CARL H. CLARK
The great increase during the last few years in the use of automobiles, power boats, and other units of small power, has led to the expenditure of a large amount of time and money in the development of the gas and gasoline engine. These engines are particularly well suited to their present uses, being as they are, self-contained, easily installed and operated and reasonably reliable when in good hands. Since there is no external generator or boiler, and no fuel to be handled, they are easily kept clean, and as the supply of fuel is automatic and continuous, they run with the minimum amount of care, and little labor is required other than the lubrication and the regulation of the fuel supply.
The principle upon which the operation of the gasoline engine depends is the fact that a body of gas tends to expand upon the application of heat, and if allowed to do so, has the power of doing work. The heat is contained in the fuel as potential energy and is freed by the combustion, causing a rapid rise of pressure in the body of the gas. When this body of gas is confined in the cylinder of the engine it is capable of doing work upon the piston.
The gas engine is of the type technically known as internal combustion engines. This name originates from the fact that the combustion of .the fuel and the consequent generation of heat take place directly in the cylinder of the engine instead of in a separate chamber or boiler, as in the steam engine. In the latter engine the heat is generated in the boiler, raising the pressure of the contents, which are carried to the engine and allowed to expand, thus utilizing the heat. The steam engine is thus complicated by the boiler and extra piping, and the necessary care to feed the fuel and maintain the proper quantity of water in the boiler. These duties usually require the entire attention of one man, while with the gasoline engine one need only give an occasional oversight.
Gas or gasoline engines may be divided into two general classes, the two cycle, and the four cycle, the principles of operation of which are quite distinct. The former, or the two cycle type, is the simpler in operation and will be considered first. In either type of engine there are four operations to be accomplished; (1) drawing in the fresh charge, (2) compressing and firing the charge, (3) expansion of the ignited charge, and the absorption of its power, (4) expulsion of the burnt gases. The completion of this series of events is termed the "cycle"
The general outline of the twocycle engine is shown in Fig. 1, where A represents the cylinder, B the piston, E the connecting rod, CI) the crankshaft and pin. G is the crank case, which must be air tight. F is the inlet opening into the crank case, which is provided with a valve allowing the gas to enter, but not allowing it to return. II is a passage leading from the base into the cylinder at the inlet port I, which is
above the piston when the latter is in its lowest position, as in Fig. 2; at K is another port called the exhaust port which opens into the air; at L is some means for producing an electric spark to ignite the charge. For the operation, suppose the piston to be at the bottom of its stroke, and to ascend; this action will create a suction in the air-tight crank-case and draw in a charge of vapor, as shown in Fig. 1. On the next downward stroke the non-return valve on the inlet F prevents the charge from being forced out again, and it is compressed slightly, banking up the pressure in the passage HI, the outlet of which is shown closed by the piston in Fig. 3. When the piston nearly reaches the bottom of its stroke it uncovers the inlet port J, and the charge rushes in and fills the cylinder, as in Fig. 2. Before, however, any of the new charge can escape through the exhaust port K, which is also open, the piston has begun its next up stroke and covered the inlet port, so that the cylinder is now full of fresh gas. The upward stroke continuing, the charge is compressed into the space above the piston, until the latter reaches its highest point, when the compressed charge is ignited by the spark at L, Fig. 1. This ignition produces a powerful impulse due to the heat generated by the combustion, which drives the piston down. When the piston has nearly reached its lowest point, as in Fig. 3, it uncovers the exhaust port K and allows the burnt gases to partially escape. A moment later in the stroke, the inlet port I
is uncovered and a fresh charge is admitted from the crank case which fills the cylinder, as before, and drives out the remainder of the burnt gases. This new charge is then compressed, and a new supply drawn into the crank case, and the operation continues.
Following the sequence carefully it will be seen that the cycle is completed during every revolution, or for every two strokes. It is thus called the two-stroke cycle, or as commonly stated, the "two cycle." This
cycle gives an impulse or working stroke during each revolution. The momentum of the flywheel is depended upon to carry the piston up on the idle or compressive stroke. The projection on the top of the piston is a deflector or shield surrounding the inlet port to deflect the gas upwards and prevent its rushing directly across the cylinder and out of the exhaust port. The exhaust port is directly opposite and somewhat higher than the inlet port, in order that the pressure may be reduced and the burnt gases partially escape before the fresh gases are admitted. The relative size and position of these two ports is the key to the success of the two cycle engine. It will be seen that the piston acts as its own valve, so that this engine, from its very principles, is valveless. The claims of some engine builders as to the "valveless" featuresof their engines are therefore entirely superfluous, as it would be difficult to build this type of engine in any other manner. In the four cycle type of engine the admission and exhaust of the vapor are controlled by mechanical means. In Fig. 5, A is a valve opening into the cylinder from the admission chamber C. B is a similar valve opening from the cylinder into the exhaust chamber D. The.other portions of the engine are substantially the same as those of the two cycle engine, with the exception that the crank case does not require to be air tight. The valves are controlled from the main crank shaft.
For the operation, suppose the piston to be a the top of its stroke and to travel downwards, as in Fig. 4; the inlet valve A is open and the suction draws in a charge of fresh gas, filling the cylinder. On the next upward stroke, shown by Fig. 5, both inlet and exhaust valves are closed, and the gas is compressed into the space above the piston. When the piston reaches the top of its stroke the compressed charge is fired by an electric spark, or other means, and it expands, driving the piston down as in Fig. 6 and furnishing the working impulse. On the next up stroke, illustrated by Fig. 7, the exhaust valve B opens, and the burnt gases are forced out by the piston through the exhaust port. The cylinder is now clear and ready for the admission of another fresh charge on the next downward stoko of the piston.
This cycle is completed in two revolutions, or four strokes, and is called the four stroke cycle, or "four cycle". There are three idle strokes and a working stroke for each cycle, thus giving an impulse for every alternate revolution. The flywheel must be heavy enough to carry the piston over the three idle strokes.