Torque to Power

The torque an engine produces at a given rotational speed is a basic engine specification. The power output is the torque multiplied by the rotational speed.

where

P = Power [kW]
T = Torque [Nm]
N = Engine rotational speed [rpm]

1 PS = 0.73549875 kW = 0.986320 hp (SAE)

Some example engine torque and rpm numbers are interesting to gauge the range of values. You can convert the torque to power given the quoted engine speed.

However, most engines generate peak power at a different engine speed to peak torque. Therefore, be careful when using torque and rpm values to calculate power as it will be just at the peak torque engine speed.

Torque

The torque or turning moment acting on a body is defined as the product of the magnitude of the force and the perpendicular distance of the line of action of the force from the axis about which the body is being rotated.

T = Fd

where

• T = torque Nm
• F = magnitude of the force N
• d = perpendicular distance from the line of action of the force to the axis of rotation m

Torque has the same units as work or energy, however, it is a different physical concept and so to show the difference it is measured in newton metres rather than in joules.

Some typical torque values are given in the examples table below. Note: these are only indicative values.

Bench Running a Small Glow Plug Engine

Running the OS FS-20 engine on glow fuel is interesting. This engine may only have a peak torque of 0.2Nm, but if it backfires under compression when starting it hurts.