Go Back   Physics Help Forum > High School and Pre-University Physics Help > Energy and Work

Energy and Work Energy and Work Physics Help Forum

Reply
 
LinkBack Thread Tools Display Modes
Old Aug 19th 2016, 02:24 PM   #1
Junior Member
 
Join Date: Aug 2016
Posts: 1
IR heat reflectivity and engines

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. These engines put out about 65kW.

The engine compartment is a semi-closed system for VW bugs, which are air-cooled engines. They have vents coming in to 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 engine 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.
apensity is offline   Reply With Quote
Old Aug 22nd 2016, 06:18 AM   #2
Physics Team
 
ChipB's Avatar
 
Join Date: Jun 2010
Location: Morristown, NJ USA
Posts: 2,287
I'll take a stab. You are correct that chrome (or any "shiny" plating) tends to reflect IR back toward its source, whereas dark colored coatings tend to absorb IR energy. So chrome plating on things like valve covers would tend to reflect external heat away from the engine (e.g. heat in the engine compartment air and the heat in the metal of the trunk), but would reflect internal engine heat back into the engine. On balance I would think that since the internal temp of the engine is greater than the air of the engine compartment having chrome plated engine parts would tend to raise the internal temp of the engine.
ChipB is offline   Reply With Quote
Old Aug 22nd 2016, 06:36 AM   #3
Senior Member
 
Woody's Avatar
 
Join Date: Jun 2016
Location: England
Posts: 524
I think that another factor would be that the chrome plating will (generally) be much shiner on the outside surface than on the inside surface.
The inside surface will be the interface between the chrome plate and the underlying casting.
This will tend to be quite rough, partly due to the innate nature of the casting process, and partly to ensure good adhesion of the chrome.
Woody is offline   Reply With Quote
Old Aug 22nd 2016, 06:54 AM   #4
Senior Member
 
Woody's Avatar
 
Join Date: Jun 2016
Location: England
Posts: 524
Re-reading the initial post, I realised you were talking about the enclosing engine compartment, rather than the components of the engine itself.

Here I would suggest that one would want to encourage heat transfer from the air to the metal, and thence ideally to the outside of the car.
This would seem to count against a shiny chrome finish to the inside of the engine bay.

I would also suggest that one would want to encourage natural convection currents when car is stationary and there is no forced air flow cooling.

If you want to be "fancy" you could perhaps include an automatically opening engine bay cooling flap,
possibly something similar to the automatic ventilation openers used in greenhouses.
Woody is offline   Reply With Quote
Old Aug 22nd 2016, 08:16 AM   #5
Physics Team
 
ChipB's Avatar
 
Join Date: Jun 2010
Location: Morristown, NJ USA
Posts: 2,287
I guess I misread the post - I thought the OP was talking about chrome-plated engine parts, such as valve covers and exhaust headers. But if he's really asking only about the inside surface of the engine compartment then definitely making those surfaces with shiny plating would cause increased retention of heat in the engine. If instead the inside of the engine compartment was flat black then the compartment material would absorb more IR radiated from the engine surface, which has the effect of sucking heat out of the engine. Of course the metal of the engine compartment would in turn heat up a bit, but that's just proof that heat is being transferred out of the engine itself.
ChipB is offline   Reply With Quote
Reply

  Physics Help Forum > High School and Pre-University Physics Help > Energy and Work

Tags
engines, heat, reflectivity



Thread Tools
Display Modes


Similar Physics Forum Discussions
Thread Thread Starter Forum Replies Last Post
Operation of heat engines - otto cycle stuckinthemiddle Advanced Thermodynamics 0 Apr 18th 2016 06:32 AM
Diesel-Electric Engines Vector Kinematics and Dynamics 1 Apr 22nd 2011 10:04 AM
Heat Engines 11rdc11 Thermodynamics and Fluid Mechanics 1 Nov 30th 2010 12:24 PM
Specific latent heat and heat capacity Punch Thermodynamics and Fluid Mechanics 0 Apr 23rd 2010 06:51 PM
Help with Ideal Gases and Engines mortalapeman Thermodynamics and Fluid Mechanics 0 Sep 10th 2009 12:30 PM


Facebook Twitter Google+ RSS Feed