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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.