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piston top. This entering stream of fresh gas strikes a deflector plate interposed between the inlet and exhaust port across the topof the piston so the fresh charge of gas is directed to the upper portion of the cylinder instead of passing out through the now fully opened exhaust port. The piston is moved up to the top of the cylinder again by means of momentum or energy stored in the engine flywheel and operations of compressing the gas above the piston top and charging the lower portion of the engine base with gas are again effected. When the piston reaches the top of the cylinder the gas is again exploded and this cycle of operations continues as long as fuel is supplied the cylinder and a spark is provided to ignite it at the proper time. (See Fig. 12.)

Q. What is the four-stroke cycle principle?

A. The four-stroke cycle engine, commonly termed the "Four cycle" or Otto cycle, differs from that previously described in that four strokes of the piston are necessary to obtain an explosion in the cylinder. The operation of this form of engine is clearly outlined at Fig. 13. The first stroke of the piston which is shown at A is a down stroke or movement from the closed to the open end of the cylinder. During this suction or intake stroke a vacuum or suction is produced in the cylinder and an inlet valve member opens to admit a charge of gas which is drawn in by the pumping action of the moving piston. This inlet valve remains open during the entire first down stroke, and closes when the piston reaches the end of its downward movement.

The second stroke of the piston is depicted at B. This is an upstroke in which the piston moves from the open to the closed end of the cylinder. The intake valve, which has been open on the first stroke, is closed, as is also the exhaust valve which is utilized to close the passage through which the burnt gases are expelled. During this upward movement the charge of gas which has been previously drawn into the cylinder is compressed. At the end of this stroke, an electric spark takes place to explode the gas.

The third stroke which is outlined at C is termed the "explosion or power stroke." In this the piston is driven down by the expanding gas with a force depending upon the area of the piston top and its position in the cylinder. At the instant of combustion a pis

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Fig. 13.-Diagrams Illustrating Four-Cycle Motor Action.

ton with an area of 100 square inches will receive an impulse varying in value from 10 to 15 tons. As the piston goes further down on its stroke, and the burnt gases expand more, the pressure becomes reduced to a point where the gases have so little pressure that it is better to dispose of them than to take further advantage of their expansive force.

The power of a gas or gasoline engine is usually figured on a basis of mean effective pressure of 80 to 100 lb. per square inch, which is considered as existing during the entire stroke of the piston. When the piston reaches approximately seven-eighths of its stroke following the explosion, the exhaust valve is raised by a cam member and the inert gases escape through the open exhaust port by virtue of their pressure. The next stroke of the piston is outlined at D and is termed the "exhaust" or scavenging stroke, and it is devoted exclusively to clear the cylinder of the burnt gases. These are pushed out by the upward movement of the piston, and when the piston reaches the end of its stroke the exhaust valve closes and the inlet valve opens again to admit a fresh charge of gas during the suction stroke.

It will be seen that in a four-cycle engine the piston must make twice as many strokes as in a two-cycle. This is because all the work is done in the upper portion of the cylinder and by the piston top. One entire piston stroke is necessary to fill the cylinder, and a full piston stroke is devoted to clearing the cylinder of burnt gas. In a two-cycle engine, these two operations are performed at practically the same time.

Q. Name advantages of two-cycle engine.

A.

Owing to the more rapidly occurring series of explosions, a two-cycle engine has a much more even torque, which is synonymous to more uniform power application. A two-cycle engine is much simpler than four-cycle, as in its simplest form it comprises practically only the essential elements absolutely necessary to secure power from the explosion of gas. There are but three moving parts, namely, the piston, connecting rod and crankshaft. As the ports through which the gas enters and leaves the cylinders are opened and closed by the piston the use of valves which are necessary with a four-cycle engine is dispensed with.

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Q. How many types of two-cycle engines are used?

A. Three main types

of two-cycle engines are generally used. These are the two-port, three-port and differential piston forms.

Q. What is a twoport, two-cycle engine? A. A two-port twocycle engine is shown at Fig. 12 A. In this form of motor the gas enters into the crankcase, through a passageway in that member, which is closed by an automatic valve of the mushroom type. It is called a twoport engine because but two ports are provided in the cylinder walls. One is for disposition of the exhaust and the other permits the fresh gas to enter the cylinder. Both of these ports are uncovered or fully open when the piston reaches the bottom of its stroke, or when approximately in the position shown.

Q. What is the three-port, two-cycle engine?

A. The three-port, two-cycle engine is so called because there are three ports in the cylinder wall instead of two. One of these ports

serves as a means for gas entrance to the engine crankcase and is fully opened only when the piston reaches the top of the cylinder or the end of the up-stroke. The other two ports are exactly the same in location and perform the same functions as do those in the twoport engine. The three-port is the simplest form and is a truly valveless motor, inasmuch as the two-port engine utilizes an automatic inlet or check valve to prevent the gas from escaping from the crankcase interior when the piston is moving down. In a three-port engine the piston wall itself covers the intake or charging port to which the carburetor is attached as it descends on its power stroke and no check valve is necessary in the engine base.

Q. What is a "differential" piston, two-cycle engine?

A. This form of motor utilizes a piston which has an enlarged lower portion so that it is virtually a double member. The lower portion of the cylinder bore is enlarged to fit the larger diameter of the piston and a pumping chamber is thus formed. The inlet gas is drawn into this chamber and compressed therein prior to transfer into the upper or working end of the cylinder instead of into the engine base.

It is claimed that it is easier to keep a pumping chamber of cylindrical form tight than it is to have an absolutely tight engine base. As the bearings of the ordinary form of two-cycle engine wear, a certain amount of air is drawn in through the worn bearings and dilutes the mixture in the crankcase and at the same time the amount of compression which insures prompt transfer of the gas charge from the engine base to the cylinder is reduced because of this leakage. In a differential piston engine, piston rings are depended on to keep the lower piston tight in its pumping cylinder and the engine operation is not affected by bearing depreciation.

Q. What are the disadvantages of two-cycle motors?

A. Two-cycle motors are not as efficient as the four-cycle forms, because it is practically impossible to expel the burnt gases and fill the cylinder with fresh gas at the same time without serious mixing. The dilution of the fresh gas by the inert burnt products reduces its power and it will not explode as readily as pure, fresh gas will. Then

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