38 ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 
fair market prices given, varies widely, 
lying between 200,000 heat units per dollar 
and 10,000,000 heat units per dollar. It 
is possible to buy eight times as much 
energy for a given amount of money in 
the form of cheap coal as in the form of 
low-priced gasoline, or 25 times as much 
as in the form of high-priced gasoline or 
kerosene. This being true, it might seem 
to a casual observer as rather strange that 
gasoline should be used at all, and the 
fact that it is used in competition with 
fuel of one-eighth to one twenty-fifth 
its cost shows clearly that either the gaso- 
line engine has some characteristics not 
possessed by an engine or plant using 
coal, which makes it able to do things 
the other can not do, or that more of the 
heat it contains can be transformed into 
energy for useful work. Both of these 
things are true. 
Thermal Efficiency 
As was pointed out before, the different 
kinds of machinery used to generate pow- 
er render more or less of the fuel energy 
into useful work; all systems do not give 
equal returns for equal amounts of heat 
supplied. If all the heat energy in fuel 
were transformed into work with no losses 
whatever in the mechanism, the machin- 
ery would be said to have a thermal ef- 
ficiency of 100 per cent, and it would re- 
quire 2,545 heat units per hour to main- 
tain an output of one horsepower. If half 
of the energy in the fuel were lost in the 
machinery, its thermal efficiency would be 
said to be 50 per cent, and there would 
be required 5,090 heat units per hour. If 
only one per cent of the heat energy in 
the fuel were transformed into useful 
work, the efficiency ot the machinery or 
power plant would be said to be one per 
cent and there would be required 254,500 
heat units per hour to maintain one horse- 
power. 
Steam plants in use represent a great 
variety of styles or types, but in general 
it may be said that the more complicated 
and refined the plant the larger its size 
the more efficient it is, because the com- 
plication exists only as evidence of an 
attempt to minimize the losses of heat in 
the macLinery. Similarly the more stead- 
ily the plant works at the output for which 
it was designed the higher the efficiency 
of the plant, and, conversely, the smaller 
the plant the simpler the apparatus, or 
the more intermittently it works, the 
lower its efficiency. Steam-power plants 
are built today to do every conceivable 
sort of work, and range in size from one 
horsepower to 100,000 horsepower. For 
purposes of comparison neither the larg- 
est nor the smallest should be used, nor 
the best performance nor the worst per- 
formance of these plants, but a figure rep- 
resenting a fair average for the conditions 
named should be taken. Large steam 
plants in their daily work seldom use less 
than two pounds of poor coal per hour 
for each useful horsepower (known as a 
brake horsepower), which is equivalent to 
about 25,000 B. T. U. per hour, and which 
corresponds to about 10 per cent thermal 
efficiency. Small steam plants working 
intermittently, such as hoisting engines, 
may use as high as seven pounds of coal 
per brake horsepower, which is equivalent 
to about 100,000 heat units per brake 
horsepower hour, or 2.5 per cent thermal 
efficiency. Some plants will do better than 
the above with proper conditions, and 
some may do worse, but in general it may 
be said that the performances of steam 
plants lie between the limits of 2.5 and 10 
per cent thermal efficiency. 
Plants consisting of gas producers for 
transforming coal into gas for use in gas 
engines have in general a much higher 
thermal efficiency than steam plants do- 
ing the same work. They are, however, 
not built quite so small as steam plants, 
the smallest being about 25 horsepower, 
and in general they have not been built 
so large, the largest being only a few 
thousand horsepower. Their efficiency, 
however, does not vary so much as is the 
case with steam plants. It may be fair to 
say that under the same conditions as 
above outlined these plants will use 144 
to 2 pounds of coal of fair or poor quality 
per brake horsepower hour, which gives 
a thermal efficiency ranging from 18 to 10 
per cent. These plants can be made to do 
much better than this, and perhaps may 
do worse, although the variation is not 
nearly so great as for steam plants. 
