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. For these reasons, this implementation of the Atkinson cycle is only suitable for hybrid vehicles, where the engine - because it is not directly linked mechanically to the wheels - works only in its best EOP. This paper analyzes the losses in TCE of internal combustion engine ( ICE ) - especially for the Atkinson cycles - in detail, and a proposal is made for their reduction for aspirated and especially for high pressure supercharged engines.