Testing wheel skirts A-B-A
 

Last weekend a student and I put wheel-well skirts (one coroplast and one cardboard) onto my Civic. You can see them as tested in the attached pic.

We tested them B-A-B around my usual 18 miles mostly flat freeway test route. From noon to 130pm, and after thoroughly warming up car and tires, in temperatures that were between 62 and 64 the whole time, with slowly falling humidity, gradually clearing skies, and wind at a steady 3mph from one direction with gusts up to 5mph, we made three complete runs around a circuit at a steady throttle position of 15.3.

I'm disappointed in the results and I'm redesigning the skirts for another test but I would like constructive criticisms and input.

The one run without the skirts reached 52.1 mpg and the average of two with the skirts was 51.3 mpg. The skirts seemed to HURT the FE. An execution and a design error seemed to have contributed to the result:

1 - Execution. I had the alternator enabled the whole time, but it seemed to cut on much more often during the runs with the skirts on (I have a dash-mounted volt meter). Since I had depleted my deep cycle battery fairly heavily on my way to the site with the alt disabled, I think this might have been a significant confound.

2 - Design. To keep them from rubbing the wheels and keep them easily removed, the sections were duct taped into place with a little bowing for tension. The bowing is what I wonder about. I wounder if it had a parachute effect.

So the lesson: I redesign the skirts so they have the minimum clearance from the wheel and plan a shorter test route of 10 miles so that I can run the tests easily for three or four rounds with my alternator disabled.

Did I miss anything? What else should I try to account for?

I had the same result, but only a .3 mpg difference, less with skirts.

Why not just leave the alternator on for all the runs? Current consumption should be the same if you don't use the radio.

ConnClark -

That's what I thought at first. However, I can also see the logic of using the topped-off deep-cycle battery to take the alternator out of the equation and therefore "isolate" the A-B-A testing towards aero benefits.

CarloSW2

That's a good question, and Carlos/cfg83 has part of my answer. But it's also true that this Civic has the ELD/Alt set up that allows the ECU to modulate the power output of the alternator, and it does not seem to be completely regular under otherwise identical conditions. Maybe with a starting battery it would be, but not with my deep cycle. I watched the alt charge sometimes and not at others. And it was not always putting-out the same amount of power. Isolating the alt seems like my only way to control this factor. I have been looking more at some of the other test on here again too. The tendency is to build the skirts as close to the wheel as possible. I made a mistake leaving room there, I bet.

This is just me but why not cut your circle in half. The first half run it with the alternator off. the second half run it with it on. this way it will charge up on the back half.

then with the data you get with it off you can get a better idea of the side skirts.

and at the same time you can get some data on the alternator stuff too.

you are a few steps ahead of me on mods so i would love to see it. my trip to work is 30 miles of flat texas highway. so the aero mods and alternator delete really get to me

Not to add to the confusion but if I have a say in this,I would vote for the alternator enabled for all runs and hope that the voltage regulator plays a fair game for all runs.

When the alternator is disabled and you start with a freshly charged battery,the fuel pump is running at a higher output,using more amps,producing higher fuel rail pressure and the car could run a tad richer than in later runs where the battery voltage is sagging and the fuel pump output is significantly less than with a fresh battery! ( think of how the headlights dim with sagging voltage).
I realize your car has fuel pressure regulator and the ECU has control over fine tuning fuel ratio but it still has an effect.

Story: Modern vehicles actually use this method as a desperate attempt when something goes wrong. I worked on a GMC Yukon recently (forgot year) that ran rough on a cold start. Long story short it thought that the fuel alcohol content was 87% so the PCM richened the mixture and then it was fun to watch the fuel pump control module dropping the pressure from a steady 55psi down to 45psi as an attempt to achieve the correct ratio. Reprogrammed it for 10% ethanol and problem was gone.(and fuel pressure back at 55psi).
Just my opinion!

Barna

That's an idea, but I think it would be better to do both legs with the alt disabled and then charge the battery some by idling for a few minutes in between runs.

The problem was that it was not running equally. And I'm not confident it would next time. I know their are parameters for the ECU but the charging cycles for the alt looked unequal watching my volt meter run after run.

How low do you think voltage would have to go to have such an effect? Across the full 30-40 miles I would drive, I might expect battery voltage to dip from 12.5v - 12.0v or maybe 11.8v. Still, my alt delete seems associated
with a 3+ mpg improvement. Could fuel rail pressure really rival that impact?

If I did something to recharge the battery in between runs, that would deal with the problem entirely, correct? How much recharging to you think? What voltage level should I look to maintain?

I'm really not sure on the answers to any of the questions because of all the variables from car to car. However despite the fluctuations you see on your volt meter,I still think the alternator would do a better job at keeping the average load even across the runs than you could achieve trying to set the charge level manually. I say that because surface charge (or discharge)can make it impossible for you to set the same state of charge level based on voltage readings. That is why they recommend to let the battery sit for at least 12 hours before a SOC voltage measurement as to allow time for the voltage to stabilize.(after charging or discharging)
Barna

i´m not a native speaker. did i understand this right? do u drive with the same amount of throttle?
that means a higher speed if the skirts working. this make the difference smaller. maybe i´m totaly wrong, but can imagine, higher revs and shorter driving time is the killer in this case.
what about a coast downhill test?

Kurzer has a good point. It seems like you're driving at the same throttle level, rather than the same speed. That, in itself, introduces more variables into the equation.

I think you should wait to draw conclusions until you've found a methodology that will yield accurate results.

But if I drive at a steady 15.3 throttle position, speed should vary in a regular way--everything else being controlled as effectively as possible. Therefore, if I cover the same terrain at a higher speed and in less time as a result of a mod my gal/hr. should be lower and my MPG higher. Seems logical to me. What's the flaw, exactly?

i think your gal/hr would still stay the same because you throttle position is at the same point. Your MPG would also stay the same but you get to your destination faster. Drive at the same speed and i think you will see an increase in your MPG

I do think you are correct that a steady speed would make differences more apparent. I just can't see the steady throttle making the numbers necessarily the same. If I set a steady fuel consumption rate (15.3 throttle) and achieve the same distance (18 miles) in less time, I should see higher MPG. But maybe the differences in fuel economy would not be as apparent with steady throttle as they would be if I held a steady speed. Does that make sense?

So a modification of my test method: on a shorter route (a 5.2 mile stretch of freeway I'll drive in each direction) the ground is flatter still and I'll be able to hold a steady speed. I'll do that. Thanks.

it isn´t so easy. higher revs at same throttle, are sucking more air into the engine and more fuel. i´m curios to see the results with constant speed.

Just FYI, steady throttle does not necessarily mean steady fuel consumption. There are many other variables in the fuel consumption equation.

BTW, did you monitor vehicle speed during the test? If the wheel skirts allowed a slightly higher speed at the same throttle position, then you have less aero drag.

There's no time factor in MPG, only miles and gallons. You need to aim for equal speed so the aerodynamic load is equalized. Otherwise the skirted run might be faster, but has a quadratically higher drag load.

I'm puzzled by the result. Can you do a coast-down to verify the drag difference? It won't give hard MPG numbers, but it will show if the problem is in the test or in the mod itself.

No I understand clearly. Thanks for sticky with me on the explanation. BTW, I used the steady throttle method and wrote about it in my WAI test last summer and I don't recall any such objections. I might need to do that test again with steady speed instead of throttle, no? As for this test, since I have already redesigned the skirts, making them hug the body more closely, leaving minimal clearance with the wheels, I can't do a coast down tests on the same design, but I can do coast downs with the new design. Wouldn't I need some pretty high speeds, though? Could I get reasonable results from a coast down between say 55mph to 30mph using specific points on the road for reference?

The best coast down tests are done by coasting down a hill from a chalk starting line. That way you can always have the exact same energy to deal with for each test.

I ran skirts over a year. Results didn't show up in tank-to-tank records. Skirts are now off. I prefer to be able to check and air up the rear tires without the hassle.

I have the perfect hill. A mile and a quarter or more, no lights, no stops. Right next to campus. I'll mark a spot. Record speed at a specific second spot. A "coast-up" test. I'll do that first, being the less costly option in terms of time and gas.

I hear you, Frank. But I can deal with the slight hassle of the air pressure check, and I actually like the look a lot. As long as they are not a net loss aerodynamically, I'll keep at it until I have a good looking, well-fabricated pair of 'em.

For accuracy it would be best to start from a dead stop and coast to a dead stop and look at the distance. Do maybe 3 test for each configuration.

but starting from a dead stop would require a large hill to get speed up enough to affect aero.

Rolling to a stop from highway speeds won't be an option around here. Too many lights and stop signs. What I have is a hill that gets me from a total stop to 55mph in 0.5 miles. That's 10mph above the posted speed limit with half a mile of hill still to go. I would have to brake to avoid going farther beyond the speed limit. That's not good enough? I figure I pick a spot and compare the speeds I reach by the time I get there. That's too inaccurate?

I've never done a coast test before but 55mph should be fast enough to notice a change.

Start lower down the hill then.
Yes that is too inaccurate. It involves too much human interaction and relies on your ability to time it right.

If want the test to prove conclusively that you can go farther on the same amount of energy then you want to measure it directly by going farther on the same amount of energy.

In college I did on the road aero experiments and testing. I found that a gust of wind, passing truck, anything could screw up the results. Statistically, one needs to make hundreds of runs to mitigate environmental effects. Good luck.

That sounds perfect to me!

But like Mr Lee suggested, it may take several runs to get a good sample group.

On a side note, you could use a stop-watch to measure the amount of time it takes to go from zero to the speed limit, to avoid law enforcement issues.

I would think that, the higher the speed, the more you can see differences in aero mods?
Yes, I know that an true ecomodder don't want to ruin any tank mileage by some high speed testing.:rolleyes:

Maybe he could do it at 3am when the winds are calm and there is no traffic.

I have a strip of roadway 1.1 miles long that I identified today. The slope is also more gentle. From a dead stop today I coasted to 45mph during the coast, then slowed to about 43mph before I will have to brake for a light. 45mph is not really all that fast, but the distance is better. And that slowdown from 45mph to 43mph near the end would change notably if the drag were changed significantly. I need a rout by Saturday or Sunday. And yes, I'll do multiple runs. I figure A-B-A-B with three separate runs for each "A" and each "B". Twelve total. Should I use the 45mph 1.1 mile hill or the 55mph 0.5 mile hill?

You should use the one that lets you coast from to a stop to a stop or find yet another hill. It will eliminate any subjectivity or bias (conscious or unconscious) from effecting the results. This is the scientific method. Set a good example for your students.

Oh now you did it! Set a good example for my students, huh! ;) Now that's checkmate! :o Ugh... looking for another hill I go...

Sorry to play dirty ;) I'm sure there are some back roads some where near you. You might also be able to tie your test in as an educational experiment and get cooperation with the city and the police to shut down a street temporarily to allow you to do the test. There would be some paperwork but it would be good publicity for the school and an educational experience for the students.

California98Civic -

I'm about 2 hours away, but I don't have the time :

http://ecomodder.com/forum/showthrea...nia-12053.html

Have to call ahead to make sure it's not closed due to rain or having an event.

CarloSW2

(Bump)

Any updates?

Yes, two sets of figures. We tested a colleague's 1992 Corolla, taking it just as it was. We drove it for a baseline fuel economy (got 30.09 mpg mostly highway).

Then we slapped several mods on it and did a coast down test: pumped up tires to 50psi, grill block, huge air dam, wheel skirts. All cardboard, duct tape, and blue painter's tape. She was ugly!

To test these mods we had to use the less-than-scientific hill because of time constraints. We did three runs without mods and then three with mods on a one mile course, recording speeds at two points down the hill. First we recorded peak speed and then we recorded residual coasting speeds at a marked point along a flat straight-away. The mods yielded +4.1% top speed reached and a +9.1% in final coasting speed retained. The students were amazed. I was very clear with them about the limits of our method, and that the results could not be regarded as "scientific" even though methodically reached.

After the coasting test, we did another MPG test. But we first added to the equation several hypermiling techniques on-top of these mods: weight reduction (taking two students out of the car), driving a slower freeway speed (55mph instead of 60), and mild EOC hypermiling in the city routes leading to the freeway. The new MPG rating, baring pump error--an issue I taught the students about--was 45.26 mpg.

All of this was methodical though clearly not "scientific" and I made them aware of the difference.

Broadly, what they learned was that using a variety of techniques and mods, you could possibly improve your MPG by 50% virtually overnight.

They're forming a student club to pursue the issues, histories, and techniques more than a mere three week course allows...

It was fun. Thanks for your input.