I would like to buy a Volkswagen Polo 1,6 manual. According to VW, it delivers 77 kW at 5 250 r/min and 155 N.m of torque. Can you explain the technical jargon?
Laurence Mahlangu, via email
Answer: Let’s start with the measurable of torque, as it is easier to understand. Engine torque is the amount of rotational force that an engine can deliver at its crankshaft (flywheel) and is measured in Newtons per metre. For example, if you have a metre-long torque wrench connected to a wheel nut and press down with 1 kg (9,81 N) of force, the wheel nut experiences torque of 9,81 N.m. The engine’s maximum-torque curve is a measure of its maximum rotational force over the entire engine-speed range. This is done on an engine dynamometer, where the engine is braked at specific engine-speed intervals to measure the torque reaction in N.m at a wide open throttle. If this is performed at several engine-speed intervals spanning the entire engine- speed range, a torque curve can be drawn up. In the case of the Polo, the maximum value of 155 N.m occurs at 3 500 r/min. A gearbox can be seen as a torque multiplier because the torque value at the crankshaft is multiplied by the gear ratio and again with the final drive ratio of the vehicle, resulting in the torque value on the driven axle. As first gear has the highest ratio, it results in the highest torque value on the driven axle.
Power (measured in Watts) is the rate at which work is done. Therefore, in an engine, it is a function of the torque multiplied by the engine speed. The full equation reads power [kW] = (2 x the engine speed [r/min] x 3,14 x torque [N.m])/60. So, if the full torque curve of an engine is known, the power curve can be easily calculated. The Polo’s maximum power is delivered at 5 250 r/min. By using the equation and calculating rearwards, it is found that, at 5 250 r/min, the maximum torque delivered by the engine must therefore be 140 N.m. Power is not multiplied through a gearbox and therefore will be the same at the driven axle as at the crankshaft if power-train losses are ignored.
