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. Consequently, diesel emissions can be drastically reduced if HCDC enables low-emission combustion, even in the partial-load operation range.In this study, therefore, we attempted to improve combustion and reduce emissions in the low- and medium-load operation range by means of DI fuel injection timing control and EGR. For the full load conditions, we tried to optimize HCDC by using diesel fuel with an oxygen-containing compound having a high anti-knock property.Our results indicated that NOx emissions can be substantially reduced, without increasing smoke emissions, under half-throttle and low premixed fuel ratio conditions by retarding (optimizing) the DI fuel injection timing. This NOx reduction effect was equivalent to that obtainable under a high premixed fuel ratio condition. We also found that NOx emissions can be reduced without increasing smoke emissions through optimization of premixed fuel ratio and EGR rate.Furthermore, we were able to confirm the reduction of both NOx and smoke emissions when HCDC was made effective over the entire engine operation ranges through mixing of MTBE into diesel fuel. Additional reductions in NOx and smoke emissions were achieved through retarding of DI fuel injection timing.