THE ECONOMICAL OIL ENGINE Since gas is not always at hand and gasoline is quite an expensive fuel, makers of internal combus tion engines turned their attention long ago toward using cheaper fuels, such as kerosene and even crude oils, and their efforts along this line have been highly successful.
Fuel oils are the cheapest kinds you can buy, but kerosene is the best of the fuel oils, especially for small engines, and as it costs only about one-half as much as gasoline and only about one-half the amount is used for the power produced, it is very economical, you can see.
Difference Between Oil Engines and Gasoline En gines.—Stationary, portable, and mobile oil engines are built in all sizes, but the last named type is used only for marine and traction work.
They are made in both the two and four stroke cycle kinds, the two cycle engines being largely used because they do away with the valves in the cylinder and this gets rid of the camshaft, gears, and other accessories that are always found on four cycle en gines.
_ _ Again, mixing valves are used on oil engines very similar to those on stationary gasoline engines, and this makes for simplicity as against the more com plicated carburetors that are a part of all mobile gasoline engines.
Further, many oil engines, especially the larger sizes, are of the Diesel and semi-Diesel types, that is, the fuel charge is fired either by the heat devel oped by compression of air alone or by the heat of the explosion which keeps a hot ball, or bonnet, at a temperature high enough to fire the fuel charge. Either of these simple expedients gets rid of a lot of ignition trouble.
Finally a spray of water is sometimes injected into the cylinders at the same time that they take in air, and the valve for the water so used will be con sidered presently.
Oil Fuel Feed Oil Supply Tank.— In very small engines the fuel tank often sets close to the cylinder and the oil is drawn into the latter through a nozzle by the suction stroke of the piston.
In some engines, up to 20 horsepower, the fuel tank is placed in the base and is large enough to hold a sufficient quantity of oil to run the engine for a stretch of 24 hours, and, where the rules of the Fire Underwriters will permit, there is no danger in using an oil supply of this kind. For engines of
more than 20 horsepower it is better in every way to have a large storage tank outside of the engine and sunk in the ground.
In either case the oil from the supply tank is forced into a small auxiliary fuel tank close to the engine by (1) a simple plunger pump, (2) by an air compressor pump, or (3) by a vacuum formed by the suction stroke of the piston in the cylinder of the engine.
Why Fuel Mixing Valves and Injectors are Used. —No carburetor has yet been invented which can successfully break up kerosene, or the heavier fuel oils, into a spray so that it will mix with air before it enters the cylinder except that these oils are highly heated first.
When a fuel oil is preheated for this purpose it must not cool down between the carburetor and the cylinder, and so the air must also be heated before it enters the cylinder. But when the air is preheated it expands and this decreases the weight of it, and consequently of the fuel charge, with the result that the engine drops in power.
For this reason the kerosene, or other heavy fuel oil, is sucked or injected, as the case may be, through a nozzle into the cylinder.
Types of Fuel are two distinct types of fuel nozzles used on oil engines, namely, those in which (1) the oil and air are drawn by suc tion through the same nozzle, when it is called a mixing valve, and (2) the oil alone is forced by pres sure through the nozzle, when it is called a fuel injector.
Kinds of Mixing Valves.—A Simple Mixing simplest kind of a mixing valve is merely a nozzle through which the fuel oil and air are drawn at the same time by the suction stroke of the piston.
When the fuel oil and air are drawn into the cyl inder, the oil is broken up into fine particles by striking the inlet valve, or some other metal surface inside the cylinder, when it mixes with the air; the heat of the cylinder then vaporizes it, when it makes a suitable fuel mixture. This is the kind of a mix ing valve that is used on the oil engine, and is shown in Fig. 57.