(part 1's ending).......The higher the compression ratio, the greater the mean effective pressure will be. The compression ratio is limited by the quality of the fuel burned; that is, with higher compression ratio, where the gasoline mixture is tightly compressed in the combustion chamber, a gasoline of higher octane rating (later discussed) must be used so as to prevent detonation.

...(continued from part 1)

FIG. 122. At the time the gas mixture is compressed in the cylinder its temperature rises to about eight hundred degrees Fahrenheit. At the instant of the explosion, the temperature of the mixture rises as high as thirty-five hundred degrees Fahrenheit, and as the piston approaches the end of the explosion stroke it drops to about nineteen hundred degrees.

Figure 122

Only 42 per cent of the gasoline heat energy is used in the form of power in the motor. The balance of 58 per cent is thrown out in the form of heat through the exhaust and radiated through the cylinder fins to the free atmosphere. Even the 42 per cent of heat energy cannot be delivered, in the present-day engines, in the form of power at the end of the crankshaft, as 12 per cent is being consumed in the work of compressing the gas mixture. About 4 per cent is lost to overcome mechanical friction and only 26 per cent is left for our use. When we attach the propeller at the end of the crankshaft we have still greater energy losses, as the propeller is only a little over 80 per cent efficient (this will be explained later), thus causing the actual heat energy of the gasoline transmitted in the form of propeller thrust to be only 20 per cent of the total heat energy content of the gasoline.

Figure 121 (re-presented from part 1)

FIG. 123. While the power of the engine depends basically on the factors shown in Fig. 121, there are numerous other factors which determine the power output of a particular engine. One of them is the proper gas-mixture distribution in all the cylinders with the minimum friction losses caused by the walls of the intake manifold. I am mentioning this only to suggest that although the gas mixture is in a volatile form, nevertheless it has viscosity, which in turn causes, more or less, rubbing against the walls of the intake manifold and a resultant retarding effect on the gas mixture in its travel toward the combustion chamber.

Figure 123

The volume of gas mixture drawn into the cylinder always weighs less than it did at the time it left the carburetor jets. This difference determines the volumetric efficiency of the engine.

Figure 124

FIG. 124A. The gas mixture drawn into the cylinders of the engine consists of a definite proportion (in weight) of gas and air. This mixture can be varied and adjusted in such proportion as to give complete combustion. With the rise of temperature of any gas (Fig. 124B) its volume increases; therefore, for equal volumes, the gas of lower temperature will be heavier. This is an important point in the operation of gasoline engines. If the motor is overheated, which means the intake manifold as well, the eight of gas mixture drawn into the cylinders will be less, resulting in loss of power.

Figure 125

FIG. 125. In this illustration you can clearly see how the horsepower of the motor is affected by the proportions of gas and air in the mixture. One weight of gas mixed with twenty of air will make the leanest mixture, resulting in the minimum power. Should this proportion be changed so as to be one to eight—the richest mixture—the motor power still will fall off, because in this case there is not enough air in the mixture, not enough oxygen to produce perfect combustion. Some of the gas particles are thrown out through the exhaust. The same illustration shows the engine power variations between the leanest and richest mixtures and their effects on the horsepower output.

Figure 126

FIG. 126. When the gas mixture is sucked into the cylinder of the engine it fills all the space A as soon as the piston has completed the suction stroke. At the end of the compression stroke, the gas mixture is compressed to smaller volume, B. The ratio between volume A and volume B gives the compression ratio of the particular engine. The higher the compression ratio, the more B.T.U. are utilized for actual work. In other words, with a higher compression ratio, the engine becomes more efficient.

To illustrate the effect of the compression ratio to motor power: an engine with compression ratio five to one develops one hundred and fourteen horse power with a fuel consumption of .53 pounds for each horsepower per hour (specific fuel consumption). Should we increase the compression ratio of the same engine, making it seven to one, the output will be about one hundred and thirty-five horsepower with specific fuel consumption of .44 pounds, or it will burn 9.9 gallons of gasoline per hour for one hundred and thirty-five horsepower as against 10.7 gallons for the engine of the lower compression ratio and for an output of only one hundred and fourteen horsepower. The compression ratio is greatly limited by the anti-knock quality of the fuel, which I shall explain later.

Figure 127

FIG. 127. The gas-air mixture ratio should be maintained at all times in the proportion shown in Fig. 125 as the best for power output. If the engine is functioning constantly on the ground and at the same elevation, it will continuously draw air for the mixture with practically the same density, which means the same weight per given volume. But the airplane engine does not function in air of the same density as it flies at various altitudes, and therefore the gas-air ratio changes unless we maintain the desired ratio either manually or automatically. The air density decreases with altitude, and at eighteen thousand feet is about one-half the density of air at sea level. Should the engine be operated at sea level and without any adjustment for mixture control, and should it then be raised to higher and higher altitudes, the mixture (gas-air) will begin to become rich at two thousand feet and will be noticeably richer at four thousand. The mixture, in other words, will contain too many gas particles and not enough air particles for complete combustion. This will not only cause the power of the motor to fall off very rapidly, but will also cause a considerable amount of fuel to be burned not inside of the motor but out in the free atmosphere where it is thrown through the exhaust.

The correct gas-air proportion can be maintained either by restricting the amount of fuel-flow from the carburetor (Fig. 127A) or admitting more air in the intake manifold, as in Fig. 127B. The fuel restriction in the first case is caused by the decreased atmospheric pressure above the gas level in the carburetor, according to whether the small or the large opening is closed, or both. When the two openings are closed we have a condition of full lean mixture. In Fig. 125, notice the valve-like air-mixture control, through which the atmospheric pressure in the carburetor float-chamber can be regulated, by having the valve either entirely closed or sufficiently open if richer mixture is required for a given altitude.

When the mixture control is being regulated manually, as is the case with most low-powered motors, the procedure is to set the throttle at a given position and notice on the tachometer the indicated revolutions of the crankshaft per minute. Then start to lean the mixture until the revolutions per minute, as shown by the tachometer, begin to drop. Now increase its richness until the tachometer indicates the same R.P.M. as in the beginning of the procedure.

With an ordinary engine, although you can adjust the mixture for best power at a given altitude, still, with height—because of decreased air density—the power of the engine will steadily drop. This drop of power increases a little faster than the decrease of the air density. For each pound of gasoline the motor burns at sea level, it needs thirteen pounds of air. At eighteen thousand feet, with the mixture control adjusted for the same ratio, the weight of the volume of air sucked into the cylinders will be half the weight of the volume sucked in at sea level, as the air density at this altitude has dropped one-half. With such thin air we cannot burn one pound of gasoline, but half a pound, if we wish to maintain the best mixture ratio. Consequently, while the engine is operating at its maximum efficiency under the thin-air condition, it cannot develop the same power as at sea level. If, at the altitude in question, we maintain sea-level pressure in the intake manifold, the engine will be capable of burning the same amount of fuel as at sea level, developing sea-level power. Increase the pressure beyond that of the sea-level atmosphere, and even greater power can be maintained at high altitudes. This is done with the aid of superchargers, with which I will familiarize you a little later.