CHAPTER VI
TWO STROKE CYCLE ENGINES
(30) The Junker Two Stroke Cycle Engine.
The Junker two stroke cycle engine stands unique among the large stationary units not only in the principle of its working cycle but in its construction as well, and while it may be considered freakish when compared to standard practice it has proved its value in many European installations. The combustion occurs in the center of an open ended cylinder between two pistons that are forced in opposite directions by the expansion of the gas, and as there is a single acting piston in each end of the cylinder at the end of the stroke, there is no need of stuffing boxes, cylinder heads or valves.
It is apparent that by moving the pistons in opposite directions, the effective piston velocity is twice that of the actual velocity of either of the pistons, and that it is therefore possible to gain a high heat efficiency at high piston velocities with a low rate of rotation. The double pistons increase the scavenging effects, reduce the losses to the cooling water and increase the efficiency at light loads. A marked reduction in weight over the four stroke cycle engine is made possible because of the absence of valves and valve gear.
This engine is of the injected fuel type that is the fuel is sprayed into the combustion chamber after the completion of the compression stroke in a manner similar to the Diesel engine. By prolonging the injection of fuel after the piston has started on the outward working stroke it is possible to maintain the maximum pressure due to the combustion for a considerable period. This gives an indicator card that is very similar to that of a steam engine as the flat top of the Junker’s card due to the continued combustion and pressure corresponds to the admission line of the steam engine. As ignition is caused by the high temperature of the compression, almost any low grade oil may be used even down asphaltum oils and coal tar.
In Fig. 8 five piston positions corresponding to five events are shown by the diagrams a, b, c, d, e. From the diagrams one may also get an idea of the arrangement of the principal parts of the engine and their relation to one another. P and P2 are the two pistons, C the open ended cylinder, G the connecting rod of the inner piston P, H-H the two connecting rods of the piston P2, I-I the side rods of the piston P2, and V is the three throw crank shaft which is acted on by the three connecting rods H-H-G. The piston P2 is connected to the side rods through the yoke Y. It will be noted that the crank throws controlling the piston P2 are 180° from the crank connected to piston P, which causes the pistons to move in opposite directions.
With the pistons together at the inner dead center, the space between them is filled with highly compressed air from the previous combustion stroke. At this point the fuel is injected into the highly heated air, and the expansion of the charge begins, the combustion proceeding under constant pressure during the first part of the stroke, or during that part of the stroke in which the fuel is admitted to the cylinder. When the supply of fuel is cut off the working stroke continues by the increase of volume, or expansion of the gas, the gases being reduced to nearly atmospheric pressure at the end of the stroke with the pistons at the position shown by diagram (b). At this point the piston P is just opening the edge of the exhaust port M, allowing the products of combustion to escape to the atmosphere through the annular exhaust passage that surrounds the port M.
As the pistons continue to move outwards the gases continue to issue from the exhaust port at practically atmospheric pressure until the position shown by diagram (c) is reached by piston P2. At this point P2 is just opening the inlet port N allowing fresh air to enter the cylinder for the purpose of scavenging the engine. The passage of the air through the intake port N and out through the exhaust port M continues until the pistons pass the outer dead center, shown by diagram (d), and begin to come back on the return stroke. In diagram (e) the pistons have traveled far enough to close both ports, and as the space between them is filled with pure air from that furnished by the port N, the pistons will continue to move toward one another on the compression stroke. When they have reached the end of their travel as shown by diagram A, the fuel is injected into the cylinder and combustion occurs due to the temperature of the high compression temperature.
This is the complete cycle of events made in two strokes, and it will be noted that the cycle has been accomplished without the use of valves. The compressed air for scavenging the cylinder is provided by air pumps that are driven from the connecting rods by a link motion. One low pressure pump for the scavenging and one high pressure pump for spraying the fuel into the cylinder against compression are provided. As the inside of the piston is always exposed to the atmosphere through the open ends of the cylinder and is never exposed to the heat of combustion, perfect cooling is secured, and as a matter of course, perfect lubrication.
In the two cylinder engine in which four pistons are used, the cylinders are arranged in tandem with the two adjacent pistons, and the two outer pistons connected respectively. In fact the second cylinder pistons are duplicates of those just shown and are connected to the linkage in such a manner as to have the corresponding pistons in one cylinder act with the corresponding pistons in the second.
(34) Koerting Two Stroke Cycle Engine.
One of the most prominent of the two stroke cycle scavenging engines built for heavy stationary service is the Koerting engine. Because of its peculiar scavenging arrangement, and as it is of the double acting type, it will serve to illustrate the cycle of that class of engine equipped with independent air pumps. Several of these engines are in use in Europe that have an output of over 4,000 horse-power, the general arrangement of which is the same as shown in the accompanying diagram Fig. F-11.
Since the engine is double acting, two similar combustion chambers are provided at each end of the piston as shown by C and C_{1}, and as each of the chambers gives one impulse per revolution because of the two stroke cycle, the single cylinder shown in the figure delivers two impulses per revolution to the crank-shaft. In order to have one exhaust port serve for both combustion chambers, the annular port E is placed in the center of the cylinder so that it is alternately opened to C and then C_{1} as the piston travels to and fro, the port being covered by the piston at intermediate points in its travel. As the piston must cover the port for a considerable portion of the stroke, it is made very long, nearly as long as the stroke. The piston rod R that connects the piston with the crank passes through the cylinder head of chamber C_{1}, surrounded by a gas tight packing that prevents the leakage of the charge from C_{1}.
Unlike the ordinary type of two stroke cycle engine, the two combustion chambers are provided with mechanically operated inlet valves, V-V_{1}-V_{2}-V_{3} that are opened at definite points in the stroke by the lay shaft X which is driven from the crank shaft. As the exhaust port E serves all of the functions of an exhaust valve, there are no valves provided at this point. Exhaust pipes connected to E carry the burnt gases to the atmosphere.
Two auxiliary air pumps of the double acting type are provided, shown at A and A_{2}, one pumping gas and the other air. They are driven from the crank-shaft through the connecting rod Y, and are proportioned so that together they force a mixture of the correct proportion for complete combustion into the working cylinder at a pressure of about ten pounds per square inch. Air and gas are compressed on one side of each pump piston in the spaces B and B_{2}, and the air and gas are drawn in on the other side as at H and H_{2}. The connections from the compressor cylinders to the working cylinder are arranged so that the two crank ends of the compressor cylinders discharge into the crank end of the working cylinder, and the front ends of the compressors discharge into the front end of the working cylinder, the exact moment of discharge being controlled by the inlet valves V-V_{1}-V_{2}-V_{3}. The pumps are arranged so that only pure air is admitted at first in order to force the products of combustion through the exhaust port so that they will not contaminate the following mixture of air and gas. The inlet valve opens immediately after the piston of the working cylinder uncovers the port E and reduces the pressure of the burnt gases to that of the atmosphere.
By the action of the admission control, the scavenging air first admitted, is prevented from mixing with the residual gas from the previous explosion, and in the same way the device prevents the loss of fuel through the exhaust ports, thus overcoming the principal objections of the simple two stroke types described earlier in this chapter. The compressor cylinders provide only enough air and mixture for one stroke and no reservoir is provided for a surplus of air or mixture.
As the piston moves forward, on the compression stroke and covers the exhaust port, the inlet valves also close, and the compressor pistons arrive at the end of their stroke so that no more air or mixture is delivered to the inlet valves. At the end of the compression stroke ignition occurs and the expansion or working stroke begins. The piston again moves to the right on the working stroke until the front edge uncovers the port E where the exhaust gases escape to the atmosphere.
The valve gear on the gas compressing cylinder is arranged so that no gas is delivered to the inlet valves of the working cylinder until the air cylinder has provided sufficient air to insure perfect scavenging of the products of combustion, this preventing the fuel from becoming contaminated with the burnt gas. Speed regulation for varying loads is effected by shifting the valve gear of the gas pump so that the gas is delivered at an earlier or later period in the stroke of the working piston, thus causing a variation in the quantity of gas delivered to the working cylinder. This is controlled by the governor directly on the valve gear of the pump or upon a by-pass in the pump cylinder or both. The by-pass, when open returns all of the gas in the passage leading to the inlet valve, that is beyond a certain pressure to the cylinder, so that the gas is delivered to the cylinder at a constant pressure, and therefore in proportion to the load and point of cut off.
This method of governing produces a mixture that varies in richness with the different loads that are carried by the engine, but as the air enters the cylinder first and is prevented from mixing to any extent with the gas by the shape of the cylinder heads, the igniting value of the mixture is not disturbed particularly as the rich gas remains in the cylinder heads and in contact with the igniters.
Like all large engines, the Koerting is started by compressed air taken from a reservoir. A special starting valve is provided for each end of the cylinder which is operated from the cam shaft by means of an eccentric. The air valves may be thrown in or out of gear by a clutch.
(57) Two Stroke Cycle Rail Motor Cars.
A unique application of the two stroke cycle motor will be seen in Fig. 56 which shows a Fairbanks-Morse two stroke cycle motor direct connected to the driving wheel of a railway motor car. The three cylinders are mounted between the driving wheel with the ends of the axle terminating in the crank cases of the motors. Access to the bearings is had through a cover on the crank-case. The simplicity of this motor and its freedom from valves, cams, springs, gears, and other trouble causing parts makes it particularly adapted for the service that it performs in the hands of unskilled track laborers. As there is no water to freeze or leak, and as the lubricant is mixed with gasoline, the car needs very little more attention than the old type hand car.
The car is started by opening the gasoline supply cock, closing the ignition switch, and pushing the car along the track until the first explosion occurs. The speed is controlled in the usual manner by means of the spark advance and throttle. As the motor is of the two stroke cycle type, it may be reversed by simply changing the position of the timer without the use of the gears. The speed is the same in either direction. By the use of three cylinders, three impulses are obtained per revolution which gives a distribution of power equal to that of the ordinary six cylinder, four stroke cycle automobile motor.
For larger cars built for carrying large gangs of men, a three cylinder motor is used which drives through a clutch and gears, similar to that used on automobiles. It is located near the center of the axle and is supported on a frame that is independent of the car proper. This motor unit is easily removed from the car for inspection with all of the parts intact. A universal coupling is provided on the motor shaft to prevent strains due to changes in the alignment from being thrown into the motor. The motor of this car is started with a crank, and may be left standing with the motor running. As with the two cylinder car, the engine is reversible, and is lubricated by mixing the lubricating oil with the gasoline.
(58) Rotating Cylinder Two Stroke Cycle Motor.
An unusual type of two stroke cycle engine is that designed by M. Farcot for aeronautic work. It is of the rotating cylinder type in which the cylinders rotate about a stationary crankshaft, and unlike all previous two stroke motors, whether of the revolving or stationary cylinder type, no initial compression is performed either in the crank-case or otherwise.
Undoubtedly the two-cycle rotating multi-cylinder engine has a future when some of the particularly difficult designing problems involved in its production have been successfully tackled. Crank case compression has had its devotees, but so far it has entailed the use of a low compression, owing largely to the difficulties involved in lubricating the bearings and maintaining gas-tight joints, besides other defects. Some of these barriers appear to have been surmounted in this design.
Fig. 63 of the accompanying drawings is a sectional side elevation of the engine, which, it will be seen, is similar in general disposition to the usual arrangement of the rotating cylinder type. In this particular case, however, the short end A of the stationary crankshaft is reduced in diameter at B, and on this part are mounted ball bearings C carrying the circular casing of a rotating centrifugal blower D. To the inner end of the hub of this blower is attached a gear wheel E, the teeth of which mesh with small intermediate pinions carried on a spider F attached to the crankshaft. These pinions are in turn driven by an internally toothed ring G attached to the hub of the crank case H. Thus the blower D is driven in the opposite direction to the crank-case and at a higher speed. In the interior of the blower casing radial blades K are provided.
A hollow annular casing L is bolted to the cylinders, and communicates with their interiors by means of inlet ports M covered and uncovered by the pistons.
The blower casing D has on either side circumferentially flanged rings N, which are a running fit in circular register slots provided in the annular casing L and its cover plate P, in order to provide a gas-tight joint between the opposite revolving casings D and L. Fan blades Q are also provided in the casing L to accelerate still further the incoming gas. The arrangement of the two sets of blades is made clear in the sectional sketch (Fig. 64). It will be realized that by means of this compound blower device a considerable pressure can be attained.
The crankshaft is drilled to provide a feed for the gasoline, which is atomized by a device R in the large central opening of the blower casing D by means of pressure fed from the annular casing L through suitable leads S.
As each piston nears the bottom of its stroke, exhaust ports T, provided with expansion cones for the purpose of increasing the velocity of the exhaust gases, are opened. The inlet port M is then uncovered, and the compressed charge rushes into the combustion chamber.
The general design of the engine is made plain by Fig. 63, but there is one other point to which reference should be made, and that is the provision of rings V, one on either side of the cylinders, to enhance the strength of the construction.
Although the difficulty of compression appears to have been cleverly tackled in this invention, the possibility of the compressed mixture in the inlet casing and blower becoming ignited at the moment of admission by a residue of exhaust gas in the combustion chamber still exists. However, the effect of such a backfire should not prove quite so serious as in some designs. Apart from other considerations, owing to the large area of the blower intake, such an occurrence should merely have a more or less elastic braking effect.
(60) Gnome Radial Two Stroke Motor.
The builders of the famous Gnome four stroke cycle rotary motor, Sequin Frères, have recently developed a radial two stroke cycle motor that bids fair to supplant their original type. Referring to the diagrammatic cross-sections which show only a single cylinder unit, a very long tubular piston will be seen that is divided into two independent chambers, A and B. Both chambers are placed in communication with the outside space, C and D.
The upper end of the piston is continued above the top division head of the chamber A, and the extension is provided with the slot F. Near the center of the piston, the walls of the piston are run out into a flat circular plate or trunk piston E, which is the actual piston head that receives the force of the explosion. The piston E reciprocates in the large cylinder H, which is reduced at its upper end to the diameter of the main piston barrel, for which it affords a sliding support, or guide, and also serves to aid the exhaust port closure. The lower end of the cylinder H is enlarged in diameter as shown by K so that a clear annular space is left between the cylinder walls and the piston head E, when the latter is at the bottom of the stroke. The cylinder diameter is then reduced to the diameter of the main piston barrel.
The motor operates as follows:
Suppose the piston to be ascending (Fig. 1), compressing the mixture above the piston head in the cylinder E, and at the same time the volume of the space M, below E, is being increased until the piston reaches the position shown in Fig. 2.
Referring to Fig. 1; the interior chamber A of the piston is in direct communication through the holes C with the space M, consequently as the piston goes up, a partial vacuum will be formed in these two chambers. When the piston reaches the top of its stroke as shown in Fig. 2, the holes D in the lower end B of the piston are uncovered as they rise into the increased diameter of the cylinder, and therefore the mixture is sucked in from the crank case until the chambers A and M are filled to atmospheric pressure.
The spark now occurs at the plug S, and the explosion takes place, driving the piston downwards as shown by Fig. 3, just before the exhaust takes place. The volume of the chamber M has now been decreased with the result that the mixture will have been compressed into the chamber A.
In Fig. 4, the piston has now reached the bottom of the stroke, and the ports F have opened as the slots carry below the upper end of the cylinder where the bore is increased. At the same time, as the piston plate E passes the bottom of the cylinder H into the enlarged diameter K, the compressed mixture in A and M rushes through the annular space opened around E into the combustion chamber and drives out the residual burned gases which still remain after the explosion. On starting the second revolution the piston rises and the cycle repeats as shown by Fig. 1.
This engine may be built with any number of the cylinder units described, preferably with an uneven number, as in the case of the Gnome radial four stroke cycle, and with twice the number of impulses of the four stroke type a very uniform turning movement should be had.
Since the valves are the parts that give the most trouble in the four-stroke cycle Gnome, this motor should be better adapted for aviation than the original type of Gnome.
(62) Variable Speed Two Stroke Motor.
A variable speed two stroke cycle motor is described by C. Francis Jenkins in the _Scientific American_ that seems to solve many of the problems encountered in designing a two stroke cycle motor for automobile purposes. As is well known, the present design of the crank-case compression type is wasteful of fuel, and ignites irregularly at low speeds and light running, and as nearly all automobiles are well throttled for a greater portion of the time it means that this type of motor is working under the greatest disadvantage.
Since the greater part of the trouble is due to the dilution of charge by the residual gases, and as the spark plug of the motor is situated in the most diluted portion of the gas, it would seem that a change of spark plug location, or a change in the circulation of the fresh mixture in the cylinder would be a great aid in remedying the difficulty. With the spark continually in contact with fresh undiluted mixture it would be possible to run it as low speeds as with the four stroke motor, with a corresponding increase in the efficiency, and opportunity to run with a constant advance of the point of ignition. This is accomplished by any or all of the following conditions:
(1.) By keeping good gas separate from bad.
(2.) By placing the spark near the intake port.
(3.) By leaving the plug in its present position and deflecting the fresh gas to meet it.
(4.) By changing the location of the inlet port.
In the motor invented and described by Mr. Jenkins, the method given by (4) is adopted as shown by Fig. 66, in which the spark plug is placed at the point of admission of the gas and in a confined passage. The operation of the motor is as follows:
Carbureted gas is drawn into crank-case from the carburetor (not shown) in the usual manner, i. e., by the upward movement of the piston; and by its downward movement is forced through the rectangular port in the wall of the piston into the combustion passage within the water-jacket when the port in the piston wall registers with the lower end of this combustion passage, and drives ahead of it the bad gas remaining after the previous explosion. If the throttle is wide open the combustion space above the piston will be completely filled, and on the ignition of the charge the maximum pressure will be exerted on the piston. If, however, the throttle is but slightly open, the combustion passage only may be filled and none overflow into the combustion space above the piston. This small charge will be just as efficient in proportion to its volume as was the large charge, for it was compressed to practically the same extent and none was mixed with the bad gas of the previous explosion. It will, therefore, be obvious that the spark-plug is always swept by the fresh charge, be it large or small, and the ignition will be just as certain in one case as in the other, although the charge and consequent impulse may be only just sufficient to keep the engine turning over, and without missing a single explosion.
In the motor built to test and demonstrate this design, provision was made for a second spark-plug to be located in the top of the cylinder for speed work, if this was found necessary. No opportunity has yet been had for making track tests, though without regret, as this two-cycle motor will run idle without missing or “stuttering,” which was the thing heretofore impossible.