Re: [DML] Engine cover screens - necessary or not?

["Martin Gutkowski" <martin@xxxxxxxxxxxxxx>]

I hate to talk "average temperature" in something such as this as it's not entirely helpful. 

The average engine temperature will rise but the energy level *at the point of shut off* won't. Here's why - including the surrounding air, you're assuming the engine to be a closed system and it isn't. When running the system includes the coolant circuit, when switched off it becomes just the engine, therefore the energy that would have been transported away via forced convection in the coolant content of the engine suddenly becomes part of the closed system that is just the engine and it must now reach thermal equilibrium by dissipating heat through free convection and radiation to the air around it. You've taken a large system in dynamic equilibrium and suddenly switched to a smaller system which will then move to a static equilibrium. In short, it's not a simple question - and I agree the hottest parts of the engine will not get hotter, but many sensitive parts of the engine, eg heads and gaskets, block etc will.

You can define "heat soak" any number of ways but I don't think you'll find the expression in any thermofluids text book.

Martin

.
Sent from my BlackBerry®

-----Original Message-----
From: Peter Lucas <lucas@xxxxxxxx>
Sender: dmcnews@xxxxxxxxxxxxxxx
Date: Sun, 24 Feb 2013 15:02:01 
To: <dmcnews@xxxxxxxxxxxxxxx>
Reply-To: dmcnews@xxxxxxxxxxxxxxx
Subject: Re: [DML] Engine cover screens - necessary or not?


On Feb 24, 2013, at 2:16 PM, Harold <hmcelraft@xxxxxxx> wrote:

> I think you might want to read again what your Lotus friends said - Martin is right on!
> 
> I think a heat issue in the DeLorean is the muffler. When you stop and air movement stops around the engine, the heat soaks belts , alternator, bearings, etc. especially if there is no natural venting via the open areas in the engine cover
> 

All true.   But Martin said "for a period, the engine actually gets hotter."   Assuming that by "the engine" he means the average temperature of the entire engine (or the entire car, for that matter), this is impossible.  The belts etc can certainly get hotter, but not "the engine".  That was my only point.   Forgive me if you consider this to be nit-picking. But a lot of people really do believe that this is what "heat soak" means, and it drives me crazy.

--Pete Lucas
 VIN #06703

> 
> Harold - 3354
> 
> --- In dmcnews@xxxxxxxxxxxxxxx, Peter Lucas wrote:
> >
> > No, that is not exactly what "heat soak" means. The term is properly used in two ways:
> > 
> > 1) "Something absorbing heat from something else until it stops functioning properly". So, for example, an intercooler can stop doing its job if its local ambient temperature gets too high.
> > 
> > 2) "Heat moving from a hotter part of a system to a cooler part, making the cooler part hotter, and the hotter part cooler". So, for example, if you turn off an engine, the coolant stops flowing, so the parts of the engine that were being cooled by the coolant can get significantly hotter by heat soak from deeper in the engine, but only because those deeper parts are getting *cooler*. This is simply the system moving toward thermal equilibrium. The Second Law of Thermodynamics ensures that the *average* temperature of the engine constantly goes down, starting the instant you turn it off. 
> > 
> > If you disagree with this, I ask again: If the engine is off but getting hotter, where is the heat coming from?
> > 
> > You can read about heat soak here, courtesy of our Lotus friends:
> > 
> > http://www.lotustalk.com/forums/f25/please-explain-heat-soak-50397/
> > 
> > --Pete Lucas
> > VIN #06703
> > 
> > On Feb 22, 2013, at 2:12 AM, Martin Gutkowski wrote:
> > 
> > > It's called heat soak. The moment the engine is switched off, the coolant stops flowing and for a period the engine actually gets hotter. It's why I don't like turning the car off when the fans are running.
> > > 
> > > Some cars have some pretty complex electrically driven water pumps to prevent heat soak issues
> > > 
> > > Martin
> > > 
> > > Sent from my BlackBerry®
> > > 
> > > -----Original Message-----
> > > From: Peter Lucas 
> > > Sender: dmcnews@xxxxxxxxxxxxxxx
> > > Date: Thu, 21 Feb 2013 17:28:46 
> > > To: dmcnews@xxxxxxxxxxxxxxx>
> > > Reply-To: dmcnews@xxxxxxxxxxxxxxx
> > > Subject: Re: [DML] Engine cover screens - necessary or not?
> > > 
> > > 
> > > On Feb 21, 2013, at 6:08 AM, jtrealtywebspannet wrote:
> > > 
> > >> Once the motor stops the heat builds up quickly.
> > > 
> > > I'm a little skeptical about that one, Dave. Sounds thermodynamically impossible. If the motor isn't running, where is this heat coming from? None is being generated, so the temperature can only go down. It is possible that the *compartment* could get hotter when the car isn't *moving*, but I don't see any mechanism by which the engine running or not would matter in a D. Now, an engine with a mechanical fan might be another matter...
> > > 
> > > Am I missing something?
> > > 
> > > --Pete Lucas
> > > VIN #06703
> > > 
> > > 
> > > 
> > > 
> > > 
> > > [Non-text portions of this message have been removed]
> > > 
> > > 
> > > 
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> 

--Pete



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