Looking at wikipedia,

For a passenger-car engine, idle speed is customarily between 600 rpm and 1,000 rpm. For buses and trucks it is approximately 540 rpm. In case of many single-cylinder motorcycle engines, idle speed is set between 1200-1500 rp

Does anyone know if 540 is the actual minimum rpm or if it could be lower in some vehicles?

  • No, it is not a theoretical limit – Zaid Jul 28 '15 at 13:14
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    I know they arent cars but supertankers have an Idle RPM of 30-40 rpm. Their flywheel is obviously much larger than a cars. – Mauro Jul 28 '15 at 14:31
  • @Mauro Interesting, when you say supertankers do you mean boats or some kind of really big heavy goods vehicle? – SeanJ Jul 28 '15 at 15:14
  • Boats - around 300m long ones. – Mauro Jul 28 '15 at 15:36
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    @Mauro OK, good to know but no really something I'll expect to encounter in the garage! – SeanJ Jul 28 '15 at 16:12

Idle speed is limited by the weight of the flywheel. A heavier flywheel allows for lower RPM idling and vice versa. If the flywheel is too light, it doesn't have enough inertia to keep the engine turning when you're not using the accelerator pedal. But if the flywheel is too heavy, the car won't accelerate fast enough. At 540 to 1000RPM idle speeds, you can assume the manufacturer found a good balance between performance and economy (lower RPM means less fuel consumption).

  • Any idea what the lowest RPM that I could reasonably expect to handle might be? – SeanJ Jul 28 '15 at 13:34
  • What do you mean by "handle"? I expect that reasonable speed to be the quoted 540. Any lower and the flywheel has to be really big and heavy. – Captain Kenpachi Jul 28 '15 at 14:13
  • each engine will have its own idle speed, wikipedia is telling you of the lower end, however some cars may be happy idling at 600rpm (probably diesel) while smaller european petrol engines will not and will only happily idle at 850rpm and above... – Mauro Jul 28 '15 at 14:31
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    @Mauro Unfortunately, it's not so easy, it's a device that's intended to be installed in any vehicle, (so any truck any model). – SeanJ Jul 30 '15 at 10:34
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    from memory of my mechanics days each engine model had an acceptable idle speed range - you'd have to look it up for the specific model you are installing and set the device accordingly. – Mauro Jul 30 '15 at 10:45

The idle speed is determined by fuel flow and air mixture based on either computer or carb/tbi - Flywheel does not determine the idle speed but only influences how easily the engine maintains its current rpm and how quickly it can increase or decrease (the answer above is slightly inaccurate)

Idle speed differs by make and model and transmission type and is typically set based on the lowest RPM allowed to maintain enough inertia (based on flywheel weight) to allow: 1. The engine to stay running when the clutch is engaged in manual cars 2. Be in harmony with the stall speed of a torque converter in automatic transmissions (in order to allow the car to idle without resistance from the drive train - this rpm is lower than the stall speed of the converter ie. 700-800 RPM for a 1200RPM converter)

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    I believe you are on track with your explanation, but would suggest you modify your torque converter statement. Torque converter stall speed, even for stock drivetrain, is still much higher than idle speed. If stall were set at idle speed, it would kill the engine ... which kind of negates what the TC was designed to do in the first place. Stock stall speed in most stock vehicles is around 1700-1800rpm, while idle speed is usually no greater than 700-800 rpm. Good answer overall though! – Pᴀᴜʟsᴛᴇʀ2 Jul 28 '15 at 18:12

There is nothing saying that 540 RPM is the minimum theoretical speed for engines.

The idle speed is usually controlled by carburetor settings or engine computer in case of fuel-injected engines.

The speed below which the engine cannot sustain itself is highly-dependent on its design and application. Usually the manufacturer will leave a little bit of wiggle room.

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