Study on Realization of Dual Combustion Cycle by Lean Mixture and Direct Fuel Injection

The purpose of this study is to realize dual-combustion cycle for gasoline engines. For the purpose, lean combustion and direct fuel injection were applied to small diesel engine. The lean gasoline-air mixture was provided and was ignited by small amount of pilot diesel fuel injection (constant volume combustion). Then, diesel fuel was injected by main injection and was burned with the remained oxygen after the lean combustion (diffusion combustion). The equivalence ratio 0.3, 0.4 and 0.5 of mixture were used to avoid the spontaneous compression auto-ignition. The total equivalence ratio with supplied gasoline and diesel fuel was adjusted to 1.0. The base pilot injection timing was selected as the ignition of pre-mixture took place at T.D.C. and pilot injection timings were changed 2 degree before and behind of base timing. The main fuel injection timings were 50, 75 and 100% of the duration between pilot injection timing and T.D.C. The test engine was an air cooled four-stroke single cylinder direct injection diesel engine. The compression ratio was 17.3, and engine performance and exhaust gas emissions were measured at engine speed of 2000 rpm. The pressure of constant volume combustion was rapidly raised for equivalence ratio of 0.5 and was low for equivalence ratio of 0.3, therefore equivalence ratio of 0.4 was preferable in this study. When the constant volume premixed combustion occurred near T.D.C. and main fuel injection was injected after T.D.C., the constant pressure diffusion combustion appeared and high thermal efficacy was obtained. The high THC concentration was detected, because the entire remained air after pre-mixed combustion did not contribute to diffusion combustion, however the smoke concentration was hardly detected. The NOx concentration increased for early pilot and main injection timings due to rapid premixed combustion. However, the pilot and main fuel injection timings hardly influenced engine performance.