Fig. 15.-Part Sectional View of Single Cylinder T Head Air-Cooled Tri-Car Motor of English Design.

again, a certain amount of the fresh charge invariably flows out of the open exhaust port before this is closed by the upwardly moving piston. The two-cycle engine will not throttle down as well as a four-cycle, nor will it attain as high speeds. Two-cycle engines are rather sensitive to mixture variations, and instead of developing twice the power that a four-cycle of the same piston displacement or cylinder dimensions will give, they only deliver about 1.5 times the

power and use twice as much fuel. Two-cycle engines are not as flexible as the four-cycle form and do not respond as readily to throttle control. They are not as satisfactory for general automobile service as four-cycle engines are, and have not received very wide application as automobile power plants.

Q. Where are two-cycle engines widely used?

A. Two-cycle engines are entirely suitable for use where a wide range of speed variation is not desired, and have been successfully applied as stationary engines and motor boat power plants.

Fig. 16.-Simple Two-Cylinder Opposed Water-Cooled Motor. Q. What are the advantages and disadvantages of four-cycle engines?

A. When a four-cycle engine is compared to the two-cycle form it is apparent that it is more complicated, inasmuch as it has many small parts that are liable to make trouble which are not found in a two-cycle motor. At the other hand the four-cycle form is the more flexible and economical of fuel, as a full stroke of the piston is devoted to charging the cylinder with gas and the inlet valve closes as soon as the cylinder is filled, so there is no opportunity for the fresh gas to escape or to become diluted with exhaust or dead gas.

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.

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.

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?