Virtual Boattail 0n 05 Honda Civic
 

It's been in the mid 80's quite bit, so I've been cranking the fan in my car, with the AC off. I decided to put all that air to work and vent it out the back of my Civic LX trunk's low pressure area.

I took out the lights from above the license plate and mounted them an inch higher. That way the light still shines out, but there are two large openings. When the fan is cranked up, the air pours out of the holes. It's pretty impressive. I don't expect the have huge FE improvements, but I know it will help. I'm also aware of the danger of exhaust coming into the cabin if I don't have positive interior pressure.

I'll run some tests late tonight if I can get a nap in. Again, I suspect the difference will be hard to measure.

What keeps the exhaust fumes out of the cabin when the fan isn't cranked up? Do you die on the first long drive on a nice day where you don't have the fan on? What happens if the interior windows are just cracked, or the sun roof? Seriously, dropping dead is a poor trade off for possibly saving 0.1 MPG.

One Way Valve
 

You may be able to buy (or make) a one way valve so air goes out, but not back in. You should be able to make it work without a problem. Like you say, the results should be minuscule, but I like your thinking.

Yeah, I understand; that's why I mentioned the need for positive air pressure. The air coming out the holes is coming out just as hard as it is out of the vents on the dash. I'm in less danger than if I had done nothing and kept the fan off.

Blow on your hand as hard as you can. It's just a little weaker than that, but with more volume because of the size of the holes - each about the size of a half dollar coin.

Yeah, I thought about the valve thing, also using a separate fan. I'm not taking it too seriously yet, but if I had a car that i didn't mind cutting up I might make a bigger vent using air from the front side of the firewall and exiting out behind the license plate through a large diameter hose (front to rear). Better yet, I could draw air from beneath the rear of the car.

Assume the area of the back of your car dragging the low pressure region is A. If there is a negative pressure back there of -1 inches of water, how high, in inches of water, will the pressure of the air coming out of the two hole have to be to for the force of the air coming out of the holes to equal the force on the back of the car due to negative pressure? This isn't exactly the right calculation, but it is one we can do quickly.

Let A equal 1, which is at least order of magnitude for a passenger car. The diameter of a half dollar coin is, um, 5 cm(?), which is .05m. Area of each coin is pi * .05 * .05 / 4 ~= (3 * 2.5 / 4) *10^4 ~= .0002 m^2. You have two such holes, so .0004 m^2. See where this is going? To completely negate the drag pressure the outgoing pressure would have to 10000/4 = 2500 inches of H20. Assume your car fan is really powerful and you have 10 inches of H20 pressure difference (this is pretty doubtful), at most you negate 1/250th of the drag.

Detector?
 

No. But I never suggested it would. Rather, I said it probably couldn't be measured by any tests I am willing to run. You quote me responding to your comment about "what keeps the fumes out" as if I was discussing the aerodynamic advantages. I wasn't. Instead, I was answering your question, which was already answered in my very first post in the thread.

So here are your first two questions, which I answered before you asked them:

1. What keeps the fumes out?

2. Will it make a measureable difference?


Yes, it will keep the fumes out (see first post). You would have to keep the fan on. And yes, it can be measured, but I probably won't get enough samples to capture the difference (see first post), so it won't be measurable thereby. Logic tells us that it will make a difference. A lot of small differences can make a big difference.

I'm surprised that modern cars and trucks don't have a CO detector by now... :(;)

Anyone wanna loan me their parakeet?