Re: Underhood Temperatures

["Swayze" <kswayze@bellsouth.net>]

TEC (thermo electric cooling (peltier effect)) like the toyota(arrgh) 12V
6-pack coolers. only specs I've seen on them  as far as max. temp is around
150degC, so I guess general purpose it wouldn't work.
Temp measurement underhood I guess is the way to go and try to find a spot
where the rejection temp would be low enough to keep the electronics cool
enough. thanks for the link.

ya see in my plymouth, underdash space is at a premium(especially if I
install AC), my main reason for wanting to go with the '727. for that matter
underhood space is at a premium too.

as far as you producing a SAT to P&H device, buying vs. building ,I would
lean towards the under $100 range for eight injectors ( and I don't see how
you could do it ).

byE
Mike
Swayze
mswayze@truswood.com
kswayze@bellsouth.net
----- Original Message -----
From: "Garfield Willis" <garwillis@msn.com>
To: <gmecm@diy-efi.org>
Sent: Thursday, May 25, 2000 4:31 PM
Subject: Underhood Temperatures


> On Thu, 25 May 2000 09:05:37 -0400, "Swayze" <kswayze@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@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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