TECHNICAL FIELD

The present invention relates to a control device for an engine having a cylinder in which a combustion chamber is formed.

BACKGROUND ART

Conventionally, in the field of an engine, various measures for preventing occurrence of knocking have been taken. Specifically, under conditions where engine loads are high and the temperature in the combustion chamber is high, self-ignition combustion of an air-fuel mixture of fuel and air occurs in the outer periphery of the combustion chamber and the like separately from main combustion, and high pressure waves are generated, thereby causing knocking, that is, vibrations of the cylinders and pistons. Since occurrence of knocking may increase noise and damage the pistons and the like, knocking needs to be prevented.

For example, Patent Literature 1 discloses an engine that retards the ignition timing when knocking occurs.

When the ignition timing is simply retarded to avoid knocking as in the engine in Patent Literature 1, under operating conditions where the basic ignition timing is set on the retarded side of the MBT that is the ignition timing at which the engine torque is maximized, the engine torque is reduced because the ignition timing is retarded. Particularly when the compression ratio of the cylinder is high, the engine torque may be significantly reduced. Specifically, when the compression ratio is high, knocking is likely to occur and combustion occurs steeply, thereby causing increase in combustion noise and the amount of NOx production. Accordingly, when the compression ratio of the cylinder is high, the basic ignition timing (basic ignition timing according to the operating range of and the operating environment state of the engine) is set to a timing considerably retarded from the MBT and the combustion tends to start after the compression top dead center. On the other hand, the amount of change in the piston position with respect to the change in the crank angle at the beginning of the expansion stroke when the compression ratio is high is larger than the amount of change when the compression ratio is low. Accordingly, when the compression ratio is high, even if the ignition timing is retarded by a small amount from the basic ignition timing, the piston position during combustion becomes significantly lower (the amount of separation from the position at the compression top dead center becomes significantly larger), thereby significantly reducing the engine torque and furthermore the fuel efficiency performance.

CITATION LIST

Patent Literature

Patent Literature 1: JP-A-2008-291758

SUMMARY OF INVENTION

An object of the present invention is to provide a control device for an engine that can suppress knocking while ensuring high fuel efficiency performance.

An control device for an engine according to an aspect of the present invention is a control device for an engine having a cylinder in which a combustion chamber is formed, the device including fuel supply means for supplying fuel containing gasoline to the combustion chamber; ignition means for igniting an air-fuel mixture of air and the fuel supplied to the combustion chamber by the fuel supply means; and control means for controlling the ignition means, in which, if knocking occurs when ignition is performed at a reference ignition timing set on a retarded side of the MBT that is an ignition timing at which an engine torque is maximized in a high-load region in which an engine load is larger than a predetermined load, the control means performs ignition advance control that causes the ignition means to perform ignition at a timing on an advanced side of the reference ignition timing.

