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An entire stroke of the piston and part of the power stroke is devoted to clearing out the burnt gases so that the cylinder of a four cycle engine is more thoroughly scavenged than is that of a two-cycle motor. A four-cycle engine responds readily to throttle control and its speeds may be accelerated from a few hundred revolutions per minute to several thousand without lack of uniformity or the backfiring usually present when a two-cycle engine is accelerated too quickly. A four-cycle engine will run faster than the two-cycle form and it is also able to run slower when the supply of gas is reduced.

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Fig. 17.-Inlet Side of Typical Four-Cylinder Water-Cooled Motor.

Q. How many cylinders do gas engines have?

A. Gas engines used for automobile power plants usually have four or six cylinders. Formerly simple types of one and two cylinder motors were very popular, but at present these have been largely

displaced by the multiple cylinder form. Racing automobiles have been made with eight, twelve, and sixteen cylinders, but eight cylinders have been the greatest number ever used on a stock touring car. Q. What is the advantage of a one-cylinder engine?

A. The main advantage of a single cylinder motor is that of simplicity. Practically no one-cylinder automobiles are on the market at the present time, though many one-cylinder cars of early vintage are still giving satisfactory service.

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Fig. 18.-Exhaust Side of Four-Cylinder Water-Cooled Motor Used on Peerless Automobiles.

Q. Why are multiple cylinder engines superior to simple types?

A. Multiple cylinder engines are smoother running because they deliver power more uniformly than do the simple one and two cylider engines.

Q. How many explosions per crankshaft revolution are obtained with various forms of four-cycle engine?

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Fig. 19.-Part Sectional View of Four-Cylinder Motor Showing Radiator and Clutch. Note Three Bearing Crankshaft and Cylinders Cast in Pairs, Also Large Water Manifold.

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Fig. 20.-Sectional View of Six-Cylinder Motor Used on Pierce Arrow Cars. Note Seven Bearing Crankshaft and General Simplicity and Strength of Design.

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Fig. 21.-End Sectional View of Motor With T Head Cylinder Showing Important Parts.

A. The crankshaft of a single cylinder four-cycle engine will receive one impulse every two revolutions. That of a two cylinder form will receive an impulse every revolution. A three cylinder crankshaft will receive three impulses in two revolutions or one every two thirds crankshaft revolution. Four explosions are obtained for every two revolutions of a four cylinder crankshaft or two each

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