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Rh after the piston has begun to descend. This system is the simplest, but its effective use depends very largely on the skill and experience of the driver. Where this system is adopted, a throttle is also used, which enables the operator to regulate the quantity of mixture, and thus alter the power of the explosion. For example, when the maximum power is required, the throttle is open to the fullest, and the sparking advanced to the utmost that the engine will take.

(2) Exhaust-Valve Lifter.—This operates by preventing the exhaust valve from closing after the exhaust gases have escaped. When the exhaust valve is held fully up, no explosive charge is taken into the cylinder. If, however, the exhaust valve is held up to a very slight degree, a reduced charge will be admitted, and the force of the explosion consequently minimised.

In the case of cycles, an exhaust valve lifter is frequently fitted, in order to enable the rider to effect an easy start.

(3) Exhaust-Valve Closer.—This system is adopted on the De Dion voiturette, and consists in regulating the lift of the exhaust valve, but does not prevent the valve from closing in the usual course. It is also used in conjunction with the sparking advance. When the maximum power is required, the exhaust valve is not interfered with, and opens to its fullest compass. When less power is required, the exhaust valve is prevented from opening to that extent. Consequently the exhaust gases are not fully expelled, and as they partly occupy the space in the combustion chamber, a full charge of mixture cannot be admitted through the induction valve, and the explosion is weakened.

(4) By Regulating the Lift of the Induction Valve.—By operating a lever, the induction valve is prevented from opening to its fullest extent, and consequently the largest possible charge is not admitted to the combustion chamber.

(5) By mechanically Governing the Exhaust Valves.—This is effected by preventing the cam from raising the exhaust valve after an explosion has taken place. This system is only adopted on two- and four-cylinder engines, and is the one in