From: "Norm Needham" To: <80_series@sgiblab.sgi.com> Subject: Re: [80] Letting a diesel Idle Date sent: Fri, 15 Oct 1999 13:49:35 +1000 Send reply to: 80_series@sgiblab.sgi.com From: Tricia Crichton >> dear all >> this being my first diesel, i have been told to let the car sit and >> idle before actually turning the ignition off. What is the purpose >> of this exercise? Answer From: Coote, Michael MA >I'm no expert on this, but I believe it is called "normalising" and is >a good habit to develop whether the engine is diesel or petrol. > >Dunno if this is sensible, but at speed an engine is not at the same >temperature all over - there are hot and cold spots. This is not a >problem during running, though if you switch the engine off suddenly it >can lead to uneven cooling. This can cause stress concentration and >cracking due to the uneven contraction of engine parts as the engine >cools. > >By idling the engine, it makes the engine temperature more even >throughout and allows more even cooling, less stress and less potential >for cracking. > Full marks Michael, Your belief as a professed "non" expert is pretty much correct. Any engine working hard will build up localised "hot spots", particularly around the combustion chamber area. A short time spent idling (or driving) under little or no load will allow the flow of coolant to equalise temperature throughout the engine. The popular belief that turbos need a cool down is valid to some degree (and a good practice). Turbos can run glowing red hot under heavy load, and while the modern materials will withstand being dunked in cold water at this temperature, a premature shutdown can boil the lubricating oil within the turbo. This is not as dramatic as it sounds, because, contrary to popular belief, the great majority of turbos do NOT use bearings on their shaft. They use a bush, or more commonly a "half" bush. That is, two bushes inside each-other to reduce their relative speed. They rely on a film of oil to hold them concentric (commonly called a hydrostatic bearing). Some very high performance (and very very expensive) turbos use bearings, but these units are extremely delicate, usually small for small petrol engines, and their treatment in terms of clean oil and proper cooling is critical. In short, a cool down period to equalise temperatures is beneficial to the WHOLE system. Cheers * Norm Needham * * Traction 4 / ARB Northside * Sydney, Australia * Trac4@bigpond.com From: Couyant_George@emc.com To: 80_series@sgiblab.sgi.com Subject: RE: [80] Letting a diesel Idle Date sent: Fri, 15 Oct 1999 00:45:02 -0400 Send reply to: 80_series@sgiblab.sgi.com > this being my first diesel, i have been told to let the car sit and idle > before actually turning the ignition off. What is the purpose of this > exercise? > [GC says] A number of reasons. If the motor has been working *really* hard, and I'm talking flat out for while, it pays to let it cool before shutting down (to avoid heat soak). In that case, you *don't* just let it idle. At idle, the fan's pulling minimal air with little cooling effect. You need to leave it revving at a couple of thousand rpm for a minute or two. It may also be good to push the transfer case lever into neutral and engage 2nd gear to keep the gearbox oil pumping and turning. With turbos, again if they've been working hard, the same applies. Modern turbos though don't seem to suffer from oil starvation when shutting down. It doesn't hurt to let it spin down for 10 sec. I was browsing through some figures just back from high speed testing in Saudi Arabia on a turbo V8 100 Series. An hour with the foot to the floor at 145mph in 40 deg C heat. The engine (oil), gearbox and diff temps were up marginally over stock (at slower speed of course). Transfer case was up a bit more. You'd never think things would need to cool down. It's when you shut down that oil and water stops flowing. The fluids no longer transfer heat away from the hot zones and you get localized hot spots. Cooling fluid for example will happily bubble away when left stationary near a combustion chamber at several hundred deg C. Cheers gc '97 HDJ80 - Melbourne Oz