When I turn on my car, stop at a light, or just have it in park it hovers around 750 rpm (my older car sat at like 1-1.2k rpm). But why? It’s not moving the car or anything. I know some modern cars turn off the engine if it idles for too long but that doesn’t really explain why. Also when I coast on the highway it will idle at even higher rpm’s. I would have expected the automatic transmission to shift down when it’s not applying any acceleration.
Furthermore, do electric engines idle at a rpm? If not does this make electric cars more energy efficient?
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A lot of the reason idle speed is a bit higher on modern cars than old ones, is that the emissions equipment (catalytic converters, combustion chamber, spark plugs, sensors) need to be kept above a certain minimum temperature to function. If you run an engine too slow at idle, not enough heat is created or moves through the exhaust. This is especially a problem with direct injection and diesel engines.
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