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.