--WebTV-Mail-8772-525
Your reply (which appears below) was a good one, Paul, and much
appreciated. I have to say you forced my mind to work just a bit! Here
is my best take:
The thermostat will adjust its position (which regulates the flow of
coolant) as required to hold the coolant temperature at some preset
(i.e., prespecified) value. The thermostat is really acting to bring
into balance two items or processes.
First of these is the production of heat by and within the engine. This
varies depending upon how hard the engine is working. More heat is
produced by an engine labouring to move the car uphill than by the same
engine when you are headed down a hill.
Second of these items is the ability of the cooling system at any moment
to cast off heat. This ability is also variable, as for example the
casting off of heat is easier to accomplish on a cold day than on a very
warm day. Heat also becomes progressively easier to cast off as we
increase vehicle speed.
The thermostat will always ultimately assume a position which takes into
account the above items (processes) and nevertheless holds the coolant
temperature at a particular targeted constant value. Because it works
in real time, the position of the thermostat will vary as required
depending upon ongoing conditions as you drive along.
The temperature of the coolant will not deviate significantly from the
targeted constant value unless a situation is encountered where more
heat is being added to the system than is being cast off. At that point
the temperature of the coolant will rise AND REMAIN above the target,
regardless that the thermostat is fully open.
(in the above I am ignoring overshoot, undershoot, damped oscillations,
and second order and transient conditions which occur on the way to
"steady state" operation)
I guess my bottom line is that, provided the thermostat is in a
regulating mode and not wide open, the coolant temperature will hold
(what is for all practical purposes) a steady value.
Beyond regulating mode I am less certain of what is going on. I know
the radiator can cast off more heat at higher coolant temperatures, but
I do not have a good feel for any nonlinearities which may exist as we
drive the car into this (from my standpoint) undesirable area. Thus, as
the needle creeps above the "N" are we headed for a runaway situation or
is there some stability up there? I do not know. This is fodder for
mechanical engineers which I ain't! I do know it makes me NERVOUS when
I am driving along and this happens.
My gut just tells me if at all possible to avoid running the car when
the thermostat is no longer in regulating mode. Hence came my original
inquiry regarding the shroud.
Bruce Burrows
'59 MGA basket case
'60 Daimler SP 250
'61 Daimler SP 250
'73 MGB driver
My Dart website (not my cars!):
http://community-2.webtv.net/guardian45/THEDAIMLERSP250DART/
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From: "Paul Hunt" <paul.hunt1@virgin.net>
To: "Bruce Burrows" <bburrows@webtv.net>, <mgs@autox.team.net>
References: <15054-38FB9708-4458@storefull-151.iap.bryant.webtv.net>
Subject: Re: Shroud Question
Date: Wed, 19 Apr 2000 08:45:02 +0100
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I would expect your thermostat to still be in 'regulating' mode even when
the needle is slightly above N, since the thermostat is not a precise
instrument but has an element of hysteresis (e.g. a slightly higher opening
temp than closing temp). You can ususally see the effect of this when
warming up - it cycles up and down slightly about the N as cold coolant is
switched on and off by the thermostat until the whole system has reached a
similar temperature. How much it cycles varies from installation to
installation, and if you have a V8 with a blocked steam pipe the temp gauge
has been known to go right round to 60psi on the oil part of the dual gauge!
PaulH.
----- Original Message -----
From: Bruce Burrows <bburrows@webtv.net>
To: <mgs@autox.team.net>
Sent: Monday, April 17, 2000 11:58 PM
Subject: Re: Shroud Question
>
> --WebTV-Mail-12586-2439
>
> When my thermostat is running in "regulating" mode my temp gauge needle
> is perhaps an eighth of an inch to the left of the "N". As it becomes
> necessary for the system to dissipate more heat the thermostat will
> depart from "regulating" mode, remaining fully open, and the coolant
> temperature will rise. It would be unusual for my temperature gauge to
> indicate as much as an eighth inch to the right of the "N", although it
> can and does easily _reach_ the "N".
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