Torque is the name given to a force applied through a radius to produce a turning moment. It also can be defined as the tendency of a force to rotate an object around a pivot.
If you have a spanner 0.1m long and apply a force of 10N at the end of the spanner to tighten a nut you are applying a torque of 0.1 × 10 = 1Nm. Or if you apply a force of 5N to the end of a spanner 0.2m long again you apply a torque of 1Nm.
Or in imperial units if you have a spanner 1ft long and apply a force of 10lb to the end of the spanner to tighten the nut, you are applying a torque of 10ft lbf, and if you apply a force of 5lb to the end of a spanner 2ft long again you apply a torque of 10ft lbf.
Let us return to our horse turning the mill stone (see horsepower)
The use of reduction gearing to increase the torque of an engine is easily demonstrated in a car. It is almost impossible to pull away smoothly from a standstill in a high gear, because the torque is low, and the engine will stall. Using first gear gives a high torque but at the sacrifice of a low speed.
On a marine diesel engine used for propulsion, then there must be sufficient torque to turn the propeller at a low enough speed to prevent excessive cavitation and maintain efficiency. This is why medium speed engines rotating at 400 RPM + are geared down using reduction gears.
Torque also comes into play when we consider the crank of the engine. At TDC there is no turning moment, and though the gas load on the piston is high, there is no torque and thus no power is being transmitted.
As the crank turns past TDC there is now a turning lever (the distance OA in the diagram below). If this is multiplied by thrust in the con rod (the turning force) the torque at that particular crank angle can be calculated. When the piston is moving up the cylinder on compression the torque is applied by the rotating crank so for the purpose of this explanation is given a negative value.
Because the torque is varying infinitely throughout the engine cycle depending on crank angle, so the power being developed is constantly varying. However if the torque is averaged for the whole of one engine cycle, then the power developed can be calculated.
To illustrate this, open this exel spreadsheet, for which a screenshot of the first few rows is produced below
It can be seen that whilst the torque varies depending on the crank angle and the cylinder pressure, the average torque, (total torque/360) is 481.7kNm. If the power is now calculated (Torque ×revs/sec × 2p) The answer comes out as 5069.1kW
Now compare this with the computer generated pressure/crank angle indicator diagram taken from the same engine. Looking at the figures in the table it gives the indicated power (pind) as 6779 hp. This is 5069kW.