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Hybrid electric vehicles (HEVs) are considered as the most commercial prospects new energy vehicles. Most HEVs have adopted Atkinson cycle engine as the main drive power. Atkinson cycle engine uses late intake valve closing (LIVC) to reduce pumping losses and compression work in part load operation. It can transform more heat energy to mechanical energy, improve engine thermal efficiency and decrease fuel consumption. In this paper, the investigations of Atkinson cycle converted from conventional Otto cycle gasoline engine have been carried out. First of all, high geometry compression ratio (CR) has been optimized through piston redesign from 10.5 to 13 in order to overcome the intrinsic drawback of Atkinson cycle in that combustion performance deteriorates due to the decline in the effective CR. Then, both intake and exhaust cam profile have been redesigned to meet the requirements of Atkinson cycle engine.

In order to improve the torque of engine full load characteristics, especially for the engine torque at high speed, the genetic algorithm combining with the weighted sum method is adopted to optimize the performance of a high-speed gasoline engine. Firstly the simulation model is built by software GT-power. The simulated values are contrasted with the tested values at the same operating condition. The results show the correspondence of the calculated values with tested values. So it proves that the simulation model is reliable. According to the importance of torque at each speed, a corresponding weight coefficient is got. Using the weighted sum method to construct an evaluation function, the multi-objective optimization problem is transformed into a single one.