Talking Torque: the secret of Saab's pulling power
Engine power is usually described as 'horsepower' but there is a measure of mechanical output that is often overlooked and also important for everyday driving: torque. But what is the difference between the two? And where does turbocharging fit in? Here Saab, the Swedish turbo pioneer, explains the importance of talking torque.
In 1977 a new instrument appeared on the dashboard of Saab's top-of-the-line model - a turbo boost gauge. Watching the needle flick round to maximum indicated the onset of fierce acceleration, delivered with a characteristic 'shove in the back', never experienced before in a normal production road car. The novel sensation that raised eyebrows was, in fact, the unique ability of turbocharging to raise torque.
Torque and Horsepower are related, but different, measures of what is commonly called 'power'. Torque provides power for acceleration, whereas maximum horsepower - in combination with effective aerodynamic design - determines a vehicle's maximum speed.
Torque is best described as the twisting or turning force that is applied to an engine's crankshaft by the power stroke of the piston when combustion releases energy from the fuel/air mixture. It is an expression of the work the engine is performing, whereas horsepower is a measure of how fast that work is being carried out. Horsepower became adopted as a unit of power because the first mechanical engineers decided, quite naturally, to compare the performance of their new steam engines to the output of a horse.
In an engine, maximum torque is always generated at a slower engine speed than peak horsepower. As engine revolutions increase, there is progressively less time for pressure in the cylinders to be fully developed. This means torque will begin to decrease, although total power output - the engine's horsepower - will carry on rising with engine speed because there will more, though less powerful, 'power' strokes in a given period of time.
Boosting power
More torque is what turbocharging is all about. Put very simply, it is a means of forcing more air into an engine so that it can burn more fuel, release more combustion energy and produce more power.
A turbocharger pumps more air into the engine by harnessing the waste energy in its exhaust gases and, in engineering terms, is an elegant solution because it makes use of energy that is otherwise lost. It essentially comprises two wheels - a turbine and a compressor - mounted on either end of a revolving shaft. The turbine wheel is driven round at extremely high speeds (up to 200,000 rpm) by the exhaust gases from the outlet side of the engine. The compressor wheel spins at the same speed and uses centrifugal force to pack pressurized air into the inlet side.
The principle is known as 'forced induction' and a supercharger works in the same way, except that its compressor is mechanically driven directly by the engine, usually off the crankshaft.
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