Surface Machinery and Methods for Oil-Well Pumping

PRIME MOVERS AND POWER PLANT MACHINERY. 87
struction described is the one generally accepted and is to be preferred on ac-
count of its greater simplicity. Engines of this type are built both horizontally
and vertically, the latter construction being preferred in multicylinder units.
The air for scavenging could also be furnished by constructing the working
piston as a differential piston, traveling with the enlarged part in an air cylin-
der, or by an independent air pump. This design would result in a more com-
plicated and expensive engine, hardly justified by its limitations,
COMPARATIVE ADVANTAGES OF FOUR-STROKE AND TWO-STROKE CYCLES.
As regards the relative theoretical advantages of the four-stroke and the
two-stroke engines, the two-stroke engine appears to be superior. For a given
cylinder size and speed, the specific duty of an engine having a two-stroke
cycle is double that of an engine having a four-stroke cycle, for it receives a
power impulse for every revolution as against one for every two revolutions
of the engine having a four-stroke cycle. Its mechanical efficiency also would
appear to be greater, as the engine with a four-stroke cycle has to make two
strokes for expelling the products of combustion and for filling the cylinder
with a fresh supply of air for burning the next fuel charge. The power for
this work has to be drawn from the flywheel, in which it has to be stored again
during the power stroke. As in addition the cyclic regularity of an engine
with a four-stroke cycle but having the same power and speed is greatly less,
a much heavier flywheel is required. The four-stroke engine is also much
heavier per horsepower than the two-stroke engine.
In practice, however, these advantages are not so apparent. Engines having
a two-stroke cycle require an auxiliary air pump for performing the work
of scavenging and of refilling the working cylinder with fresh air, work that
in the engine having a four-stroke cycle is performed by the engine directly in
the working cylinder. The air pump, proportioned to the engine it has to
supply, is a large and heavy piece of machinery, requiring considerable power
for its operation. To insure thorough scavenging of the working cylinder, a
volume of fresh air greater than the volume of the cylinder must be supplied
by the air pump. The pump is usually proportioned to supply one and three-
tenths times the working cylinder volume of air. Even then sweeping of the
working cylinder clean of the remants of the products of combustion is difficult,
and any gases remaining in the cylinder displace a proportionate amount of
oxygen needed to maintain combustion, thus lowering the possible power out-
put of the engine.
The heat exchange in an engine having a two-stroke cycle being more rapid
than in one having a four-stroke cycle, the material of the former is more
severely taxed, and it is not advisable to use as high mean effective pressures
nor piston speeds as in engines having a four-stroke cycle. High mean effective
pressure results in high terminal pressure at the time of exhausting, with a
consequent loss in power. As the exhaust ports of two-stroke engines are un-
covered by the piston, and as exhausting of the gases must be rapid, the piston
has to uncover these ports several degrees before it reaches its lower dead
center. High mean effective pressure will, therefore, cause proportionately
greater losses in an engine having a two-stroke cycle than in one having a four-
stroke cycle. These features combine to diminish the theoretical advantages
that engines having a two-stroke cycle appear to possess over engines with the
simpler four-stroke cycle.

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 December 19, 2014.