Patent Description:
A pre-chamber internal combustion engine including a spark plug in a pre-chamber disposed on an upper part of a combustion chamber has an advantage in that combustion energy is efficiently converted into pressure, and the thermal efficiency of the engine improves by causing a fuel-air mixture in a main combustion chamber to be rapidly combusted by flames ejected to the main combustion chamber from a communication hole by a configuration of igniting the fuel-air mixture flowing into the pre-chamber through the communication hole in the pre-chamber by a compression stroke. However, there has been a problem in that the combustion state becomes unstable in a specific situation.

For example, the following is disclosed in Patent Literature <NUM>. In a pre-chamber internal combustion engine, in order to deal with a problem of a fuel-air mixture which is not sufficiently mixed flowing into a pre-chamber in an excessively lean state, and causing the combustion to be unstable when fuel injection is performed immediately before an ignition timing so as to suppress preignition, ignition is performed by a spark plug in the pre-chamber when the load of the engine is less than a predetermined value, and fuel is injected at a timing later than a predetermined timing and ignition is performed by a spark plug in a main combustion chamber when the engine load is equal to or greater than the predetermined value. Furthermore, examples of methods and systems for adjusting multiplexed ignition signals in an engine system based on engine operating conditions can be found in Patent Literature <NUM>. Examples of methods and systems for a pre-chamber ignition system can be found in Patent Literature <NUM>. An example of engine system comprising a main spark plug and a sub-spark plug can be found in Patent Literature <NUM>.

However, the combustion state of the pre-chamber internal combustion engine does not always become unstable in a high load region. Instead, it tends to be more difficult to cause the fuel-air mixture to flow into the pre-chamber in a low load region in which the intake airflow rate decreases. On the other hand, there has been a problem in that the state of the fuel-air mixture flowing into the pre-chamber changes, and the combustion state becomes unstable when the immediately preceding operating point and operational state rapidly change due to the fluctuation of the load, the introduction of an EGR, and the like.

The present invention has been made in view of the points of the related art described above, and an object thereof is to prevent the combustion state from becoming unstable when an operating point fluctuates in a pre-chamber internal combustion engine.

To solve the problems described above, the present invention defines a control device for an internal combustion engine according to claim <NUM>.

By the configuration described above, the control device for an internal combustion engine according to the present invention has an advantage in that the fuel-air mixture of the main combustion chamber can be rapidly combusted by flames ejected to the main combustion chamber from the injection hole, and the thermal efficiency of the engine can be improved by executing the first combustion mode that performs ignition in the pre-chamber in the steady state, and the combustion state can be prevented from becoming unstable after a change in the operating point and the transition by transitioning to the second combustion mode that performs ignition in the combustion chamber when there is a change in the operating point and returning to the first combustion mode after the third combustion mode that performs ignition by both of the first ignition means in the pre-chamber and the second ignition means in the combustion chamber after the operating point transition is completed.

Hereinafter, an embodiment of the present invention will be described in detail and with reference to the drawings.

<FIG> shows a basic configuration of an internal combustion engine in which the present invention is implemented. In <FIG>, the internal combustion engine is a piston reciprocating internal combustion engine including a piston <NUM> that is housed in a cylinder <NUM> in a reciprocally slidable manner, the piston <NUM> is coupled to a crack shaft <NUM> via a connecting rod <NUM>, and a reciprocating linear motion of the piston <NUM> is converted into a rotational motion of the crankshaft <NUM>.

A cylinder head that defines a combustion chamber <NUM> with a crown surface <NUM> of the piston <NUM> is disposed on an upper part of a cylinder block configuring the cylinder <NUM>. A cap <NUM> that defines a pre-chamber <NUM> is disposed on an upper center part of the combustion chamber <NUM>, and a first spark plug <NUM> is provided on an upper part of the cap <NUM>.

The pre-chamber <NUM> has a sufficiently small volume as compared to the combustion chamber <NUM>, and communicates with the combustion chamber <NUM> through a plurality of injection holes <NUM> penetrating the cap <NUM>. As shown in <FIG>, the injection holes <NUM> (orifices) are diagonally arranged with an inclination angle with respect to a central axis of the cap <NUM>, and are radially provided in a plurality of locations, for example, six locations in an equiangular arrangement around the central axis.

Furthermore, the cylinder head includes an intake passage <NUM> communicating with the combustion chamber <NUM> at an intake port <NUM> on one side of a ceiling surface, an exhaust passage <NUM> communicating with the combustion chamber <NUM> at an exhaust port <NUM> on the other side of the ceiling surface, an intake valve <NUM> for opening and closing the intake port <NUM>, and an exhaust valve <NUM> for opening and closing the exhaust port <NUM>. The intake valve <NUM> and the exhaust valve <NUM> can individually control respective opening and closing timings thereof by a variable valve mechanism (not shown).

The cylinder head also includes a fuel injection valve <NUM> (injector) for injecting fuel into the intake passage <NUM>. The fuel injection valve <NUM> is diagonally disposed toward the rear surface side of the intake valve <NUM>. Although illustration is omitted, an air cleaner is connected to the upstream side of the intake passage <NUM> via a surge tank, and an airflow sensor, a throttle valve, an intake air temperature sensor, an intake airflow rate sensor, an intake air pressure sensor, and the like are disposed on the upstream side of the intake passage <NUM>.

On the other hand, an O<NUM> sensor <NUM> (or an air-fuel ratio sensor) that detects the oxygen concentration in exhaust gas and an exhaust gas purification device <NUM> that removes harmful substances contained in the exhaust gas are disposed on the downstream side of the exhaust passage <NUM>. Although illustration is omitted, an EGR passage that circulates a part of the exhaust gas from the exhaust passage <NUM> to the intake passage <NUM> is provided, and an EGR cooler and an EGR valve are disposed in the EGR passage, thereby configuring an EGR system. Pumping loss can be reduced by introducing exhaust gas into the intake passage at a partial-load region, and cooling loss can be reduced by reducing the combustion temperature.

As described above, when the immediately preceding operating point and operational state rapidly change due to the fluctuation of the load, the introduction of an EGR, and the like, the state of the fuel-air mixture flowing into the pre-chamber changes, and the combustion state becomes unstable. In particular, in a port-injection pre-chamber internal combustion engine including the fuel injection valve <NUM> in the intake passage <NUM>, the mixture of the fuel-air mixture is satisfactory. However, when the immediately preceding operating point and operational state change, the fuel-air mixture state (the ratio of air, fuel, and residual gas), the temperature, the pressure in the pre-chamber rapidly change. Thus, there may be cases in which the fuel-air mixture state in the pre-chamber is not stable and the combustion becomes unstable even when compatible values are provided for fuel and ignition in advance.

Thus, the internal combustion engine according to the embodiment of the present invention includes a second spark plug <NUM> (auxiliary spark plug) facing the combustion chamber <NUM> besides the first spark plug <NUM> in the pre-chamber <NUM>. Furthermore, as described below, ignition is performed in the pre-chamber <NUM> by the first spark plug <NUM> in a steady state (first combustion mode), and ignition is performed in the combustion chamber <NUM> by the second spark plug <NUM> when the operating point is changed (in an operating point transition period) (second combustion mode). As a result, a case in which the combustion state is unstable can be avoided.

As shown in <FIG>, the second spark plug <NUM> is preferably disposed on the intake port <NUM> side of the ceiling surface of the combustion chamber <NUM> (a ceiling surface peripheral part between two intake ports), but also may be arranged on the other side of the ceiling surface, for example, a ceiling surface peripheral part between the intake port <NUM> and the exhaust port <NUM>.

The internal combustion engine includes a crank angle sensor <NUM> that detects a crank angle and an engine speed from a timing rotor of the crankshaft <NUM>, a cam angle sensor that distinguishes cylinders, a throttle position sensor, a water temperature sensor that detects the temperature of cooling water, an ambient temperature sensor, an oil temperature sensor, a knocking sensor, and the like and more suitably includes an in-cylinder pressure sensor (combustion pressure sensor) that detects the combustion state of the engine. The detection value of each of the sensors is input to an ECU <NUM>.

The ECU (engine control device) <NUM> is an electronic control unit (computer) for optimizing the operational state of the engine on the basis of the detection value of each sensor, and includes a CPU that performs arithmetic processing, a ROM that stores therein a control program, setting data, and the like, a RAM that reads out the control program and the setting data and stores therein dynamic data and arithmetic processing results, an input/output I/F, and the like. The ECU <NUM> performs calculation on the basis of a deviation between a target value based on a driver (accelerator position) and/or output requests (a torque request and an acceleration/deceleration request) from the system and an actual value based on the engine state (the speed, the intake volume, and the air/fuel ratio) and state values such as the intake air temperature and the intake air pressure, determines control instruction values (the throttle position, the fuel injection amount, the ignition timing, and the EGR amount), and optimizes the operational state of the engine.

Then, the ECU <NUM> determines a change in the operating point of the engine in accordance with the deviation between the target value and the actual value in parallel with the output request (output decrease request) from the driver,.

As a standard for determining a change in the operating point of the engine, for example, the deviation of the actual engine speed from the target engine speed is monitored, and it is determined that there is an operating point changing request when the deviation of the actual engine speed from the target engine speed exceeds a predetermined value (for example, <NUM> rpm).

The control of the combustion mode in accordance with a change in the operating point is described below with reference to <FIG>.

The ECU <NUM> basically executes the first combustion mode that performs ignition in the pre-chamber <NUM> by the first spark plug <NUM> in the steady state in which the deviation between the output request (output decrease request) from the driver and the actual value is less than a predetermined value.

In the first combustion mode, the fuel injected into the intake passage <NUM> by an intake stroke or an exhaust stroke is mixed with air flowing into the cylinder <NUM> by the intake stroke and becomes a fuel-air mixture. The fuel-air mixture flowing into the pre-chamber <NUM> through the injection holes <NUM> by a compression stroke is ignited in the pre-chamber <NUM> by the first spark plug <NUM>. The fuel-air mixture in the combustion chamber <NUM> is rapidly combusted by flames radially ejected into the combustion chamber <NUM> from the injection holes <NUM> at high speed, and the operational state of the engine is optimized in a state with excellent thermal efficiency.

Transition is made to the second combustion mode that performs ignition in the combustion chamber <NUM> by the second spark plug <NUM> when it is determined that there is an operating point changing request in accordance with an output request (output decrease request) from the driver with which the deviation between the output request (output decrease request) and the actual value becomes equal to or greater than a predetermined value at time t1 in <FIG>, for example, during operation in the first combustion mode.

In the second combustion mode, the fuel-air mixture in the combustion chamber <NUM> is directly ignited by the second spark plug <NUM> instead of the first spark plug <NUM>. As a result, although the thermal efficiency of the engine is relatively reduced, the combustion state can be prevented from becoming unstable even when the state of the fuel-air mixture in the combustion chamber <NUM> changes due to a fluctuation in the intake airflow rate or the EGR amount.

In the second combustion mode that performs ignition by the second spark plug <NUM>, the combustion speed is lower and the peak of a heat generation rate is moved to the retarded side as compared to the first combustion mode. Thus, it is preferred that an ignition timing of the second spark plug <NUM> in the second combustion mode be advanced with respect to an ignition timing by the first spark plug <NUM> in the first combustion mode.

Then, combustion stability determination <NUM> of the engine is performed when the engine reaches the target operating point and the deviation between the output request (output decrease request) from the driver and the actual value becomes less than the predetermined value at time t2 in <FIG>. The combustion stability determination is evaluated when the rotational fluctuation during a determination period is within a predetermined value, for example, <NUM> rpm. Alternatively, evaluation may be performed when a combustion fluctuation rate (COV of IMEP) obtained by the detection value of the in-cylinder pressure sensor is within a predetermined value, for example, <NUM>%. A period that performs the combustion stability determination is not particularly limited, but it is about several cycles to several dozens of cycles.

Then, transition is made to the third combustion mode that performs ignition by both of the first spark plug <NUM> and the second spark plug <NUM> when the combustion stability determination <NUM> is completed and it is determined that the combustion in the second combustion mode is in a stable state at time t3 in <FIG>. In other words, the ignition by the second spark plug <NUM> in the second combustion mode is not immediately stopped, and the ignition by the first spark plug <NUM> is started in parallel with the ignition by the second spark plug <NUM>. As a result, the combustion state of the engine can be prevented from becoming unstable even when the ignition in the pre-chamber <NUM> by the first spark plug <NUM> does not immediately become stable.

In the third combustion mode, there is a need to retard the ignition timing of the second spark plug <NUM> as compared to the ignition timing of the first spark plug <NUM>, and to cause the second spark plug <NUM> to be ignited in accordance with a timing at which flames are ejected from the injection holes <NUM> in the ignition by the first spark plug <NUM>, in other words, prevent the second spark plug <NUM> from being ignited before flames are ejected by the ignition by the first spark plug <NUM>.

The third combustion mode is a transitional combustion mode. Therefore, when combustion stability determination <NUM> of the engine is performed at the same time as the transition to the third combustion mode and it is determined that the combustion in the third combustion mode is in a stable state at time t4 in <FIG>, the second spark plug <NUM> is stopped and transition is made to the first combustion mode that performs ignition in the pre-chamber <NUM> by the first spark plug <NUM>.

At this time, it is preferred to gradually reduce the discharge energy of the second spark plug <NUM> and not immediately stop the second spark plug <NUM>. Combustion stability determination <NUM> of the engine is performed at the same time as the transition to the first combustion mode. When the determination period elapses while the stable state is maintained, the state returns to the steady state at time t5 in <FIG>.

A case in which the combustion state becomes stable within a predetermined period after the transition is made to each combustion mode has been described in the embodiment described above, but control as follows is executed when the combustion state does not become stable after the transition to each combustion mode.

For example, as shown in <FIG>, when it is determined that the combustion state is unstable at time t3' after transition is made from the second combustion mode to the third combustion mode at the time t3, the third combustion mode is not continued in that state, and the mode returns to the second combustion mode that performs ignition with only the second spark plug <NUM> in the combustion chamber <NUM> by stopping the first spark plug <NUM> of the pre-chamber <NUM>.

The combustion stability determination is continued even after the mode returns to the second combustion mode. When it is determined that the combustion is in a stable state in the second combustion mode at time t4', transition is made to the third combustion mode that performs ignition by both of the first spark plug <NUM> and the second spark plug <NUM>. Then, when it is determined that the combustion is in a stable state in the third combustion mode at time t5', the second spark plug <NUM> is stopped, and transition is made to the first combustion mode that performs ignition in the pre-chamber <NUM> by the first spark plug <NUM>.

As shown in <FIG>, when it is determined that it is unstable by the combustion stability determination <NUM> at time t4' after the second spark plug <NUM> is stopped (the discharge energy is gradually reduced) and transition is made from the second combustion mode to the first combustion mode at the time t4, the mode returns to the third combustion mode that performs ignition by both of the first spark plug <NUM> in the pre-chamber <NUM> and the second spark plug <NUM> in the combustion chamber <NUM>. Then, in a state in which it is determined that the combustion in the third combustion mode is in a stable state at time t5', the second spark plug <NUM> is stopped (the discharge energy is gradually reduced), and transition is made to the first combustion mode.

The period of the combustion stability determination may be the same for the combustion stability determinations <NUM> to <NUM> in each mode, but the determination period of the combustion stability determination <NUM> in the third combustion mode may be longer than the determination period of the combustion stability determination <NUM> in the second combustion mode, and the determination period of the combustion stability determination <NUM> when transition is made to the first combustion mode may be longer than the determination period of the combustion stability determination <NUM> in the third combustion mode. In other words, it becomes advantageous in terms of reducing the risk of combustion fluctuation and balancing the responsiveness and stabilization of the combustion by causing the period of stability determination to be longer for transitions to the combustion mode that becomes unstable more easily.

As described above, the control device for an internal combustion engine according to the present invention:.

By control of transitioning to the second combustion mode (evacuation mode) and returning to the first combustion mode (pre-chamber combustion mode, steady state) via the third combustion mode (transition mode) when the operating point changes as described above, there is an advantage in that a reduction in thermal efficiency as the entire operation period can be kept to a minimum, and a knocking reduction effect and an improvement in thermal efficiency in the first combustion mode (pre-chamber combustion mode) can be obtained by avoiding a case in which the combustion state becomes unstable even when temporary reduction in thermal efficiency is caused.

Claim 1:
A control device (<NUM>) for an internal combustion engine, the internal combustion engine comprising:
a combustion chamber (<NUM>) defined between a piston (<NUM>) and a cylinder head;
a pre-chamber (<NUM>) communicating with an upper center part of the combustion chamber (<NUM>) via an injection hole (<NUM>);
a fuel injection valve (<NUM>) that injects fuel into an intake passage (<NUM>) leading to the combustion chamber (<NUM>);
first ignition means (<NUM>) that faces an inside of the pre-chamber (<NUM>); and
second ignition means (<NUM>) that faces the combustion chamber (<NUM>),
characterized in that the control device (<NUM>) is configured to perform:
a first combustion mode that performs ignition in the pre-chamber (<NUM>) by only the first ignition means (<NUM>) in a steady state in which a deviation between a target value for controlling the engine and an actual value is equal to or less than a predetermined value;
transition to a second combustion mode that performs ignition in the combustion chamber (<NUM>) by the second ignition means (<NUM>) and stops the first ignition means (<NUM>) when a change in an operating point is determined as the deviation exceeds the predetermined value during the first combustion mode;
transition to a third combustion mode that performs ignition by both of the first ignition means (<NUM>) and the second ignition means (<NUM>) when transition of the operating point is completed during the second combustion mode, and it is determined that combustion of the second combustion mode is stable; and
transition to the first combustion mode when it is determined that combustion of the third combustion mode is stable.