Where a plunger pump is used it is driven by an eccentric on the camshaft just as it is when used for pumping gasoline as shown in Fig. 38. Centrifugal and gear pumps are also used in water circulating systems.

The Exhaust Pipe and Muffler.-The exhaust pipe of the engine gets red-hot, nearly, and to keep it from burning the boat where it goes through it must be jacketed, that is, it must be enclosed in another pipe and be cooled by water circulating between them. The exhaust is made on the same principle as that of a motor car engine.

PART IV

AIRPLANE ENGINES

Engines for airplanes are of two types, and these are (1) the regular stationary cylinder type and (2) the revolving, or gyro type.

The first kind of engines are built like motor boat and motor car engines, only very much lighter for the power they develop. They are made with from four to twelve cylinders; they use carburetors, and have magneto ignition, radiators with water circulating pumps, and force feed oiling systems.

The Famous Liberty Motor.-The need of a light. and powerful airplane engine that could be made in large quantities was felt as soon as the United States. got into the war.

The work of designing this engine, or motor, as it was popularly called, was begun in the early part of 1915; by fall work was started on its construction, and in the spring of 1916 the first engine was completed. This engine was a twin-six, that is, it had 12 cylinders and was of the V type. After testing it a second time a more powerful engine was built on the same general lines. It had forged steel cylinders

instead of iron ones with water jackets which were of pressed steel, and these were welded together by the oxy-acetylene process.

After more tests with this engine a third one was built in the spring of 1917; this one was much

lighter in weight and all parts of it were designed so that the engine could be produced on a quantity basis. It is shown in Fig. 55.

The final perfected Liberty engine was the most powerful one ever built for airplane work; it de veloped upwards of 500 pounds and its weight was about 800 pounds. This engine was completed at

the Packard Motor Car Company's works in Detroit, Mich., on July 3, 1917, and it was rushed to Washington, arriving there the next day-the 4th—and this is how it came to be named the Liberty motor. Both the scupper system of lubrication, that is one in which a projecting spoon on the crank-web throws the oil in the crank-case up on the bearings, and the

force feed system was tested and the latter was finally adopted.

Several ignition systems were tried out, but the Delco was the one which proved the most satisfactory. In this system a small dynamo is used to generate the current, and after it is stepped up by spark coils it is passed on to the spark-plugs by distributors. The Gyro or Revolving Engine.-In this type of

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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

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