While the plane travels at a certain forward speed, as mentioned before, the air offers resistance—drag. The drag is overcome by the thrust of the propeller. We cannot have thrust unless the motor turns the propeller at so many revolutions per minute, which means that the motor is doing a certain amount of work by expending the heat energy of the gasoline.

Figure 111

FIG. 111. Energy can be obtained in various forms. The waterfall represents the tail-end of heat energy. We cannot use the stored energy in the waterfall in the form of heat, but we can use it to drive a water turbine installed at the lower level of the waterfall. The greater the height of the fall—or the difference of the two levels—and the greater the amount of water discharged per minute—the more power we can obtain. For the power necessary to drive our plane through the air we cannot take along the waterfall, but we can take gasoline with its stored heat energy.

Figure 112

FIG. 112. If we lift one pound one foot in one second, the work done is a unit called a foot-pound. This work was possible only when a certain small amount of energy had been expended. Now if, instead of one pound, we lift five hundred and fifty pounds one foot in one second, the work of one horsepower has been performed (Fig. 113). In the illustration, the weight of a small horse is being used as a demonstration, but not its pulling power as a unit of measure. Inasmuch as one pound is a unit of weight, any weight of five hundred and fifty pounds will serve our purpose.

Figure 113

Figure 114

FIG. 114. Should we lift thirty-three thousand pounds one foot in one minute, again the work of one horsepower has been performed, as shown in the illustration.

Figure 115

Fig. 115 illustrates how we can measure horsepower or any fraction of it. If you are pulling a wagon loaded with any weight, and a weight scale is placed between your hand and the wagon, we can always determine how much power you have developed, regardless of the speed or the distance traveled. If the scale indicates fifty pounds and you have traveled one hundred feet in two minutes, you have developed a little over seven one-hundredths of a horse power. When you reach grade C-D, however, the pull becomes one hundred pounds, and at the end of two minutes you have traveled a distance of only fifty feet, though you have developed the same horsepower as in the previous case.

The gasoline engine converts the heat energy of the gasoline into power which is available for use at the end of the crankshaft. We measure weight with the basic weight unit, the pound. Lengths we measure with inches. The measure of heat energy is the British Thermal Unit (1 B.T.U.), which is equal to the amount of heat necessary to raise the temperature of one pound of water one degree (Fig. 116).

Figure 116

The heat energy of the gasoline is converted into power by burning the gas vapor mixed with air in the cylinders of the motor. As the gasoline vapor burns in the combustion chamber of each cylinder, it expands, pushing the piston away from the top end. This linear motion of the piston is converted into rotary motion by the crankshaft, which in turn delivers power to the propeller.

With the present-day four-cycle gasoline engine, two complete revolutions of the crankshaft are made for each explosion in each cylinder.

Figure 117 and Figure 118

FIG. 117. The intake valve opens just as the piston starts to travel away from it. In the course of this travel the exhaust valve is closed and a mixture of air and gas vapors rushes inside the cylinder, filling the space. We say it is sucked in by the piston, though the scientific fact is that it is forced in by atmospheric pressure, because the pressure inside the cylinder was reduced by the traveling piston to a point below the outside atmospheric pressure.

FIG. 118. As soon as the piston has completed the suction stroke, the intake valve closes (the exhaust valve remains closed) and the compression stroke is under way. The piston starts to travel toward the closed valves with all the gas mixture now trapped in the cylinder.

Figure 119 and Figure 120

FIG. 119. At the end of the compression stroke the gas mixture has been compressed to such an extent that it exerts a pressure—on the top of the piston and on all the area surrounding the trapped mixture— of about a hundred and twenty pounds per square inch. This pressure varies with the different motors, according to their design. Upon the completion of the compression stroke, the mixture is exploded by a spark from the spark plug, and at that instant the burning gas mixture exerts a pressure of about six hundred pounds per square inch and pushes away the piston, thus delivering the power to the crankshaft. Following the explosion stroke, the exhaust valve opens just before the piston reaches its full travel (Fig. 120). With the exhaust valve open, the burning gas mixture escapes at the rate of about a hundred miles per hour into the outside atmosphere.

The action of the gas engine above described is assumed to take place at normal sea-level atmospheric pressure of 14.7 pounds per square inch.

At the moment of the explosion of the gas mixture, as stated before, the pressure exerted by the burning gas increases considerably above the pressure at the time of the end of the compression stroke. As the piston is being pushed away, the initial pressure at the instant the gas mixture is ignited, begins to drop, and at the end of the explosion stroke the pressure has become much less than the initial pressure of six hundred pounds. In calculating the motor’s horsepower, we take the mean effective pressure of the burning gas mixture, which is the happy medium between the maximum pressure at the beginning of the power stroke and the minimum at the end of that stroke.

Figure 121

FIG. 121. The indicated horsepower, after all, is nothing but the foot-pound work the engine can deliver per minute; in other words, the number of B.T.U. expended and converted into work during one minute.

The illustration depicts the formula which will give us the indicated horse-power.

There is a difference between indicated horsepower and the actual power we receive at the end of the crankshaft. This latter power is smaller than the indicated horsepower because, by the time the power from the expanding gas mixture has been transmitted from the piston to the crankshaft, some of it has been absorbed to overcome the mechanical friction of the moving parts of the motor. The greater the brake horsepower of a given motor, the greater its efficiency will be. The brake horsepower is not calculated, but is obtained by an actual test of the motor under running conditions.

The mean effective pressure in the cylinders of the engine largely depends upon the weight of the gasoline mixture drawn into them, the correct proportion of gas to air necessary for perfect combustion, and also the compression ratio. 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.

• How Engines Make Power
  Part 1
• How Engines Make Power
  Part 2.