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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It takes energy to move the engine components, the more they weigh, the more energy is needed. If the engine does not idle at a high enough rpm, the energy generated will not be enough to keep all those parts moving effectively and engine will stall.
It takes energy to move the engine components, the more they weigh, the more energy is needed. If the engine does not idle at a high enough rpm, the energy generated will not be enough to keep all those parts moving effectively and engine will stall.
Electric cars don’t idle, yes they are more efficient and it shows up on efficiency numbers by having higher city mpg(e) estimates vs highway, where ICE (gas) cars are the other way around
Some ICE cars have stop & go systems to pause the engine idling
Most ICE cars keep the engine running so that it can keep oil circulating / the engine stays “warm” (i.e. hot to the touch) so the oil works properly and reduces wear & tear – also this allows auxiliary systems like AC or heating to have a constant supply of power from the alternator (which also needs some minimum rpm to function at useful levels)
Electric cars don’t idle, yes they are more efficient and it shows up on efficiency numbers by having higher city mpg(e) estimates vs highway, where ICE (gas) cars are the other way around
Some ICE cars have stop & go systems to pause the engine idling
Most ICE cars keep the engine running so that it can keep oil circulating / the engine stays “warm” (i.e. hot to the touch) so the oil works properly and reduces wear & tear – also this allows auxiliary systems like AC or heating to have a constant supply of power from the alternator (which also needs some minimum rpm to function at useful levels)
An engine needs to be at a minimum regime to run continuously. There are losses from friction, etc. and to be able to go when you want it to. If you get below that, the engine will stall and you’d have to restart it manually.
Regarding going on the highway, your engine isn’t idle. It’s running at the RPM it needs to keep your speed constant. A transmission has limits in the gear ratios it has. You’re basically in the gear with the most fuel efficient ratio already, so you can’t shift to a more efficient gear.
By the way, when accelerating, the transmissions downshifts to gear ratios that are not as fuel efficient, but will give you better acceleration.
As for electric, they are a completely different beast in how they operate. It’s no longer internal combustion, it’s coils of wires and magnets. They don’t idle like an internal combustion engine would.
An engine needs to be at a minimum regime to run continuously. There are losses from friction, etc. and to be able to go when you want it to. If you get below that, the engine will stall and you’d have to restart it manually.
Regarding going on the highway, your engine isn’t idle. It’s running at the RPM it needs to keep your speed constant. A transmission has limits in the gear ratios it has. You’re basically in the gear with the most fuel efficient ratio already, so you can’t shift to a more efficient gear.
By the way, when accelerating, the transmissions downshifts to gear ratios that are not as fuel efficient, but will give you better acceleration.
As for electric, they are a completely different beast in how they operate. It’s no longer internal combustion, it’s coils of wires and magnets. They don’t idle like an internal combustion engine would.
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.
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.
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.
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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