Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(26) Two Stroke Cycle Engine.

Two stroke cycle engines perform the five events of aspiration (suction), compression, ignition, expansion and release in two strokes or one revolution. Providing that these events are performed as efficiently as in the four stroke cycle engine, it is evident that with equal cylinder capacity, the two stroke cycle engine would have twice the output of a four stroke cycle since it gives twice the number of impulses per revolution. Unfortunately it is impossible to attain twice the output of the four stroke cycle type with the small two stroke engines built at the present time because of their imperfect scavenging and poor fuel economy. In the larger two stroke engines, the pumps and blowers used for scavenging the cylinders consume a considerable percentage of the output.

Fig. 5. Diagram of Two Port—Two Stroke Cycle Engine, Showing the Events in the Crank-Case and Cylinder.

A general classification of the two stroke cycle engine is not so simple a matter as that of the four stroke because of the differences in construction of large and small sizes. This difference between the large stationary engine and the small type commonly used on boats is due to the efforts of the builders of the large engine to obtain great fuel economy, while the chief endeavors of the builders of small engines is to build a simple and reliable engine for the use of inexperienced persons. While the smaller type of two stroke engine (less than 25 horse-power) has not been used in stationary practice to any extent, owing to the defects just named, or on automobiles, it has been widely used on motor boats, a service for which it is peculiarly adapted. Its extended use on boats is due to the fact that in such service it runs at practically a constant speed and works against a steady load, the conditions that are most favorable to the type. With automobiles where the motor speed is constantly varying, as well as the load, this type of motor is not flexible enough to meet the continually varying conditions.

The small two stroke motors are divided into two principal classes, the two port and three port type, depending on the method by which the charge is transferred to the cylinder. No valves are used in the cylinders of either type for the admission or release of the gases. As the two strokes of the cycle are the compression stroke and working stroke, it is evident that the charge must be introduced into the cylinder by means other than by the suction of the piston and at a time when there is no pressure in the cylinder. This is accomplished by a preliminary compression of the charge in the crank case which places the mixture under sufficient pressure to force it into the cylinder at the end of the working stroke and at the same time to displace the burnt gases left from the previous explosion. It should be noted that the incoming mixture is a substitute for both the suction and scavenging strokes of the four stroke cycle engine.

A diagrammatic view of a two port, two stroke cycle engine is shown by Fig. 5, in which P is the piston, C the crank case, I the transfer port, V the inlet valve, E the exhaust, and S the spark plug for igniting the charge. It should be noted that there are no valves in the cylinder and only three moving parts. The cycle of events for the two port type is as follows:

STROKE 1. We will consider the piston to be moving up on the compression stroke as shown in view (A), compressing the mixture in the combustion chamber D. While moving upwards in the direction of the arrow, the piston creates a vacuum in the crank case C drawing fresh mixture into the crank case. The piston at this time is covering the opening of the transfer port I and the exhaust port E so that the compressed mixture in the cylinder cannot escape. On reaching the end of the compression stroke, a spark occurs at S which drives the piston down and turns the crank towards the right as shown by the arrow.

STROKE 2. When the piston uncovers the exhaust port E on its downward working stroke as shown by view B, the exhaust gases being under pressure rush out into the atmosphere as shown by the arrows, and relieve the pressure in the cylinder. Some of the burnt gas remains in the cylinder at atmospheric pressure as there is no scavenging action up to this point. While the piston has moved down on the working stroke it has compressed the mixture in the crank case ready for admission to the cylinder. The valve V prevents the escape of the gas during the compression.

On reaching the end of the stroke the piston uncovers the transfer port which allows the compressed mixture in the crank case to rush into the cylinder through I, as shown by view C. Owing to the shape of the deflector plate Z on the piston head, the stream of mixture issuing from I is thrown up toward the top of the cylinder, as shown by the arrows, and consequently sweeps the remainder of the burnt gas before it through the exhaust port E. In this way the fresh mixture from the crank case scavenges the cylinder and fills it in one operation. Being filled with gas, the piston now moves up on the compression stroke for the next explosion as shown by view A.

Unfortunately the scavenging action of the incoming gas is not complete for the whirling motion of the charge causes it to mix with the residual gas to a certain extent which, of course, reduces the heating effect of the fuel and reduces the power output. Another factor that reduces the output of this type of engine is the loss of explosive mixture through the exhaust port at low engine speeds with an open throttle. In this case, the piston speed being low, part of the mixture has time to pass over the deflector plate and through the exhaust opening before the piston closes the exhaust port. At very high speeds the charge is diluted by a considerable quantity of burnt gas which has not had time to escape through the port causing a further loss of power. With the throttle nearly closed on a light load, the impact of the incoming mixture is so slight that the percentage of exhaust gas left in the cylinder is very high. This dilution is so great that with moderately low speeds (easily within the capacity of the four stroke cycle engine) it is either impossible to ignite the charge or it is impossible to ignite two in succession.

In marine service where the loads are constant, and the speeds fairly uniform, there is but little trouble from the last mentioned source, and as the fuel is usually a smaller item than the repair bill, the simplicity of the small two stroke engine with its freedom from mechanical troubles usually gives satisfactory results in the hands of the novice.