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.

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