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# Motor Car Operation Terminology

## force, torque, power, engine, speed, body and pressure

MOTOR CAR OPERATION TERMINOLOGY In the development and testing of automobile equipment cer tain engineering terms are used that are frequently misunderstood and misinterpreted. (See INTERNAL COMBUSTION ENGINES.) These terms are : mean effective pressure, work, torque, power, thermal efficiency, mechanical efficiency and volumetric efficiency.

Pressure is generally used in referring to the force of the gases on the piston, and the force between two bearing surfaces. It is commonly measured in pounds per square inch. For some pur poses it is convenient to refer to a theoretical average pressure such that, if it acted on the piston throughout the expansion stroke, would produce the same amount of work as the actual varying pressure. This theoretical pressure is known as the mean effective pressure (m.e.p.). Work denotes the transfer of energy from one place to another. It is defined as a product : of force and distance. The engineering measures of work are the foot-pound and inch-pound ; i.e., force in pounds multiplied by distance, in feet or inches, through which the body, on which the force is applied, moves. From a scientific point of view no work is done unless the force applied to a body moves the body. In other words, no energy is transferred unless there is movement of the body into which we are trying to put the energy. Torque is merely a technical name for twist. It is different from work in that it does not take into consideration the distance through which a force acts. Torque might also be defined as a special case of linear force where the force is so placed with respect to the body as to produce only rotary motion of the body. Its measures are the pound-foot and the pound-inch. At the speed of maximum power the torque is only about two-thirds or three fourths as great as its maximum at a lower speed, generally 900 2,500 r.p.m. American automobile engines develop from 06-foot pounds of torque to over 32o foot-pounds.

## Power.

Power may be defined as the rate of doing work.

The common measure of power is the horse-power, which may be defined as the rate of doing work which is equal to 55o f oot pounds in one second or 33,00o foot-pounds in one minute. An

other way of saying this is: the rate energy is transferred so many foot-pounds per second or per minute, or so many horse power. Power (rotary motion) must consider force, distance and time, which is expressed for convenience as torque (force) and revolutions per minute (distance and time). If the torque of an engine should remain constant, the power developed would be twice as great at double the speed. The power of an engine is commonly measured on a dynamometer (q.v.), which in some cases is a special electric generator with certain arrangements to permit measuring the torque developed at the crankshaft. This torque, combined with the speed of the engine, gives a measure of the power available for doing work, which is called the brake horse-power (b.h.p.). The b.h.p. of an engine increases with speed up to a certain speed, above which it decreases. For Ameri can cars this speed varies from 3,200 r.p.m. to about 4,200 r.p.m., the majority being about 3,60o r.p.m. The maximum b.h.p. de veloped by these engines varies from about 15 to over 185.

Brake horse-power is sometimes confused with the horse-power rating often used for taxation purposes. Various countries use a formula based on the physical dimensions of the engine to deter mine a theoretical horse-power figure for computing the tax. In the United States and Great Britain the same formula is used (called the A.M.A. formula for the U.S., and the R.A.C. for mula for Great Britain) based on the diameter and number of cylinders : h.p. when D is the diameter of the cylinder in 2.5 inches, and N is the number of cylinders. This formula gives a figure which is from one-third to one-fourth of actual b.h.p. developed by modern engines. Other formulas also are used in the United States, one of which is : , where S equals the length Io of the stroke.

## Efficiency.

Thermal efficiency is the ratio of the work out put of the engine, expressed in terms of heat, to the total amount of heat in the fuel used to secure that amount of work. A normal engine has a thermal efficiency of from 15 to 22% at full load.

(See MOTOR CAR : Fuels and Combustion; INTERNAL COM