THE TWO-STROKE MOTOR
In the ordinary "Otto-cycle" motor an explosion occurs once in every two revolutions of the crank. With a single cylinder the energy of the explosion must be stored up in a heavy fly-wheel to carry the engine through the three other operations of scavenging, sucking in a fresh charge, and compressing it preparatory to the next explosion. With two cylinders the fly-wheel can be made lighter, as an explosion occurs every revolution; and in a four-cylinder engine we might almost dispense with the wheel altogether, since the drive is continuous, just as in a double-cylindered steam-engine.
The two-stroke motor, i.e. one which makes an explosion for every revolution, is an attempt to unite the advantages of a two-cylindered engine of the Otto type with the lightness of a single-cylindered engine. As it has been largely used for motor boats, especially in America, a short description of its working may be given here.
In the first place, all moving cylinder valves are done away with, their functions being performed by openings covered and opened by the movements of the piston. The crank chamber is quite gas-tight, and has in it a non-return valve through which vapour is drawn from the carburetter every time the piston moves away from the centre. There is also a pipe connecting it with the lower part of the cylinder, but the other end of this is covered by the piston until it has all but finished its stroke.
Let us suppose that an explosion has just taken place. The piston rushes downwards, compressing the gas in the crank chamber to some extent. When the stroke is three-parts performed a second hole, on the opposite side of the cylinder from the aperture already referred to, is uncovered by the piston, and the exploded gases partly escape. Immediately afterwards the second hole is uncovered also, and the fresh charge rushes in from the crank case, being deflected upwards by a plate on the top of the piston, so as to help drive out the exhaust products. The returning piston covers both holes and compresses the charge till the moment of explosion, when the process is repeated. It may be said in favour of this type of engine that it is very simple and free from vibration; against it that, owing to the imperfect scavenging of exploded charges, it does not develop so much power as an Otto-cycle engine of equal cylinder dimensions; also that it is apt to overheat, while it uses double the amount of electric current.