David Campbell, age 13, of Providence, R.I., for his question:
WHAT EXACTLY IS HORSEPOWER?
Horsepower is a unit used to express the power or rate of doing work of an engine in the customary system of measurements. The term "horsepower" was first used by a Scottish engineer named James Watt. He used it to compare the power of a steam engine to the power of a horse.
Today the term horsepower is used to express the power of such devices as automobile engines, jet engines and electric motors. Even the power of an atomic reactor is figured in horsepower.
One horsepower is defined as 550 foot pounds of work per second, or 33,000 foot pounds of work per minute. One foot pound is the work needed to lift one pound of weight a total distance of one foot.
The metric unit of power is the watt. One horsepower equals 745.7 watts.
If an engine lifts a 550 pound object to a height of two feet in one second, it is working at a rate of 1,100 foot pounds per second. This engine is delivering two horsepower.
If a 150 pound person climbs to a height of eight feet, that person does 13,200 foot pounds of work. If the person makes this climb in one minute, the working rate is four tenths horsepower. A person who is accustomed to hard work can work at a rate of between one tenth and one eighth horsepower continuously during an eight hour day.
The power of an engine can be measured by indicated horsepower, brake horsepower and S.A.E. horsepower.
Indicated horsepower is a measurement of the power produced inside the cylinders of an engine while brake horsepower, sometimes called effective horsepower, is the amount of power available at the engine's shaft.
S.A.E. horsepower is a calculated rating approved by the Society of Automotive Engineers. Its most important use is in determining the licensing fees for automobiles in some states.
The formula used to calculate S.A.E. horsepower is not exact and because of this, is rarely used.
The differences among horsepower ratings are evident when all three methods are used to determine the horsepower of a single engine. Take an engine with four cylinders, each with a diameter of 3.5 inches.
Say that the average pressure on the pistons is 60 pounds per square inch and each piston has a stroke of five inches. The crankshaft turns at 2,500 revolutions per minute. Since this is a four cycle engine, the number of power strokes per minute by each piston is half of 2,500.
The power produced inside the cylinders is calculated by multiplying together the average pressure on each piston in pounds per square inch, the area of each piston in square inches, the length of the stroke in feet, the number of power strokes per minute and the number of cylinders. The indicated horsepower of this engine is 36.4. A dynamometer measurement of this engine gives a value of 29 brake horsepower.
With the S.A.E. rating, horsepower is computed by squaring the diameter of the cylinders, multiplying this value by the number of cylinders and dividing by 2.5. This gives the engine 19.6 S.A.E. horsepower.