Heat exchanger automotive.

[Chris S]

For a long time now this question has bothered me. I have a classic car, and I want to know how the rate of flow of coolant affects cooling. Could I make a simplified model to show how engine temperature varies with coolant flow? Obviously zero flow would halt the cooling effect. But does engine temp continue to reduce with increase in flow or is there a point where it stabilises or even drops?  As an electrical engineer I dont have the tools to make such a model.  

[Bufofrog]

The simplest model is just to look a the cooling loop.  This is where you have a heat exchanger and a heat source.  The heat exchanger is the radiator and the heat source is the engine.  You can write a simple equation for the amount of heat removed by the heat exchanger as follows:  \[\dot{Q}=\dot{m}c_p(T_H - T_C)\]

Where \(\dot{Q}\) is the rate of heat transfer

\(\dot{m}\) is the mass flow rate

\(c_p\) is the heat capacity of the coolant.

so you can see that increasing the delta T or increasing the coolant mass flow will increase the heat removed by the radiator.

The actual equation for the heat transfer is very complicated.  Consider this:  as you increase your speed your engine produces more heat but the heat production is not linear and as the speed increases the ability of the radiator to remove heat increases from the air flow and again it is not linear.

The car maintains the engine temperature quite easily with a thermostat.  As the engine heats up the thermostat heats up and a bellows connected to a valve expands which allows the coolant to circulate through the engine when the engine cools the bellows contract and shuts the valve decreasing coolant flow.  The thermostat is designed to allow the engine to run at the desired temperature.  The radiator is designed to be able to cool the engine when running at the highest rated steady state speed during a predetermined highest ambient temperature.  What that means is the radiator has excess cooling capacity, so if the thermostat valve was always fully opened then the engine would run cold all the time, it could not heat up to the optimum operating temperature. 

Hope that answers your question.

Edited May 6, 2019 by Bufofrog