88 SURFACE MACHINERY AND METHODS FOR OIL-WELL PUMPING.
To avoid any misconception it is well to emphasize that the difficulties and
the considerable fuel losses during exhausting experienced with the usual two-
stroke gas engine, chiefly on account of the successive scavenging and loading
of the engine cylinder, are absent in the two-stroke Diesel engine. In this en-
gine the same air pump does the scavenging and the charging, whereas in the
two-stroke gas engine separate pumps are used for scavenging and for charg-
ing the cylinder with the gas-and-air mixture. These pumps consume con-
siderable power, and it is difficult to prevent a part of the fresh gas-air charge
from being expelled with the products of combustion during exhausting. Hav-
ing to deal with inert air (air not mixed with any combustible gas) and a
liquid fuel, the working process of the two-stroke-cycle Diesel engine is much
simpler than that of the gas engine having a two-stroke cycle.
Engines with a four-stroke cycle, in sizes from 50 to 800 horsepower, have
been developed to a high degree of perfection, and are so simple in operation
that little justification for the adoption of two-stroke engines in these sizes ex-
ists at present. The field of two-stroke types lies chiefly in engines of very
small and very large powers, the latter from 1,000 horsepower upward.
ESSENTIAL QUALITIES OF ENGINES OF SMALL POWER.
Engines of small power should combine lightness with the greatest simplicity
in design, to assure low manufacturing costs and ease of operation in inex-
perienced hands. In low-powered engines fuel economy and refinements in de-
sign are sacrificed to low first cost of engine. Nor is fuel economy of so much
moment in small engines, as the total fuel consumption is a relatively small
source of expense. Explosion engines having a two-stroke cycle best meet these
requirements. Engines of this type have merely self-acting disk valves for ad-
mitting air to the crank case of the engine, and a fuel pump delivering a meas-
ured amount of fuel to the fuel-injection nozzle. This construction insures
simplicity, but also results in low volumetric and mechanical efficiency and low
fuel economy. Diesel engines, to insure high fuel economy, demand mechani-
cally operated valves, and a high-pressure air compressor, which, together with
the high compression and resulting high temperatures used, demand the highest
grade materials anl workmanship, involving a high cost per horsepower.
Although in European countries Diesel engines are manufactured in sizes
as small as 15 horsepower there is little demand for this size in America, where
fuel prices generally range lower, and where Diesel engines of 75 horsepower
indicate the probable commercial limit in size. For large powers, from 1,000
horsepower upward, engines having a two-stroke cycle are preferred, as in
these sizes limitations are put on the size of cylinder practical for engines hav-
ing a four-stroke cycle by the high temperatures and pressures produced in
such engines. An increased use of material to withstand the greater total pres-
sure in large cylinders merely accentuates the difficulty of cooling the cylinder,
the cylinder head, and the piston, and of carrying off the heat fast enough
through thick cylinder and cylinder-head walls. Stresses due to unequal ex-
pansion and contraction may easily lead to ruptures of vital engine parts,
such as cylinder heads, pistons, and cylinders. Successful building of large
Diesel engines must, therefore, be fortified by a great amount of practical ex-
perience, particularly in the rational design and selection of casting mixtures
with correct chemical and physical properties.
Twenty-four inches in cylinder diameter represents the probable upper limit
with air-cooled pistons and 30 inches with water-cooled pistons for Diesel
engines having a 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 March 28, 2015.