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Emissivity and an engine

Posted by KilleRabbit, Apr 5, 2018

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  1. KilleRabbit

    KilleRabbit Proven Member

    Joined Jul 3, 2016
    Deltona, Florida
    I am posting this in the misc. tech article section because im not entirely sure where you guys might classify this information.

    Let me start this by saying that I am not an expert, I have a decent amount of knowledge on how the following concepts work and this topic is definitely up for discussion and debate. I am simply introducing the concept and leaving the floor open to questions and ideas. If you have questions or an argument against anything I say in this post please voice your opinion. I will note that my knowledge of this concept comes from what I do for a living so I do have an idea of what I'm talking about.

    Here is the deal:

    Heat is huge factor in the efficiency of any machine. Friction and combustion create heat as a byproduct. How can the properties of heat, heat transfer, and heat dissipation be utilized to more efficiently reduce the heat in certain components in the engine bay to improve performance of the engine?

    Allow me to introduce, to those who may be reading this who aren’t familiar with the concept already, emissivity. Emissivity is the measure of how efficiently a material surface emits (or radiates) heat. It is measured on a scale from zero to one; zero having no heat emittance and one having perfect heat emittance also known as a black body. A perfect emitter has yet to be discovered. Human skin has an emissivity of roughly .98 and bare aluminum has an emissivity of .02-.4 depending on the condition of the surface.

    Changing emissivity does not change the thermal conductance or capacitance of a material. Changing the emissivity of the surface of a material will only change the way that surface radiates heat. So it impacts heat being expelled not heat being absorbed. This is important because this is all this write up will be diving into. Changing components to materials that have more efficient thermal capacitance and conductance has been common practice for a long time and requires the changing of components and considerable investment to achieve goals. People often times do this without realizing they are doing it. The LS series motors are a good example of this, changing to an all-aluminum engine doesn’t just have a weight reduction improvement; aluminum has thermal properties that make it more efficient dealing with heat as well.

    What I am getting at is altering a surface to make it radiate heat more efficiently. Some components should retain their heat to prevent transfer of heat into areas where it is unwanted, while others should release it as efficiently as possible to keep temperatures down.

    Polishing a surface reduces emissivity and roughening, painting, or coating a surface raises it.

    Intakes should prevent heat transfer in order to prevent air flowing through them from heating up reducing the efficiency of the incoming air when mixing with fuel. Engine blocks should emit as much heat as possible to keep internal temperatures down to prevent failure.

    Polished metal surfaces are terrible emitters, they will retain a lot of heat. If you look at a polished metal surface that is 200 degrees with an infrared camera it will read a much lower temperature than the actual temp. This is because the surface is not radiating its own heat, only reflecting heat from objects around it. For this reason I am, from a performance standpoint, completely against polishing the exterior of any metal component that comes into contact with the head or block.

    A cold air intake system that is polished on the inside will reduce the amount of heat transferred from the piping to the air passing through it. That being said I would not polish the interior of the intake manifold or intake ports on the inside of the head because a certain amount of turbulence is required to efficiently mix the air and fuel and polishing these surfaces reduces turbulance.

    Coated or painted surfaces emit heat fantastically. For this reason I am an advocate of powder coating blocks, heads, and manifolds to allow them to release heat more efficiently and in turn reduce the temperature of the motor.

    I will note here that while painting and coating can help surfaces emit heat better, it probably is not a good idea to do this to a radiator. We wouldn’t want to reduce the space for the air to travel through between the fins. If you have another opinion on this please feel free to chime in, this is only an opinion generated by looking at the functionality at face value and I don’t have any data to support the claim that coating or painting a radiator is a bad idea.

    It should also be noted that making surfaces within an engine bay radiate heat more efficiently will make the engine bay hotter, so ventilation will be a factor for keeping the bay, and in turn the engine, cooler. Again this is another common concept where people install vented hoods and larger fans to move more cool air to keep the radiator and engine bay cooler.

    I am bringing up this concept because i think having a good understanding of how heat radiates can be beneficial but some of you might have more ideas than what I have mentioned that might make this information more useful.

    By the way, just so everyone is aware, your radiator does not cool by means of radiation. It actually cools by convection, just a fun fact.

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    1998 Mitsubishi Eclipse Spyder GST
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