Torque and Horsepower

The energy generated by a vehicle's engine is usually discussed in terms of torque and horsepower. In order to better understand what people are talking about when they discuss torque or horsepower, each must be properly defined.

Torque is a measure of force in a circular direction; that is, a twisting force. Specifically, it is described as the amount of force (F) applied tangentially at a given distance (d) from the center of rotation. When you turn a wrench or screwdriver on a bolt or screw, you are applying torque to it. The greater the force you apply, the higher the torque. 

Horsepower could be described as the result of torque. It is a measure of actual work being done and how fast it is being done. If the bolt or screw turns when a torque is applied, work has been accomplished. From a physics point of view, the amount of work done is related to force, distance (how far the bolt was turned) and time (how long it took to turn it that far). So, if you applied a certain amount of torque to a bolt and it turned 1/2 circle in 1 second, you would have generated a certain amount of power. An increase in torque will result in either turning the bolt farther in the same amount of time, or turning it the same distance in less time.

Mathematically, torque is measured in pounds-feet (lbs-ft), or newton-meters if you use metric measurements. Since most US cars use US measurements, we will stick to lbs-ft as our unit of measurement. 

Caution should be used when dealing with torque, since the lbs-ft unit is also used to describe work when dealing with linear movement. Torque is not work; it is force. Force times movement equals work. 

Horsepower is related to torque in that it uses torque as part of its equation. Horsepower equals the amount of torque applied times the distance traveled, divided by the time it took to travel that distance. In other words, the amount of work completed in a period of time equals power.

So, how does all this relate to the automobile? Shown below is a graph of the typical horsepower and torque of a given engine. 

Notice how the torque rises to a peak, after which it falls off. This is due to the engine's being "tuned" to operate most efficiently in a given RPM range (see intake system). This range is selected when the engine is engineered to match the goals of the manufacturer's designers. It is costly to design an engine that has a wide, flat torque curve. It isn't possible to built an engine that provides the same torque over its entire operating range, though it may be acheivable in theory.

Now, notice how the horsepower's peak is acheived at a higher engine speed than the torque peak. This is due to the relationship between horsepower and torque. Recall that horsepower is related to torque, distance, and time. You will note that as torque and distance rise, horsepower will also rise. Also, as the time it takes to cover a given distance decreases, horsepower must be higher. Now, even though torque is lower at these higher engine speeds, the increased distance traveled as well as the shorter time required more than overcome the decrease in torque, up to a point. The relationship between torque, RPM, and horsepower is best described by the following formula:

The constant, 5252 has been derived from converting circular motion to linear motion to satisfy the definition of horsepower. It also takes into account any units conversions that need to be corrected.

Matching Gears to the Power Curve

It can be seen now how the choice of gears is related to the engine's torque and power curves. Ideally, the engine should be operating in a range that supplies ample torque to deal with day to day driving requirements regardless of vehicle speed. 

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