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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Inline 4 cylinder motor has to spin the crankshaft using only 2 cylinders per revolution (the other 2 are sucking a new fuel-air mixture in the meantime). That means they can only “kick” the crankshaft only once per 180 degrees of spin cycle. It is efficient enough if you are spinning at 1000RPM rate. If you have more cylinders (let’s say inline 6 cylinder) you have 3 pistons that work every cycle , so you push the crankshaft once per 120 degrees- it’s easier so engine doesn’t have to run so fast- it idles at 800RPM. Old tractors idle at ~100- 200RPM because they have a huge and heavy flywheel connected to crankshaft which keeps the spinning going without stalling at such RPMs.
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