汽车发动机缸内热力循环是与热力学第二定律紧密相联的,而能量的相互转换时具有明显的方向性,如机械能可全部转化为热能,理论上转换效率近百分之百。这类可无限转换的能量称为,机械能全部为。因而,习惯上将“有用功”作为“可无限转换能量”的同义。但是,反方向的热能转换为机械能却不可能全部转换,转换能力受到热力学第二定律的制约。所以,从技术性和经济性角度,前者质量更高,更为宝贵。本文主要对照汽车发动机的几种循环情况的研究。 The thermodynamic cycle of a car engine is closely linked with the second law of thermodynamics, and the energy conversion has obvious directionality. For example, all mechanical energy can be converted into heat energy, and the conversion efficiency is theoretically nearly 100%. This type of energy that can be infinitely converted is called  and its mechanical energy is . Thus, it is customary to use “usefulness ” as the synonym for “infinitely convertible energy ”. However, it is impossible to convert all the energy in the opposite direction to the mechanical energy, and the conversion ability is restricted by the second law of thermodynamics. So, technically and economically, the former is of higher quality and more valuable. In this paper, the main control of the car engine several cycles, the situation of research.