| A × P × 2R × L × N × O | |
| H.P. = | |
| 33000 × C |
| When | A = | Area of piston in square inches. |
| P = | Average or mean effective pressure per square inch. About 75 pounds for Gasoline Engines. See Table on Page [31]. | |
| R = | Revolutions per minute. | |
| L = | Stroke of piston in feet. | |
| N = | Number of cylinders. | |
| O = | 2 when engine is double acting, that is when explosions occur on both sides of the piston. | |
| C = | Number of strokes per explosion. C = 4 in a four cycle engine, and 2 in a two cycle. |
It should be specially noted that the area of the piston is given in square inches and the stroke of the piston in feet. The number of revolutions per minute, R, is multiplied by two in order to obtain the number of strokes, as there are two strokes per revolution. When the engine governs its speed by dropping explosions to meet varying loads, the quantity C should be omitted and the explosions counted.
Due to the fact that the incoming charge of the mixture is expanded by the heat of the passages, a full charge computed at atmospheric temperature is never obtained in the cylinder and for this reason the gas should be kept as cold as possible before entering the passages in order to obtain the maximum output. Friction due to restricted passages and valve openings also reduces the amount of mixture available. Small exhaust valves and pipes prevent the gases from escaping freely to the atmosphere and produces a back pressure on the piston which cuts down the effective pressures. All of these items are recorded by the indicator and makes it possible to make alterations that will increase the output of the engine.
Because of the reduced atmospheric pressures at high altitudes the output and compression are reduced for every foot of elevation above sea level. As the weight of the atmosphere is reduced, less mixture is drawn into the cylinder. Taking the output of the engine as 100 per cent at sea level, it is reduced to less than 62 per cent at an elevation of 15,000 feet.
CHAPTER II
FUELS AND COMBUSTION
(7) Combustion.
The phenomenon called combustion by which we obtain the heat energy necessary for the operation of the internal combustion engine is a chemical combination of the air with the fuel. This process results in heat and some light which is equal in quantity to the energy required to separate the fuel compound into its elements or to build it up in its present form from the original elements. If the process is comparatively slow, the compound is called a fuel, if it is instantaneous it is called an explosive. Some substances produce mechanical force through an instant, without the evolution of much heat, due to the disintegration of an unstable compound. The effect of the latter type of which dynamite is an example is static, that is to say, it is not capable of producing power, but only pressure. For this reason, compounds having an instantaneous effect without the ability to produce the pressure through a distance, or an expansion, are not considered as suitable fuels for a heat engine.
A fuel is essentially a substance which is capable of generating heat, which is a form of energy, and not static pressure. The heat engine is an instrument which transforms this energy into power which is again dissipated into heat through the friction of the engine itself and by the load that it drives. This is an illustration of the physical law that “energy can neither be created nor destroyed,” that is, the heat energy developed by the fuel is converted into mechanical energy which is again transformed into heat energy through friction.