The two cycle engine has the advantage of extreme simplicity, as there are no valves or other external moving parts which are likely to require adjustment. Since it receives an impulse for each revolution, more power may be obtained from the same size engine than is possible in the four cycle type; it might seem that since the two cycle engines receive twice as many impulses as the four cycle, twice as much power should be obtained, but this is by no means so, as owing to the superior regulation of the four cycle types the difference is much less. The more frequent occurrence of the impulses does, however, allow a lighter fly-wheel and produces a smoother running engine with the least vibration.

The valveless feature of the two cycle type gives rise to some uncertainties and irregularities in the action of the engine. The action of the gases in the cylinder is more or less uncertain, as it is hardly to be expected that the inflow of gases will continue until just the right time to fill the cylinder and no more; it is entirely possible either that some of the exhaust may not have time to escape, or that some of the fresh gases may pass over and out of the exhaust. It is hardly possilble, also, for the new charge to entirely scour the upper parts of the cylinder, and some waste gas is sure to be caught in the cylinder, thus diluting the new charge. The driving out of the burnt gas by the fresh mixture while some combustion may still be going on, frequently results in the ignition of the new charge and the explosion of the reserve in the base, producing a back explosion, causing irregular action, and even stopping the engine.

There are also some disadvantages which might be termed structural ones. While the working parts are very simple, they are entirely enclosed and not easily open to examination. Since the crank case requires to be air tight any leakage around the crank shaft bearings from natural wear, causes a loss of crank-case pressure, and consequent loss of power. Any leak around the piston and rings will allow the partially burnt gases to pass down and deteriorate the quality of the fresh gas in the crank case. If the workmanship on the engine is originally poor, as in some of the cheap engines, these troubles are likely to occur soon, and in any case they are apt to occur after prolonged use.

The four cycle type, although more complicated,is surer and more certain in its action, as the behavior of gas is exactly controlled. The idle stroke allows the cylinder a very short time to cool between explosions.

As the flow of gases takes place only on each alternate revolution, and then during a whole stroke instead of in a puff, the four-cycle engine may be run at a

Higher rate of revolution. Owing to the mechanical regulation, there is less chance for loss of fuel and the economy is greater. No enclosed crank case is necessary, and the working parts can be more easily cared for. On the other hand, the three idle strokes require a very heavy flywheel, and since the impulse occurs on each alternate revolution, the four cycle engine must for the same power, be larger and heavier than the two cycle. Each explosion or impulse is much heavier than in the two cycle, and tendency to vibration is consequeutly much greater.

The two cycle engine, in spite of the more or less theoretical disadvantages, has reached a high state of perfection, both as to reliability and economy, although in the latter respect it is probably not the equal of the four cycle. As a general conclusion it may be stated that for small, light engines where economy is of small moment and especially for those of one cylinder, which as a rule receive little care, the two cycle type is to be preferred; while for engines of larger size, where economy of fuel is a consideration, together with great reliability, the four cycle type should be employed.