Belly pan + Fan concept?

[meanderingthemaze]

So, I'm converting a 1969 Datsun pickup truck to battery electric and need to aero-mod this puppy quite a bit.

I am definitely planning a belly pan, that seems a no brainer. However, I'm not sure if I can block off the grill, since the system is air-cooled and I will need some airflow through the engine compartment.

So, I am wondering if this idea is supported by any aerodynamic principles. I was thinking of putting a fan at the bottom of the engine compartment mounted on the belly pan, so as to blow air through a hole in the belly pan down underneath the car. In this way I am hoping it will draw the air that is coming through the grill and force it out under the car. I would imagine it would also increase the cooling of the electrical components.

Does this make sense?

[Bicycle Bob]

You can probably arrange things so that the fan can be off while you are moving. The front is a high-pressure zone, and the underflow can work as a venturi to draw air out of a shaped duct.

[lunarhighway]

i imagine you'll just need some cooling airflow over certain components, so ducting the intake air might reduce the overall drag of cooling. also you could considder creating hood vents so that air will be drawn out when the car is moving, but also as hot air rises it will naturally have a way to escape. a fan is always a safe bet, but perhaps link it to some sort of temperature sensor of manual switch so it doesn't have to be on all the time

also if the radiator is gone you'll have much less flow restriction so i'm sure you can block off some portion of the grill, especially if you'll have the fan as a safeguard

[Rokeby]

The battery cooling air, like radiator cooling air (in a perfect world!) exiting
through the belly pan should be ducted/directed such that it is moving as
parallel to the pan as possible. BMW has done some work on this. Here's a
picture:



IIRC, taking the easy way out and just shooting the air straight down acts like
a barrier to the air moving under the car. This acts like additional frontal area
and also greatly disturbs the flow under the car.

Off topic: Look at the size of those front tire deflectors, 2 or 3 times the size
of the little OEM things you can see on lots of cars.

[MadisonMPG]

Why do you need to aeromod it? Electrics only go 40ish.

[meanderingthemaze]

Originally Posted by Bicycle Bob

You can probably arrange things so that the fan can be off while you are moving. The front is a high-pressure zone, and the underflow can work as a venturi to draw air out of a shaped duct.

Interesting, so I wouldn't need the help of the fan while moving. Cool!

[meanderingthemaze]

Originally Posted by lunarhighway

i imagine you'll just need some cooling airflow over certain components, so ducting the intake air might reduce the overall drag of cooling. also you could considder creating hood vents so that air will be drawn out when the car is moving, but also as hot air rises it will naturally have a way to escape. a fan is always a safe bet, but perhaps link it to some sort of temperature sensor of manual switch so it doesn't have to be on all the time

also if the radiator is gone you'll have much less flow restriction so i'm sure you can block off some portion of the grill, especially if you'll have the fan as a safeguard

Thanks, that's very useful to know. Great ideas1

[meanderingthemaze]

Originally Posted by Rokeby

The battery cooling air, like radiator cooling air (in a perfect world!) exiting
through the belly pan should be ducted/directed such that it is moving as
parallel to the pan as possible. BMW has done some work on this. Here's a
picture:



IIRC, taking the easy way out and just shooting the air straight down acts like
a barrier to the air moving under the car. This acts like additional frontal area
and also greatly disturbs the flow under the car.

Off topic: Look at the size of those front tire deflectors, 2 or 3 times the size
of the little OEM things you can see on lots of cars.

Well, this is why I posted. So you think that the fan flow will actually worsen the situation.

I haven't done the internal layout of components under the hood yet, but perhaps I'll do it in such a way as to make it possible to run ducts.

So would a long and thin rectangular opening at the bottom of the compartment be better than circular?

[meanderingthemaze]

Originally Posted by MadisonMPG

Why do you need to aeromod it? Electrics only go 40ish.

Obviously, someone told you some nasty lies about electric cars. Google "Tesla Roadster" and your world will expand.

I plan to take this truck on the freeway at speeds of up to 70mph, but will most likely cruise at 55-60mph to squeeze more range out of the battery pack. But for the few miles that I'm on the freeway, modding it should help some.

[Rokeby]

Originally Posted by meanderingthemaze

... So would a long and thin rectangular opening at the bottom of the compartment be better than circular?

The issue here isn't the shape of the cooling air exit opening. It is the most
effective -- or least harmful -- direction for the air to be moving as it passes
through the belly pan.

In the picture I posted, notice the large radius on the aft edge of the ducting.
Its purpose is to allow the radiator cooling air in this case to entering into
the under car air flow smoothly, kind of like idealized traffic flow at an
interstate entry ramp. Although we can't see it, I suspect the other side of
the duct towards the front of the car also has this curve in it.

There is another interesting element in the picture, the vertical air dam in
front of the exit opening. I suspect this is to make a low pressure area
behind it for the exiting cooling air to fill.

(Experts, PLEASE, jump in here as I'm well beyond what I can say with
confidence! )