1999 Acura EL or Civic EX sedan - dropping Cd from 0.33 to below 0,20

[California98Civic]

Thanks for all the pictures and thinking about what you are doing. I looked at them all and your notes. To remind: I am not expert and I have no formal training. I just own a car that I have modding and testing on the basis of research and my own and others testing.

I think your photos confirm for your specific car configuration what we broadly know for these three box sedan and coupe car bodies. A large turbulent wake develops behind the car, especially behind the trunk (and the rear window, though dust won't show it). The wheel wells and the underside in general are major contributors to such turbulent separation.Your dust shows that the angle of the curves on the body panels at the rear, such as at the driver and passenger sides of the rear bumper cover, become too sharp, too fast, and very quickly. At first, there is perhaps no dust because there is relatively little turbulence, and then the dust deposits show up as the turbulence grows.

Better separation edges should help, but despite the dust, these cars were pretty well optimized from the factory. So, testing experience here with 6th and 7th gen Civics (similar to your Acura) has shown it is hard to improve at the tail with small changes. A rear flat spoiler, a box cavity, wheel well covers, side skirts, grill blocking, an airdam, or more ambitiously a full undertray with a carefully tuned rear difuser angle. I think it is important to study placement and shape of the mods and to test them as best you can. Go bigger than a small separation edge; try a box cavity of some sort on combination with wheel well skirts and a diffuser.

I am planning to test a flat rear decklid spoiler again this summer. But I have a ton of work and family responsibility to wade through before that it really possible.

[ACEL]

Before to fabricate and install the "definitive" wheel covers, two mock-up were tested. First was a open perimeter wheel cover. Second was an almost full wheel cover with the wheel center open (pics). Tape mock-up was installed on all four wheels to get aerodynamics information.

The goals are :
1. Find the most appropriate wheel cover for this 1999 Acura EL car in terms of actual fuel economy and aerodynamics behavior.
2. Determine which wheel cover will allow brakes heat to escape.
3. Aesthetic constraint: the final wheel covers have to look good on the car and not disfigure the good looking wheels (wifey requirement)

To measure the parameters to satisfy goal 1, Torque Pro App was used to log : throttle opening (%), acceleration (x), mileage instant, RPM, Speed GPS. The data series were logged every half second.

To satisfy goal 2, simple observation and "touch feeling" were used

Goal 3 is satisfied when "wifey" says it looks good. She did. See pics.

Testing of the mock-ups were conducted "back-to-back" on the same morning and road segment. The previous day, on the same road and similar conditions a series of runs without any cover were done in order to obtain a baseline data for comparison.

The metrics for comparison were the top speed achieved at a constant throttle opening according to the "throttle stop" method proposed by Julian Edgar on Youtube. The instant mileage averages over the few seconds that the speed remained constant at the said throttle opening were also compared.

Tests were conducted in the following order:
0. Baseline test, no covers (32 Celsius)
1. Open perimeter mock-up (17 to 20 Celsius)
2. No perimeter mock-up (17 to 20 Celsius)
3. Final wheel cover installed

Results and observations:

1. Open perimeter mock-up : allowed a top speed increase of 1 to 3 % over the base line even though the air temperature was lower by more than 10 degrees Celsius.
2. No perimeter mock-up : did not allow an increase in top speed as the open perimeter did. On the contrary, it decreased the top speed obtained by the open perimeter mock-up by at least 13 %. Also, the tape on the wheel perimeter felt hot to the touch on one front (disc) and one rear (drum) wheels. For mileage, the open perimeter mock-up used at least 7 % less fuel than the full perimeter blocked wheels. The final wheel covers will be with an open perimeter like the Tesla "aero" wheel covers and the design will have to match the underlying wheel.

3. Final wheel covers (17 Celsius : same road): Projected wheel covers fabricated with aluminum pizza pans and decorated with back sticking lettering vinyl was okay’ed for aesthetic.

These final wheel covers did not allow for a significative increase in top speed for a given throttle opening over the mock-up open perimeter wheel covers. On the other hand, fuel economy was improved by 2 to 5 % over the said mock-up. No particular heat accumulation was felt on the perimeter of the aluminum pizza pans.

Conclusions

Cheap modification mock-up is a good way to test various alternatives to orient the search for the appropriate modification.

Definitive wheel covers design retained are appropriate to the 1999 Acura EL and are proved to be a worthy modification that saves fuel.

Addition

Final wheel covers had to be removed from wheels because the attachment method was wrong and the covers moved causing wheel balance problems.

First attachment method was simply with zipties attached to the “spokes”

Second method was with toggle bolts: removing and reinstalling the covers made the toggle part move between the spokes thus promoting cover movement and wheel imbalance.

Next wheel cover attachment method will be to use longer lug nuts, drill their center, stick a nut on hit with epoxy glue and use fine pitch machinery screws to secure the cover to the wheel. (this attachment method is proposed on ecomodder.com for a Tesla owner).

Happy and best success in EcoModding !

Acel

[California98Civic]

I like the decorated Pizza pan approach, too. Make sure your vinyl is the kind that can take the sun and the heat without pealing or fading. Also, the same for the paint. Drilling the longer lug nuts is a good, tired and true method. Good stuff!

[ACEL]

Vortex generators trials and final installation

Two vortex generators disposition were trialed from scientific literature results. Five locations were trialed until the appropriate one was determined by rear window tuft test observation. Tufts movements were filmed and a frame by frame examination allowed to get the story about geometry and location.

As for many car models from the end of the 1990’s, the rear window showed a center bubble flow recirculation thus making vortex generators an appropriate modification to reduce the drag induced.

1rst trial : geometry location proposed by

Gopal P. and Senthilkumar T., 2013, Influence of Wake Characteristics of a Representative Car Model by Delaying Boundary Layer Separation, Journal of Applied Science and Engineering, Vol. 16, No. 4, pp. 363_374 (2013)

1rst Position : Rear of VGs lined at 100 mm (4 inches) from the car rear window top edge.

The vortex generators are of the “airplane” type and are rectangular 4 inch long by ¾ inch high. Rear window was tufted so tufts movements could be observed and photographed from inside the car.

[ACEL]

2nd Position : Rear of VGs were lined along the roof edge (rearward by about 100 mm – 4 inches

VGsTufTest 2nd

Some improvement in the tufts movements. One center tuft is pointing forward and probably sticking to the masking tape. The bottom of window tufts are still very excited. The tufts along the window side are also moving a lot. This first geometry is not promising so, for the next trial, the VGs are put in the second geometry starting along the roof end.

[ACEL]

3rd Position, 2nd geometry along the roof end

2nd geometry location proposed by

Dheeraj Sagar et Al, 2010, Aerodynamic Effects of Rear Spoiler and Vortex Generators on Passenger Cars, Proceedings of ICTACEM 2010 , International Conference on Theoretical, Applied, Computational and Experimental Mechanics, December 27-29, 2010, IIT Kharagpur, India

This geometry was adopted for remaining trials.

Some improvement in the tufts movements along the window side edges. The tufts on the window center and bottom still move some.

[ACEL]

4th Position, 2nd geometry, 2nd VG type

Since previous trials demonstrated some improvement, shark type VGs which will be the definitive ones installed are trialed along the roof edge. The number of available shark VGs is only 8 and 12 are required for a complete row from the car left to right.

The window center tufts stabilize. The bottom ones still move. The side edges ones move pretty much.

[ACEL]

5th Position, 2nd geometry, VGs type mix
As experienced and reported by Julian Edgar in his book on a 2007 Honda Legend, vortex generators are positioned on the rear window glass. In my case, along the top edge. I am not tall enough to have my rear sight hindered. The “airplane” type VGs are installed on each sides to replace the missing shark type ones to make a complete row from left to right.

Ah! Finally! The window center, bottom and side edges tufts all stabilize. No picture needed! I proceed to a “clean” definitive install. Some “Dollar Store” two sides transparent acrylic gel tape was used to secure the “airplane” type VGs to the glass. The commercial shark fin type VGs have their own two side tape. The “airplane” type VGs will be replaced when the shark fin ones ordered come in in August. The acrylic gel tape glue is easily removed when soaked with WD-40, like may other glues.

Aerodynamics result

The recirculation flow bubble on the car rear window causes drag. So, using the “throttle stop” method (Julian Edgar, 2019, Youtube) against the last modification results data to date, the maximum speed allowed at a given throttle opening is increased by 2 %. The fuel economy measured at 2,5 %.

In the end, adding up small economies make up some big economy!

[ACEL]

EDIT: I will add this photo from my dust test last summer:


Ridiculous how similar the shape of our cars are. In this shot you can see that without the rear spoiler, dust settled heavily on my bumper cover. When I added the decklid spoiler mockup, this was the result.
[/QUOTE]

California98Civic your trunk lid flat spoiler mock-up is clearly reducing the wake size as the "dead water zone" of little air movement is diminished thus allowing less dust deposition on the bumper cover. The "flat spoiler" seems to move away further rear the "dead water zone". Also, some air probably coming from under the car is moving up along the bumper cover.

The flat spoiler seems like a good modification. Fuel economy figures or "throttle stop" speed increase figures would be informative and very interesting if coming from your next flat spoiler trial.

I look forward to see your modifications.

Regards

[ACEL]

As seen on “Aerocivic” and some other ecomodded cars, rear wheel boat tails may help in car fuel economy. In the end, small economies a make large economy. So, I decided to try some.

I had some vinyl corners that were used as airplane type vortex generators and I had some vinyl gutter leftover from the side skirts fabrication.





The vinyl corners were first attached to the rear part of the belly pan with self tapping screws.



The boat tail is made. Note that the front of the boat tail is angled so that when installed air moved by wheel is deflected under the car. Also, note that the rear part of the boat tail will be opened like the no bottom box concept used on car rear ends.







The boat tail is attached to the corners support with self tapping screws on both sides. Et voilà!



Results

For constant throttle opening of 17,65 %, the average top speed increase is 4,3 Km/h from 110,7 Km/h. A 3 % average top speed increase at 18,43 % to reach about 116 Km/h.

Now fuel economy. That is what we are here for! The Boat tails behind the rear wheels seem to reduce fuel consumption by about 1 L/100 km or a 20 % mileage improvement at speeds between 100 and 120 Km/h. That is what the data says!