Heat Dissipation with a Belly Pan

[drrbc]

I'd read a belly pan can cause heat retention in the differential, exhaust, etc.

Granted, turbulent airflow is a good means of convection- but I doubt this is what the engineers at Ford had in mind for my F-150 (I would expect to see "fins" of some sort attached to vital structures to increase surface area).

Conduction of heat to attached structures would lower the overall temperature and increase surface area. This would help warm fluids and bring parts up to their operating temperatures, but after that is the increased surface area adequate to dissipate the heat OFF of the vehicle should the temperature go too high? Maybe- I don't know?

What if the main mechanism of heat dissipation is radiation? In that case acrylic, coroplast, the pink stuff (board) would all be bad ideas (though clear polyethylene probably would be ok).

How about this:
1. Fashion the under tray from aluminum.
2. Spray the top of the Al belly pan (the side exposed to the vehicle) with Krylon Flat Black 1602 (high absorptivity of long wave), and the bottom (exposed to the road) with Krylon Flat White 1502 (highest emissivity and reflectance). Before painting black, bolt or TIG some Al u-channel along the length to increase the surface area.
3. Drive your car/truck to normal temperatures. Stop, crawl underneath and map out the hot spots with your IR thermometer.
4. Clean and prep the hot spots and paint the bottom exposed surfaces of the hotspots (those facing the road) with the Flat White 1502.
5. Install the belly pan.

Testing may be a bit tough once the belly pan is on. I don't know what a string of thermocouples would cost. An option may be a string (along say 25-30' of a single Cat6 ethernet cable) of "one-wire" temp sensors ($3-4 per in packs of 10) that could just be super-glued to the hotspots. The ethernet cable could be run into the cab and DAQ done on a laptop.

Thoughts?

[aerohead]

I went online and looked around.
One site mentioned that mineral oils were okay to 250-300-degrees F,while synthetics could see 450-700 degrees F without catastrophe.
Some members are wrapping their exhaust to limit exposure along with heat-shields.
Up to the limits,the hotter the lubes the better for economy.Furnace Creek,Death Valley,California temps would be okay for all oils on a daily basis unless under extreme service.
Low drag achieved through streamlining would lower the road load of the vehicle,and with it,the friction-heating effects of power transmission viscous shearing.So extra heat would actually be welcome.
You might fully warm the vehicle and do a temp survey with I.R. pyrometer,then cobble a low-cost pan together and compare the effects.
In a worst-case scenario,just compromise the pan a bit to allow some strategic airflow,or do some dedicated NACA submerged inlets with ducting to critical areas with outlets, as done in some high end supercars and racing vehicles.

[mcrews]

Drove my infiniti Q45 100k+ miles with Belly pans. see link in sig. never had an issue w/ 'heat'.

you make a interesting discussion. And one we all should remember.
But it's hard to remember we drained the swamp when we are up to our asses in alligators!!!!

unintended consequences........

[drrbc]

Originally Posted by mcrews

Drove my infiniti Q45 100k+ miles with Belly pans. see link in sig. never had an issue w/ 'heat'.

you make a interesting discussion. And one we all should remember.
But it's hard to remember we drained the swamp when we are up to our asses in alligators!!!!

unintended consequences........

I was kind of thinking of this fellow here: "ecomodder.com thread belly-pan-danger-belly-pan-owners-must-read-26401" (won't let me post the URL)

I want thinking so much about oil and gears as much as heat build up in places not intended, i.e.- gas tank, the pickup bed, trunk, the cabin, and the dangers this may present.

If heat is lost mainly by convection, a well sealed belly pan although desirable Eco wise- is unsafe.

But could the correct selection of materials (if heat dissipation is mainly radiant) allow a tight seal AND be safe?

Either an air dam/skirts or belly pan was to be my next project. So I'm just thinking it through. Currently I'm just trying to finish my "Burning Man hexayurt" inspired removable camper top.

BTW- love the gator analogy. It's the story of my life.

[aerohead]

Here is an image of the pan on my truck.It's fiberglass and aluminum.
I've taken no care to accomplish any special shielding or insulating and at around 7-years now,have had no issues with lubes or fuel.
I have leakage at the front suspension.The coooling air can escape to the front wheel wells,and also,there is a full-span gap at the rear axle ahead of the diffuser.
So far,deep water is the only thing which has impacted the truck.I drove slowly through a flooded alleyway and the force of the water was enough to rip the diffuser off because of it's sealed rear bulkhead.
http://www.evworld.com/images/pknox_bellypan.jpg

[drrbc]

aerohead-

Yeah dude- verrry cool. I've followed links all over the internet about your work and your truck.

My dilemma- great with science; bad with tools and fabrication.Worse, I suffer w/ OCD. Lots of stuff I can't compete due to lack of skill and frustration.

On retrospect, I'm considering a belly pan may actually HELP moderate vehicle temps, especially an aluminum one. I figure up to about 75ºC convection would be the main for of heat loss. Past that I'm thinking radiation would dominate. A large Al belly pan would disperse the concentrated heat energy over a larger potentially improving convection. Additionally, the emissivity of weathered Al approaches 0.95, so I doubt you could expect a worse case of even a 5% temp increase.

BUT-- I would also stand to reason that you would get to operating temps much quicker in the above case. IOW, I wouldn't be freezing for the first 10 miles, only 5.

Then the 2 questions that tempt me are:
1.) after the belly pan, did you notice any change in how fast your truck warmed up, and
2.) if you ever consider bolting or TIG'ing some Al U-channel lengthwise along the bottom (not that you would or need)- I'd love to know if there were any changes.

Me? I'm thinking of one of the aftermarket steel pans just so I don't knock my underside off. Pretty sure anything I try to fabricate would be tore to shreds. I was looking at some 1/8th X 8" rubber for the dam and skirts, but I'd need a way to unbolt them easily where the highway ends, cause I still got that 60 mile each way highway to deal with.

And thanks for the input. Like your rig a lot and appreciate y'alls work in the area of saving me money (also the Aerocap guy, and the guy with the little chevy truck that did the spoiler, chin, and dam). And almost forgot the red F-350 getting 27mpg!

[aerohead]

Originally Posted by drrbc

aerohead-

Yeah dude- verrry cool. I've followed links all over the internet about your work and your truck.

My dilemma- great with science; bad with tools and fabrication.Worse, I suffer w/ OCD. Lots of stuff I can't compete due to lack of skill and frustration.

On retrospect, I'm considering a belly pan may actually HELP moderate vehicle temps, especially an aluminum one. I figure up to about 75ºC convection would be the main for of heat loss. Past that I'm thinking radiation would dominate. A large Al belly pan would disperse the concentrated heat energy over a larger potentially improving convection. Additionally, the emissivity of weathered Al approaches 0.95, so I doubt you could expect a worse case of even a 5% temp increase.

BUT-- I would also stand to reason that you would get to operating temps much quicker in the above case. IOW, I wouldn't be freezing for the first 10 miles, only 5.

Then the 2 questions that tempt me are:
1.) after the belly pan, did you notice any change in how fast your truck warmed up, and
2.) if you ever consider bolting or TIG'ing some Al U-channel lengthwise along the bottom (not that you would or need)- I'd love to know if there were any changes.

Me? I'm thinking of one of the aftermarket steel pans just so I don't knock my underside off. Pretty sure anything I try to fabricate would be tore to shreds. I was looking at some 1/8th X 8" rubber for the dam and skirts, but I'd need a way to unbolt them easily where the highway ends, cause I still got that 60 mile each way highway to deal with.

And thanks for the input. Like your rig a lot and appreciate y'alls work in the area of saving me money (also the Aerocap guy, and the guy with the little chevy truck that did the spoiler, chin, and dam). And almost forgot the red F-350 getting 27mpg!

I was never able to instrument the truck properly and document a database of large enough volume so as to scientifically assess thermal dynamics.
My time working around HVAC equipment did suggest that without the airstream licking the oilpan,transmission housing,differential/axle,that warmup would come sooner and also heat retention after parking.
The dead air space would act as a partial insulation.
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Ford Motor Company incorporated an insulated exhaust pipe,along with coolant tubes within an extruded aluminum channel which was integrated into their 1984 Probe-IV concepts belly pan.The insulation mitigated heat transfer to the coolant lines,and the additional heat sink area provided by the cooling fins of the extrusion helped reject a little heat flux between the engine and rear-located radiator and back.
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Your selective surface coatings for the radiant loads is a good idea.Air-cooled motorcycles used this strategy with cylinders and heads,and racers used the white coatings on the underside of intake manifolds to help shield radiation from the heads and rocker valley.

[arcosine]

Yea, heat, the car get hot inside in the summer with a full belly pan. Put some vents around the gas tank to help keep it cool, or the gas might boil and build up pressure. I wrapped the exhaust pipe and muffler with fiberglass cloth and foil. Also, need to add vents to let the radiator air flow out underneath the car (truck). i run thinner synthetic oil, 5w less than oem spec.

[Thenorm]

some basic simplified heat transfer equations

highest heat transfer is conduction = Area x thermal conductivity x delta T

convection is next = A x convection coefficient x deltaT
forced convection is ~ 10 times as effective as natural convection

radiation is least effective unless its really hot as it varies by temperature to the 4th power. = emissivity x A x (deltaT)^4, so I would assume it to be negligible at the temperatures we're dealing with.

ya you can feel heat radiating if you put your hand near a radiator, but you would feel it a lot more if the fan was blowing hot air, or I dumped hot coolant on your arm, if you get what I mean.

[drrbc]

@arcosine- and hence the problem with being a fat dude in texas during the summer

@Thenorm- so as airflow/convective loss decreases, radiant loss will become the dominant mechanism. My thoughts then would be to either avoid interfering with airflow, or maximize radiation.