Tractor Principles / The Action, Mechanism, Handling, Care, Maintenance and Repair of the Gas Engine Tractor

by Roger B. Whitman

Thus, the piston drives the crank shaft during the power stroke, and the crank shaft drives the piston during the dead strokes.

To start an engine, the crank shaft is revolved to make the piston suck in a charge of mixture and compress it; then the charge is burned, the power stroke takes place, and the engine runs.

A clear idea of what goes on inside of the cylinder is quite necessary in order to take proper care of an engine and to get the best work out of it. The following description applies to any cylinder, for the action in all cylinders of an engine is the same.

Inlet Stroke.—During the inlet stroke ([No. 1, Fig. 1]), the piston moves outward; the inlet valve is open, and the exhaust valve is closed. This movement of the piston creates suction, and if there are leaks in the cylinder, air will be sucked in and will spoil the proportions of the charge. This will prevent the proper burning of the mixture, and the engine will lose power.

The piston moves at such high speed that the mixture cannot enter fast enough to keep up with it; mixture is still flowing in when the piston reaches the end of its stroke, and even when it begins to move inward on the next stroke. The more mixture there is in the cylinder, the more powerfully the engine will run; the inlet valve is therefore held open for as long a time as the mixture continues to enter.

Fig. 1.—The Gas Engine Cycle

In slow-speed 1-cylinder and 2-cylinder engines the valve closes when the piston reaches the end of its stroke; on high-speed engines the valve does not close until the piston has moved ¼ inch or ½ inch on the compression stroke.

Compression Stroke.—During the compression stroke ([No. 2, Fig. 1]) the piston moves inward, and both valves are closed. This movement places the piston in position to move outward on the power stroke. As the outlets to the cylinder are closed, the charge of mixture cannot escape, and is therefore compressed into the space between the piston and the cylinder head when the piston is at the inner end of its stroke. This space is usually about one quarter the volume of the cylinder; the charge is therefore compressed to about one quarter of its original volume.