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Old 09-28-2012, 10:17 AM   #1 (permalink)
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Electric AC Cobra (looking to reduce high energy consumption - 489 Wh/mi)

Hi forum,

Just found this forum today and hoping that I can gain some improvement on my 'fuel' economy.

My car is a Factory Five Mk4 kit car, that I have built as 100% electric. The goal of a Cobra replica is mostly performance and fun. And it has plenty of both.



Now that it is (finally!) on the road, I am finding that it is horrible in terms of watt hours per mile. (a metric used by EVs to measure the electricity consumption rate)

Typically EVs are anywhere from 200 to 350 wh/mi, but in my first measurement I am getting a miserable 489 wh/mi. This test was done with a very light throttle and then staying at about 50 mph.

I realize that the dual goals of high performance and high economy are conflicting, but I'm looking to improve economy anyway. The high wh/mi is causing my range per charge to be very low.

I have seen comments on other forums comparing the coefficient of drag of a Cobra to a 4x8 sheet of plywood or a hummer. I now see why.

I'm looking for suggestions on how to improve it but without changing the appearance of the vehicle, or going with skinny tires. ( I can see readers rolling their eyes now!!)

About the EV aspect of my car: it has a WarP 11" HV DC motor, it is direct drive (no transmission), the rear end is a 4.1 ratio and is using lithium battery packs. So the car is essentially in '4th' gear all the time. The motor can easily propel it with 400+ ft/lbs of torque.

I am still building more battery packs for it, but at the moment, the car is 120V and will make 1,400 amps. Once the packs are all done, it will be 320V, 1400A.

As it sits now with the 120V pack, the car weighs only 1,928 lbs.

The car can be seen at Wayne's 2010 FFR Mk4

Cheers,
Wayne

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Old 09-28-2012, 10:39 AM   #2 (permalink)
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Sounds like a fun car Wayne.

#1 on the list is slow down when you're not showing off. The aero is horrible on that car, so anything you can do to slow down will help. Also, you don't have regen, so avoiding brake use is another huge benefit, even (actually especially) at highway speeds.

As for mods, I'd start with finding an increased tire pressure you're happy with. Then I'd make a full grill block. If you don't want to effect looks, do the blocking from the back. The next thing I'd look at is some sort of air dam for the front bottom side of the car. Unfortunately after that you're going to start having to effect looks which I doubt you want to do.

Also, what are your rpms at highway speeds? If the motor isn't spinning fast enough you'll have a reduction in efficiency.
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Old 09-28-2012, 11:09 AM   #3 (permalink)
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Hi Wayne - cool car!!

(Hope you don't mind I added a pic from your EV Album entry, and moved this thread over to the Fossil Fuel Free section where I think you may get some more eyeballs.)

Quote:
Originally Posted by ev99saturn View Post
Typically EVs are anywhere from 200 to 350 wh/mi, but in my first measurement I am getting a miserable 489 wh/mi.
Yikes! That's pretty bad.

Track it:

First off, I'd recommend starting a "fuel economy" log, because maybe - hopefully? - that one-off measurement will turn out to be an outlier. (You can enter & track kWh units in this site's fuel log tool, btw.)

Wow - bad aero!!!

Quote:
I have seen comments on other forums comparing the coefficient of drag of a Cobra to a 4x8 sheet of plywood or a hummer. I now see why.
Holy cow, yes, it's unusually horrible:

Quote:
The value of a drag coefficient, estimated by A-C, is Cd = 0.5
(source: http://www.automobile-catalog.com/ma..._iii/1966.html)

Use our calculator:

If you know (or can measure, estimate, or look up) the frontal area of the car, you can enter the car's particulars in our rolling & aero drag efficiency calculator, and it will give you accurate estimates of what energy consumption you can expect to see at various speeds. See: Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

I'll be really curious to find out what you learn from the calculator. (Also: please capture the link to your vehicle's specifics when you fill it out and post it so we can see the numbers as well - once you submit your details, the link to save or share results appears further down that page.)

The tool will also show you exactly what you need to do to reduce your consumption & increase range. Not to ruin the surprise, but as Daox already pointed out, the primary options are:

- slow down (easiest, doesn't affect looks)
- improve aero (harder, a few mods can be done without affecting looks)
- decrease rolling resistance (possibly harder, depending on LRR tire options in your tire size)

(Decreasing weight isn't really an option - it's pretty pared down already.)

Get better feedback:

The other thing I would strongly recommend: put instrumentation in the car to give you instant & "trip" feedback on your consumption. Ideally, that would show instant & resettable average Wh/mi. It would help keep you aware of YOUR contribution to consumption (driving technique).

EDIT: I see you have the Torque app in the car. Not too familiar with it, but are you getting decent feedback there?

In my car, it's more basic: an LED array that shows the voltage of each battery in the pack. Since voltage sag is a function of current draw and it's very visible on a lead acid battery, the gauge is a useful visual reminder of how much I'm demanding from the pack at any moment (and also helps me prevent battricide by keeping voltage out of the danger zone, since I don't have a fancy BMS nanny watching over my batteries).



(from: http://forkenswift.com/electric-car-parts.htm )

cheers-
Darin
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Old 09-28-2012, 11:20 AM   #4 (permalink)
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Another thought: on the topic of tracking consumption, one of the "good" things about EV's is even if you don't have on-board kWh/mi instrumentation, their relatively low range means lots of "fill-ups", so monitoring consumption can happen at a higher rate/resolution than most ICE vehicles.
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Old 09-28-2012, 02:03 PM   #5 (permalink)
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Nice looking car wayne!

500Wh/mi is indeed pretty poor efficiency.
An open-top roadster is horrible for aerodynamics, the rollbar probably makes it even worse, and that big grill opening..

If you get this even at 50mph might have some type of drag going on.
Check the brakes. Check motor for drag.
With fat tires be sure alignment is correct, fully inflate the tires.

There has been a big trend to get a "big block" e-motor to go fast.
An 11" motor is overkill for such a light car.
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Old 09-28-2012, 02:05 PM   #6 (permalink)
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I have considered blocking the oval grill area with a sheet of clear lexan, just on the inside of the body opening. There is a small radiator to cool the motor controller, but it has fans and would be 6-12 inches away from the lexan sheet. So it can pull air as it needs regardless of a blocked grill. I didn't think that it would help, but would do the opposite and hurt. But perhaps it's worth a try.

Currently the opening allows for air to flow off to the sides and into the wheel wells (all behind the body).

Another (possibly dumb) thought would be to put flat sheet under the car, covering as much of the length and width as is reasonable. Would that help reduce air friction and improve wh/mi?

I also thought about trying to tilt the windshield back a little, as I do have a bit of room to play there.

At 50 mph, the motor is turning about 2600 rpm.


Quote:
Originally Posted by Daox View Post
Sounds like a fun car Wayne.

#1 on the list is slow down when you're not showing off. The aero is horrible on that car, so anything you can do to slow down will help. Also, you don't have regen, so avoiding brake use is another huge benefit, even (actually especially) at highway speeds.

As for mods, I'd start with finding an increased tire pressure you're happy with. Then I'd make a full grill block. If you don't want to effect looks, do the blocking from the back. The next thing I'd look at is some sort of air dam for the front bottom side of the car. Unfortunately after that you're going to start having to effect looks which I doubt you want to do.

Also, what are your rpms at highway speeds? If the motor isn't spinning fast enough you'll have a reduction in efficiency.
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Old 09-28-2012, 02:16 PM   #7 (permalink)
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Tires are new and properly inflated. On level ground, I can push the car with two fingers, so I'd like to think that the brakes are not dragging. Also up on jack stands, all 4 wheels spin easily.

And I just had the front end aligned a few days before I did the first measured mileage test, so that should be spot on.

I know that the 11" motor is a lot of motor for a 1900 lb car, but I was initially thinking about putting a WarP 13" in it!. For a Cobra, there is no such thing as overkill! Too much power is not quite enough!!

When driving at a steady 50 mph versus a steady 40 mph on a level road, I see a significant increase in motor current. So in my humble estimation it is mostly air resistance that I'm fighting.

Quote:
Originally Posted by nimblemotors View Post
Nice looking car wayne!

500Wh/mi is indeed pretty poor efficiency.
An open-top roadster is horrible for aerodynamics, the rollbar probably makes it even worse, and that big grill opening..

If you get this even at 50mph might have some type of drag going on.
Check the brakes. Check motor for drag.
With fat tires be sure alignment is correct, fully inflate the tires.

There has been a big trend to get a "big block" e-motor to go fast.
An 11" motor is overkill for such a light car.
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Old 09-28-2012, 02:20 PM   #8 (permalink)
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How many amps are you drawing at 50mph?
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Old 09-28-2012, 02:28 PM   #9 (permalink)
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MetroMPG, Thanks for the many suggestions. I have added my first measured run to logs on this forum and will plan to update it frequently.

I am still considering additional instrumentation, but at the moment, I don't have any instantaneous wh/mi display.

Torque is a really nice application. It runs on a Google Android tablet and is wirelessly connected to the Orion BMS. It displays every cell voltage, high & lo cell resistance, hi & lo battery temperatures, amps, volts, gps speed & coordinates, SOC, SOH, and a ton of other stuff. Very nice.

Here is a shot of my dashboard with the tablet, taken a few weeks ago, before the carpeting was added.


..... oops, I can't upload an image yet but will show it in a subsequent post.
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Old 09-28-2012, 02:28 PM   #10 (permalink)
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Ok, I have 5 posts now so should be able to add the photo....

Here is a shot of my dashboard with the tablet, taken a few weeks ago, before the carpeting was added.


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