Why is it bad for an engine to idle for long periods? Surely a car’s engine should be designed for this condition, especially since many cars have to spend a great deal of their lives in heavy traffic?
I’d like to know if there is something I can do to prolong an engine’s life, because I spend three hours a day in stand-still traffic.
Lastly, does a diesel engine fare better, or get less damage while idling, than a petrol engine?
What about extreme off-road work where the progress is really slow and the engine idles for long periods of time?
DANIEL BROOKE – By e-mail
An engine is said to be idling when there is no load on it. The moment the car moves, there is a load on the engine and the internal conditions change.
On a modern engine, the ECU (electronic control unit) tries to keep a petrol engine’s mixture strength close to the chemically-correct value for most of the time, because this will allow the catalytic converter to operate efficiently.
However, a petrol engine will not idle reliably unless the mixture is significantly richer than the chemically-correct value.
This is because at such a small throttle opening there may not always be sufficient combustible mixture in the vicinity of the spark, unless a rich mixture is supplied.
A rich mixture will increase the carbon build-up as well as resulting in unburnt fuel diluting the oil, thus reducing its lubricating qualities.
These conditions are at their worst when idling with the car stationary. In slow traffic, there are normally enough spurts of acceleration to reduce the time that the engine is running rich. Under these conditions the oil also gets hot enough for most of the fuel to evaporate.
This evaporation is important, so that it’s a good idea for cars that are mostly used for commuting to be taken for runs on the open road from time to time for at least an hour. This will get rid of any fuel or water in the oil. This will also clear out some of the carbon build-up inside the combustion chambers.
By contrast, diesel engines always run very lean. At maximum output the mixture is still about 20 per cent lean, and at idling the mixture may be more than 80 per cent lean, so diesel engines do not have this problem.
This explains why in Europe diesel taxis are often left to idle at the kerbside without causing any damage when the temperature gets close to freezing.