It will be seen from the example just given that the heat engine performs mechanical work by dropping the working medium from a high to a low temperature, as it receives the medium at a high temperature from the lamp and rejects it at atmospheric temperature after delivering a small percentage of useful work. This may be compared to a water wheel which receives the working medium (water) at a high pressure and rejects it at a lower pressure. Carrying this comparison still further, it is evident that an increase in the range of the working temperatures (high and low) would increase the output of the heat engine in the same way that an increase in the range of pressures would increase the output of the water wheel. The temperature at which the engine receives the working medium and the temperature at which it is rejected determines the number of heat units that are available for conversion into mechanical energy, and therefore, if the range be increased by either raising the upper limit of temperature or by reducing the lower limit, or by the combined increase and decrease of the limits, the available heat will be increased. Based on the temperature range, the maximum possible efficiency of the heat engine may be expressed by the ratio— =Reception Temperature—Rejection Temperature= E = ——————————————————————————————————————————— =Reception Temperature= This maximum defined by Carnot establishes a limit that can be exceeded by no engine, whatever the construction or working medium. According to the methods adopted in applying the heat of combustion to the working medium, heat engines are divided into two general classes, (1) External combustion engines, (2) Internal combustion engines. The expressions “Internal” or “External” refer to the point at which combustion takes place in regard to the working cylinder, thus an internal combustion engine is one in which the combustion takes place in the working cylinder, and an external combustion engine is one in which the combustion takes place outside of the working cylinder. The steam engine is an example of an external combustion engine, the fuel being burned in the furnace of a boiler which is independent of the engine cylinder proper. As the fuel is burned directly in the cylinder of a gas engine it is commonly known as an internal combustion engine.