Engine torque

This is the turning-effort about the crankshaft’s axis of rotation and is equal to the product of the force acting along the connecting-rod and the perpendicular distance between this force and the centre of rotation of the crankshaft. It is expressed in newton metres (N m);

i.e. T = Fr

where T = engine torque (N m)

F = force applied to crank (N) and

r = effective crank-arm radius (m)

During the 180 crankshaft movement on the power stroke from TDC to BDC, the effective radius of the crank-arm will increase from zero at the top of its stroke to a maximum in the region of mid-stroke and then decrease to zero again at the end of its downward movement. This implies that the torque on the power stroke is continually varying. Also, there will be no useful torque during the idling strokes. In fact some of the torque on the power stroke will be cancelled out in overcoming compression resistance and pumping losses, and the torque quoted by engine manufacturers is always the average value throughout the engine cycle.

The average torque developed will vary over the engine’s speed range. It reaches a maximum at about mid-speed and decreases on either side.