22 POWER AND THE PLOW 



of one pound of pure water from 62 to 63 F. The amount of 

 heat units in fuel or food can be determined by exploding a 

 given quantity in a bomb-like vessel known as the calorimeter, 

 and noting the rise in temperature of a measured quality of 

 water surrounding the vessel. The calorimeter derives its 

 name from the French heat unit, the calorie. This represents 

 the amount of heat to raise one kilogram of water (2.2.pounds) 

 one degree on the Centigrade scale, or 1.8 degrees Fahrenheit. 



By careful experiments, it has been found that one B.t.u. is 

 equivalent to 778 foot-pounds of work. In other words, the 

 energy required to raise one pound of water one degree Fahren- 

 heit in temperature is equivalent to the energy required to lift 

 a weight of 778 pounds one foot vertically. One calorie, then, 

 is equivalent to 3.97 B.t.u., or 3090 foot-pounds of work. Since 

 the energy delivered by an engine and the energy in the fuel 

 may thus be reduced to a common basis, it is possible to de- 

 termine the proportion of the heat that has been recovered 

 in work. 



For example: Let us assume that an internal-combustion 

 engine delivers 1 h.p. at the flywheel for the period of one hour 

 for every pint of kerosene consumed. By previous calculations 

 we have found that a pound of this fuel contains 20,000 B.t.u. 

 and that the fuel weighs 6.7 pounds per gallon or .8375 pound 

 per pint. Then for each h.p.-hr. of work recovered we will 

 supply to the engine .8875 x 20,000, or 16,750 B.t.u., equiv- 

 alent to 13,031,500 foot-pounds of energy. In one h.p.-hr. 

 there are 33,000 x 60 = 1,980,000 foot-pounds of work. The 

 ratio of foot-pounds of fuel energy supplied to foot-pounds of 

 work recovered is, therefore, 1:0.152, which is equivalent to a 

 thermal efficiency of 15.2 per cent. The thermal efficiency of 

 an internal-combustion engine usually ranges from 12 to 25 per 

 cent., though an efficiency of 37 per cent, has been realized. 

 Steam engines in large central plants, even of a compound 

 condensing type, seldom reach a thermal efficiency higher than 

 12 per cent. A steam traction engine under the best conditions 



