half-turn, and this is the stroke that develops the power. During this stroke both the inlet and the exhaust valves stay closed.

The Exhaust Stroke.-The instant the power stroke is completed the exhaust valve opens and as the piston moves up it pushes the burnt gases out of the cylinder and into the open air. At the end of this stroke the exhaust valve closes, the inlet valve. opens and the cycle of strokes begins all over again. The position of the valves, the piston and the crankshaft during the exhaust stroke is shown at D.

From the above you will see that there is only one power stroke, to every four strokes of the piston, or cycle of the engine, and as the crankshaft makes half a turn for each stroke of the piston it is evident that there are two revolutions of the crankshaft to every power stroke, which means that power is applied to the crankshaft in turning it around only onequarter of the time.

How the Inlet and Exhaust Valves Work.-Since the inlet and exhaust valves open only once in every four strokes of the piston and since there are two revolutions of the crankshaft to every four strokes, it is clear that an eccentric, or another crank, on the crankshaft could not be used to open them, as these devices do to move the slide valve in a steam engine.

Instead, a camshaft is geared to the crankshaft with gears in the ratio of 2 to 1, that is, the gear on the camshaft has twice as many teeth on it as

the one on the crankshaft with which it meshes. This reduction gear makes the camshaft turn only half as fast as the crankshaft, with the result that every time the camshaft makes a complete revolu tion, which is once in every four strokes, the valves are raised. When the cams have moved on past the valve rods the springs force the latter down and so close the valves which are fixed to them.

How the Electric Spark Igniter Works.-The simplest kind of a jump-spark igniter consists of (1) an induction coil, or jump-spark coil as it is called, which changes the low pressure current generated by a dry battery into a high tension current that will jump across an air-gap between two metal points about of an inch apart, which are set in a spark-plug.

The spark coil is connected to the spark-plug in the cylinder through a timing device, or timer as it is called. This device is simply a make and break contact switch which is worked by a cam on the camshaft, and this makes and breaks the electric circuit once in every four strokes of the piston at the instant when the charge must be fired. It is shown schematically in Fig. 12.

How the Engine is Made to Run Smooth.—As you quite likely know, every stroke of the piston of a steam engine is a power stroke and, as you have just seen, only one stroke in every four of the piston of a single cylinder four stroke cycle gas engine is a power stroke.

How IT IS TIMED

The result is the steam engine works smoothly and the gas engine has a tendency to run unevenly. Now there are three ways to make a gas engine run with a steadiness comparable to that of a steam engine, and these are (1) by putting one, or better, two heavy flywheels on the crankshaft; (2) by fitting a governor to it, and (3) by connecting up the pistons of two, often four, sometimes eight, and occasionally twelve cylinders to a single crankshaft, and some engines adopt all of these expedients to get the best result.

What the Flywheel Does.-The flywheel as it rotates gathers energy and when it is turning fast enough its inertia1 tends to keep up the speed of the engine and to make it more nearly uniform.

How the Governor Works.-The governor of a gas engine works on the same principle as that of a steam engine. The simplest type, see Fig. 13, is called a flyball governor, and it consists of a vertical shaft, called a governor shaft, geared to the camshaft which makes it revolve at a goodly speed.

A pair of arms, each of which carries a ball on its free end, is pivoted to a support while the upper ends of the arms press on the top of the shaft. A sleeve is fixed to the shaft and a lever with a fork at one end sets in a groove in the sleeve and this

When a body is at rest it tends to remain so and when in motion to keep on going, and this property of matter is called inertia.

opens and closes the gas valve which admits the fuel into the cylinder.

As the engine increases its speed the balls revolve faster around the shaft and fly farther and ever farther apart. In so doing the arms press down on the shaft and, as the sleeve moves down with it, it works the forked lever and this in turn cuts off

GAS INLET

BUTTERFLY
VALVE

FIG. 13. A SIMPLE FLYBALL THROTTLING GOVERNOR

the supply of gas; the engine then slows down, the balls fall and the gas valve is again opened.

The Purpose of Multi-Cylinders.-There are two ways open to get a power stroke oftener than once in every two revolutions, and these are (1) to connect two or more pistons to a single crankshaft and (2) to use a two stroke cycle engine which has one or more cylinders.

Where multi-cylinders are used the pistons are connected to the crankshaft in such a way that the power strokes will be delivered in succession, and