Air temperature inside the car bonnet?

[jak9]

hello friends,
I want to know the air temperature inside the car bonnet. So i thought that it would be found out by considering several factors like the environment temperature, heat dissipated by the engine. So is it right to say that the air temperature inside the car bonnet is;
Airtemperaturebonnet= Heat dissipated by engine-Environment temperature
?
If yes then how the heat dissipated by the car engine can be found?
thanks

 

[nasu]

hello friends,

Airtemperaturebonnet= Heat dissipated by engine-Environment temperature

This is not right. Heat and temperature are different quantities (with different units) so tehre is no meaning in subtracting one from the other.

You may have in mind something like
Heat dissipated by engine- Heat dissipated to the environment
This would account for the temperature increase of the medium under the hood (bonnet).
You may be able to estimate the heat dissipated by the engine.
For the rest, I think it would be much easier and a lot more accurate to put a thermometer under the hood and read the temperature.

 

[jak9]

@nasu...thanks a lot for the reply
but i can't use thermometer or any other temperature sensor because i am developing a simulation model in simulink.
what you want to suggest by heat dissipated to the environment?? because what i think is that i have to take into account all components heat under the hood. But i don't want a complicated model its ok if i get 70% of the result
so any ideas??

actually the battery is quite close to the engine, so how to calculate the heat absorbed by the battery post and i think it will also depend upon the distance.
thanks

 

[jak9]

@nasu...well your idea seems to be far correct because the ambient temperature inside the hood would be heat dissipated by the engine-heat given out to the environemt
so how the heat given out by the engine can be calculated? i think it is something related to mass, area...but is their a direct way of calculation?
and heat given out to the environment would be from the radiator right?? if yes then is it right to calculate this given out heat to the environment from the coolant temperature?

 

[mrspeedybob]

The air under the hood is not just sitting there, it is being constantly exchanged with outside air. In most vehicles air enters through the grill and flows through the radiator, It then circulates around the engine compartment for a very short time before being forced out the bottom of the engine bay by new incoming air.

In order to calculate temperature You'll need to know how much air is flowing in and out of the engine bay as well as the heat transferred from the radiator and directly from the engine. At a stop, air circulation will be determined primarily by the fan that is situated behind the radiator. When you are moving the cross sectional area of the grill and the speed of the vehicle will also play important roles.

One rule of thumb for gas powered vehicles is the 1/3 of the energy from the gasoline is dissipated by the radiator. 1/3 is used by the engine itself as well as to overcome friction, drive accessories, gets blown out the exhaust, etc. 1/3 is used to drive the wheels. Since a lot of the friction happens under the hood I would start with 1/2 being converted to heat in the engine bay.

You will probably find that there are a lot of variables for which you will just have to guess at values. When you get done run a simulation on your own car and maybe a few of your friends cars. Then take a thermometer and go check the actual temperatures. Tweak the numbers you made up until the simulation agrees with reality and you will likely have a decent simulator.

 

[jak9]

thanks friend you mean i have to know the amount of air flowing in in terms of its volume or temperature?

 

[mrspeedybob]

thanks friend you mean i have to know the amount of air flowing in in terms of its volume or temperature?

Both

 

[jak9]

ok thanks a lot

 

[xxChrisxx]

You simply won't get any meaningful answer by 'calculation'. This type of thing would require a full simulation just to get a value for heat transfer.

Just make an assumption and put it at a set value. Say 15% of all energy burned is lost through the block as heat. The rough surface area dissipating heat is... I don't know 0.75-1m^2 or something.

You then have a value of heat flux to the surrounding air. To make it easy i'd just say the air in the engine bay is static for now. You can then work out the temperature increase (you'll have to put a sensible stop value on the increase though).

 

[jak9]

thanks friend