They do it, probably to run the heat, A/C, or other accessories.  But
it's not good for the engine.  A diesel at idle is not producing much
heat, so it tends to run below ideal operating temps and thus is more
prone to wear.  Newer rigs have an auxiliary generator to power
accessories and heat the cabin while the truck is parked overnight.
Saves wear on the drive engine, and uses even less fuel.

Allan

On Nov 25, 2007, at 8:22 PM, Jake de Villiers wrote:

> Really?  Have you never overnighted in a Rest Area or truck stop?
> I'm pretty sure the noise I hear is idling diesels.   :)
>
> On Nov 25, 2007 2:43 PM, Allan Streib <streib@cs.indiana.edu> wrote:
> Yes, but diesels already idle much more efficiently than gasoline
> engines.  It's not particularly good for them to idle for prolonged
> periods, but at idle speed they do consume relatively little fuel.
>
> Allan
>