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To secure the greatest efficiency, durability, and power of the engine, it is necessary, as can be easily understood, to be able to control the speed—in other words, to regulate the number of revolutions of the fly-wheel per minute—and there are various devices for accomplishing this object. The range of speed of different engines varies very considerably. With the small single-cylinder engines it is necessary that the speed should be very great in order to secure sufficient power, and also to reduce vibration, while with the two- and four-cylinder engines this is not necessary, and consequently they are much more durable. As a rule the speed varies from about 750 revolutions per minute for the latter class to 2,000 for the small engines fitted to motor-cycles.

There are various successful methods of governing, which we shall now proceed to enumerate and describe.

(1) Timing of ignition.

(2) Exhaust-valve lifter.

(3) Exhaust-valve closer.

(4) Regulating lift of induction valve.

(5) Mechanically governing exhaust valves.

(6) Throttle.

(7) Governing both exhaust and throttle.

(8) Governing by variable induction and throttle valve.

(1) By Advancing or Retarding the Sparking.—This is the method adopted for single-cylinder engines, and consists in altering the time at which the spark occurs in the combustion chamber by the manipulation of a small lever. It will be easily understood that if the full force of the explosion occurs in the combustion chamber at the moment when the piston is at the highest point, that its effect will be greatest. This is due to the fact that the compression is then at its maximum, and the force of the expanding gas has a longer time to act on the piston. The compression and duration of pressure on the piston can be reduced by altering the timing of the spark, so that it occurs