Underhood Temperatures

[Garfield Willis]

On Thu, 25 May 2000 09:05:37 -0400, "Swayze" <kswayze at bellsouth.net>
wrote:

>no answer,

If you're referring to your earlier question, I saw it, but didn't
really know how to answer; it's not really as simple as just looking at
the max temps on the data sheets. Guess I get stuck with emitting
another laborious tome, but you seem to be insistant...

>will a TEC device work or are they only good to 150degC also?

I dunno what a TEC device is, but here's the best I can say about your
inquiry:

On Tue, 23 May 2000 22:41:36 -0400, "Swayze" <kswayze at bellsouth.net>
wrote:

>I reread (quickly) the .pdf's for the LM1949 and L584(548?) and saw that the
upper end of their operating temp range is 105 to 120 degC with an
absolute max around 150degC, so, what would need to be done to make it
(them)  heat survivable at the injector or close to it.


I dunno where you got those numbers, perhaps temps at which tests were
made for some of the datasheet parameters. Did you find them in "test
conditions" at the top of the page? I thot so. :).

The LM1949 really does have the full -40degC to +125degC of most any
automotive part. The 150degC max doesn't apply, that's the limit of
*storage* temp. The part must not be operating when it's at that temp.
The numbers you want to look at are always the "max operating
temperatures".

In either case, the real problem is NOT in these predriving devices
anyway, it's in the final drive devices, which may have to dissipate
some significant amounts of heat to ambient.

ALL semiconductors effectively have a maximum junction temp of 150degC
[ALL of them; that's EVERYTHING that's EVER been under a hood, OK? Oh
gawd, I just know someone's gonna point out an exception :]; that's how
hot their very innermost crannies can get without turning into silicon
mush. If the ambient was already 150degC, ANY slight power dissipation
in operating the device would raise the internals beyond 150degC, so the
practical operating limit is ALWAYS going to be less than 150degC. And
125degC is as close as anyone has cared to push the envelop so far.
That's why I said, almost all the devices made for the automotive market
are, at least on the hot side, pretty much the same as the old mil-aero
specs used to be. [The Mil-Aero temp range is historically -55degC to
+125degC, but are you gonna sweat the diff. between -40degC and
-55degC?...do you have ANY idea how COLD that is?? Wuh Brrrrrr].

I can't go thru the entire process onlist, it's too long/laborious, but
basically what one does with a thermal design problem, is you first
estimate the maximum ambient you're going to be dealing with. You then
estimate the max amount of power your devices inside the thermal shell
have to dissipate out to that ambient. Then you start looking for a
case/sink and any mounting techniques for the thermal shell that can
achieve a low enough total thermal resistance (that's usually referred
to as "r-theta", and is spec'd in degC/Watt), so that when dissipating
the estimated amount of power, the internal device temps don't exceed
their maximums. This is done by working back from ambient, and adding
the temperature rise expected, which you get by multiply the estimated
power dissipation times the overall r-theta.

The design fun comes from the fact that the overall thermal resistance
from the junction of the device to ambient is a series of thermal
resistances, some of which can only be estimated. So you pad with some
margin; add to that, the estimated ambient can be sloppy (certainly hard
to predict engine topside), so you want some margin there; add to that,
you don't want to allow the device to operate long periods of time right
up to it's max temps, so you try to add some margin there. After you've
added all these safety margins, you may find you've used up ALL (and
then some) of your available heat dissipation capacity. So you iterate
on both the margins you shoot for, and the size/thermal-resistance of
the shell.

Point about all the above, besides the fact that it's really nothing
more than alot of iterative arithmetic and tradeoffs, is that it's NOT
merely a matter of looking up the device's "max operating temp". So you
can't just look for a temp spec, compare it to ambient, and then say
"OK, that'll work". The process is more involved than that, as I tried
to sketch above.

In the case of engine topside, the most difficult problem is that the
temps are already so high (and vary widely), pick a bad location and you
may have precious little delta between ambient and max temps. But there
ARE some places you can rely on the engine/cooling system designers
having already had to spend some design time to keep as cool as
possible; one is around the fuel rails, the closer the better, because
they have to be kept cooler than just about any other area, otherwise
vapor lock raises it's ugly head, and of course that area is also being
cooled by the fuel flow in the rails as well while running. I also can't
think of a better place as far as heat soak avoidance, than near the
fuel rails (assuming you can't mount your gizmo near some opening for
convective cooling). They're (fuel rails) usually left as uncluttered as
possible to make the most of any convective cooling once the engine is
shut down. That's one of the reasons you'll see them carefully separated
and stood off from other things. Take a look at the newer non-return
type fuel rails (which can't benefit from bypassing alot of cool fuel),
and you'll often see those areas where they put them right out in the
open, and very tidy and clean all around them, with some breathing
space.

The only definitive way to go about this is to shoot the engine
compartment with a well-focused infrared gun, or better yet, mark it up
with some templsticks to see what kind of temps you're dealing with,
with the hood down, and following heat soak. You can get those
indicators from 125degF all the way up to 2500degF!!, but they have a
nice set from 125degF to 800degF. Take a look at:
	http://www.tempil.com/tempilstik.htm

They're NOT cheap, tho. [You knew that was coming didntya?] The 20 pen
set is around $150, but they have this neat deal where you can specify a
custom set of 10 temps, and the 10-pen set is only around $50 IIRC. That
way, you can select some more closely spaced temps from 125degF to say
300degF, rather than get them all the way up to 800degF [no engine
compartments live at those temps anyway, at least not more than once :].

This is how the big-boys do it if they can't shoot the scene with an
infrared gun. If you just want a rough idea, and don't wanna do a full
study, you can use some of their "bar" style labels you can place in
different locations. They turn black up to the highest temps achieved,
in the place you stick them. Take a look thru their site to see all the
various goodies they have there. Tempil pretty much owns the market in
this field.

HTH Mike, toldya it wasn't simple :),

Gar


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