Internal combustion engines, in common with all heat engines, derive their capacity for work from a cycle of operations which comprises the supply of heat at a high temperature followed by the rejection of heat at a much lower temperature. The difference between the two quantities of heat represents the maximum amount of energy which can be converted into useful work. In the familiar piston-type internal combustion engine, the heat supply is maintained by burning a suitable fuel in air, and heat is rejected largely in the exhaust gases. With heat engines in general, it is true that the higher the temperature of the heat supply, the greater the efficiency of the engine. There are, however, limitations to the temperature at which an engine can operate. These are imposed by the properties of the materials used in its construction, and by the necessity of maintaining satisfactory lubrication in all circumstances.