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?

I finished the experiment, and I must say I'm pretty excited. I did 6 runs from Hartland (H) to Oconomowoc (O), WI on Hwy 16. I had originally planed an additional 6 runs which would have been on the return trip from O to H, but the cops pulled me over on the return route the first time, so I abandoned that aspect of the testing because they were all over me on each return trip.

Here are the results in the order in which they occurred in A-B-A-B-A-B pattern, where "A" is a test with the fan turned up to its highest setting and the two light holes open and "B" is a test with the fan turned up to its highest setting and the two light holes covered up with tape.

Results:

A1: 50.3 mpg
B1: 49.6

A2: 50:1
B2: 50:0

A3: 50:2
B3: 49:9

A couple of notes:.

The stats on the Scanguage were reset at the same specific points (road signs) at the beginning of each run. Resulting stats were recorded at the same specific points (road signs) at the end of each run. The car was up to speed (50 mph each time) and set on cruise at least a quarter of a mile before the beginning of each run. On the "B" runs the taped holes were checked to make sure they were still covered at the end of each run.

This experiment took place between 1:30 AM and 4:00 AM CST. I stayed in the right lane at all times, except where the road narrowed to one lane, due to construction. I kept the radio off and did not use the wipers, so the headlights and the fan were the only devices I had on.

The engine fan was running quite a bit throughout the test. It was a variable that I could not control. Normally I would turn the heat up just a bit to reduce the water temperature, but I couldn't do it because I was testing. However, I do want to add that under normal conditions turning the heat on just a little keeps the water temp below 206 degrees, where the fan normally turns on. And with that much air flowing, the temp change in the cabin is hardly noticeable.

Weather:

The road was totally dry.

UV Index: 0 Low
Wind: From WSW at 5 mph
Humidity: 95%
Pressure: 29.89 in.
Dew Point: 68°F
Visibility: 9.0 miles

Final comments:

I may do more experiments once the guilt of wasting all that gas and ruining by mileage by driving without hypermiling wears off. I'm looking forward to others trying this out.

One more thing: I drove each return trip from the end of each test to the beginning of the next test with the fans off. I didn't smell anything, which is what I expected. If the high pressure is in the front of the car and the low pressure is in the back, the air will tend to flow to the back of the car and out the holes. Of course, the car isn't always moving, so that is always a concern - hence the fans staying on.

I wouldn't worry about CO much. First off it takes a lot of it to do damage, and second is that modern cars emit very little of it. Carbed cars were a lot worse. You'd get a killer headache before anything happened.

An interesting, but not convincing, set of observations. They do trend in the desired direction, which is good. However, in terms of experimental design there are two problems. (These are common problems in this sort of test, I'm not singling you out.)

1. The data is limited. The means differ by .37 mpg but that is not much different than the variation within the measurements, so we can't say with much confidence yet if it the measured difference is statistically significant.

2. You knew the state of the car for each test, which opens up the possibility of subconscious changes in driving style which could bias the results. If you do repeat the experiment it would be good if you could have somebody else flip a coin before each run and then open or close holes without telling you which they did. You can determine which it was at the end of the run.

I'm still skeptical - it doesn't seem right that the tiny amount of air coming out of those two small holes could significantly affect the large and continuously generated volume of low pressure air behind the car.

I didn't have time to compute whether or not it is statistically significant yet. Given the trajectory, a few more samples would easily make the difference.

Blind studies are not reasonable in this situation. In fact, if you only trust blind studies, all posts on this forum are useless to you. This information is made public for others to think about, test for themselves, and possibly design better and safer solutions. I will be doing that myself, too.

Although my background is in experimental psychology, I spend my free time messing with tangential bypass vacuums. I know from experience that a small leak in the system affects waterlift in ways that seem disproportionate. I'm used to dealing with enclosed systems where air dynamics don't play much of a role in a system that is designed for larger volumes of air than are actually used, so I didn't exactly know what to expect in this situation. The results of this test, then, are in line with my experiences with enclosed systems involving waterlift.

Yours was one of the few experiments here that would be easy to blind since the modification was small and isn't visible from the driver's seat.

I'm not saying it isn't possible - nobody can solve the air flow equations so there could be all sorts of surprises. It just doesn't seem likely to me that a small amount of air blown into the one to several cubic meter trailing low pressure region would do much.

I did think of another possibility. When the cabin is sealed the fan is pushing air into a closed space so the internal pressure of the car goes up. When you unplug the two back holes you reduce the cabin pressure by some amount, and the fan doesn't have to work as hard to move air into the cabin. So it draws less current, so the alternator is on less to provide that current, so the MPG goes up slightly compared to the "fan on, holes closed" case because the engine is loaded less. This may actually be a a gas saving technique (albeit a small one) via a mechanism that has nothing to do with the car's aerodynamics. If this hypothesis is correct plugging the back holes and slightly cracking a back window should give some FE improvement too. We've all seen the A/C versus windows open arguments, but I do not recall anybody measuring fan +/- windows open.

If you have the resources to do blind studies (not to mention double-blind studies), then please do so. I suspect, however, that you will say that you don't have the time. That's what I meant.

Frankly, in these situations blind studies aren't done. Popular Mechanics, Consumer Reports, Car and Driver don't do blind studies. They do easily replicable tests and publish their results for people to judge and peers to examine, and possibly retest.

However, if you do have the resources, please test it yourself. Keep in mind that a proper scientific test requires a random sampling of the available population (meaning both cars and drivers). If you can also get a dust-free wind tunnel, then that's even better. Keep us updated on your progress.