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As a method for reducing exhaust emissions from diesel engines, we have experimented on a homogeneous charge diesel combustion technique (HCDC) whereby a portion of fuel is supplied into the intake port to form a homogeneous premixture, this is then fed into the cylinder from the intake port before ignition of the diesel fuel, which is injected directly into the cylinder. Our results have indicated possibilities of substantially reducing both NOx and smoke emissions. If diesel fuel is premixed with air, the premixture under-goes excessively early self-ignition, making it difficult to maintain ignition timing near top dead center and hence limiting the engine operating conditions. While an important target in emission reduction is to realize stable low-emission combustion during a high-load operation, the actual operation of diesel engines mostly involves partial-load conditions.

Under heavy load condition, single fuel operation with diesel fuel was studied experimentally for the homogeneous charge diesel combustion (HCDC) method. HCDC engine, in which pre-mixture was formed by fuel injected into an intake manifold and mixed with air beforehand then ignited by small amount of fuel directly injected into a cylinder, can reduce NOx and smoke simultaneously from the diesel engine. In HCDC the higher the premixed fuel ratio was, the lower the emissions were. Accordingly, it was indicated that homogeneous pre-mixture contributed to improvement of exhaust emissions. However, a diesel knocking due to uncontrolled self-ignition may occur under high premixed fuel ratio conditions in the case of operating heavy loads. Thus, the maximum amount of premixed fuel was restricted by these knocking limits.

An experimental and a numerical analysis wereconducted based on the concept of homogeneous charge diesel combustion (HCDC), in which most of the fuel is supplied for pre-mixed homogeneous charge which is compressed in the cylinder and then ignited by small amount of diesel fuel directly injected into a cylinder. At the previous report, It was indicated that simultaneous improvement of NOx and smoke were possible. Especially under a certain condition, NOx was extremely reduced. This report describes the preliminary analysis for the cause of this emission improvement with HCDC method. As result, direct optical observation of the combustion phenomena and numerical analysis using KIVA2 code suggested that low NOx combustion may be caused by lowered combustion temperature and reduced combustion period due to the uniform lean combustion.