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California98Civic 01-24-2012 05:43 PM

Testing wheel skirts A-B-A
 
1 Attachment(s)
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?

BHarvey 01-24-2012 07:11 PM

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

ConnClark 01-24-2012 07:51 PM

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

cfg83 01-24-2012 07:57 PM

ConnClark -

Quote:

Originally Posted by ConnClark (Post 282131)
Why not just leave the alternator on for all the runs? Current consumption should be the same if you don't use the radio.

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

California98Civic 01-24-2012 08:06 PM

Quote:

Originally Posted by ConnClark (Post 282131)
Why not just leave the alternator on for all the runs? Current consumption should be the same if you don't use the radio.

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.

deathtrain 01-24-2012 08:36 PM

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

3dplane 01-24-2012 10:34 PM

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

California98Civic 01-24-2012 11:09 PM

Quote:

Originally Posted by deathtrain (Post 282158)
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.

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.

Quote:

Originally Posted by 3dplane (Post 282186)
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.

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.

Quote:

Originally Posted by 3dplane (Post 282186)
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.

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?

3dplane 01-24-2012 11:56 PM

Quote:

Originally Posted by California98Civic



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

kurzer 01-25-2012 05:48 AM

Quote:

Originally Posted by California98Civic (Post 282109)
...at a steady throttle position of 15.3.

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?


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