JP2008184968A - Control device of gasoline engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain even the deterioration in exhaust emission, while securing combustion stability in transition, when switching an operation mode of an engine 1 between a self-ignition mode (HCCI combustion) and a spark ignition mode (SI combustion). <P>SOLUTION: When switching the mode, fuel is injected by a direct injection injector 18 in a negative overlap period of intake-exhaust valves 11 and 12, and an activated air-fuel mixture having high ignitability is formed. The fuel is injected by a port injector 19 in an intake stroke, and is supplied in a cylinder 2 together with intake air, and a substantially uniform premixed gas is formed. A small quantity of fuel is injected by the direct injection injector 18 in a compression stroke, and a stratified air-fuel mixture is formed around a spark plug 16, and self-ignition of the premixed air-fuel mixture is induced (assisted) by burning by igniting this premixed gas. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a gasoline engine in which a premixed gas in a cylinder is compressed and burned by self-ignition, and more particularly, in a low load and low rotation side operation region where self-ignition hardly occurs, the exhaust stroke or intake of the cylinder. This relates to a structure in which a negative overlap period for closing both the intake and exhaust valves is provided in the stroke to increase the temperature in the cylinder.

  In recent years, a new combustion mode has been proposed in which premixed gas in a cylinder is compressed and burned by self-ignition in order to further improve fuel efficiency and clean exhaust of gasoline engines. In general, premixed compression ignition combustion is proposed. (Hereinafter also referred to as HCCI combustion). In this new combustion mode, unlike conventional combustion by spark ignition (hereinafter also referred to as SI combustion), the premixed gas self-ignites all at once in a number of locations in the cylinder and starts combustion. Becomes extremely high.

  In addition, even in the case of an ultra-lean premixed gas that cannot realize conventional SI combustion or a premixed gas diluted with a large amount of EGR, self-ignition is performed if the temperature in the cylinder compressed by the piston becomes higher than a predetermined value. Thus, although the combustion period itself is short, it does not become vigorous combustion, so that the generation of nitrogen oxides is remarkably reduced.

  However, when the engine is in a relatively low load and low speed operation region, the temperature of the premixed gas may not rise to the self-ignition temperature even near the compression top dead center (TDC). In the gasoline engine described in Patent Document 1, a negative overlap period is provided in which both the intake valve and the exhaust valve are closed from the exhaust stroke to the intake stroke of the cylinder to leave a large amount of burned gas (hereinafter also referred to as internal EGR). Therefore, the temperature in the cylinder is increased.

  On the other hand, in the so-called high-load, high-speed operation region, the premixed gas self-ignites at a premature timing, for example, around the middle of the compression stroke (so-called knocking), and HCCI combustion cannot be realized. In the engine, SI combustion is performed in an operating region on a high load or high rotation side. That is, a uniform premixed gas having a substantially stoichiometric air-fuel ratio is formed in the cylinder, and this is ignited and burned by propagation of the flame surface.

  By the way, when SI combustion is performed at a high load or high rotation side, a large amount of internal EGR gas should not be present as in HCCI combustion. Therefore, when switching between HCCI combustion and SI combustion, an intake / exhaust valve is required. The amount of internal EGR gas must be changed drastically by changing the overlap state of the engine, and during the switching, the internal EGR gas is insufficient for HCCI combustion, while the internal EGR gas is large for SI combustion. It will be too much.

With this point, in the engine of the conventional example (Patent Document 1), when switching between HCCI combustion and SI combustion, the fuel is injected only in the compression stroke of the cylinder to ignite the stratified mixture around the spark plug. In other words, so-called stratified combustion is performed. In this way, even when the amount of internal EGR gas is excessive, it is easy to ensure ignition stability, and the amount of fuel injection is relatively small, so that knocking does not easily occur.
JP 2001-152919 A

  However, when stratified combustion is performed transiently at the time of switching of combustion as in the conventional example, the amount of nitrogen oxide produced is temporarily reduced when a slightly rich mixture around the spark plug is ignited and burned. From the viewpoint of exhaust emission, there is still room for improvement.

  In view of the above, an object of the present invention is to prevent combustion instability and knocking during a switching transition in a gasoline engine that switches between HCCI combustion with a large amount of internal EGR gas and SI combustion with a small amount of it. While preventing, it is in suppressing the deterioration of the exhaust emission in that case.

  In order to achieve the above object, in the engine control apparatus according to the present invention, the stratified mixture around the spark plug is ignited and burned transiently when the combustion state is switched, so that the premixed gas in the cylinder is burned. A self-ignition assist mode is provided to induce self-ignition.

  Specifically, in the first aspect of the invention, a negative overlap period for closing both the intake valve and the exhaust valve is provided in the exhaust stroke or the intake stroke of the cylinder so that the burned gas remains, and the air-fuel ratio after the end of the compression stroke. The gasoline engine is operated by switching between a self-ignition mode for self-igniting a lean premixed gas and a spark ignition mode for igniting a premixed gas having a substantially stoichiometric air-fuel ratio in the cylinder. This is intended for control devices.

  The engine is provided with a fuel injection valve that directly injects fuel into the cylinder, and when switching from one of the self-ignition mode and the spark ignition mode to the other, the negative overlap period is provided, After forming a substantially uniform premixed gas in the fuel, fuel is injected by the fuel injection valve in a compression stroke to form a stratified gas mixture that is unevenly distributed around the spark plug, and this stratified gas mixture is ignited. In this way, self-ignition of the premixed gas is induced (the control means at the time of switching transient).

  With the above-described configuration, first, in the self-ignition mode, both the intake valve and the exhaust valve are closed during a predetermined period (negative overlap period) in the exhaust stroke or intake stroke of the cylinder, and a large amount of burned gas is contained in the cylinder. (Internal EGR gas) remains, thereby increasing the temperature in the cylinder. When the premixed gas is compressed as the piston rises during the compression stroke, the temperature further rises and combustion (HCCI combustion) occurs due to self-ignition.

  On the other hand, in the spark ignition mode, the pre-mixed air with a substantially stoichiometric air-fuel ratio in the cylinder is ignited and burned (SI combustion). At this time, the amount of internal EGR gas is greatly reduced compared to the HCCI combustion. The intake efficiency can be increased and high output can be obtained.

  When the self-ignition mode and the spark ignition mode are switched, the internal EGR gas amount is transiently insufficient for HCCI combustion, but is too much for SI combustion. As in the case of, a substantially uniform lean premixture is formed in the cylinder, but in addition to this, a stratified mixture is formed around the spark plug, and this stratified mixture is ignited and burned. Triggers self-ignition of the premixed gas (self-ignition assist mode).

  In other words, the combustion of the stratified mixture around the spark plug assists the self-ignition of the premixed mixture, and can prevent problems such as instability of combustion, and thus the necessary engine output can be obtained by the self-ignition combustion. Therefore, the amount of fuel for forming the stratified mixture is much smaller than that of the conventional example (Patent Document 1), and the amount of nitrogen oxides generated by the combustion is also reduced. Deterioration is suppressed.

  From the viewpoint of suppressing the generation of such nitrogen oxides, the control at the time of switching transition as described above should be as short as possible. For example, according to the operating state of the engine when the operating mode is switched. Therefore, it is preferable to shorten the period during which the control is performed at the time of switching transition as the load is relatively high or the rotation speed is relatively high.

  In other words, if the operating state is relatively high in load and rotation speed, the temperature of the cylinder is also high, so the premixed gas is likely to self-ignite, and accordingly, the control period during switching transition is shortened accordingly. This minimizes the generation of nitrogen oxides due to combustion during transients. It should be noted that the period for performing the control during the transition may be set according to the time from the start of mode switching or the number of combustion cycles.

  On the other hand, in the operation state with a relatively low load and rotational speed, the temperature of the cylinder is correspondingly lowered. Therefore, even if assisting by ignition of the stratified mixture as described above, the premixture is stabilized. May not be able to self-ignite. That is, for example, when switching from the self-ignition mode to the spark ignition mode, the amount of internal EGR gas in the cylinder gradually decreases, and accordingly, the premixed gas gradually becomes difficult to self-ignite. During the period when the amount of EGR gas is less than or equal to the predetermined value, so-called stratified combustion may be performed as in the engine described in the conventional example (Patent Document 1).

  In other words, when the engine is in the low-load low-rotation region, when the mode is changed, the amount of residual burned gas in the cylinder is compressed by the fuel injection valve without forming a substantially uniform premixed gas during a period when the amount of residual burned gas in the cylinder is below a predetermined level Preferably, fuel is injected in the stroke, and the stratified mixture around the spark plug is ignited and combusted. (Claim 3) In this way, instability of combustion at the time of switching even in a low load and low speed region Can be prevented from occurring.

  Further, in the engine control device as described above, it is preferable that fuel is directly injected into the cylinder by the fuel injection valve in the negative overlap period to form an activated air-fuel mixture having high ignitability ( In this way, self-ignition can be promoted by the activated air-fuel mixture contained in the pre-air mixture.

  Also, it is preferable to provide a fuel injection valve so as to inject fuel into the intake passage separately from the fuel injection valve that directly injects fuel into the cylinder, thereby forming a substantially uniform premixed gas in the cylinder. The fuel is injected by the other fuel injection valve.

  If the fuel is injected into the intake passage, the fuel can be injected before the intake valve is opened. Therefore, it is easy to secure the time for atomization of combustion compared to the direct injection into the cylinder. . Further, as described above, a small amount of fuel for forming the stratified mixture is preferable, but the fuel injection amount for forming the premixed mixture does not increase in accordance with the output required for the engine. For this reason, it is impossible to perform both using the same fuel injection valve. From this point of view, it is better to provide a plurality of fuel injection valves having different flow characteristics.

  As described above, according to the control device for a gasoline engine according to the present invention, the internal EGR gas amount transiently changes in the HCCI combustion when switching between the self-ignition mode (HCCI combustion) and the spark ignition mode (SI combustion). On the other hand, even if it becomes too much for SI combustion, the stratified mixture around the spark plug is ignited and burned, assisting the self-ignition of the premixed mixture and destabilizing the combustion Etc. can be prevented, and generation of nitrogen oxides at that time can be suppressed, and deterioration of exhaust emission can be suppressed.

  In particular, if the period of control during switching transition is changed according to the operating state of the engine, the generation of nitrogen oxides due to combustion during transition is minimized, and deterioration of exhaust emissions is minimized. it can.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. It should be noted that the following description of the preferred embodiment is merely illustrative in nature, and is not intended to limit the present invention, its application, or its use.

(overall structure)
FIG. 1 shows an overall configuration of an engine control apparatus A according to the present invention, and reference numeral 1 denotes a multi-cylinder gasoline engine mounted on a vehicle. The main body of the engine 1 has a cylinder head 4 disposed on a cylinder block 3 provided with a plurality of cylinders 2, 2,... (Only one is shown), and a piston 5 is fitted in each cylinder 2. A combustion chamber 6 is formed between the top surface of the cylinder head 4 and the bottom surface of the cylinder head 4. The piston 5 is connected to the crankshaft 7 by a connecting rod, and a crank angle sensor 8 for detecting the rotation angle (crank angle) is disposed on one end side of the crankshaft 7.

  An intake port 9 and an exhaust port 10 are formed in the cylinder head 4 so as to open to the ceiling portion of the combustion chamber 6 for each cylinder 2. The intake port 9 extends obliquely upward from the ceiling of the combustion chamber 6 and opens on one side of the cylinder head 4, and the exhaust port 10 opens on the other side opposite to the cylinder head 4. The intake port 9 and the exhaust port 10 are opened and closed by an intake valve 11 and an exhaust valve 12, respectively. These intake and exhaust valves 11 and 12 are cams of a valve mechanism 13 disposed in the cylinder head 4. The shaft (not shown) is driven in synchronism with the rotation of the crankshaft 7.

  The valve mechanism 13 includes a known variable lift mechanism 14 (hereinafter abbreviated as VVL) capable of continuously changing the valve lift amount on the intake side and the exhaust side, and a phase angle of the valve lift with respect to crank rotation. And a known phase variable mechanism 15 (hereinafter abbreviated as VVT) that can be continuously changed are incorporated, and the lift characteristics of the intake and exhaust valves 11 and 12 are changed by their operation to It is possible to adjust the amount of intake air and the amount of residual burnt gas (internal EGR gas). For VVL14, for example, those described in Japanese Patent Application Laid-Open Nos. 2006-329022 and 2006-329023 may be used.

  In addition, a spark plug 16 is disposed with an electrode facing the ceiling of the combustion chamber 6 of each cylinder 2 and is energized by the ignition circuit 17 at a predetermined ignition timing. On the other hand, a direct injection injector 18 that directly injects fuel into the cylinder 2 with its tip facing the peripheral edge of the intake side of the combustion chamber 6 is disposed. The direct injection injector 18 has a small capacity capable of controlling the flow rate with high accuracy when a relatively small amount of fuel is injected. With this, when a small amount of fuel is injected after the middle stage of the compression stroke of the cylinder 2, An air-fuel mixture mass (stratified air-fuel mixture) is formed in the vicinity of the electrode of the spark plug 16.

  In this embodiment, a port injector 19 (another fuel injection valve) is disposed so as to inject fuel facing the intake port 9. The port injector 19 has a large capacity capable of injecting a large amount of fuel corresponding to the maximum torque of the engine 1. By injecting fuel from the compression stroke to the expansion, exhaust and intake stroke of the cylinder 2, A sufficient injection time can be secured even in the rotation range. The fuel spray thus injected flows into the cylinder 2 together with the intake air, and is widely dispersed in the cylinder 2 whose volume increases as the piston 5 descends to form a substantially uniform premixed gas.

  Although not shown, high pressure and low pressure fuel supply lines are connected to the injectors 18 and 19 for each cylinder 2. The low-pressure supply line is supplied with the fuel sucked up from the fuel tank by the low-pressure fuel pump, and the high-pressure supply line branched from the low-pressure supply line is provided with a high-pressure fuel pump for boosting and sending the fuel. Yes.

  In the figure, an intake system is disposed on one side of the cylinder head 4 located on the right side of the engine 1, and an intake passage 20 communicates with the intake port 9 of each cylinder 2. The intake passage 20 is for supplying air filtered by an air cleaner (not shown) to the combustion chamber 6 of each cylinder 2 of the engine 1. An electric throttle valve is provided in the common passage upstream of the surge tank 21. 22 are arranged. The intake passage 20 is branched for each cylinder 2 downstream of the surge tank 21 and communicates with the intake port 9.

  On the other hand, an exhaust system is disposed on the other side of the cylinder head 4, and an exhaust passage 25 (exhaust manifold) branched for each cylinder 2 is connected to the exhaust port 10 of each cylinder 2. A sensor 26 for detecting the oxygen concentration in the exhaust gas is disposed at the collection portion of the exhaust manifold. In addition, a catalyst 27 for purifying harmful components in the exhaust is disposed in the exhaust passage 25 downstream of the exhaust manifold.

  In order to control the operation of the engine 1 configured as described above, a powertrain control module 30 (hereinafter referred to as PCM) is provided. As is well known, this includes a CPU, a memory, an I / O interface circuit, and the like. As shown in FIG. 2, the signals from the crank angle sensor 8, the oxygen concentration sensor 26, etc. are input, and the intake passage 20 A signal from an air flow sensor 31 that measures the flow rate of air in the vehicle, a signal from an accelerator opening sensor 32 that detects an operation amount (accelerator opening) of an accelerator pedal (not shown), and a vehicle speed sensor 33 that detects the traveling speed of the vehicle. And at least a signal from.

  The PCM 30 determines the operating state (for example, the load state and the engine speed) of the engine 1 based on signals from the various sensors, and the VVL 14, the VVT 15, the ignition circuit 17, and the direct injection injector 18 according to this. The port injector 19 and the electric throttle valve 22 are controlled. That is, the PCM 30 adjusts the lift amount of the intake / exhaust valves 11 and 12 mainly by the operation of the VVL 14 to control the amount of intake (air) charged into the cylinder 2 and mainly controls the intake / exhaust valve 11 by the operation of the VVT 15. , 12 are adjusted to control the internal EGR gas amount.

  The lift curves Lin and Lex of the intake and exhaust valves 11 and 12 are continuously changed from the minimum lift to the maximum lift as schematically shown in FIG. 3 by the control of the VVL 14 and VVT 15. The lift amount of the intake / exhaust valves 11 and 12 becomes larger as the load (target torque) and the rotational speed of the engine 1 are higher, and an overlap period (positive overlap period) is generated accordingly. On the other hand, on the relatively low load and low rotation side, a negative overlap period in which both the intake and exhaust valves 11 and 12 are closed occurs, and the amount of internal EGR gas is considerably increased.

  Thus, since the amount of intake air charged into the cylinder 2 can be changed over a wide range mainly by controlling the VVL 14, the engine 1 of this embodiment can control the output regardless of the control of the throttle valve 22. . Therefore, the throttle valve 22 provided in the intake passage 20 is mainly for fail-safe, and is normally fully opened even in the partial load region of the engine 1 to reduce the pumping loss.

  In addition, the PCM 30 switches the air-fuel ratio and the mixture formation state in the cylinder 2 by operating each of the two injectors 18 and 19 at a predetermined timing as will be described later, and mainly uses the VVT 15 as described above. By controlling the internal EGR gas amount in the cylinder 2 by the operation of, and further switching the operation state of the spark plug 16, the combustion state of the engine 1 is switched between HCCI combustion and SI combustion described below.

(Outline of engine control)
Specifically, as shown in an example of the control map in FIG. 4, in the operation region (I) on the relatively low load and low rotation side, the premixed gas formed in the cylinder 2 is not directly ignited. This is compressed by the rise of the piston 5 and self-ignited (self-ignition mode). At this time, basically, in the intake stroke of the cylinder 2, the fuel is injected into the intake port 9 by the port injector 19 and supplied into the cylinder 2 while being mixed with the intake air to form a substantially uniform premixed gas. At this time, the fuel injection amount is controlled so that the air-fuel ratio in the cylinder 2 becomes lean according to the intake charge amount into the cylinder 2 obtained by a signal from the air flow sensor 31 or the like.

  Further, in the exhaust stroke or intake stroke of the cylinder 2, a period from when the exhaust valve 11 is closed to when the intake valve 12 is opened (a negative overlap period in which both the intake and exhaust valves 11 and 12 are closed) is provided, and a large amount of internal EGR is provided. By increasing the temperature in the cylinder 2 with gas, self-ignition of the premixed gas is promoted. If the negative overlap period is relatively long, the amount of internal EGR gas also increases, and the self-ignition timing is advanced.

  Such self-ignition by compression of the premixed gas is conventionally called HCCI (Homogenious Charge Compression Ignition). As schematically shown in FIG. 5, this combustion by HCCI is considered that the premixed gas is self-ignited almost simultaneously at a number of locations in the combustion chamber 6 in the cylinder 2 and starts combustion. Compared with conventional combustion by flame propagation (Spark Ignition: SI combustion), the combustion period is shortened and the thermal efficiency is increased.

  In addition, HCCI combustion in which the premixed gas is self-ignited can be realized even with an ultra-lean premixed gas that is difficult to realize SI combustion or a premixed gas diluted with a large amount of internal EGR gas. As described above, since the combustion temperature is low even if the combustion period is short, the generation of nitrogen oxides is very small. On the other hand, a premixed gas that is not very lean or a premixed gas with a low degree of dilution makes the timing of self-ignition too early, and HCCI combustion cannot be realized.

  That is, HCCI combustion is realized in a premixed gas mixture that is considerably lean or a premixed gas diluted with a large amount of EGR, and originally, a very high output cannot be obtained. As shown in the control map (FIG. 4), SI combustion is performed in the operation region (II) on the relatively high load or high rotation side. That is, fuel is injected into the intake port 9 by the port injector 19 during the compression stroke or intake stroke of the cylinder 2 and supplied into the cylinder 2 while being mixed with intake air to form a substantially uniform premixed gas (spark ignition mode). . At this time, the fuel injection amount is controlled so that the air-fuel ratio in the cylinder 2 becomes substantially the stoichiometric air-fuel ratio.

  Further, the operation region (III) on the lower load and low rotation side than the HCCI region (I) is a region where the idling of the engine 1 and the vicinity thereof are very infrequent, but here as described above. Even if the temperature in the cylinder 2 is increased by a large amount of internal EGR gas, stable self-ignition is difficult. Therefore, in this operating region (III), the pre-mixing of the substantially stoichiometric air-fuel ratio is performed as in the operating region (II). The spark is ignited and SI combustion is performed (spark ignition mode). Hereinafter, the operation region (I) is referred to as HCCI region (I), and the operation regions (II) and (III) are both referred to as SI region (II) and (III).

  In the HCCI region (I), the region on the relatively low load and low rotation side (the region shown with diagonal hatching in FIG. 4) is directly connected during the negative overlap period of the intake and exhaust valves 11 and 12. Fuel injection (first injection) is performed by the injector 18. In this way, the fuel spray exposed to the high temperature internal EGR gas immediately vaporizes, and the chain of molecules breaks to generate radicals or the partial oxidation reaction proceeds to form an activated gas mixture that easily ignites. It is considered.

  By the way, as shown by the white arrow in FIG. 4, when the operating state of the engine 1 changes and shifts between the HCCI region (I) and the SI region (II) (III), In order to switch between HCCI combustion (self-ignition mode) and SI combustion (spark ignition mode), the fuel injection mode, air-fuel ratio, or internal EGR gas amount must be changed. Is gradually changed by the operation of VVL14 and VVT15, and cannot be switched instantaneously as in the injection mode or air-fuel ratio.

  For stable HCCI combustion, a large amount of internal EGR gas having an EGR rate of about 60% or more is required, while SI combustion of uniform premixed gas is performed with an EGR rate of less than about 30%. Therefore, when switching between the two, the engine 1 has a combustion cycle of 2 to 5 cycles, and the EGR rate is transiently about 30 to 60%, that is, stable HCCI combustion is difficult. To do so, the amount of internal EGR gas is too large.

  Therefore, in this embodiment, when the combustion state is switched as described above, the self-ignition of the lean premixture is induced by ignition combustion to the stratified mixture. That is, as shown in an example in FIG. 6, even when the negative overlap period of intake and exhaust changes at the time of combustion switching, fuel injection (first injection) by the direct injection injector 18 during the negative overlap period. ) To form an activated air-fuel mixture that easily ignites.

  Then, fuel is injected by the port injector 19 in the intake stroke (second injection) to form a substantially uniform premixed gas in the cylinder 2, and then a small amount of fuel is injected by the direct injection injector 18 at the end of the compression stroke. (Third injection), an air-fuel mixture mass (stratified air-fuel mixture) that is unevenly distributed in the vicinity of the electrode of the spark plug 16 is formed, and this is ignited and burned at a predetermined timing immediately after the compression top dead center (TDC).

  In this way, self-ignition of the premixed gas is induced by ignition and combustion of the stratified gas mixture, thereby stabilizing the lean premixed gas even when the EGR gas amount in the cylinder 2 is insufficient at the time of switching transition. Can be self-ignited. The amount of the third injection may be a minimum amount necessary to form a stratified mixture capable of spark ignition, and the required engine output is obtained mainly by the self-ignition combustion of the premix. Hereinafter, the engine operation mode that induces (assists) self-ignition of the premixed gas is also referred to as a self-ignition assist mode, and the combustion at that time is distinguished from HCCI combustion and is referred to as SCCI (Stratified Charge Compression Ignition) combustion.

(Specific control procedure)
Next, a specific procedure of engine control will be described based on the flowcharts of FIGS. 7 to 9. First, in the flow of FIG. 7, determination of switching between HCCI combustion (self-ignition mode) and SI combustion (spark ignition mode) is performed. In step SA1 after starting, signals from the crank angle sensor 8, the airflow sensor 31, the accelerator opening sensor 32, the vehicle speed sensor 33, etc. are input. In step SA2, the required torque (load) to the engine 1 is obtained. Obtain the engine speed. That is, the engine rotational speed may be directly calculated by a signal from the crank angle sensor 8, and the required torque is determined based on, for example, the vehicle speed and the accelerator opening or the internal EGR based on the signal from the air flow sensor 31 and the engine rotational speed. What is necessary is just to calculate in consideration of the amount.

  Based on the required torque and the engine rotational speed thus obtained, in step SA3, it is determined whether or not the engine 1 is in the HCCI region (I) with reference to the control map of FIG. If this determination is NO, since it is in the SI region (II) (III), the process proceeds to step SA5, which will be described later. If the determination is YES, the process proceeds to step SA4 to determine whether HCCI combustion is being performed this time. . If this determination is NO, switching from SI combustion to HCCI combustion is in progress, and thus the flow proceeds to the flow of FIG. 8, while if the determination is YES, control for HCCI combustion is continued.

  That is, first, the operation timings of the intake and exhaust valves 11 and 12 are controlled so that a negative overlap period is generated by the control of the VVL 14 and the VVT 15. For example, on the basis of the target torque and the engine rotation speed, the overlap amount of the intake / exhaust valves 11 and 12 that determines the required internal EGR amount is determined with reference to a map set experimentally in advance. What is necessary is just to control VVT15 mainly so that it may become.

  The VVL 14 may be controlled so as to have a lift amount determined by referring to a map set experimentally in advance based on the target torque and the engine speed. In this map, the lift amount of the intake / exhaust valves 11 and 12 is determined and set in advance by experiments or the like so as to have an appropriate air-fuel ratio corresponding to the fuel supply amount to the cylinder 2.

  Thus, a negative overlap period is provided for the operation of the intake and exhaust valves 11 and 12, the temperature in the cylinder 2 is increased by a large amount of internal EGR gas, and fuel is injected by the port injector 19 in the intake stroke. A substantially uniform lean premixed gas is formed inside. Then, after the end of the compression stroke, the premixed gas is ignited without being ignited and burned. On the other hand, if the load is relatively low and the rotation speed is low, fuel is injected by the direct injection injector 18 during the negative overlap period to improve the ignitability of the premixed gas.

  On the other hand, in step SA5, which has been determined to be NO in step SA3 and advanced to step SA4, it is determined whether SI combustion is being performed as in step SA4. If this determination is NO and switching from HCCI combustion to SI combustion is being performed. For example, while proceeding to the flow of FIG. 9, if the determination is YES, SI combustion is in progress, and control for that is continued. That is, fuel is injected into the intake port 9 from the compression stroke to the intake stroke of the cylinder 2 by the port injector 1 to form a uniform air / fuel mixture having a substantially stoichiometric air / fuel ratio in the cylinder 2 and ignited by the spark plug 16.

-Switching to HCCI combustion-
Next, switching from SI combustion to HCCI combustion will be described. First, in step SB1 of the flow of FIG. 8, VVL 14 and VVT 15 are controlled so that the operation timing of intake and exhaust valves 11 and 12 suitable for HCCI combustion is reached. . That is, the operation timing of the intake valve 11 is retarded mainly by the operation of the VVT 15, while the operation timing of the exhaust valve 12 is advanced, the negative overlap period gradually increases, and the amount of internal EGR gas also increases. To do.

  Subsequently, in step SB2, it is determined whether or not stratified combustion is performed from the operating state of the engine 1. In short, this is to determine whether to shift to the HCCI area (I) from the SI area (II) on the high load / high rotation side or from the SI area (III) on the low load / low rotation side. Yes, if the combustion is switched at the relatively high load and high rotation side, the temperature of the cylinder 2 is high, and it is not necessary to perform stratified combustion (determination is NO), so the process proceeds to Step SB7 described later.

  On the other hand, when the combustion is switched at a relatively low load and low rotation side, the temperature of the cylinder 2 is also low, so that the amount of internal EGR gas that gradually increases due to the operation of the VVT 15 or the like is a predetermined amount (eg, EGR rate) In the following period, it is difficult to cause the premixed gas to be stably self-ignited anyway. In this period, the combustion is stabilized by stratified combustion. That is, first, in step SB3, the amount and timing of the third fuel injection in the compression stroke of the cylinder 2 are determined, and if the injection timing comes, the third injection is executed in step SB4. In step SB5, the stratified mixture is ignited and burned by the spark plug 16.

  The amount of the third injection may be determined with reference to a map set experimentally in advance based on, for example, the target torque and the engine speed. The injection timing may be set to an appropriate timing in the latter half of the compression stroke corresponding to the injection amount, the ignition timing, and the like, which is also referred to a map set experimentally in advance based on the target torque and the engine speed. To do so.

  In step SB6, it is determined whether or not the timing for switching from stratified combustion to SCCI combustion has been reached. If this determination is NO, the flow returns to step SB3 to continue stratified combustion. On the other hand, if the internal EGR gas amount is gradually increased by the operation of the VVT 15 or the like and exceeds the predetermined amount, the self-ignition of the pre-mixture can be induced (assist) by the spark ignition of the stratified mixture. (Determination is YES), the process proceeds to step SB6, and the self-ignition assist mode is switched to the following.

  The determination as to whether or not the switching timing has come can be made based on the passage of time from the start of control of VVL14 and VVT15 in step SB1, or the combustion cycle from the start of control of VVL14 and VVT15. You may make it determine by counting a number.

  In step SB7, which proceeds after determining that the switching timing has been reached (YES), the fuel injection amounts for the three times by each of the injectors 18 and 19, that is, the direct injection for forming the activated air-fuel mixture are performed. The first injection amount by the injector 18, the second injection amount by the port injector 19 for forming the premixed gas, and the third injection amount by the direct injector 18 for forming the stratified mixture, respectively. It is determined by reading from an injection amount map set experimentally in advance.

  This injection amount map is also an experiment in which optimum values for the first, second, and third injection amounts are set in advance corresponding to the target torque and rotational speed of the engine 1, and detailed description thereof is omitted. For example, the first and second injection amounts may be increased in accordance with the increase in the target torque, respectively, while the third injection amount is a minimum amount necessary for forming a stratified mixture capable of spark ignition. Is preferred.

  In step SB7, the first, second, and third injection timings (target injection timings) are also determined by reading from an injection timing map that has been experimentally set in advance. For example, the timing of the first injection is such that the valve opening period of the direct injection injector 18 is included in the negative overlap period of the intake and exhaust valves 11 and 12, and the timing of the second injection is from the middle stage of the intake stroke. Set until the is opened. The timing of the third injection is preferably set at the end of the compression stroke.

  In step SB8, at the first, second and third injection timings determined in step SB7, the direct injection injector 18 is set at the first injection timing, and the port injector 19 is set at the second injection timing. Further, the direct injection injector 18 is operated again at the timing of the third injection. In the following step SB9, the ignition circuit 17 is operated at a predetermined ignition timing in the vicinity of TDC (desirably immediately after TDC), the ignition plug 16 is energized, and the air-fuel mixture formed in the vicinity of the ignition plug 16 by the third injection. Ignite and burn (stratified mixture).

  Subsequently, in step SB10, it is determined whether or not the timing for switching from the SCCI combustion to the HCCI combustion is reached. If this determination is NO, the process returns to step SB7 to continue the operation in the self-ignition assist mode. If the amount of EGR gas becomes sufficiently large and the premixed gas is stably ignited even without assistance (determination is YES), the routine proceeds to step SB11, where the above-described control for HCCI combustion is executed. Then return.

  Note that the timing for switching to the HCCI combustion can also be determined based on the passage of time and the number of combustion cycles since the control of the VVL 14 and VVT 15 is started in the step SB1, similarly to the determination in step SB5. Further, with respect to the time lapse and the number of combustion cycles serving as the determination criteria, depending on the operating state of the engine 1 at the time of switching the combustion, the relatively short time (the number of combustion cycles) is shorter on the higher load side or higher rotation side. It is preferable to set.

  This is because the temperature of the cylinder 2 is high when the engine 1 is in a relatively high load and rotation speed, and the premixed gas is likely to self-ignite, so that the amount of internal EGR gas is relatively large. This is because HCCI combustion can be stably realized even in a state where the amount is small. As a result, it is possible to shorten the period during which the transitional control of the combustion switching is performed, and the generation of nitrogen oxides associated with the SCCI combustion and the stratified combustion during the transition is minimized.

  As described above, when switching from SI combustion to HCCI combustion is transient, HCCI combustion has a small amount of internal EGR gas and stratifies when premixed gas cannot be stably ignited automatically. Self-ignition is assisted by spark ignition of the mixture. Further, in the situation where the self-ignition of the premixed gas is difficult even if the assist is performed as described above by switching in the low load and low rotation region, stratified combustion is temporarily performed.

-Switching to SI combustion-
Next, switching from HCCI combustion to SI combustion will be described with reference to the time chart of FIG. 10 based on the flow of FIG. The control procedure in this case is basically the reverse of the switching from SI combustion to HCCI combustion shown in FIG. 8, and a detailed description of the same procedure is omitted.

  First, in step SC1 of the flow of FIG. 9, the VVT 15 is mainly controlled in the same manner as in step SB1 of the flow of FIG. 8, and the intake and exhaust valves 11 and 12 are controlled so that the internal EGR gas amount suitable for SI combustion is obtained. Control the operation timing. As a result, as shown in FIG. 10 (b), the lift curve Lin of the intake valve 11 is displaced toward the advance side from time t0 to time t2, while the lift curve Lex of the exhaust valve 12 is displaced toward the retard side. The negative overlap period gradually decreases, and the internal EGR gas amount gradually decreases as shown in (d) of the figure (in the figure, expressed by the EGR rate).

  In response to the decrease in the internal EGR gas amount, in steps SC2 to SC4, the engine 1 is operated in the self-ignition assist mode in the same procedure as steps SB7 to SB9 in the flow of FIG. As a result, the combustion state becomes SCCI combustion as shown in FIG. At this time, in order to instantaneously switch the air-fuel ratio to the theoretical air-fuel ratio (A / F = 14.7) in consideration of exhaust emissions (see (e) in the figure), the fuel injection amount (mainly the second injection amount) is increased. At the same time (see FIG. (F)), the lift amount of the intake valve 11 is decreased by the operation of the VVL 14 (see FIG. (C)) in order to suppress the fluctuation of the torque due to this.

  Subsequently, in step SC5, it is determined whether or not stratified combustion is performed in the same manner as in step SB2 of the flow of FIG. If it is not necessary to perform stratified combustion at the time of switching the combustion on the high load, high rotation side (determination is NO), the process proceeds to step SC12 described later, while the combustion switching on the low load, low rotation side is performed. If it is necessary to perform stratified combustion (determination is YES), the routine proceeds to step SC6.

  In step SC6, as in step SB6 of the flow of FIG. 8, it is determined whether it is time to switch to stratified combustion. That is, as shown in FIG. 10 (d), the amount of internal EGR gas that gradually decreases is equal to or greater than a predetermined amount (indicated by an asterisk in the figure), and stable self-ignition of the premixed gas is possible with assistance. Uchiha (determination is NO), the process returns to step SC2 to continue SCCI combustion.

  On the other hand, if the internal EGR gas amount is further reduced to be less than the predetermined amount (YES at step SC6 at time t1 in FIG. 10), stable self-ignition cannot be expected even with assistance, so steps SC7-9 Then, the engine 1 is operated in the stratified combustion state in the same manner as Steps SB3 to SB5 in the flow of FIG. That is, the third fuel injection is performed in the compression stroke of the cylinder 2, and this is ignited and burned.

  In the subsequent step SC10, as in step SB10 in the flow of FIG. 8, it is determined whether or not the timing for switching to SI combustion has come. If this determination is NO, the process returns to step SC7 to continue stratified combustion. When the amount of internal EGR gas gradually decreased as in 10 (d) becomes suitable for SI combustion (determination is YES at time t2), the process proceeds to step SC11 to switch to the control for SI combustion described above. Then return.

  Further, in step SC12 which proceeds after determining that stratified combustion is not performed (NO) in step SC5, it is determined whether or not the timing for switching to SI combustion is reached as in step SC10. If the determination is NO, step SC12 is performed. Returning to SC2, SCCI combustion is continued. Then, although not shown in FIG. 10, if YES is determined in step SC12 as the internal EGR gas amount decreases, the process proceeds to step SC11 to switch to control for SI combustion, and then returns.

  Even when switching from HCCI combustion to SI combustion as described above, it is preferable to make the period for performing the transitional transition control as short as possible as in the case of switching from SI combustion to HCCI combustion. For this purpose, for example, depending on the operating state of the engine 1 at the time of switching of combustion, the control at the time of switching transition is started on the relatively high load side or high rotation side (that is, switching from HCCI combustion to SCCI combustion). The EGR gas amount is set to be small, and the time elapsed until the control at the time of transition is finished (that is, switching to SI combustion) and the number of combustion cycles may be set to be small.

  8 and 9, when switching between the self-ignition mode and the spark ignition mode, a negative overlap period of the intake and exhaust valves 11 and 12 is provided, and fuel is injected in the intake stroke by the port injector 19. A substantially uniform lean pre-mixture is formed in the cylinder 2, and then fuel is injected in the compression stroke by the direct injection injector 18 to form a stratified mixture around the spark plug 16, which is ignited and burned. Thus, the switching transient control means for inducing the self-ignition of the premixed gas is configured.

  The switching transition time control means of this embodiment injects fuel by the direct injection injector 18 during the negative overlap period of the intake and exhaust valves 11 and 12 (first injection) so as to improve the ignitability of the premixed gas. I have to.

  Further, the switching transition time control means of this embodiment is such that when the engine 1 moves between the HCCI region (I) and the low load, low rotation side SI region (III), the internal EGR gas amount is transiently changed. In a state where the amount is less than a predetermined amount and stable self-ignition cannot be expected even when assisting, the engine 1 is operated in a stratified combustion state to ensure combustion stability.

  7 to 9 is realized by executing a control program electronically stored in the memory of the PCM 30. In this sense, the PCM 30 itself constitutes the switching transient control means. It can be said that.

  Therefore, according to the engine control apparatus A according to this embodiment, the internal EGR of the cylinder 2 is transiently switched when the operation mode is switched between the self-ignition mode and the spark ignition mode in accordance with a change in the operation state of the engine 1. Even if the amount of gas is insufficient for HCCI combustion but too much for SI combustion, fuel is injected into the high-temperature internal EGR gas to form an activated mixture and stratify around the spark plug 16 A self-ignition assist mode (SCCI combustion) that induces (assist) self-ignition of the premixed gas by forming a combusted gas mixture and igniting it to prevent combustion is prevented. be able to.

  In the self-ignition assist mode, the torque required to maintain the operating state of the engine 1 is obtained mainly by the self-ignition combustion of the premixed gas, and the fuel injection amount for the self-ignition assist can be spark ignition. Therefore, the amount of nitrogen oxides generated by combustion of this fuel is extremely small, and the deterioration of exhaust emissions during switching transients can be sufficiently suppressed. .

  In addition, the period during which the switching transition control as described above is performed is relatively shortened from the high load side to the high rotation side in accordance with the operating state of the engine 1, thereby generating nitrogen oxides by combustion during the transition. Can be minimized.

  On the other hand, in an operation state with a relatively low load and rotational speed, combustion stability may not be ensured even in the self-ignition assist mode transiently at the time of switching. Can be secured.

(Other embodiments)
The configuration of the present invention is not limited to the above-described embodiment, and includes various other configurations. That is, in the above-described embodiment, in the SCCI combustion at the time of switching between the HCCI combustion and the SI combustion, the first injection for forming the activated air-fuel mixture is performed by the direct injection injector 18. Depending on the case, the first injection may be unnecessary.

  In the above-described embodiment, the second injection for forming the premixed gas is performed in the intake stroke. However, since this is due to the port injector 19, the exhaust stroke or the expansion stroke before that is performed. You may make it carry out by a compression process.

  Alternatively, the first injector, the second injector, and the third injector can be performed only by the direct injector 18 without providing the port injector 19 in the engine 1. However, from the viewpoint of suppressing the generation of nitrogen oxides, a small amount of the third injection amount is preferable, but the second injection amount may have to be considerably increased in consideration of the maximum output of the engine 1. Therefore, it is impossible to perform both with a single injector, and it is better to use two injectors 18 and 19 having different flow characteristics as in the above embodiment.

  Furthermore, in the above-described embodiment, the lift characteristics of the intake and exhaust valves 11 and 12 are continuously changed by the operation of the VVL 14 and VVT 15, but not limited to this, either the lift amount or the phase angle is It is good also as a structure which switches in steps. Needless to say, a valve operating mechanism that opens and closes the intake and exhaust valves 11 and 12 individually by an electromagnetic actuator may be used.

  As described above, according to the present invention, in a gasoline engine that switches between HCCI combustion and SI combustion, while ensuring combustion stability at the time of switching transition, it also suppresses deterioration of exhaust emission at that time. It is useful because it can.

It is a figure showing the whole engine control device composition concerning an embodiment of the present invention. It is a block diagram which shows the outline of control. It is explanatory drawing which shows the change of the lift characteristic of an intake / exhaust valve. It is explanatory drawing which shows an example of the control map which switches a combustion state. It is an image figure of HCCI combustion. It is explanatory drawing which shows the aspect of the fuel injection by two injectors. It is a flowchart figure which shows the determination procedure of switching of a combustion state. It is a flowchart figure which shows the determination procedure at the time of the switching transition from SI combustion to HCCI combustion. FIG. 9 is a diagram corresponding to FIG. 8 at the time of switching transition from HCCI combustion to SI combustion. It is a time chart which shows each change of intake / exhaust overlap, intake valve lift, EGR rate, air-fuel ratio, and fuel injection amount at the time of switching transition from HCCI combustion to SI combustion.

Explanation of symbols

A Engine control device 1 Gasoline engine 2 Cylinder 9 Intake port 11 Intake valve 12 Exhaust valve 16 Spark plug 18 Direct injection injector (fuel injection valve)
19 Port injector (another injector)
30 PCM (switching transient control means)

Claims (5)

In the exhaust stroke or intake stroke of the cylinder, a negative overlap period is provided to close both the intake valve and the exhaust valve so that the burned gas remains, and the air-fuel ratio lean premixed gas is self-ignited after the end of the compression stroke. A control device for a gasoline engine that is operated by switching between a self-ignition mode to be performed and a spark ignition mode for igniting a pre-air-fuel mixture having a substantially stoichiometric air-fuel ratio in the cylinder,
The engine is provided with a fuel injection valve that directly injects fuel into the cylinder,
When switching from one of the self-ignition mode and the spark ignition mode to the other, the negative overlap period is provided, and a compression stroke is generated by the fuel injection valve after a substantially uniform premixed gas is formed in the cylinder. In this case, the fuel is injected to form a stratified mixture that is unevenly distributed around the spark plug, and the stratified mixture is ignited and burned, thereby providing a switching transient control means for inducing self-ignition of the premixed mixture. A control device for a gasoline engine, comprising: The control device according to claim 1,
The switching transition time control means is characterized in that the period during which the switching transition control is performed is shortened relatively toward the higher load side or higher rotation side according to the operating state of the engine at the time of mode switching. Gasoline engine control device. In the control device according to claim 1 or 2,
When the engine is in a low-load low-rotation region, the switching transition time control means performs fuel switching without forming a substantially uniform premixed gas during a period in which the amount of residual burned gas in the cylinder is below a predetermined level when switching the mode. A control apparatus for a gasoline engine, wherein fuel is injected by a compression stroke by an injection valve, and a stratified mixture around an ignition plug is ignited and burned. The control device according to any one of claims 1 to 3,
The switching transition time control means directly injects fuel into a cylinder by a fuel injection valve during a negative overlap period to form an activated mixture with high ignitability. Control device. The control device according to any one of claims 1 to 4,
Another fuel injection valve is provided to inject fuel into the intake passage,
The control device for a gasoline engine, wherein the switching transition time control means is configured to inject fuel by the separate fuel injection valve to form a substantially uniform premixed gas in the cylinder.

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