Although the injection rate increases with the adoption of high-pressure fuel injection, the ignition lag, which is the delay from the start of injection to the beginning of combustion, cannot be shortened to less than a certain period of time. Therefore, the quantity of fuel injected until ignition takes place increases (the initial injection rate is too high), resulting in explosive combustion simultaneous with ignition, and an increase in NOx and sound. To counteract this situation, pilot injection is provided to keep the initial injection at the minimum requirement rate, to dampen the primary explosive combustion, and to reduce NOx and noise.
