Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(5) Efficiency and Heat Losses.

Up to the present time we have considered an engine in which there is no heat loss or loss from friction, but in the actual engine such losses are large and tend to materially reduce the values of heat and pressure to be obtained from a fuel with a given calorific content. Applying the rule for heat engines given in a previous section where the efficiency is—

	T – t
E =
	T

We have the theoretical efficiency of a gas engine, neglecting friction, loss to the cylinder walls, and loss through the rejection of heat with the exhaust gas, equal to—

	1960 – 520
E =		= 73.5 percent.
	1960

In substituting the numerical values in the above calculation it was assumed that the temperature of the burning mixture would be 1500° F above zero, and that the exhaust temperature would be as low as 60. Since the calculation is made from absolute zero, which is 460° below the zero marked on our thermometers, the temperature of the burning charge, T = 1500 + 460° = 1960° above absolute zero. Similarly the absolute temperature of the exhaust would be, t = 60 + 460 = 520° absolute. The application of the absolute temperatures will be seen from the calculation for efficiency. The value given, 73.5 per cent, it should be understood is the theoretical efficiency and is at least 20 per cent above the best results obtained in practice. The best record that we have had to date, is that established by a Diesel engine which returned 48.2 per cent of the calorific value of the fuel in the form of mechanical energy. In order that the reader may have some idea of the losses that occur in the engine, and their extent we submit the following table. These are the results of actual tests obtained from different sources and represent engines built for different services and of various capacities:

Losses—Data	Automobile Motor	Stationary Engine	Stationary Engine
Horse-power	30.	200	1000
Heat lost to jacket water	35.8%	31.0%	2970 B.T.U.	Loss at per Horse-power in B.T.U.’s
Heat lost in exhaust	24.6%	30.0%	2835 B.T.U.
Friction loss	8.6%	6.5%	810 B.T.U.
Heat lost by radiation	15.4%	8.2	540 B.T.U.
Heat available as power			2700 B.T.U.
Efficiency (per cent)	15.6%	24.3
Fuel	Gasoline		Producer Gas

The remarkable efficiency of the Diesel engine is due principally to the extremely high compression pressure, which was from 500 to 600 pounds per square inch. When this is compared to the 60 to 70 pounds compression pressure used with automobile engines it is easy to see where the Diesel gains its efficiency. It is evident that as much depends on the manner in which the fuel is used in the engine as on the calorific value of the fuel.