eNV200 aero mods
 

Hi !

I am new to this forum, a few words about me:
French engineer working in satellites industry near Toulouse.

Recently bought an EV, Nissan eNV200 Evalia (40kwh) to drive all the family (3child) in replacement of a (too small) Toyota Yaris hybrid.

Given the car might probably hit the bottom of the best aerodynamics ranking, I decided to go on with some aero mods, to test how far I could go (and possibly make some mods permanent so that we can go see grandparents at 110km/h instead of 90km/h (400km trip) !.
https://ecomodder.com/forum/member-v...cientific.jpeg

Up to now, I did:
- rear wheel cover
- rear undertray and diffuser
- side mirror profiled casing

and now about to test boat tail I just completed.
https://ecomodder.com/forum/member-v...ront-view.jpeg

I have some pics and videos from tuft testing (for rear wheel cover and side mirro profiled casing - there is also a Citroen C4 of by brother in law that we tuft tested together to see what was going on on his car), I will find out how to provide them later.

Problem is that by my side there is always wind, and no long flat road, so it is quite hard to assess drag reduction below 5%. and also I don't have long commuting trip so averaged MPG on long distance is not possible.

I got the LeafSpy Pro app and with the LE Link2 adapter I am able to retrieve 5samples/seconds logs of drawn battery power (basically, battery HV volts * amps).
What I do is processing measured battery power and GPS speed data logged by LeafSpyPro with Scilab, and I compute the experimental SCx (CdA) by removing expected rolling resistance, slope effect, wind effect from measured battery power. This allow to get rid from cruise control not being always spot on targeted speed.

I hope I will be able to test boat tail tomorrow. Will do drag reduction assessement as well as tuft testing with a camera-on-stick approach.

Vianney

here are some pics of w/o undertray
https://ecomodder.com/forum/member-v...ite-awful.jpeg
and with undertray. purpose of the undertray was to avoid having the large transverse bar facing the flow, so the udnertray is composed of two parts, one kind of deflector in front, and the bottom diffuser.
https://ecomodder.com/forum/member-v...-diffuser.jpeg

my plan for this is to perfom tuft testing to check attached flow on the diffuser. (once i will have the boat tail attached to the end of the car this will be even more important!)

and a pic of the boat tail structure (which is fitted on the car by means of bike holder hooks)
https://ecomodder.com/forum/member-v...structure.jpeg

I see members are allowed to post links when they have posted more than 12 posts, when this will be done I will put the tuft testing videos, showing the rear wheel cover works quite well, and the profiled side mirror casing also good (altough less than expected).

Thanks for the pics. They prompt some questions.

Will you seal the edges of that waferboard, if not the surface. Newer product isn't as vulnerable to moisture, but the underbody is a harsh environment.

For an example, BamZipPow's Dark Aero He started with dimensional wood, but gradually replaced it all as it deteriorated.

Will the boat tail lift with the tailgate? Will it underlap the bumper to prevent a step abaft the diffuser?

yes since I took the pic, the board has been covered with varnish both sides and edges. This is OSB3 so baseline water-resistant but yes I added some additional protection.


for prototype: first question, no and the second, yes: in that prototype the boat tail skin front edges needs to be taped to the car body every side. However in this version, since the boat tail joins with the outer car surface it masks the braking lights, which means for a production version I will need to add some repeater lights like in some mods presented in this website.
if I go for a production version, I think I will probably make it not overlap the lights and the bumper so that it is possible to open the tailgate yes, so there would be a step, at least at lights level. but maybe that step could be radiused anyway if the skin is rigid enough (I'm thinking of building the production version using the prototype as a mould and filling some thickness of expansive polyurethane foam inside the skin)

A few observations.

- The rear diffuser will work only if the floor ahead of it is also smooth. ie the airflow pattern ahead of the diffuser is critical.

- Measuring surface pressures will tell you at least as much as tufting.

- Trying to ascertain Cd accurately by road testing is, I think, impossible. Just go on changes to the total power (A * V), averaged over a long distance in two directions. Also, ensure that when you make a deliberate change to drag (eg by having the windows down), that change is clearly measurable. Otherwise, you may be measuring just noise.

If you're productizing the result, that's 10x the effort; so good on you, here's hoping....

It's worth following Julian Edgar's advice. I think this is the best 'local' boat tail, I think it's on a different Insight now. Pic title says 'jims tail'.

https://ecomodder.com/forum/member-f...-post1-img.jpg

Would your expected customers have a trailer light socket available? That would reduce rewiring effort.

Honda Insight Tail Extension Project
https://ecomodder.com/forum/showthre...ect-13533.html
http://forums.pelicanparts.com/uploa...1622352671.JPG

eNV200 aero mods
https://ecomodder.com/forum/showthre...ods-39423.html
http://forums.pelicanparts.com/uploa...1622352671.jpg

Looks like a fun project, good luck.

yes actually the rear diffuser goes along with a plate in front of the low hanging transverse bar. the undertray is flat from front bumper to battery end (approx 2/3 of the vehicle length), so this latter plate fills the gap betwen the battery end to the transverse bar, and then the diffuser from that bar to rear bumper.

For the Cd yes I just want to have relative variations of the CdA, not the absolute value one. the thing is on short test tracks if I don't have perfectly the same speed (cruise control is not always spot on) it can very rapidly pollute results. thus this approach allows to get partly rid of this variations (altough I would prefer use relative wind speed instead of gps speed..)

definitely. at this stage I want to assess the benefits only. if on an Insight which is already quite good in terms of wake given existing roof slant angle, boat tail allows reducing drag by 10-20%, I imagine that on this car this would be even more !

and yes if i go for the same version than prototype, i will install the trailer light socket so the mod is "plug and play" :)

and I bought the book of Julian, inspiring indeed :) !

So, after a afternoon of testing with the boat-tail:
https://ecomodder.com/forum/member-v...side-view.jpeg

SCx shows an visible improvement, of around 0.1 to 0.15, which is not quite bad (around 15% drag reduction). Visible on both back and forth runs as expected.

red plots are without boat tail, and black ones with boat-tail installed. Abscissa is number of samples at target speed (90km/h)

https://ecomodder.com/forum/member-v...-boat-tail.png

https://ecomodder.com/forum/member-v...-boat-tail.png

So there is improvement, but not as much as expected.
Tuft testing reveals that flow is properly attached throughout the top side of the boat tail (12° angle), but on the sides this is not as good, and on the bottom side flow is completely detached, with strong recirculation bubble (tufts angled toward front)

tuft test videos (remove space between https and :// after pasting) are here :
https ://drive.google.com/drive/u/0/folders/145GXU43Vr1g5UUUGFSZIjnWNE0dYWV1b

I redid the tuft testing after having installed the undertray and the rear wheel cover (assuming the undertray would provide more stable flow undercar, and same for rear wheel cover on the side).
However the tufts remained the same.
For the car side, I had checked previously that flow are conveniently attached up to the rear of the car.
So most probable cause is:
- for underside, the undertray is not efficient and/or the boat tail angle is too important (it is around 15°, while literature consensus is more around 7°). Problem of the undertray is that the transverse bar is so low that if I want the diffuser to go from the bottom of that bar up to the rear bumper it requires an angle of 15° for the diffuser, which is too large according to literature.
- for side, maybe the slant angle is too large (I think it is around 20° in average). there is not recirculation (tufts general direction is aftwards), but it is visible that the flow is perturbated by something.

I will attempt with some friends to add smoke in the boundary layer to see how it develops over the car.

15% is not bad if there's obvious room for improvement.

If you flatten the angle of the undertray, it will approach the departure angle. Could the it run above the traverse bar, with an airfoil section added to it?

If the plan is a flat bulkhead at the truncation, there exist boxed cavities (my favorite example it the Cobra Daytona Coupe). Even if the gain is marginal a concave surface would allow lightening the structure. Maybe a little harder to fabricate.

Good job on tufting.

I wouldn't worry too much about the side flows, but obviously the underside is unacceptable. Why not put a temporary underside panel that is horizontal and see if it gets attached flow? If it doesn't, you then know the issues are ahead of it, not its angle.

' angles'
 

https://www.pinterest.com/pin/168533211025982771/https://www.pinterest.com/pin/556827941407233444/Technically, you want 'curves' not angles, at least until you've transitioned from the straight plank sides into the critical slope angle. Just as you've drawn for the side elevation.
The sides need to be exactly like the top, when viewed from above.
Only you can tell how much the diffuser angle will have to be compromised.
18% body length diffusers are never more than around 4-degrees upsweep.
The low drag diffusers are even 'slower' in angle if 'straight-flat'. They can withstand higher angles if gently and progressively curved, as in the Volkswagen 2000 concept of 1980. Much like your drawing.
I realize that is complicates fabrication by orders of magnitude, but in order to protect the boundary layer, there cannot be any 'kinks' or pressure 'spikes'.
The Evalia is reported at Cd 0.31. It's frontal area is going to be in the neighborhood of 2.9 meters-squared.
It's rated at 165- km highway in 'warm' weather.
391- Wh/mile ( 242.4 Wh/ km )
If we knew the exact speed at which your Evalia registered 242 Wh/km, that would be a good test speed.
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Dirty math suggests:
* Cd 0.31 OEM
* Cd 0.304 with rear skirts
* Cd 0.269 with smooth rear belly section and 'slow' diffuser
* Cd 0.17 with boat tail.
* a 45% drag reduction
* maybe 195-km range
You'd have to have the more complex geometry.
Here's how Union-Pacific Railroad handled the more complex surfaces in 1934. See link at top

Getting good flow on the bottom of your boattail may be difficult in the real world. If you ever have to drive up or down a ramp the back end of the boattail can drag on the ground.

https://en.wikipedia.org/wiki/Trailer_tail

These are sometimes used on semi truck (tractor-trailers) trailers here in the U.S. It is compromise toward utility over aerodynamic optimization.

here is the excel sheet I used to calibrate drag coefficients (rolling resistance, cd, etc)

docs.google.com/spreadsheets/d/1wZTI-nX2AWMz4ppH0gTCxHLHz6gaMt8Z/edit#gid=190811041

average consumption at 103km/h (true speed) is 244Wh/km (avg of 3 measures). The Cd I get is more around 0.37 (actually 0.31 is the Cd of the leaf which is much more aerodynamic than the env200, frontal area set apart, so 0.31 I thought anyway it was a bit underestimated, altough there may be as well some economies of scale).

I will try to tuft-test the undertray to see if flow is detached already at this stage. Problem is that the transverse bar is very low hanging and if I attempt to put a 5° rigid slope upwards I'm pretty sure it will scratch at every bumper :(. But I could setup something flexible so that it the shape can be designed for target speed and adapt in other situations.

I also added in the tuft test videos folder the one done with boat tail and wheel skirt.

compared to without wheel skirt, the tufts located just below the low horizontal tape line behave much better (at least they do not move in the forward direction ^^)
I should do some data logging in this configuration to see what drag reduction i achieve.

perturbed side flow
 

Since you mentioned this, I re-visited the research done on the Ahmed body.
Some data points:
* when the rear is modified, this new 'aft-body' constitutes 21.8% of total body length
1) @ zero-degree rear downslope only, Cd 0.25
2) @ 5-degree downslope only, Cd 0.233
3) @ 9-degrees downslope only, Cd 0.229
4) @ 12.5-degrees downslope only, Cd 0.23
5) @ 15-degrees downslope only, Cd 0. 236
6) @ 20-degrees downslope only, Cd 0.253
7) @ 25-degrees downslope only, Cd 0.287
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8) #7 plus a 10-degree upswept diffuser, Cd 0.262
9) #7 and #8, plus 10-degree plan-view boat-tailing. Cd 0.214
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* As the numbers reveal, the top rear slant initially lowers drag until just before the 20-degree angle, then rises above the original Cd.
* Adding the diffuser lowers drag, although it's still above the original Cd.
* It isn't until the side boat-tailing is introduced that, drag finally falls below the original Cd, for an overall 14.4% drag reduction, @ Cd 0.214.
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* This 25-degree, 10-degree, 10-degree tail relies on a series of vortices, in 3- planes, attacking one another, to burst each other.
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If it were true that the side flow on your tail WAS compromised, the above illustration might provide insight into the performance.
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I did a rough calculation using the as-shown, truncated W.A. Mair boat tail:
* Ignoring the wheels, the wake area was reduced to 15.38 square-feet ( 1.428 meters-squared. A 51% reduction.
* With rear belly pan, 'slow' diffuser, rear wheel skirts, and tail, we have a probability for a 45% drag reduction.

Vans are more aerodynamic than people think, my Renault Trafic had an ECO version that was 0.31 vs the stock version of 0.33. 0.37 was the old boxy shape Ford Transit. There's almost no way the ENV is that bad.

I'm currently trying to find an ENV of my own, I'll be doing an aero trailer rather than a boat tail.

Vans & Cds
 

The 1990 Pontiac Trans Sport minivan was Cd 0.30.
The 1991 GM HX3 concept van was Cd 0.258.
NASA's 1977 truncated boat-tail van was Cd 0.242.
Coventry University was able to get the van-like, 'Kamm-back' 1999 Audi A2, from Cd 0.288, down to Cd 0.204.
The van-like 2008 Mercedes-Benz Bionic Boxfish would be approx. Cd 0.20 with a set of Tesla-esque mirrors.
The van-like, 1985 Renault Vesta-II, with an extra side mirror would be approx. Cd 0.191.
These are some of the vehicles I benchmarked for AeroStealth's Chevy Bolt modifications. We're targeting Cd 0.21, from an OEM Cd 0.308. I don't see any technological impediments. We're looking for 300-miles range, HWY.