Making a heat shield for my car's exhaust

[matt621]

Hello,

I'm just wanting to make sure I remember this correctly.

I have a hot exhaust pipe I want to shield from fuel lines.

So I have a shield (stainless) that is going 1/2 way around the exhaust, with an air gap of about 1". I want to sand blast the side facing the heat and polish the side facing the fuel. Right? (I will also insulate around the fuel lines.)

Did I remember this correctly?

Thanks

 

[Baluncore]

So I have a shield (stainless) that is going 1/2 way around the exhaust, with an air gap of about 1".

The shield should not be too close to either exhaust or to the fuel lines. 1" clearance suggests they are 2" apart, with the shield in the middle.

I want to sand blast the side facing the heat and polish the side facing the fuel. Right?

I see no reason why the sides should be different. The shield will be cooled by airflow. It might be an advantage to mirror the side closest to the exhaust pipe, which would reduce the temperature of the shield, while increasing the temperature of the exhaust pipe.

Insulation of fuel lines is a separate issue. The fuel lines are cooled by internal circulation of the fuel, and externally by airflow.

 

[matt621]

I'm considering radiant energy.

 

[Baluncore]

The shield operates by blocking direct IR radiation, heating the stainless steel shield, that is then cooled by both airflow and some minor radiation. Cooling the shield by convection is much greater than cooling by radiation, so the shield has a low temperature compared to the exhaust pipe surface. It will be warm if touched. An air-cooled shield is sometimes called a "stove".

Stainless steel does not rapidly accumulate dirt and rust that might reduce reflection, or insulate the metal shield from airflow.

Dry grass that is caught between the exhaust pipe and the shield can start fires. That is especially true where catalytic converters are involved.

It should not matter which way the energy is crossing an impedance mismatch. The transmission and reflection coefficients will be of the same magnitude.

 

[berkeman]

I have a hot exhaust pipe I want to shield from fuel lines.

Are you custom-building a vehicle? If this is a standard production vehicle, why would you want to make this modification? What is the motivation for changing a proven design?

 

[matt621]

To berkeman:

I'm adding an aux. fuel tank and lines to an existing vehicle. I also have a race car (with turbo) that is fabbed. So this issues comes up more than once. :-)

To Baluncore:

Yes, thanks. My thought was to make one side of the heat shield more like a black body and the other side a "white body". Ie, limit the radiation on the fuel side. Reradiating the energy back to the exhaust side is preferred to radiating it to the fuel side. There is also a fuel tank in that area so I want to keep it as cool as possible. And yes, air flow is good in this area.

Thanks for the answers. Best forum on the net for science questions.

 

[Baluncore]

My thought was to make one side of the heat shield more like a black body and the other side a "white body". Ie, limit the radiation on the fuel side.

The heat transfer from exhaust to fuel lines will be minimised if you polish both sides of the stainless steel.