Combustion noise characteristics of an engine are said to generally correspond to an output, which is a product of torque and speed. That is to say, as shown in FIG. 9, output contours and combustion noise contours of an engine on a speed-torque plane, i.e., constant-output lines Pp and constant-combustion-noise lines Cp correspond to each other within a predetermined range, i.e., virtually overlap with each other with a generally common peak point existing at central sides of the approximately arc-shaped constant-output lines Pp and constant-combustion-noise lines Cp, thus providing substantially similar characteristics.
On the other hand, in a vehicle equipped for example with manual transmission means, an operating locus Dp when an accelerator pedal is depressed without any gear shift during driving moves in such a way that first the torque increases rapidly and then the speed increases, as shown in FIG. 9.
In this case, combustion noise increases rapidly in a short time in an initial stage of the acceleration as surrounded by an ellipse in FIG. 10, which may impair driving comfort depending on the degree of the acceleration.
As a technique to prevent combustion noise, a pilot injection is known, in which prior to a main injection, a trace amount of fuel is preliminarily charged. According to this pilot injection, a trace amount of fuel preliminarily charged first burns to be a pilot burner, so that fuel ignitability in the main injection is improved and an initial combustion in the main injection becomes slow. As a result, an explosive combustion due to an ignition delay is avoided and accordingly the combustion noise of the engine can be suppressed.
On the other hand, since performing a pilot injection in a high-load and high-speed range or the like makes it easier to produce black smoke, it is necessary to stop the pilot injection at any point of time during output increase.
[Patent Document 1] Japanese Patent Application Laid-open No. 2002-206447
However, if the stop of the pilot injection is carried out concurrently in all cylinders, the combustion noise rapidly increases and this brings discomfort to a driver, thus impairing driving comfort. To deal with this problem, Patent Document 1 discloses the following technique: an operating range determined by speed and torque is divided into two ranges, i.e., an operating range requiring a pilot injection and an operating range requiring no pilot injection, and at the transition from the operating range requiring a pilot injection to the operating range requiring no pilot injection, the pilot injection is stopped at a different timing from each other cylinder to prevent a rapid increase of combustion noise.
With the technology described therein, however, since the pilot injection is performed uniformly across a predetermined speed-torque range, the rapid change of combustion noise at a rapid acceleration or the like still occurs, which may impair driving comfort depending on the degree of the acceleration.
It is therefore an object of the present invention to prevent a rapid change of combustion noise in a rapid operation for improving driving comfort.