takes place in it and, hence, this part is called the combustion chamber. It is shown at D.

The Air, Gas and Exhaust Valves.-These valves are called poppet valves, and are kept closed by spiral springs. They have flat, steel heads with beveled edges so that they will seat, that is fit tight, into the inlet and exhaust ports of the cylinder.

AIR OR GAS
INLET

-POPPET VALVE

E AND F

TO CYLINDER

FIG. 20-E AND F. THE AIR INLET VALVE AND THE GAS VALVE ARE MADE AND WORK THE SAME

The Air Inlet Valve.-The air and gas are taken into the cylinder through separate valves, when they mix inside of it to form the fuel mixture. The air valve is an automatic suction valve, that is, it is a valve that is opened by the suction of the piston at the same time that the gas inlet valve is opened, and it is kept closed during the other strokes by the spiral spring and the internal pressure on it. It is shown at E.

The Gas Valve.-This valve sets close to the air valve in the cylinder head, and it admits the gas into the cylinder at the same time the air is taken in by the suction stroke. It is opened at the right time by a cam working on the camshaft. All that is needed is to connect a gas pipe, with a gas cock WATER COOLING

SPACE

TO CYLINDER

POPPET VALVE

WATER
COOLING
SPACE

EXHAUST

G

FIG. 20-G. THE EXHAUST VALVE

in it, to regulate the supply of gas to the valve, and turn on the gas. It is the same as the air valve and is shown at F.

The Exhaust Valve.-As the burnt gases exhaust through this valve it gets very hot and it should therefore be water cooled. It is timed to open by and is worked from the camshaft by a cam and an exhaust lever. It is shown at G.

The Piston and Piston Rings.-The piston is made of the same kind of iron as the cylinder and is accurately ground to fit it.

Different from a steam engine, the piston of a gas engine is long and hollow, with one end closed and the other open; again, instead of being packed with hemp or asbestos it has grooves cut in it and packing rings, or piston rings, as they are called, are

FIG. 20-H, I, AND J. THE PISTON, PISTON RINGS AND CROSSHEAD PIN

slipped into them. These rings are also of iron and make the piston fit so closely to the cylinder, that the fuel mixture cannot leak by it.

The piston has a crosshead pin, called a wrist pin, set across its center and on this the small end of the connecting rod is secured. The piston and piston rings are illustrated at H and I, and the wrist pin at J.

The Connecting Rod.-Since the piston is long no piston rod, guides and crosshead such as are used in a steam engine are required, hence the piston is usually coupled direct to the crankshaft with a connecting rod. How it is made is shown at K.

The Crankshaft and Flywheels.-This shaft, see M, changes the reciprocating motion of the connecting rod into rotary motion. It usually carries two

FIG. 20-K AND L. THE CONNECTING ROD WITH WIPER OIL

FEED

flywheels, one on each end, and these are made very heavy, which help to keep the speed uniform. The crankshaft also has a pulley and a spiral, or worm

gear on it and the latter meshes with another spiral gear on the camshaft.

The Cam or Gearshaft.-In vertical and some horizontal engines the valves and igniter for firing the fuel charge are worked by a reciprocating rod,

while in other horizontal engines the valves and igniter are operated by a rotating camshaft. It is shown at N.

This shaft is driven by the crankshaft through a pair of spur or spiral gears. A bevel gear is keyed to the shaft which drives the governor, and the cams on it work the gas and exhaust valves and the igniter.

BEVEL GEAR FOR

SPIRAL OR WORM
GEAR

GOVERNOR

THE CAM OR

GEAR SHAFT

FIG. 20-N. THE CAM, OR GEAR SHAFT

A horizontal Otto gas engine assembled is shown in Fig. 21.

About the Gas Supply. Since the gas does not flow into the cylinder in a steady stream, but by the intermittent suction of the piston, the supply pipe must be sufficiently large to deliver enough gas for each charge.

Not only this, but a rubber bag, as shown in Fig. 22, is connected in between the gas supply pipe and the gas inlet valve of the engine. This bag acts as a reservoir and supplies all the gas that is needed for each fuel charge. For a small engine the bag should be protected by a wooden box, but for a large engine it should be enclosed in an iron casing.