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This study has experimentally clarified the effects the pilot injection fuel quantity and pilot injection timing have on the mode of combustion of the pilot spray and exhaust emissions. The result shows that one of the points to effectively reduce exhaust emissions by pilot injection is to reduce the emissions produced by the pilot combustion itself. For that purpose it is effective to advance the pilot injection timing and to increase of pilot quantity. In this case, dividing the pilot injection into multiple small-quantity shots is a solution to avoid cylinder wall wetting.

Investigations of Homogeneous Charge Compression Ignition (HCCI) combustion have been actively conducted as a new combustion technology to substantially and simultaneously reduce NOx and soot to comply with the future stringent exhaust emission regulations. In the past, a method of injecting fuel at the initial stage of the compression stroke has been proposed, but it is known that fuel adheres to the cylinder wall, causing a decline in combustion efficiency and oil dilution. The authors have developed Premixed Compression Ignition (PCI) combustion as a technology of solving the above problem as well as simultaneously reducing NOx and soot. In PCI combustion, fuel is injected into a combustion chamber in the vicinity of the top dead center for preventing fuel from adhering to the wall, and pre-mixture, which is formed shortly before ignition, is burnt.