In physics, torque can be thought of informally as 'rotational
force'. Torque is often listed as part of the basic specifications
of an engine. The power output of an engine is expressed as torque
multiplied by rotational speed.
Internal-combustion engines generally produce useful torque over
a limited range of rotational speeds, usually from around 1,000
- 6,000 rpm for a standard car.
The varying torque output over that range can be measured with
a dynamometer, and shown as a torque curve. The peak of that torque
curve usually occurs somewhat below the overall power (horsepower)
peak. The torque peak cannot, by definition, appear at a higher
rpm than the horsepower peak.
Understanding the relationship between torque, power and engine
speed is vital in automotive engineering, concerned as it is with
transmitting power from the engine through the drive train to
the wheels. The gearing of the drive train (transmission) must
be chosen appropriately to make the most of the motor's torque
Below is an example dyno test showing peak torque and horsepower
Practical Example of Torque:
You generate a torque any time you apply a force using a wrench.
Tightening a bolt is a good example. When you use a wrench, you
apply pressure to the handle. This force creates a torque on the
bolt , this torque is what causes the nut to turn.
Units of torque are usually measured in pound-feet (lb ft), pound-inches
(lb in) or kilogram meters (Kgm). You will notice that torque
measurements contain a distance and a force. To calculate the
torque, multiply the force applied by the distance from the center.
In the case of the bolt, if the wrench is a foot long, and you
put 100 pounds of force on it, you are generating 100 lb ft of
torque on the bolt. If you use a 2-foot wrench, you only need
to put 50 pounds of force on it to generate the same amount torque.