POWER STROKE
The increasing size of the combustion chamber as the piston moves outward on the power stroke permits the gases to expand, and in doing so the temperature will fall, the pressure decreasing in consequence. A further decrease in pressure is caused by the hot gases being in contact with the metal cylinder and piston, which absorb heat. The more slowly the engine runs, the longer the gases will be in contact with the cylinder walls, and the more opportunity there will be for loss of heat from this cause; at higher speeds, there will be less time for heat to be absorbed by the cylinder walls, and more will be utilized in expanding the gases and producing work.
Even at the outmost position of the piston, the combustion space will not be large enough to permit the gases to expand until their pressure has dropped to that of the atmosphere, so that they will still be exerting pressure. By opening the exhaust valve, the gases will have an outlet for expansion, and will begin to rush out. While the pressure might be utilized against the piston to the end of the power stroke, it has been found that better results are obtained by opening the exhaust valve before the piston reaches the end of the power stroke. There is then a higher pressure forcing the gases out than there would be later in the stroke, and the greater quantity of gases that escapes leaves less to be expelled during the exhaust stroke.
EXHAUST STROKE
The inward movement of the piston pushes out of the open exhaust valve the gases that have not escaped through their desire to expand. The exhaust valve remains open for the entire stroke, but when the engine is running at high speed the piston moves so rapidly that the gases cannot escape fast enough to prevent their being slightly compressed. When the piston is at its inmost point, the gases are still flowing through the valve because of this slight compression, and if the valve closed, a portion would be retained. The best results come from the closing of the exhaust valve not at the end of the exhaust stroke, but a short time after the piston has begun to move outward again, during which period the compression forces the gases out. The exhaust valve closes at the point when the slight compression has been reduced to the pressure of the atmosphere by the escape of the gases and the enlargement of the combustion space.
If the piston completely filled the combustion space when at its inmost point, all of the burned gases would be expelled, but the necessity for leaving a space in which combustion may take place renders this impossible, and a small portion of the burned gases therefore remains in the cylinder. The space between the cylinder head and the piston when at its inmost point, called the clearance, should be as small as possible, in order that the amount of these gases remaining in the cylinder may not be sufficient to contaminate the fresh charge and weaken the pressure of its combustion.
The passages through which the burned gases are led away from the cylinder must be large and free from obstructions, for if a free flow is not permitted back pressure will be set up, which will prevent the largest possible amount of gases from escaping, and leave a greater portion to contaminate the fresh charge.
The power that a gasoline engine is capable of developing depends on the size of the cylinder, the pressure acting on the piston, and the speed at which it operates. A steam engine, which obtains its pressure from a boiler, can do work as soon as the steam is turned into the cylinder, but a gasoline engine must be running before it can be called on to deliver power. Because of the cycle of events on which its operation depends, the piston must be forced to perform the inlet and compression strokes before pressure can be developed, and it is necessary to revolve the crank shaft by outside means until a charge of mixture has been taken into the cylinder, compressed, and ignited, when the engine begins to work by the pressure from the combustion, and takes up its cycle. Not until this has been done can it be called on to do work.
A steam engine can be made to deliver more power than it is built for by increasing the pressure acting against its piston, and the full pressure of the boiler can be utilized when extra work is necessary. The power developed by a gasoline engine being greatly dependent on its speed, and there being no reserve by which greater power can be developed in emergencies, it is necessary for an engine of this type to be perfectly adapted to the work that is desired of it. At excessive speeds the piston acquires great momentum, which must be overcome at each end of a stroke by the crank shaft, and while a speed above normal may be attained, it results in the quick destruction of the bearings and the severe straining of the engine. The best results in efficiency and long life accompany the running of the engine at the slowest speed possible for the development of the required power.