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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There needs to be enough kinetic energy in the rotating mass of the engine to compress a cylinder. If you want to idle at a lower RPM, you need more rotating mass, or at least a larger moment of inertia. You also need to produce enough power to overcome friction and power any secondary loads(alternator, oil pump, water pump, A/C, etc.), but the power loss to friction in the engine is mostly proportional to RPM anyway, so that’s not as big an issue as you might think at first glance.
>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.
If it’s in gear and locked up, the idle RPM is dictated by the speed and gear ratio, just like a manual transmission. Otherwise, the RPM will depend on how much resistance the engine feels through the transmission.
Electric motors don’t need to keep moving, they can start themselves from a complete stop. If you have a hybrid, the gas and electric might both be turning while the car is stopped if the battery needs to be charged.
tl;dr: The engineer needs to make a tradeoff between idle RPM and the weight and responsiveness of the engine.
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