TECHNICAL FIELD

The present invention relates to a control apparatus for a compression-ignition type engine capable of carrying out partial compression ignition combustion to subject some of air-fuel mixture to SI combustion by spark ignition and to subject the rest of the air-fuel mixture to CI combustion by self-ignition.

BACKGROUND ART

Recently, attention has been paid to HCCI combustion. In the HCCI combustion, gasoline fuel that is mixed with air is sufficiently compressed in a combustion chamber and is burned by self-ignition. The HCCI combustion is a mode of combustion in which air-fuel mixture is simultaneously and frequently burned. Thus, a combustion velocity of the air-fuel mixture is higher in the HCCI combustion than in spark ignition combustion (SI combustion) that is adopted in a normal gasoline engine, and it is said that the HCCI combustion is extremely advantageous over the SI combustion in terms of thermal efficiency. However, the HCCI combustion has a problem that combustion initiation timing (timing at which the air-fuel mixture is self-ignited) significantly fluctuates due to an external factor such as a temperature, and also has a problem that it is difficult to control the HCCI combustion during transient operation that causes an abrupt change in an amount of a load.

For the above reason, it has been proposed to burn some of the air-fuel mixture by the spark ignition using an ignition plug instead of burning the entire air-fuel mixture by self-ignition. That is, some of the air-fuel mixture is forcibly burned by flame propagation with the spark ignition as a start (SI combustion), and the rest of the air-fuel mixture is burned by the self-ignition (CI combustion). Hereinafter, such combustion will be referred to as partial compression ignition combustion.

As an example of an engine for which a similar concept to the partial compression ignition combustion is adopted, an engine disclosed in Patent Literature 1 has been known. The engine disclosed in Patent Literature 1 subjects stratified air-fuel mixture, which is produced around the ignition plug (a spark plug) by auxiliary fuel injection, to flame propagation combustion by the spark ignition, and carries out main fuel injection to the combustion chamber at a high temperature due to action of the combustion (flame), so as to burn the fuel, which is injected by this main fuel injection, by the self-ignition.

Meanwhile, it has also been proposed to improve the thermal efficiency of the engine by another method not using compression ignition combustion. For example, Patent Literature 2 discloses a spark-ignition engine that carries out the spark ignition by the ignition plug twice per cycle. More specifically, in the spark-ignition engine disclosed in the same literature, preceding ignition is carried out to supply such low ignition energy that not the entire air-fuel mixture in the combustion chamber is ignited or burned (that a fire is locally generated) during a compression stroke, and at appropriate timing after this preceding ignition, main ignition is carried out to supply the higher ignition energy than that in the preceding ignition. Just as described, by producing the fire by the preceding ignition at an earlier stage than the main ignition, it is possible to prevent misfire of the air-fuel mixture and to increase the combustion velocity.

Here, in the partial compression ignition combustion, the combustion velocity of the CI combustion has an impact on the thermal efficiency. Since the CI combustion is a phenomenon in which a spontaneous chemical reaction of a fuel component occurs, it can be said that the combustion velocity thereof is higher by nature than that of the SI combustion in which a combustion region is gradually expanded by the flame propagation. Meanwhile, it is considered that, if a property of the fuel can be modified to show high reactivity prior to the CI combustion, for example, the combustion velocity of the CI combustion is further increased, the thermal efficiency is thereby further improved, and thus fuel consumption performance and torque performance can be balanced.

The property of the fuel can possibly be modified to have the high reactivity when a temperature of the air-fuel mixture is increased to fall within a specified temperature range, for example. That is, when the fuel component (hydrocarbons) is cleaved due to the increase in the temperature of the air-fuel mixture, an intermediate product including OH radicals with the high reactivity is produced. As means for increasing the temperature of the air-fuel mixture to modify the property of the fuel (to produce the intermediate product), the inventors of the present application considered to carry out plural times of the spark ignition, that is, to carry out the auxiliary preceding ignition prior to the main ignition so as to increase the temperature of the air-fuel mixture as in the case of Patent Literature 2 described above, for example. However, it was understood from the study by the inventors of the present application that, in the case where the preceding ignition was carried out at such timing that some of the air-fuel mixture was burned as in Patent Literature 2, a significant amount of the intermediate product was consumed by such combustion, which prevented an effect of increasing the combustion velocity of the CI combustion from being sufficiently exerted.

CITATION LIST

Patent Literature

Patent Literature 1: JP-A-2009-08778

Patent Literature 2: Japanese Patent No. 4691373

SUMMARY OF INVENTION

The present invention has been made in view of circumstances as described above and therefore has a purpose of providing a control apparatus for a compression-ignition type engine capable of carrying out partial compression ignition combustion at a high combustion velocity and with superior thermal efficiency.

In order to solve the problem, the present invention is a control apparatus for controlling a compression-ignition type engine that includes: a cylinder; an injector that injects fuel into the cylinder; and an ignition plug that ignites air-fuel mixture, in which fuel injected from injector and air are mixed, and that can carry out partial compression ignition combustion to subject some of the air-fuel mixture to SI combustion by spark ignition using the ignition plug and to subject the rest of the air-fuel mixture to CI combustion by self-ignition. The control apparatus includes: an injection control section that controls fuel injection operation by the injector; and an ignition control section that controls ignition operation by the ignition plug. When the partial compression ignition combustion is carried out, the ignition control section causes the ignition plug to carry out: main ignition in which a spark is generated in a late period of a compression stroke or an initial period of an expansion stroke to initiate the SI combustion; and preceding ignition in which the spark is generated at earlier timing than the main ignition. When the partial compression ignition combustion is carried out, the injection control section causes the injector to inject the fuel at such timing that the fuel exists in the cylinder at an earlier time point than the preceding ignition. Timing of the preceding ignition is set to be more retarded when an engine speed is high than when the engine speed is low.

With this configuration, it is possible to carry out partial compression ignition combustion at a high combustion velocity and with superior thermal efficiency.

