IR heat reflectivity and engines

[Brian Teal]

A HUGE topic of conversation on many car forums, I thought I would bring it here.
Many cars have chrome tin in the engine compartments to make them look flashy. However, for decades now, people say the chrome reflects the heat back onto the engine block and to paint the engine compartment black to absorb the heat. But if it absorbs the heat, doesn't that make it somewhat of an oven? Who's right? Remember, IR light is still present in a closed engine compartment without there having to be visible light. Let the games begin! And thank you ahead of time!

 

[Bystander]

Absorbances/emissivities are functions of wave length and temperature. You've blistered your arm on chrome trim in the dead of winter and frozen your tongue to same in the middle of summer ... not so simple after all ... is it?

 

[Andy Resnick]

A HUGE topic of conversation on many car forums, I thought I would bring it here.
Many cars have chrome tin in the engine compartments to make them look flashy. However, for decades now, people say the chrome reflects the heat back onto the engine block and to paint the engine compartment black to absorb the heat. But if it absorbs the heat, doesn't that make it somewhat of an oven? Who's right? Remember, IR light is still present in a closed engine compartment without there having to be visible light. Let the games begin! And thank you ahead of time!

I'm confused by the question- the engine compartment is filled with lots of objects, moving air, and is not a sealed compartment. Of what significance is the heat load on the engine block? Are you trying to perfect the car-b-que?

 

[Brian Teal]

Arrghhh, my well-thought-out reply didn't post, so here's an abbreviated version...

The engine compartment is a semi-closed system for VW bugs, which are air-cooled engines. They have vents coming into bring in fresh air, and then vents leaving to pull the hot air out. However, when in traffic or when the outside ambient temperature is high, it does not cool well. Because of this we have to be very sensitive to keeping the temperatures down inside the engine compartment.

The engine compartment is made with "tin" surrounding it, and creating a "pressurized box" which is sealed except for the incoming and outgoing vents. This "tin" is often chrome, in order for engines to look fancy. However, there is a large following now for many years that say the chrome is responsible for reflecting the engiine heat back on itself and thus creating engines to overheat. If people were to paint or powdercoat these tins, then the black color would absorb the heat and keep the engine from overheating.

My question is revolving around if the chrome actually reflects the IR heat/light back onto the engine, as I would think that the BLACK tin would act more like an "oven" and keep the engine compartment hot because of absorbing and retaining the heat. I read an article in the NASA archives mentioning how they use various metallic materials in order to reflect IR/radiation to keep instruments cool. This would lead one to believe then that the chrome would reflect IR/heat/light back to the darker engine block which would increase the temperature of the engine and oil and create overheating issues.

So I brought it here for you "educated folk" (I have my Masters in Bio-Psych, not Physics) to discuss, rather than the arm-chair mechanics who "swear" they know the answer without any explanation based on the physics of the problem.
Attached is an image of what I mean.

 

[Twigg]

Obviously can't give you exact answers here, but for an engine at 100C (note: this is for the temperature of the outer wall of the engine, not the hot stuff on the inside) you get ~11W/m^2 of power emitted, which is pretty tiny compared to the massive heat flux you get from the air flow through the engine compartment. Say the engine is putting out 10kW of power at an efficiency of 10%. That means it's putting out 90kW as heat. Taking the surface area of the outer engine wall to be on the order 1m^2 (probably on a fraction of that, but close enough). I was lazy and picked easier numbers to work with (engine is probably putting out a lot more power on average, and the surface area is probably less), but that just goes to show how small 11W/m^2 is compared to the hundreds of kW/m^2 you'd expect from convection cooling.

Incidentally, the 11W/m^2 for the 100C blackbody you get out happens to be around a wavelength of 7.8 microns, which is inside an atmospheric absorption band (infrared near those wavelengths gets partially absorbed by water in the atmosphere). So not even all of that is going to make to the compartment wall to begin with, much less after a reflection. Granted, some of it can form a standing wave by incoherent addition, but the initial intensity off the engine is around ~0.01% of the convective heat flux. I just don't see it being more than 0.1%, in a worst-case scenario. If that kind of tolerance matters, then maybe it's worth taking steps to alleviate it.

As far as painting the walls of the engine compartment black, sure why not, if it doesn't cause other problems (paint peeling, rust, etc.). It would reduce the amount of infrared floating around in the compartment, but that level of radiation probably wouldn't matter in the first place. I doubt it's going to make the engine compartment act like an oven, because the engine compartment has a giant heat sink attached to it: the rest of the car, plus all the air surrounding the car. NASA has to worry about things like IR reflection in instruments in space because they have no atmosphere up in space to keep things cool and stop infrared from bouncing around forever. As a rule of thumb, in large volume systems filled with inert gas like air with no extraordinarily hot things in it (~1000K is where things start to get "extraordinary"), convection tends to be the main mechanism of heat transfer. At smaller volumes and for fluids with higher Prandtl number (like oil and such), conduction plays a big part too. IR radiation is the weakest of the bunch.

But as far as practical advice, I would say do what's been proven to work and don't change nothin' unless you're being paid to do R&D.

 

[berkeman]

Thread closed for Moderation...

 

[berkeman]

Thread re-opened. Thanks for your patience.

 

[Brian Teal]

Obviously can't give you exact answers here, but for an engine at 100C (note: this is for the temperature of the outer wall of the engine, not the hot stuff on the inside) you get ~11W/m^2 of power emitted, which is pretty tiny compared to the massive heat flux you get from the air flow through the engine compartment. Say the engine is putting out 10kW of power at an efficiency of 10%. That means it's putting out 90kW as heat.

Great explanation! Though I am guessing you meant 100kW of power for the engine? Other than that, I can see what you mean (though the wavelength over 7.7 microns and info on the absorption band I only thought of as a "outer space" related issue ;^) Glad to see it "brought down to earth" in this application.

Anyone else want to chime in?

 

[Twigg]

Great explanation! Though I am guessing you meant 100kW of power for the engine? Other than that, I can see what you mean (though the wavelength over 7.7 microns and info on the absorption band I only thought of as a "outer space" related issue ;^) Glad to see it "brought down to earth" in this application.

Anyone else want to chime in?

I actually did mean 10kW, though in retrospect I noticed that's probably small for a car. Whoopsies! As far as the math, I got the 90kW from the %10 engine efficiency. If 10kW was just 10%, then there's 100kW of power going into the engine (energy from combustion, flow through valves, etc.), and only 10kW being used, so 90kW wasted as heat. Despite my numbers being a little small, I think the point still stands about how small the IR radiation contribution is.

Also, there was something you mentioned I didn't consider before. If you're stuck in traffic so there's no air flow, that changes things a bit. If there's not enough cooling, the engine temp will just keep on rising (until convection catches up). Say it hits 200C. Since the radiation power goes with $T^{4}$ (in Kelvins), then you start emitting 28W/m$^{2}$. It's a positive-feedback process. Not to mention, as the temperature goes up the IR wavelengths will be pushed out of the absorption band. If you're in traffic for hours on end on a hot day, then having IR radiation bouncing around the engine compartment certainly doesn't help. My guess is that it wouldn't be the root cause of overheating, but it would probably make it happen slightly faster. The big question is how "slightly" is slightly?