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Most automobile manufacturers have developed hybrid vehicles that combine an internal combustion engine and an electric motor, fusing the advantages of these two power sources. For example, Toyota, in its Prius II, uses a highly efficient gasoline engine based on a modified Atkinson cycle featuring a variable valve timing management. This implementation of the Atkinson cycle is not the optimal solution because some of the air is first sucked from the intake manifold into the cylinder and subsequently returned. This oscillating air stream considerably reduces the thermal conversion efficiency of this cycle. This paper analyzes in detail the loss of thermal conversion efficiency of an internal combustion engine -especially for modified Seiliger and Atkinson cycles - and a proposal is made for the improvement of aspirated and supercharged engines.

Most recent implementations of the Atkinson cycle are not ideal from the point of view of thermal conversion efficiency ( TCE ). For example, Toyota has put a gasoline engine in its Prius II which should achieve high efficiency by using a modified Atkinson cycle based on variable intake valve timing management. Firstly, this implementation of the Atkinson cycle is not the ideal solution because some of the air is first sucked from the intake manifold into the cylinder and subsequently returned back there. Consequently, the oscillating air stream reduces the thermal conversion efficiency of this cycle to a considerable extent. Secondly, this implementation of the Atkinson cycle only reaches low levels of indicated mean pressure ( IMEP ) and, thirdly, it is not suitable for part load engine operating points ( EOP ) because of the lower TCE.