Surface Machinery and Methods for Oil-Well Pumping

PRIME MOVERS AND POWER PLANT MACHINERY. 39
COMPARATIVE ECONOMIES OF DIESEL AND OF EXPLOSION OIL
ENGINES.
The use of explosion oil engines should be dictated entirely by their over-
all economy. Although they are materially cheaper in first cost, they con-
sume considerably more fuel and lubricating oil than Diesel engines, and
their fuel consumption at fractional loads increases at a greater rate than
does that of Diesel engines.
The difference in economy between explosion oil engines and Diesel engines
is due not so much to a difference in thermodynamic cycles as in construc-
tional differences, which decidedly favor the Diesel engines.
The higher the initial temperature and the lower the terminal temperature
the more perfect will be the heat utilization. High initial temperature is
a function of pressure. If the compression were carried as high in the ex-
plosion engine as in the Diesel, the explosion oil engine theoretically would
show a slightly higher thermodynamic efficiency than the Diesel engine. The
final pressure, when the fuel is burned at constant volume, would, however,
be greatly increased above the compression pressure, or to about 60 at-
mospheres, with an accompanying rise in temperature far beyond that prac-
ticable with the materials of construction available. These limitations im-
pose a lower compression pressure on explosion oil engines than on Diesel
engines, so as to keep the maximum pressure and temperature within safe
limits of permissible engine construction.
Thus with a compression pressure of 150 to 250 pounds per square inch
the explosion pressure becomes 270 to 500 pounds per square inch; and with
the instant ignition and burning of the previously vaporized oil common to
explosion oil engines, a temperature of 2,300 to 3,1500 F. is reached.
The Diesel engine has a higher but more gradually increasing compression
pressure (450 to 500 pounds per square inch), which does not subject the
engine to sudden shocks, with a resulting increase in temperature from
atmospheric to about 1,000 F. The fuel is gradually injected as the piston
moves from its top center (inner dead center) downward, the pressure re-
maining practically constant during the time of fuel admission. With a
decrease in load, the time of fuel admission is also shortened; that is, the
fuel supply is shut off sooner. Therefore, the increase in temperature due to
the burning of the fuel at constant pressure does not exceed that reached
in an explosion engine, notwithstanding the higher compression pressure used
in the Diesel engine, and the higher initial temperature caused by this com-
pression. Thus, the temperature in a Diesel engine seldom exceeds 2,6000 F.
and reaches 3,000* F. only when the engine is overloaded.
If, then, the working cycles of the two types of engines are compared on
the basis of compression pressures used, the Diesel engine is found to have
a greater thermal efficiency, because it can work successfully with the higher
compression pressure. This superiority is confirmed by comparative entropy
diagrams.
Whereas in explosion oil engines the efficiency is influenced by the compres-
sion ratio alone, in the Diesel engine the efficiency is influenced by the com-
pression and the cut-off ratios, the efficiency increasing with a decrease in
the length of the cut-off or constant-pressure line. Thus, at fractional loads,
the indicated thermal efficiency of Diesel engines increases, which partly
offsets the loss in mechanical efficiency; that is, the increased fuel consump-
tion for performing the internal work of the engine. This accounts for the
very "flat" fuel-consumption curve of Diesel engines, which maintain an

George, H. C. Surface Machinery and Methods for Oil-Well Pumping. Washington D.C.. UNT Digital Library. http://digital.library.unt.edu/ark:/67531/metadc12407/. Accessed August 21, 2014.