Frank N. Martin.

The extensive increase within the last few years in the use of gas engines as generators of power for automobiles and launches, and to almost as great an extent for stationary purposes, has developed a general interest in their design and use, and it will be the purpose of this series of articles to so present the subject that, the reader will, at their completion, have sufficient knowledge to be able to clearly understand the general features of the different types, and the proper workings of their path.

The designation of "Gas Engine " as here used includes all engines using gas or oil vapor for producing an explosion in the cylinder of the engine, but owing to the limitations of space, only the more commonly used types will be described.

It is well to mention, however, that engines using kerosene or petroleum vapor, as well as acetylene gas, are receiving much attention at the hands of inventors, and successful results have already been achieved. In the near future, therefore, we may expect important developments in these other types of engines.

Gas engines require a mixture of inflamable gas or vapor and air, the mixture of the two being effected before introduction to the cylinder and subsequent combustion. Air is an important necessity for two reasons ; that of providing the necessary amount of oxygen to render combustion possible, and to secure the expansive effect due to the heat generated by the combustion or explosion. In fact, the greater the amount of air used in effecting the mixture, the more economical the engine, from the greater expansive effect of the larger volume of heated air.

The working process of a gas engine is as follows : - The mixture of explosive gas and air is introduced into the cylinder, then compressed explosion follows, generating a high heat and consequent pressure forcing the piston outward. The products of combustion, gases of a different character, are then expelled from the cylinder, which is then recharged with a fresh mixture and the process repeated.

The working cycle of an engine is seen, therefore, to consist of four parts; charging, compression explosion and expansion, and exhaust. When these are effected by four movements of the piston, two outward and two inward, or two complete terms of the crank, it is termed a " Four Cycle" engine. When only two movements of the piston, one in each direction are necessary, it is a " Two Cycle " engine. With the four cycle engine, during the first outward stroke of the piston, the mixture is drawn into the cylinder at atmospheric pressure by the suction of the piston, the inlet valves closing at the end of the stroke. The first inward stroke compresses the gas to a high pressure and occupping but a small portion of the space at the inner end of the cylinder. Combustion follows generally just previous to completing this inward stroke. The pressure caused by the heated gases and air then causes the second outward or impulse stroke, the return stroke clearing the cylinder of the gases produced by the combustion, and completing the cycle.

With the two cycle engine, the compression of the gas and air mixture is effected in an external chamber, usually the crank case, which is made strong enough to withstand considerable pressure, and small enough so that the piston on its outward stroke will cause the compression of the mixture therein contained. When the piston, on its outward travel, has reached a certain point, an inlet valve admits the compressed mixture in the crank case to the cylinder, the entry being rapid owing to its compression. The inward stroke of the piston then begins, the inlet valve closes, the mixture is compressed, then exploded at or about the end of the inward stroke when the compression is greatest, the high heat causes the outward or impulse stroke. At a certain point in the outward stroke an exhaust valve opens, the gases escape in part, so that the pressure is greatly reduced. This is quickly followed by the opening of the inlet valve, the incoming mixture of fresh gas assisting to force the exhaust gases through the exhaust opening, and the process is repeated as before. In this type of engine the several parts of a cycle more or less lap on to the one succeeding it, with the result that each part is not as fully completed as with the four cycle type, and the speed of the crank shaft cannot be as great, therefore, with the former as with the latter kind. The gain in power due to the increased number of impulses strokes is offset to quite an extent by the slower speed at which the two cycle engine must run.

The uses, therefore, for which the engine is intended, and its size and power enter considerably into the determination of the type to be selected. As a general proposition it may be stated, that for small engines of one cylinder up to 5 H. P. the two cycle type is most largely used, but for larger engines, and especially those of two, three or four cylinders, the four cycle is preferable. This has reference to marine and automobile engines. For stationary purposes, this statement must be modified by matters which will receive consideration in a future chapter.