SSS Wrote:
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> Ever been to a truck stop? Trucks there run for
> days sometimes.

While George Andrews did reply with some info, I would like to further explain:

1) In normal weather, the primary reason for big rig engines left running is for cab comfort. Most of those trucks you see sitting there has a driver sleeping/resting/etc in the cab for who knows how long. That is his home for the time being, or even permanently. Yes, more trucks are being equiped with auxillary power generators so that the engine can be shut down, but not on the big fleets (Schenider, JB Hunt, Gorden, Walmart, etc) nor on most older trucks. Why? Weight. An auxillary power generator weighs about 500 lbs total (including all the mounting hardware) and that is 500 lbs less freight capacity. While a lot of truck stops are now offering 120 volt hookups for extended stays (longer than 8-10 hours) 120 volts at 10 amps is not enough to start a AC compressor motor. (Most roof mounted AC units require at least 2000 watts to start the compressor motor.)
2) In cold weather areas (anywhere temps can go below 38 degrees) you have to worry about deseal fuel gelling up. Yes, there are additives. But the best way is to keep that engine running, as with the engine running there is a constant supply of hot fuel back into the tanks, keeping that fuel nice and warm. If the truck is equiped with an auxilary power unit, it will be designed with a fuel loop as well to keep the fuel in the tanks warm.