MTBE (Methyl Tertiary Butyl Ether) blended diesel fuel, and DME (Dimethyl Ether) with the homogeneous charge compression ignition (HCCI) system are very attractive for NO_x and PM (particulate matters) reductions in DI diesel engines, while their effects on combustion and odorous emissions are not clear. In this study, a few percent MTBE blended in diesel fuel gives a lower HCHO (formaldehyde) and an improved exhaust odor than with neat diesel fuel, while over 10% blend, a very low boiling point MTBE forms an overlean mixture and diminishes the positive effect of the O₂ content of MTBE. The study of DME achieved a 100% DME combustion, and hybrid combustion of DME and diesel fuel with HCCI system. Two peaks in heat release during combustion appear in DME and hybrid-operation under an ultra-lean engine operation. Although the homogeneous DME mixture allows the ultra-lean engine operation, It also gives a very strong exhaust odor together with higher HCHO and THC (total hydrocarbon).