# Temperature of exiting air

1. Sep 16, 2008

### chhitiz

what is the temperature of air after it has passed through the fins in an air cooled diesel engine for an automobile of 100HP?

2. Sep 16, 2008

### Staff: Mentor

3. Sep 16, 2008

### chhitiz

how can it be impossible? you only need to find the rate of heat rejected to the air, temperature of incoming air(say atmospheric temperature) and the specific heat of air.

4. Sep 16, 2008

### mgb_phys

It critically depends on the details of the airflow, which determines how efficently the heat is transferred from the metal to the engine.
To do this you need computational fluid dynamics which models the air flow in a computer - and even then it is only an approximation.

You can get a ball park guess from:
The efficency of the engine, this determines how much of the 100Hp is going into heat.
The specific heat of the air (as you say)
The rate of air flow past the engine which also controls how long the air is contact with the fins and so how much energy is transferred
The input air temperature

5. Sep 16, 2008

### Topher925

30.47 degrees Celsius.

6. Sep 16, 2008

### chhitiz

could you also tell me where i could get such information?

7. Sep 16, 2008

### mgb_phys

You can make some estimates.
The heat capacity of a diatomic gas is around 20 J/kg/K
1 litre of air has a mass of about 1.2g, so each litre/s carrys away 0.024 J/K
Or in more common units, 1 CFM is about 0.5l/s so each cfm takes away 0.012J for each deg C rise in temperature.

The engine is 100Hp = 75KW and asumming a diesel engine is 50% efficent, half this energy goes in heat, so 35000 J/s.

Now what temperature the engine reaches depends on how much air is blowing past. If there was only a gentle breeze that air would have to heat up to an incredible temperature to take all the heat - higher than the temperature of the engine, which is impossible. Alternatively if you pushed a huge amount of air past the engine it would only have to heat up a little bit to carry away all the heat.

eg .to keep the engine less than 100deg above air temprature you would have to blow 35000 /0.024 = 1.5M litre/s past it

8. Sep 17, 2008

### chhitiz

could i know the surface area of engine in contact with the air? it would really help if i find someplace where i could find the accurate technical data and not just estimates.

Last edited: Sep 17, 2008
9. Sep 17, 2008

### stewartcs

Your best hope would be to contact the manufacturer.

CS

10. Sep 17, 2008

### chhitiz

allright then, could anyone tell me estimated effective surface area(with fins and all) of engine in contact with air in an typical air-cooled 100hp diesel engine?

11. Sep 17, 2008

### Staff: Mentor

Two reasons. The first is the complexities mgb mentioned (his answer: find it experimentally). The second is that you provided no data whatsoever from which to calculate anything!