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Automotive cooling system

Automotive cooling system

Automotive cooling system

Hi All,

New to this forum and I am not sure if this has already explained in other Thread .... if so then please let me know and i will refer that.

My question is:
I am working on engine cooling system and deciding size of fan.
Air flow quantity was already given (CFM cubic feet per minute)to me as given data.
I want to verify that how much air should flow to maintain system tempreture. Can someone please let me know basic formula to verify it?
I know it can be calculated by CFD analysis but I dont have software available and also dont know how to model it in CFD.
Can someone please give me some basic idea? I want to quick verify by hand calculation.

Thanks in advance.

RE: Automotive cooling system

I bet you thought there was a nice little equation for this.

Bad luck, it's pretty complicated. Have a look at


If you have the performance curves for your radiator design then it is pretty easy, otherwise you need some way of estimating the heat transfer coefficient betwixt fluid and air.


Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Automotive cooling system

You will need much more information than just a volumetric airflow rate to design a fan capable of providing sufficient heat rejection from your cooling system to maintain adequate engine operating temperatures at all conditions required. As GregLocock noted, it's a very complicated problem that involves many variables such as air mass flow rates, pressure drop across the heat exchanger core, temperature rise/drop between the air/coolant across the heat exchanger core, etc.

RE: Automotive cooling system

Thanks Greg and Tbuelna for reply.

Yes thats true and I realize it now that there are lot of factor involve to decide cooling system components.
Can you please guide how to start at initial stage.
What is formula to calculate required airflow? also how to decide built in resistance for vehicle.
I discuss in my team they said it was done on CFD .... but I want to know basic formula for hand calculation?

Please share some documents for calculation if you can.


RE: Automotive cooling system

That book I linked to has 7 nomograms to help in system analysis, there isn't a simpler way.

Roughly speaking you need to establish your required cooling power, your top and bottom hose temps. These give an average water temp.

Then working upstream, you need to know the thermal resistance of your radiator, which is a function of materials and geometry.

then you need to look at your air side cooling power, which equals the cooling power you've already calculated, and then see if there is sufficient dT to drive that heat flux through your radiator.

I am not going to answer any further questions until you acknowledge that you have read and worked through the nomograms I linked to. Incidentally, taking a punt, FSAE has its own forums elsewhere, use them.


Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Automotive cooling system

Give us some data. What kind of cooling system does the engine use? Air cooled or liquid cooled???
What is the displacement? What fuel and what ignition system? ------> Compression ignition or spark?
What HP is it? What application? Racing, normal driving, large truck, off road etc.

RE: Automotive cooling system

Hi Dicer,

Thanks for reply. Its 6 cylinder 6.8 liter 200 HP CI diesel engine for construction machine.
Its EGR, Turbo charged liquid cooled engine.

Please let me know if you need more input or detail.

RE: Automotive cooling system

Please let me know how to start?


RE: Automotive cooling system

Why are you not asking these questions to the original engine manufacturer??

RE: Automotive cooling system

Fan will be approx. 2 feet diameter. What rpm does it run?

RE: Automotive cooling system

Go to the dealer where you bought the engine, and buy the manufacturer's Application Handbook.
Maybe you can even get on a CD inexpensively.
It should contain plenty of information about sizing radiators.
In fact, the Application Handbook for any engine that's ever used with a radiator will contain information that's valuable to you. Collect a bunch, and study them all.

Mike Halloran
Pembroke Pines, FL, USA

RE: Automotive cooling system

If the fan is 2ft in diameter, it would be safe to assume the coolant heat exchanger core would want to be somewhere close to 24"H x 24"W. Your coolant flow rate will be based on the existing water pump. And the coolant inlet temp will be based on the thermostat used. Your application involves an engine used on a construction vehicle that mostly operates in a fixed position, which means the fan will need to produce most of the airflow over the heat exchangers. But it also means it will be easier to determine the flow conditions required from the fan.

RE: Automotive cooling system

The easiest way to do this by hand involves thermocouples, 2 or 3 different plausible options for airflow, your engine, and some wrenches.

If you want to calculate it, Greg is leading you on the right track.

Its a simple concept, but it requires data that is tedious to acquire. The easiest way to get that data would be to ask the guy who gave you the CFM requirements, in which case you would effectively be checking his homework for math errors.

I would suggest the use a powerful electric fan and a thermal switch in the return coolant flow.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Automotive cooling system

It appears to OP is averse to reading...

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