[CHAPTER X]
TWO-STROKE ENGINES
In the two-stroke type of petrol engine the cycle of operations is completed in two working strokes of the piston instead of the four required by the “Otto” cycle; there is thus one explosion or power stroke in every revolution of the crankshaft. Theoretically this represents a great advance over the “Otto” cycle, but difficulties and complications arise in the practical carrying out of the cycle. The cycle on which it is desired to operate the engine is: 1st stroke—Compression; 2nd stroke—Explosion. The charge would be introduced on the compression stroke and exhausted towards the end of the explosion stroke.
Now the charge of gas required by the engine consists of a mixture of petrol vapour and air, and it must either be sucked in or pushed in under pressure. In the “Otto” cycle the charge is sucked in, and in the two-stroke cycle it is delivered to the cylinder under pressure; hence in the two-stroke cycle some form of pump is required which will suck in the charge of air and gas, compress it a small amount, and deliver it to the working cylinder at a pressure of 5 or 6 lb. per square inch above atmospheric pressure. This is where the complications commence; if we fit a separate pump for each cylinder, which is what would generally be done, or if we made one pump serve for two cylinders, we have to provide pump cylinders, pistons, rods and valves, and therefore there is practically no gain over the four-stroke engine. Hence it is that inventors all try to avoid the use of a separate charging pump and turn their attention to the production of an engine in which one or more of the existing portions is made to serve as a pump for charging the working cylinder or cylinders with gas. A favourite and fairly successful device is to make the crankchamber gas-tight and use it as the cylinder of the pump, the underside of the engine piston then forming the pump piston which draws the charge from the carburettor into the crankchamber on its upstroke and compresses it on its downstroke, delivering it to the working cylinder through the inlet port as soon as the piston has uncovered it by its downward movement.
Fig. 63.—Two Port Type of Two-Stroke
Engine with Crankcase Compression.
There is no exhaust valve, as the piston uncovers the exhaust ports a little before the inlet ports are opened. To prevent the new charge escaping directly across the top of the piston from the inlet ports to the exhaust ports, a deflector is fitted on the top of the piston equal in height to the height of the exhaust opening and situated immediately in front of and facing the inlet ports.
A two-stroke engine of the type referred to is shown diagrammatically in Fig. [63]. E is the gas-tight crankchamber, upon which the water-cooled cylinder A is mounted in the usual manner and fixed by studs or bolts. The piston P carries the deflector H, which is equal in height to the height of the exhaust opening G. The piston rings are prevented from turning by pins so arranged that the joint of the rings does not pass across the ports. The connecting rod D is of usual form, and also the crankshaft C. The carburettor, or induction pipe leading from the carburettor, would be attached to the flange L, and the automatic valve F controls the admission of gaseous mixture from the carburettor to the crankchamber. The inlet ports N are often only half the height of the exhaust ports. On the upstroke of the piston a partial vacuum will be formed in the air-tight crankchamber, which will allow the atmospheric pressure to force open the valve F against the pressure of the spring and enable the air to flow into the crankchamber through the carburettor and induction pipe, carrying the charge of petrol vapour with it. We must note, however, that no vacuum can be formed until the port N has been covered up by the piston, so that a portion of the stroke is idle. On the downward stroke of the piston the charge in the crankchamber is compressed, and as soon as the piston uncovers the ports N the charge from the crankchamber flows up into the working cylinder, displacing the burnt gases as it comes into the cylinder. Exactly what happens next it is difficult to say; the probability is that this new charge rises in the cylinder a short distance (but not a sufficient amount to displace all the dead gases from the top end of the cylinder) and that some of it gets squeezed out of the exhaust opening as the piston rises and before it has had time to cover the exhaust ports. Thus, owing to the idle portion of the stroke during admission to the crankchamber and to the low compression pressure adopted in the crankchamber, the pumping portion of the engine has what is termed a very low volumetric efficiency.
It can be proved that this type of engine which endeavours to draw sufficient gas to fill its working cylinder into the crankchamber by means of a piston having only the same diameter as the diameter of the working cylinder itself, and which cannot avoid some idle movement during the operation together with further loss from the exhaust opening, is incapable of retaining more than a little over one-half a cylinder full of fresh combustible gas at the instant when compression commences; the remainder of the contents must be dead exhaust gas. Thus, even allowing for the double number of power impulses resulting from the use of the two-stroke cycle, it is difficult to see how this form of engine could ever give more than about one and a quarter times the power of a four-stroke engine having the same bore and stroke even when the many difficulties experienced in the practical working of two-stroke engines have been overcome. To use a high compression pressure in the crankchamber would increase the volumetric efficiency, but would result in lost work during the pumping process, besides being undesirable at the delivery stage of the process; it is much better for the transfer of the gases to take place as gently as possible. If too high a delivery pressure is used the fresh gas will enter in a sharp gust and get badly contaminated by mixture with the foul exhaust products instead of gently displacing them in bulk. The use of an automatic valve is very desirable for the gas inlet to the crankchamber, but unfortunately it limits the speed of the engine and also its flexibility or ability to pull well at all speeds. An engine with an automatic valve runs best at that speed for which the tension of the spring is most suitable. If the spring is weak the speed will be low. Tightening the tension on the spring will allow the engine to speed up, but will prevent it running well at low speeds. At high speeds and with correspondingly high tension the valve does not give enough opening, and therefore limits the power of the engine. It will, therefore, readily be seen that when a two-stroke engine with automatic inlet valves is pitted against a four-stroke engine with mechanically-operated inlet valves, the comparison is unfair to the two-stroke cycle. With the position and arrangement of ports shown in the drawings, one must have a deflector on the piston head to prevent excessive loss of fresh gas through the exhaust opening. After the engine has been running for some time at a high speed this deflector becomes very hot, and as a general rule the cooling effect of the incoming gases is not sufficient to prevent it attaining a red heat on the compression stroke, thus igniting the charge before the piston reaches the top of the stroke. This defect, which is called pre-ignition, causes the engine to knock, and results in a loss of power; it may be partly overcome by admitting lubricating oil with the charge, the oil then serving to cool the deflector as the charge enters the cylinder. At high engine speeds there is great risk of the hot exhaust gases in the working cylinder setting fire to the incoming charge in the inlet ports, thus causing backfiring into the crankchamber. To avoid all possibility of backfire, some form of air scavenging must be adopted, but the general arrangement of this form of two-stroke engine does not lend itself to such an addition—it would merely reduce still further the quantity of gas reaching the cylinder.