Cycle.—See Otto Cycle.
Cylinder.—The cast-iron chamber in which the petrol gas is compressed and exploded and in which the piston travels to and fro.
Densimeter.—An instrument for testing the specific gravity (i.e., the weight as compared with water) of petrol.
Differential Gear.—The gear by which power is communicated to the driving-wheels in such a way that it is taken up automatically by either or both of them according to their respective requirements at the moment.
Dry Cell, or Battery.—A cell, or battery of cells, which works without liquid. Dry cells are generally used for motoring purposes in preference to cells containing solution.
Dynamometer.—An instrument for testing the amount of power expended by mechanical or animal effort. The dynamometer used for testing motors is often called the “pony brake.” See Horse-power.
Electric Ignition.—The ignition of the explosive mixture in the cylinder is effected by an electric spark caused by forcing an electric current through the sparking-plug. The current is obtained (1) from an accumulator or a primary battery, in which case its pressure is raised to the required voltage by passing it through an induction coil; or (2) from a magneto-electrical instrument—which is very much like a dynamo on a small scale, and is driven by the motor.
Exhaust.—The gaseous products of combustion expelled from the cylinder during the scavenging stroke of the piston.
Fan.—A rotary fan, driven by the motor, is often employed to increase the current of air passing through the radiator and thus to assist in cooling the water.
Feed.—The method of conveying the petrol from the main tank to the carburettor. If the tank is higher than the carburettor, the petrol will pass by gravity. Otherwise it must be lifted by pressure. The exhaust is generally employed for this purpose, a hand-pump being fitted to furnish the necessary pressure for starting.