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?
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).
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)
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.
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
