Underhood temperatures.

John Dammeyer johnd at autoartisans.com
Fri May 26 03:47:32 GMT 2000


Hi All,


> >Try the exhaust manifold.


I saw this and almost fell off my chair laughing.

>
> You're not serious...well, then again maybe you are. Thanks for the
> insight, we might have missed that otherwise. But the engine compartment
> (note the words *engine compartment* above?) doesn't EVER reach anything
> near the temperature of the exhaust manifold, and nobody with even a few
> beers left in their six-pack is gonna mount electronics anywhere near
> the exh. manifold. SAE J1128 wire itself is only rated to 125degC. Ya
> know, the automotive "good stuff". Think about it.
>

I just have to tell this story...

We arrived at the customers site to test some new software in our engine controllers.  I
unbolted them,.  swapped the EPROMS,  installed them back onto the firewall and we ran the
two engines,  one at a time.  (this was a Twin Hovercraft).

Each engine ran fine so then we buttoned them up and did some full power runs at 5500RPM
which translates into an EGT temperature of about 1400 degrees for 15 minutes;  and yes
the exhaust manifolds do get red.  At that point the sensor readings from the CAN bus
messages were starting to show odd values.  RPM was still alright but engine temp and MAP
sensor were going strange.  Of course we all blamed the new software.  We shut down  the
engine and opened the cowling and on a hunch I placed my hand on the extruded Al. Engine
Controller box.  After yanking my hand back really quickly I figured the temperature of
the box had to be well above 100C.  Then I looked at the mounting with a new viewpoint;
4" away from the #4 cylinder exhaust pipe.  The radiated energy had warmed up the box big
time.

We moved it to a different location and in the cool air stream before the air hit the
engine and no more problems.  Well.. there were actually.  A tether strap broke sending
the buckle through the blades which disintegrated sending bits of blade everywhere but
after the throttles were closed the engine idled nicely with the broken fan.

The funny thing was it really wasn't that near the exhaust manifold and it was in a large
engine compartment but even thought the radiated varies inversely over the square of the
distance it was hot.  My SAAB Turbo has the battery really close to the turbine but again
a large heatshield with a 3/8" thick plastic panel bolted to the shield I guess to also
slow the heat transfer.  In hindsight I think a couple of metal baffles with an air space
and some ducting would have kept the boxes cool.

That's my war story.  There are lots more and each time the blame fell on the controller
and each time it was wiring or plumbing or other external factors like salt water in the
controllers.  Don't ask.

John Dammeyer


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