A Question About the Mechanics of Cooling a Mid-engine car

[LeePeyton]

I'm new to this forum but I'm a science geek. Sorry if this is

way off topic for your forum but I thought this would have enough

science involved to ask this question and hear an intelligent debate.

 

The car in question is a MR2 Turbo.

To cool the engine bay on a MR2 turbo there is the radiator mounted up front and passive cooling over the motor; Air rushes under the car and out the vented engine bay hood. The conflict is that a common modification to the car is putting two 11 inch 850CFM fans under the hood vent.

 

My personal hypothesis on this is that the fans would restrict air flow because they can only move 850CFM thus a limited RPM. A sales rep. told me that the passive cooling would still take place the fans would just help. I just think that the fans can only spin so fast and a open free flowing area to constrict as little air flow as possible would be the best way to cool the engine bay. Essentially stating that the stock way it cools the engine bay with rushing winds dependent on the speed of the car is more effective than the fans.

 

I'm not sure how fans would work with forced passive cooling like on a vehicle. So, I wanted to see what you guys thought. If you guys need any pictures feel free to ask I'll post what I can.

 

Thanks and have a nice day.

[Spyman]

I guess we are talking about some electrical fans here and even though they have a "marked" theoretical air flow, it is calculated for air normally standing still and only being pushed by the fan. If the air already is flowing the fans will probably be able to push it up to a little higher flow than rated.

 

Electrical fans don't have a maximal RPM, they have a rating how fast the electrical current will be able to make them go and what air flow there is in the fans with that RPM. They normally don't have any mechanical breaks or other devices that will limit their maximal RPM, if the air flow increase then the RPM will also increase, up to the point where the fan will break, due to bearings failure or wings flying apart.

 

Granted there will be a maximum level where the fans won't be able to increase the flow any further and instead the fan may start to act as an generator and try to draw power from the air flow, but that can be prevented electrically. The only restriction the fans make on the airflow that remains then is the friction in the bearings and the turbulence in the fan, which I think more or less can be neglected.

 

In addition there is also times when the car is not going fast enough for producing higher air flow than what the fans support, where the fans actually raises the cooling and thus smoothes out the temperature in the car engine. More cooling before and after the high speed durations will also lower the overall engine temperature and preventing high peeks in the engine temperature will extend the engine life time.

 

I would recommend installing the fans, the restriction on the air flow during high speed is tiny and the increase in lower speeds is great.

[insane_alien]

yes, the fans likely won't become a net loss rather than a net benefit until you get up to very high speeds. the 850CFM assumes the air at the intake is staionary and if you're going 50mph then this is definitely not stationary.

 

the biggest benefit is if you are moving slowly(or not at all) the fans will be able to maintain better cooling.