that, in Fig. 2, the cylinder is closed at the end toward the crank and a piston rod k transmits the motion of the piston to a crosshead l. The pin h, which connects the crosshead to the connecting-rod, is called a crosshead pin, or wristpin. The crosshead is kept in alinement with the cylinder by guides m and n. Leakage about the piston rod is prevented by a stuffingbox o.
5. The end of the cylinder nearest the crank is called the crank end, while the other end is called the head end. The movement of the piston from the head end to the crank end is called the forward, or outward, stroke; the movement in the opposite direction is called the return, or inward, stroke. When the crank moves through the upper half of its revolution while the piston moves from the head end to the crank end, as shown, the engine is said to run over; and when the crank moves through the lower half of its revolution when the piston moves from the head end to the crank end, it is said to run under.
The valves that admit the new charge of air and fuel to the cylinder are called inlet, or admission, valves, and those which permit the burned gases to escape are called exhaust valves. These will be described later.
6. A gas engine is a type of internal-combustion engine using combustible gas as a fuel. With a few unimportant exceptions, all gas engines have been of the reciprocating type, that is, with a cylinder, and a piston that moves in a straight line. The term gas engine is also frequently applied to all internal-combustion engines, since in all cases the fuel must take the form of a gas either before or after it enters the engine.
7. A gasoline engine is a type of internal-combustion engine using gasoline as a fuel. The gasoline engine differs but little from the gas engine in its main features, the principal difference being the addition of a device, commonly called a carbureter, for vaporizing the fuel before it enters the cylinder.
