Because of the limiting length of the cylinder, and the temperature limits of the lamp it is not possible to expand the working medium and increase the temperature indefinitely, therefore there must be a point where the application of heat must cease and the temperature be reduced in order to bring the gas back to its original volume and the piston to its original position so that the expansion may be repeated. This condition results in a very considerable loss of heat and power in addition to the losses previously mentioned, as the heat taken from the medium to reduce it to its original volume is thrown away as far as the production of power is concerned. To return the piston to its former position without expending energy on the engine, the volume and pressure may be reduced either by allowing the gas to escape to the atmosphere by means of a valve, or by removing the lamp and cooling the air by the application of water, but in any case the heat of the air is lost and the efficiency of the engine reduced.
To increase the efficiency of the engine and reduce the loss just mentioned, nearly all heat engines, either steam or gas, have the working medium at the highest temperature for only a small portion of the stroke, after which no heat is supplied to the cylinder. As the pressure forces the piston forward the volume increases, and as no more heat is supplied, both the pressure and the temperature continue to decrease until the end of the stroke is reached, thus utilizing the greater part of the heat in the expansion. Since the temperature at the end of the stroke is comparatively low, very little heat is rejected when the valve is opened for the return stroke. This loss would be the least when the temperature of the gas at the end of the stroke was equal to the temperature of the surrounding air. With both the internal and external temperatures equal, there would be no difference between the pressure of the gas in the cylinder and that of the surrounding air.
=Fig. 1-a.= Fairbanks-Morse Two Cylinder, Type “R E” Stationary Engine Direct Connected to a Dynamo. ]
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.