Q. When I’m stationary for a couple of minutes at traffic lights or in a jam in my Audi A4 Avant, is it more fuel-efficient to leave my multitronic automatic gearbox in Drive with my foot on the brake, put it into Park or turn the engine off?
OH, Barford St Martin, Wiltshire
A. Many manufacturers now fit stop-start systems to their cars, which, on automatic models, turn the engine off whenever the car is stationary for more than a few seconds and restart it either when the driver releases the brake or presses the accelerator. Estimates of the fuel saving vary, but it can be as high as 8% for city driving.
Doing this yourself is more of a hassle, though. You’ll need to put the vehicle into Park or Neutral (for automatics), and in many cases the ignition must be switched off fully before the key can be turned back to the start position. So it only makes sense to turn the engine off if you know you will be stationary for several minutes.
You will hear stories that starting the engine uses extra fuel. This is true — if starting an engine from cold. But a warm engine will start after just two engine revolutions; on diesels, glow plugs will not be needed since the engine is already hot, and although a small amount of charge will be drawn from the battery, this takes just a few seconds to replace.
Putting the car in Neutral or Park with the engine running will save a little fuel on the traditional type of automatic car with a torque converter, but on your Audi, which has a mechanical clutch operated by a computer, or on a manual, this does not result in any significant fuel savings.
TIM’LL FIX IT
Tim Shallcross used to train AA patrols to fix cars. Now he advises the Institute of Advanced Motoring – read more from Tim here.
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