Homogeneous charge compression ignition (HCCI) technology is promising to reduce engine exhaust emissions and fuel consumption. However, it is still confronted with the problem of its narrow operation range that covers only the light and medium loads. Therefore, to expand the operation range of HCCI, mode switching between HCCI combustion and transition SI combustion is necessary, which may bring additional problems to be resolved, including load fluctuation and increasing the complexity of control strategy, etc. In this paper, a continuously adjustable load strategy is proposed for gasoline engines. With the application of the strategy, engine load can be adjusted continuously by the in-cylinder residual gas fraction in the whole operation range.
In this research, hybrid combustion is employed to bridge the gaps between HCCI and traditional SI and thus realize smooth transition between different load points. In order to obtain hybrid combustion, the techniques of external exhaust gas recirculation and residual gas trapping are used. Through varying the total exhaust gas fraction and proportions of internal and external exhaust gas, the different combustion mode, such as HCCI, hybrid combustion and the optimal SI combustion, could be under effective control, while at the same time the NVO strategy (Negative Valve Overlap) used for HCCI combustion mode can be maintained. Engine load is changed monotonously with the total exhaust gas fraction, and high cycle-to-cycle variation due to thermal oscillation can be avoided by reasonably adopting the combination of in-cylinder temperature and total exhaust gas fraction when load transition is carried out. When hybrid combustion is used for load transition, fuel efficiency and NOX emissions are improved.