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.

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

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.