engine, the cylinders revolve around the shaft and, curiously enough, there is no to and fro motion of either the cylinders or the pistons, but each of these turns round a different center.

The pistons are

FRAME

connected by their respective connecting rods to a hub and this in turn is fixed to a shaft.

The cylinders, which radiate from the hub like the spokes of a wheel, turn round on it, and instead of the pistons rotating the shaft, the explosions of the fuel charges act on the pistons and these turn the shaft instead. The action of the gyro engine will

be understood from the diagram A in Fig. 56, while the engine complete is shown at B.

The Cooling Scheme.-Gyro engines are air cooled, that is, the cylinders have paralleled metal rings on them to dissipate the heat exactly like aircooled motorcycle engines. They are, therefore, lighter in weight for the horsepower developed than engines with water-cooled stationary cylinders.

The Use of the Muffler.-Before the war, airplane engines were not provided with mufflers, but during the war they were used because the noise of the exhaust had to be reduced to a minimum so that the enemy could not hear the planes when they were approaching.

Self-Starters for Engines.-The latest airplane engines are equipped with self-starters. As it would not be practical to use storage batteries because of their great weight and since they contain a liquid, starting motors operated by compressed air are used instead.

CHAPTER VI

THE ECONOMICAL OIL ENGINE

Since gas is not always at hand and gasoline is quite an expensive fuel, makers of internal combustion 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 Engines. 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 engines.

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 complicated 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 developed 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 considered presently.

Oil Fuel Feed System.-The 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 Nozzles.-There are two distinct types of fuel nozzles used on oil engines, namely, those in which (1) the oil and air are drawn by suction through the same nozzle, when it is called a mixing valve, and (2) the oil alone is forced by pressure through the nozzle, when it is called a fuel injector.