Diesel and Other Internal-Combustion Engines

By Staff
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Fig. 25, Charter-Mietz low-pressure oil engine, 6 to 18 HP, courtesy of Charter Engine Company, Sterling, Illinois.
Fig. 25, Charter-Mietz low-pressure oil engine, 6 to 18 HP, courtesy of Charter Engine Company, Sterling, Illinois.
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Fig. 24, Longitudinal section of Mietz and Weiss oil engine
Fig. 24, Longitudinal section of Mietz and Weiss oil engine

The following is a chapter reprinted from the book Internal
Combustion Engines, published by the American Technical Society,
Chicago, Illinois, 1937. Sent to us by Walter A. Taubeneck, 11801
52nd Dr. NE, Marysville, Washington 98271-6225.

Classification. An oil engine may be defined as
an internal-combustion engine that uses oil for fuel, the oil being
sprayed into the cylinder or combustion chamber, during or at the
end of the compression stroke. Oil engines are of three distinct
types the low-pressure, moderate-pressure, and the Diesel
(high-pressure).

The low-pressure type of oil engine is so named because the
compression pressure is only 50 to 160 pounds per square inch. Fuel
is generally injected against hot plates, into a hot bulb, or into
an uncooled vaporizer chamber during the compression stroke. When
the atomized fuel comes in contact with the hot surface,
vaporization occurs, and as the oil is usually kept out of the
cylinder, only the expanding gases come in contact with the piston
and cylinder walls. Initial vaporization is often produced by
externally heating the chamber and after starting the engine the
surfaces retain enough heat from explosion to be continuously
effective as a vaporizer. These engines operate on the Otto cycle
and the explosion pressures are not high, seldom exceeding 250
pounds per square inch.

Moderate-pressure oil engines employ compression pressures of
200 to 350 pounds per square inch. These engines may be designed to
operate on cycles approximating the Otto or Diesel cycle. Ignition
in this type is usually secured by methods similar to those
employed in low-pressure oil engines. Injection cannot occur during
the compression stroke as in the low-pressure type, on account of
probable premature ignition. The temperature corresponding to 250
pounds per square inch pressure is sufficient to ignite most fuel
oils, but this temperature is not high enough to insure perfect
ignition at every stroke. To avoid occasional pre-ignition, the
fuel oil in moderate-pressure oil engines is injected at the end of
compression.

Low-pressure Oil Engines. The low-pressure oil
engine operates on the Otto cycle and is so named because the
compression pressure is low, seldom exceeding 100 pounds per square
inch. In this type fuel is generally sprayed into the combustion
chamber during the compression stroke. Ignition is usually secured
by a hot bulb or a hot tube; the hot bulb being heated before
starting the engine, the bulb retaining its heat from explosion to
explosion. The pressures resulting from the explosion are not high,
hardly as high as are found in some gas engines. Combustion is not
as complete as in the higher-pressure engines, and the efficiency
is correspondingly lower.

The Mietz and Weiss two-cycle low-pressure oil engine is shown
in Figs. 24 and 25. The stationary engine is manufactured in single
and two-cylinder horizontal units, while the vertical type is made
in one, two, three and four-cylinder units. The horsepowers range
from 2 to 400; the two-horsepower units operate at 600 rpm and the
400-horse-power units at 180 rpm.

The crankcase, Fig. 24, is used for compressing the charge of
air which enters through the suction port. As the piston travels on
the outward, or working stroke, the exhaust port G is uncovered
first. After the pressure in the cylinder has dropped almost to
atmospheric, the inlet port at the top of the cylinder is uncovered
and the air in the crankcase, which is under a slight pressure,
rushes in and cleans out the burned gases. Mixed with this air is
the steam which is generated in the water-jacket of the engine. The
water-jacket has no overflow, so that it is in reality a boiler
generating steam at about atmospheric pressure, the steam flowing
from E through F to the cylinder through the small inlet port.

On the return stroke, the air and steam are compressed in the
combustion chamber to about 60 pounds per square inch. During this
compression the fuel oil is taken from the reservoir A and pumped
through the pipe B onto the projecting lip of the hot bulb C. The
bulb is heated to a dull red heat by the kerosene burner D before
starting the engine, and is maintained at that temperature by the
explosions when the engine is running. The cylinder head is not
jacketed. The amount of oil delivered is regulated by an inertia
governor which controls the length of stroke of the fuel pump
plunger.

The air being compressed enters the hot bulb C, carrying with it
some of the vapor of the oil that has fallen on the projecting lip;
and near the end of the compression stroke the pressure and
temperature conditions in the hot bulb (vaporizer) will cause
ignition and explosion. The presence of the steam reduces the
exploding pressure and permits a higher compression.

The combustion in an engine of this kind cannot be as complete
as in the type where a thorough mixture of the fuel and air is
accomplished, as with a carburetor. Some of the air admitted to the
cylinder of a low-pressure oil engine will remain inactive, as it
does not get near the oil. Consequently such engines are
comparatively large for the power they develop.

  • Published on Sep 1, 1996
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