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Small Gas Engine Basics: Horsepower and Torque
These two concepts are interrelated but distinct. One horsepower is the force required to lift 550 pounds 1 foot in 1 second. Note that horsepower refers to work done over time. A 10 HP chain saw should be able to do more work than one which only develops 5 horsepower.

Torque, on the other hand, refers to the twisting motion on the crankshaft. It is measured in foot-pounds; one foot-pound is a force of one pound exerted on a one foot lever. Torque, therefore, is a measurement of instantaneous force and does not contain any concept of time.

You can understand the distinction between these two terms with the following example: two vehicles weigh the same and have identical horsepower and gearing. Both ought to reach the same top speed. But the one which develops greater torque will reach that speed sooner. In industrial applications, torque shows up in the ability of the engine to accept loads without bogging down and losing rpm.

Torque can be related to horsepower by assuming that the twisting force acts on the end of a one foot crank. The force would act over a distance of 6.28 feet-the circumference of a circle with a radius of one foot-with each revolution of the shaft. If you know the load, and the rpm, you can easily calculate the foot-pounds of work per second. To get the horsepower, multiply the rpm by the torque and divide by the constant 5252.

Slide rule calculations enable engineers to estimate horsepower with some accuracy, but the test is to mount the engine on a dynamometer, a device which can accurately load the engine. The load may be a friction brake, a generator, or a water turbine. The latter two are most commonly used because of their precision.

Normally, the engine is mounted in a flexible cradle connected to a spring scale. As the crankshaft turns, it will exert an equal and opposite force, twisting the cradle in the direction that is counter to the direction of crankshaft rotation. Torque is then read directly from the scale.

The load is adjusted to hold the engine at a given rpm with the throttle open. Then the load is reduced and the engine speed is increased to the next test point Fifteen or twenty tests across the rpm scale provide enough data to plot torque and horsepower on graph paper as a curve.

On average when the torque curve begins to fall off  the horsepower curve will continues upward. It maybe asked: how IS this possible since torque times rpm gives horsepower? The answer is that so long as torque is falling off at a rate less than the rate of rpm increase, horsepower will continue to climb. The horsepower curve will peak at the point where torque falls off in a one to one relationship to rpm increase.

The typical industrial engine must pull from low rpm and are expected to operate over a fairly wide rpm range. Highly-tuned sports engines characteristically have peaky torque and horsepower curves which attain their maximum values at high rpm.

At the medium rpm range, friction and reciprocating losses are low and the engine has time to "breathe" a full charge of gasoline and air. As the speed increases, the friction load generated by the rings, piston, and bearings increases dramatically. In most cases friction goes up as the square of the rpm: for example, at 9,000 rpm, the friction will be (32) or nine times what it was at 3,000 rpm. The same is true for reciprocating loads - the energy to move the piston and the valve up and down also increases as the square of the rpm. The carburetor, induction tract, and muffler also lose efficiency as the gas flow velocity increases.

Larger engines, bigger parts, and more and heavier reciprocating masses take their toll. This is why model air- - plane engines often develop 3 HP or even 4 HP per cubic inch while a 9 cubic inch Briggs and Stratton is rated at only 31/2 HP. The problem can be solved at least partially by going to more cylinders. A 125 cc Honda racing motorcycle had five cylinders, each no larger than a thimble.

But while small engines (or cylinders) can develop more horsepower per cubic inch, they are limited in torque. The larger engine will produce more torque simply because it burns more fuel and oxygen per cycle. There is no substitute for cubic inches-at least where mid-range performance is concerned.