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Old 08-20-2017, 11:38 PM   #1 (permalink)
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Engine bay air pressure

How much effect does lots of low pressure air in the engine area have (particularly in EV conversions with lots of open air space under the hood draining pressure into wheel wells?

One part of me figures the low pressure would pull back on the hood, the other says it would push forward on the front of the dashboard behind the hood and equalize the net effect.

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Old 08-20-2017, 11:49 PM   #2 (permalink)
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Well...on the top of my post-conversion lists is blocking off the front grill and such, but that's more to keep the weather out than anything.

I'd assume the opposite as you, though...there would be pressure buildup under the hood, and it would want to push out/up the hood, not the other way 'round.

The/my Fiero is a good example of this...if you get going fast enough, the air pressure pushes the spring-loaded pop-up headlight doors open. People who like to drive fast have to vent the pressure one way or another. (it's designed to exit beneath the car, but this isn't enough at high speeds for what's being forced in through the grill)

Anyway, there's no real point to allowing air to flow in under the hood in an EV, and multiple reasons to prevent it...making answering the question moot.
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Old 08-23-2017, 02:38 PM   #3 (permalink)
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Quote:
How much effect...
If you really want to know, get a Dwyer Magnehelic Differential Pressure Gauge (manometer). Here's an article that tells you how to make one from rubber hose and bottles:

Autospeed: Eliminating Negative Boost - Part 2
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Old 08-25-2017, 12:03 AM   #4 (permalink)
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Keep in mind, that the goal of drag reduction, at its most basic level, is to not make air move. The less you make the air move, than the less energy you expend moving it. Seems like, "Well, Duh." but most fail to keep this in mind when analyzing things.

So in this case, analyzing how air gets scooped into a dead area (engine bay) and is forced to "Slow Down" we see, that really isn't what is happening. Consider that the air was just sitting there still (Lets assume calm winds) 18" over the surface of the road, along comes the car, the air gets scooped into the engine bay, and since it is hard to escape from there, or because it runs into things, it is forced to "Speed Up" to match the speed of the car. So you took something with mass (air) just sitting there, and forced it to speed up, that requires energy, which we refer to as drag.

So the less air you scoop through the engine bay, or wheel wells, or truck bed.....the less energy you are going to require to make it speed up. This is why the template is a good tool, as it moves through the air, it minimizes how much the air has to move, and it minimizes how much the air gets energized and swirls about as the vehicle passes through the air....It keeps the air as calm as possible during and after the vehicle passing through it.

Sooooo, don't think of it as "Pressure" so much as, "How much air is being accelerated?"

Make sense? Clear as mud? Lets talk.
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Old 08-26-2017, 03:53 PM   #5 (permalink)
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engine area

Quote:
Originally Posted by mikesheiman View Post
How much effect does lots of low pressure air in the engine area have (particularly in EV conversions with lots of open air space under the hood draining pressure into wheel wells?

One part of me figures the low pressure would pull back on the hood, the other says it would push forward on the front of the dashboard behind the hood and equalize the net effect.
Typically,for streamlining purposes,and aerodynamicist,or aerodynamic engineer would be concerned with cooling system drag.
A garden-variety car of the 1960s might have a cooling system which constituted 12% of the car's overall drag.
Fully-ducted race cars of the same period had 2%-loss cooling systems.
Today,with variable grille-shutters,the drag will vary as a function of engine heat flux.
An EV with sealed grille would essentially have an 'engine' compartment at whatever the ambient barometric pressure was.All surfaces inside there would 'see' the same pressure.
Even if it were open to the outside somewhere,if it was non-porous,it would just be an area of dead air.
Given that,the flow around the car would all be at a lower pressure and the hood would want to blow off if unsecured.
Since most production engine bays are an aerodynamic torture-chamber,any thing you can do to minimize flow there cuts drag.
Some very low-drag ICE concept cars move the radiator to the rear of the car and use the inlets back there as suction slots,and the exits as blown slots.
The Ford Probe-IV can be driven at 2.5-mph on the blast from it's cooling fan.
The EV has an advantage of shunting more air around the vehicle,better energizing the boundary layer,and with less internal turbulence,has more kinetic energy to harvest as pressure recovery in the aft-body,for lower drag.
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Old 08-26-2017, 11:10 PM   #6 (permalink)
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The Ford Probe-IV can be driven at 2.5-mph on the blast from it's cooling fan.
I did not know that. Kind of like the Schlorwagen with the propellor on the back.

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