustion in which the air-fuel mixture in the cylinder is combusted by self-ignition and operates the engine body, and ignites the air-fuel mixture in the cylinder by driving the spark plug. A spark ignition mode in which the engine body is operated by performing spark ignition combustion, and at least in the compression ignition mode, the exhaust gas is introduced into the cylinder through control of the exhaust gas recirculation means. Introduced,
When the controller switches from the spark ignition mode to the compression ignition mode, the controller performs compression after performing the transient combustion mode in which the combustion gas temperature is lower than the combustion gas temperature during the spark ignition combustion before the mode switching. Switch to ignition mode,
In the transient combustion mode, the controller increases the amount of fresh air to change the working gas fuel ratio G / F or the air fuel ratio A / F to the working gas fuel at the time of spark ignition combustion before mode switching. Set leaner than ratio G / F or air fuel ratio A / F,
In the transient combustion mode, the controller prohibits the introduction of the exhaust gas into the cylinder through the control of the exhaust gas recirculation means, and switches to the compression ignition mode, and then controls the exhaust gas recirculation means. A spark ignition engine for introducing the exhaust gas into the cylinder . The spark ignition engine according to claim 1 ,
The exhaust gas recirculation means is configured to be able to introduce the cooled exhaust gas into the cylinder,
The controller is a spark ignition engine that sets the working gas fuel ratio G / F to lean by increasing the amount of cooled exhaust gas introduced into the cylinder during the transient combustion mode. The spark ignition engine according to claim 1 or 2 ,
The controller operates the engine body in the compression ignition mode when the operating state of the engine body is in a predetermined low load region, and the operating state of the engine body is lower than the predetermined low load region. When in a high load region with a high load, operate the engine body in the spark ignition mode,
The controller is also a spark ignition type engine that executes the transient combustion mode when the load of the engine main body decreases and shifts from the high load region to the low load region.

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