Underhood Temperatures

[bernie at innovative.iinet.net.au]

Garfield Willis tapped away at the keyboard with:

> 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.

One solution is to bolt it firmly (i.e. thermally conductive) to the
cylinder head which for the most part has a temperature hovering
around 100C. The actual surface temperature where you bolt the
device might be 20 degree higher - but that's not likely in an
alloy-head due to low thermal resistance.

> 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

BTW: "turning to mush" doesn't mean melting. Not at such low
temperatures. It has to do with the migration of dopants and the
resulting change in conductive properties.

[snip]

> 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.

Most manufacturers supply spec data that's useful for estimating the
temperature rise. Not always accurate but at least you get some
indication if a more accurate analysis is necessary. i.e. you don't
always have to do the detailed analysis. But you always have to do
some analysis.

> 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

Vapour lock isn't generally a problem; even at elevated
temperatures, if you can maintain a high fuel pressure.
Hence the use of check-valves and even pressure-switches 
to overcome the effects of heat-soak.

> 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 :].

Try the exhaust manifold.

> 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

The "big-boys" use a supercomputer to calculate the thermodynamics.
I won't bore you with the details.

Other aids are used to verify that some jerk hasn't screwed up the
thermodynamics by adding a whizz-gig.

-- 
Bernd Felsche - Innovative Reckoning
Perth, Western Australia
----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org