Quote:
Originally Posted by glend
At lowspeed or stopped you are only relying on the fan to move air and there is no, or neglible, pressure differential between inside and outside the engine bay.
Fan volumes can normally exit around the sides of the engine and most fan clutches do not lock up at idle speeds as the temps are too low to promote lockup of the viscous clutch.
At speeds above 50kms per hour the pressure buildup in the front of the vehicle creates a differential and more air moves through the radiator than with just a fan. At some point the fan becomes more of a hinderance to cooling than a help, if you can exit the pressure buildup under the bonnet - which is what louvres provide (a lower pressure exit point).
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I find this unlikely. If there is a contrast in heat there is always a pressure differential. And is there’s a pressure differential there’s is air flow.
A vented hood on a car at rest will allow dense cool air to enter the engine bay displacing the hot air. Because hot air has a low pressure and is lighter (hot air ballon) it will rise. As jungle juice pointed out using the strips of material. This can be made more efficient by testing the pressure differentials in the engine bay itself to complement the convection cycle of the engine bay and venting at its highest temp/lowest pressure. It would also be improved by having vents located for cold air intake and separate vents to allow the exhaust of hot air. This would allow for nice directional air flow.
So air will vent at a standstill regardless of the fan. While you are right about the fan becoming less important at high speeds due to a "ram air" effect the pressure differential becomes even more important. The hot engine will effectively suck cool air in due to the pressure differential. So venting hot air at high speeds is probably less desirable. To a degree.
Junglejuice seeingthe strips forced in to the vents at high speeds is probably a function of air speed and higher pressure at the windshield I would imagine this would have little effect on cooling as the dense cold air would rapidly sink and go beneath the car unable to displace the hot air as it has nowhere to go. As jungle juice said. The reason the temp increases due to lifted hoods is likely due to perturbed air flow this would mean the fan was unable to shift the same volume of air and reduce the heat exchange. Also it would likely limit the amount of air entering through the front as the perturbed air would act as a shield for lack of a better word. You always want air like fluids flowing in one direction.
So well located vents that close at higher speeds are probably the most ideal.
I know this is long but it looked like a fun post.
Re: Vents vs scoops
