# Heat transfer of an engine

This is the scenario..

I have a engine and the block is made of cast iron which is 1/2" thick.

The internal temperature of the engine within the cooling passages is 260° F

Is there a way to calculate an approximate surface temperature on the outside of the engine block? If so, what would the temperature be?

Say that there was another cooling passage on the other side.. if the passage behind the 1/2" of iron was 260°, what would be the second cooling passages temperature?

## Answers and Replies

So let me get this straight, as I'm not familiar with engine terms

you want the surface temperature of the 1/2 cast iron, and the air temperature a certain point above this surface?

No, not really. The cast iron will act as a insulator, so the surfice teperature on the outside of the block will be less than on the inside of the block. I'm just wanting to know how much less.

mgb_phys
Science Advisor
Homework Helper
This is a thermal resistance question.
The temperature difference depends on the thermal resistance of the material and the heat flow. You can look up the temperature difference accross a given thickness of cast iron for a given heat flow.

The complication comes when the temperature of the material on the other side isn't controlled. Suppose you want to know the temperature of air above the engine block.
Initially it is cold and you have a certain temperature difference accross the cast iron, you can then work out the heat flow. But this heat flow will heat the air and so reduce the temperature difference, which will reduce the heat flow. Eventually the air is the same temperature as the inside of the engine then there is no temperature difference and so no heat flow.
Becuase of this, the most important factor is the rate of new cold air arriving at the outside of the engine.

This is a combined heat transfer problem i.e. heat is conducted through the engine block and convected off the block by natural convection. It requires several simplifying assumptions such as:

* The engine block is modled as a horizontal plat with heated surface facing up
*The ambient room temperature is 80 degrees F
*You can neglect radiant heat transfer

q-conduction=q-convection

q-conduction= k*A/L(260-Ts)

q-convection= ho*A(Ts-80)

ho is the convective heat transfer coef (about 1.215 Btu/(hr*ft^2*F) for this problem) requires formulas for natural convection available in any Heat transfer text

k is thermal conductivity for cast iron

L is cast iron thickness in ft

Given the thin engine block wall and high conductivity of cast iron the outer surface of the engine block will be about 259 F