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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?

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  • No, it is not a theoretical limit
    – Zaid
    Commented Jul 28, 2015 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
    Commented Jul 28, 2015 at 14:31
  • @Mauro Interesting, when you say supertankers do you mean boats or some kind of really big heavy goods vehicle?
    – SeanJ
    Commented Jul 28, 2015 at 15:14
  • Boats - around 300m long ones.
    – Mauro
    Commented Jul 28, 2015 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
    Commented Jul 28, 2015 at 16:12

4 Answers 4

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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).

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  • Any idea what the lowest RPM that I could reasonably expect to handle might be?
    – SeanJ
    Commented Jul 28, 2015 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
    Commented Jul 28, 2015 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
    Commented Jul 28, 2015 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
    Commented Jul 30, 2015 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
    Commented Jul 30, 2015 at 10:45
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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 ♦
    Commented Jul 28, 2015 at 18:12
  • Ehh - I wouldn't agreed - 1200RPM is avg. although as high as 3200RPM or more which is the case in racing converters so as to allow the motor to rev higher without engagement otherwise the avg care would "rev" to 1800RPM before the car got moving which is more in line with modified street rods as per this chart tciauto.com/stall-speed-chart
    – DropHit
    Commented Aug 17, 2019 at 18:49
  • What part wouldn't you agree with @DropHit? 1200rpm is average for what? Stall speed? I think you'd to go back to your chart which shows their bottom end TC stall speed is 1500-1700 for stock. This means its for a stock application. I guess I'm wondering what you're actually stating, then, because it doesn't seem to make sense with either what I know about TCs or the chart which you provided.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Aug 17, 2019 at 19:45
  • First option on the chart is not stock - it is an upgrade to give you a higher stall speed. Please rfere here badasscars.com/index.cfm/page/ptype=product/product_id=78/… to the 2nd paragraph "Stock cars have anywhere from 1,000 to 1,400 RPM stalls from the factory. " although depending on year make model this varies greatly and is tied to vehicle weight and hp. Your avg honda 1.8 is gonna lockup at around 2500 rpm whereas a truck may swing up to 3500 or higher. Really its pretty hard to call avg. on any of it :)
    – DropHit
    Commented Aug 21, 2019 at 16:04
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    Note: Lockup is a separate thing from the stall speed ... lockup is a mechanical feature.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Aug 21, 2019 at 16:12
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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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A NASA-Crawler-Transporter engine seems to idle at 400 rpm although it may not fit your garage. https://en.wikipedia.org/wiki/Crawler-transporter http://www.cnrphotos.com/gallery2/main.php?g2_itemId=50895&g2_imageViewsIndex=1

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