Date:1 November 2011

Engines consume petrol while idling – 8 to 20 litres per hour. They consume no petrol when shut down. It takes essentially zero fuel to restart a warm engine. Do the math. That’s why hybrids and some new cars shut down the engine at traffic lights – to conserve fuel – restarting automatically when your foot comes off the brake so the car is ready to go by the time you get to the accelerator. I don’t know where this misconception came from, but it constantly comes up in my mail.

Actually, I think I may know where it came from. In days of old, when cars had carburettors and chokes, a cold start involved closing the choke to make the mixture richer, sweetened by a few pumps of the accelerator, which pumped a halfspoonful or so of raw fuel into the intake at every stroke. The extremely rich mixture ensured that enough petrol was in its vapour (not liquid) phase when the spark plugs fired; otherwise, the wet petrol drops wouldn’t ignite.

Once the engine caught, the choke had to stay on for several minutes, or the cold valves and piston would keep the petrol liquid. It would take several minutes of fast idling or choke-on driving to bring the engine up to normal operating temperature. That, of course, consumed fuel more rapidly until the engine warmed up. An inappropriate generalisation from that phenomenon may have led to the misconception that engines (modern, fuel-injected engines that are already warmed up) need substantial amounts of fuel to restart when shut off briefly.

One downside: you may need to replace your starter motor prematurely. Vehicles with idle shut-off use either an uprated starter or combine the starter function with a hybrid traction-assist motor.


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