Electric AC Cobra (looking to reduce high energy consumption - 489 Wh/mi)

[dtbaker61]

Originally Posted by ev99saturn

I'm looking for suggestions on how to improve it but without changing the appearance of the vehicle, or going with skinny tires.

you don't have to change a thing, you can trade me straight across for my electric Swift, and your efficiency will soar!


....seriously, since you've checked alignment and brake drag, and built w/o transmission you can't do much about inefficiency off the line with low rpm.

- make custom grill block. leave a duct thru controller cooler, and another straight shot to the motor intake (with filter).

- make a belly pan at least from grill valance back to torsion bar. Protect motor from grit and water and smooth out airflow.

- try getting your hands on a hardtop, or test with a cheap fabric bikini top.

- really consider narrower tires. Like old '60s vette look. at least unless showing or racing.

also keep in mind that consumption 'at the wall' measured with kill-a-watt meter will include all losses in charger during charging. I know that over 1000 miles my Swift consumes avg of 300 wattHr/mile, but the on-board cycle analyst shows battery use to be right around 225 wattHr/mile... and I KNOW I have some brake drag or bad front wheel bearings going on I havent addressed yet.

[Galane]

Full belly pan. Grille block that's flush with the front of the opening, slightly domed would do a better job than flat. Seal up the insides of the wheel wells as much as you can.

Something for going to extremes would be a filler or gasket around the headlamps to seal the gap between them and the trim rings. Might find some sort of smooth skinned foam weatherstripping that's the perfect thing. Glue it around the inside of the trim ring so it fits snugly to the edge of the bulb retaining ring. Turn an air collection spot into an air deflection spot.

Examine how the sealing at the front edge of the hood on the 1996 and up 3rd generation Taurus and Sable was done. Weather stripping to smooth out the gaps at the front and rear of the hood will help a bit. Would have to do before and after tuft testing to see what, if any effect it'd have.

Air going around the relatively smooth exterior will make less drag than air going through the rough and tumble of the mechanical bits inside the car.

Lots of little details combined to squeeze out more efficiency.

Look at the current motorhomes. They still have all sorts of greeblies stuck here and there, but many of the bits and pieces have been getting individual "slick" redesigns. Looks smoother but do they all together reduce drag any?

Can only tell by testing, same as with attempting to reduce the drag on a body style like the Cobra.

[allen_dodge]

ahh, I cant wait to build my own e-monster, and I always loved the looks of the cobra, though I was surprised to hear the drag coefficient was so high. Just a few things if I may...

Electric motors draw current directly proportional to load which is mostly determined by overall rolling resistance, but gear ratio also has a big effect so having multiple gears to choose from makes it much better for slow speed, stop and go type stuff even if you do lose a little power at the wheels due to drivetrain losses. However, for drag racing the direct drive is not only faster, it's also more durable, so it really depends on whether it's mainly gonna be a road car, or a strip car.

I've never seen anyone use an automatic in an EV conversion, probably for good reason, but you might be able to use a 2-speed powerglide and remove the torque converter, replacing it with a short driveshaft, basically the same way they sometimes delete the clutch in EV conversions. I'm sure it would require some fabrication and it might be more trouble than it's worth, but I know the basic principle is possible because I have seen it done on dirt track cars. I think it would be driveable on the road with no torque converter for the same reason you can drive an EV with no clutch, and even 2 gears is probably more than you need in that thing, but it would help with city mileage, since electric motors are happy running freely at a relatively high rpm to a gas motor.

While it is true that electric motors are more efficient at higher rpm, there is of course a sweet spot. The reasons for this are fairly simple, and it's been a while since I brushed up on it, but from what I remember the iron core of each pole on the rotor retains its magnetic alignment, and has some resistance to flipping its alignment each time the commutator advances. This creates heat in the iron core, but at low rpms its negligible. At higher rpms the iron core is having to realign its poles more often, and the extra heat generated in the core of the rotor is transferred to the copper windings which increases resistance. This is your enemy, both in terms of performance and efficiency. More resistance not only increases amp draw, but reduces power and torque at the same time. In extreme cases the increased amp draw creates so much heat that it creates a vicious cycle that can lead to thermal runaway which will melt your brushes and/or windings like a bus fuse, basically turning your motor into plasma. I'm not saying this is what's going to happen, but it would still be a good idea to get a motor temp gauge if you don't already have one just as a way to monitor efficiency. Long story short, monitor the motor temp in combination with amp draw to get an idea of how much load it's under, any excess heat is a sign that it's exceeded it's efficiency range or has just been run too hard for too long

also, drafting semis is a great way to reduce your wind resistance, combined with the front grill block, belly pan and windshield rake listed above should give significant improvement. I have the same problem in my vw rabbit (COD .44 i think, even though it's a brick on wheels it is somehow more streamlined than a cobra) and it won't affect the looks as long as you have good brakes Do you have a video camera? Would love to see some footage of it in action if you can, keep up the good work!

[few2many]

A lot of automatics are starting to be used. There is even an ev specific powerglide on the market. small, light, two speed, low for start, 1:1 high, perfect for a Cobra. Even some overdrives are being used. There are several threads on another forum dedicated to it.

[ev99saturn]

Hey thanks for the offer! I will give the Swift trade some thought... ;-)

The first thing I'm going to do is to get a better baseline of wh/mi data before making any changes. The only measured run included a very long gradual uphill slope and was maybe done a little faster than would be optimal for an wh/mi test. So a few more runs, and also a second test route with a few runs.

In the mean time, I have already started on preparing materials for a few modifications. The easy ones to start with:

- transparent wheel covers
- transparent grill block
- belly pan

- front air dam after that

I'm also building more batteries and will be more than doubling the pack voltage over the winter. Increasing the voltage will of course, proportionally reduce the amperage draw, and improve the amphour capacity a bit. Plus simply having more kWh capacity will increase the range. The pack is now only about 9 kWh and will be increasing to about 25 kWh.

An alternate set of tires has to be considered. The efficiency calculator on this site is very informative when playing with values.

Any suggestions on tire values there? I suspect that those wide rear tires are maybe above that range of up to 0.15 as shown in the example.

Lots to do and I am anxious to see how much it helps.

Originally Posted by dtbaker61

you don't have to change a thing, you can trade me straight across for my electric Swift, and your efficiency will soar!


....seriously, since you've checked alignment and brake drag, and built w/o transmission you can't do much about inefficiency off the line with low rpm.

- make custom grill block. leave a duct thru controller cooler, and another straight shot to the motor intake (with filter).

- make a belly pan at least from grill valance back to torsion bar. Protect motor from grit and water and smooth out airflow.

- try getting your hands on a hardtop, or test with a cheap fabric bikini top.

- really consider narrower tires. Like old '60s vette look. at least unless showing or racing.

also keep in mind that consumption 'at the wall' measured with kill-a-watt meter will include all losses in charger during charging. I know that over 1000 miles my Swift consumes avg of 300 wattHr/mile, but the on-board cycle analyst shows battery use to be right around 225 wattHr/mile... and I KNOW I have some brake drag or bad front wheel bearings going on I havent addressed yet.

[ev99saturn]

The headlight chrome rings are a poor fit, not very tight. Interesting idea to foam them, and along the hood gap.

I'm heading towards the bigger-bang-for-a-buck changes first, and will see how much is gained there.

I will be pulling the body back off this winter to finish & paint it. At that point, I will probably angle the windshield back a bit too.

I completely agree about test, test, test. The data tells all :-)

Putting a dome shape on to the grill block makes a lot of sense, but I'm not so sure how I would fabricate that shape and make it look decent. Already started measuring a flat one. The material cost is low, so I will start with flat. If I can manage a domed one, then it will be interesting to see the resulting numbers compared to flat.

Originally Posted by Galane

Full belly pan. Grille block that's flush with the front of the opening, slightly domed would do a better job than flat. Seal up the insides of the wheel wells as much as you can.

Something for going to extremes would be a filler or gasket around the headlamps to seal the gap between them and the trim rings. Might find some sort of smooth skinned foam weatherstripping that's the perfect thing. Glue it around the inside of the trim ring so it fits snugly to the edge of the bulb retaining ring. Turn an air collection spot into an air deflection spot.

Examine how the sealing at the front edge of the hood on the 1996 and up 3rd generation Taurus and Sable was done. Weather stripping to smooth out the gaps at the front and rear of the hood will help a bit. Would have to do before and after tuft testing to see what, if any effect it'd have.

Air going around the relatively smooth exterior will make less drag than air going through the rough and tumble of the mechanical bits inside the car.

Lots of little details combined to squeeze out more efficiency.

Look at the current motorhomes. They still have all sorts of greeblies stuck here and there, but many of the bits and pieces have been getting individual "slick" redesigns. Looks smoother but do they all together reduce drag any?

Can only tell by testing, same as with attempting to reduce the drag on a body style like the Cobra.

[ev99saturn]

The Cobra has always been my favorite muscle car. It looks like it's going fast just sitting still ;-) I am disappointed to see the low coef of drag too. It turns out that Shelby created the Daytona Cobra for this exact reason. It is far more aerodynamic, which is especially important at very high speeds in racing.

The goals of building this car were 1) maximum fun, 2) enough electric performance so I won't be embarrassed at a drag strip 3) the super energy efficiency that electric brings. Goal 1 is already met. Goal 2 is getting there, and will be reached when all of the batteries are built & installed. Goal 3 is why I'm here!

The 2 speed powerglide is the choice of my friend Jeff, who is also building the same car as me. Another choice is a Gear Vendors 2 speed. Jeff will road race his at Laguna Seca, and should do well with that massive torque.

I am not yet monitoring the exact temp of the motor, but do have 120F sensor watching for that mark. It will speed up the blower that is forcing additional air through the motor. The motor has an internal fan, which might be enough using a transmission, but my lower rpms necessitate additional coooling.

The driving that I will be doing is almost all 50 mph roads. Very little in-town 25 mph stuff. I live out in the sticks and everything is a fair distance. So motor cooling, and the higher current draw for getting up to speed is less of a concern. goal 2 is more important that goal 3 :-(

As for video footage, it's already out there. Nothing much of actual driving just yet, but a lot of short videos of the build process. Starting with a bare frame and working up from there. Search for ev99saturn on youtube to find them. (my first EV build was a 1999 Saturn)

Originally Posted by allen_dodge

ahh, I cant wait to build my own e-monster, and I always loved the looks of the cobra, though I was surprised to hear the drag coefficient was so high. Just a few things if I may...

Electric motors draw current directly proportional to load which is mostly determined by overall rolling resistance, but gear ratio also has a big effect so having multiple gears to choose from makes it much better for slow speed, stop and go type stuff even if you do lose a little power at the wheels due to drivetrain losses. However, for drag racing the direct drive is not only faster, it's also more durable, so it really depends on whether it's mainly gonna be a road car, or a strip car.

I've never seen anyone use an automatic in an EV conversion, probably for good reason, but you might be able to use a 2-speed powerglide and remove the torque converter, replacing it with a short driveshaft, basically the same way they sometimes delete the clutch in EV conversions. I'm sure it would require some fabrication and it might be more trouble than it's worth, but I know the basic principle is possible because I have seen it done on dirt track cars. I think it would be driveable on the road with no torque converter for the same reason you can drive an EV with no clutch, and even 2 gears is probably more than you need in that thing, but it would help with city mileage, since electric motors are happy running freely at a relatively high rpm to a gas motor.

While it is true that electric motors are more efficient at higher rpm, there is of course a sweet spot. The reasons for this are fairly simple, and it's been a while since I brushed up on it, but from what I remember the iron core of each pole on the rotor retains its magnetic alignment, and has some resistance to flipping its alignment each time the commutator advances. This creates heat in the iron core, but at low rpms its negligible. At higher rpms the iron core is having to realign its poles more often, and the extra heat generated in the core of the rotor is transferred to the copper windings which increases resistance. This is your enemy, both in terms of performance and efficiency. More resistance not only increases amp draw, but reduces power and torque at the same time. In extreme cases the increased amp draw creates so much heat that it creates a vicious cycle that can lead to thermal runaway which will melt your brushes and/or windings like a bus fuse, basically turning your motor into plasma. I'm not saying this is what's going to happen, but it would still be a good idea to get a motor temp gauge if you don't already have one just as a way to monitor efficiency. Long story short, monitor the motor temp in combination with amp draw to get an idea of how much load it's under, any excess heat is a sign that it's exceeded it's efficiency range or has just been run too hard for too long

also, drafting semis is a great way to reduce your wind resistance, combined with the front grill block, belly pan and windshield rake listed above should give significant improvement. I have the same problem in my vw rabbit (COD .44 i think, even though it's a brick on wheels it is somehow more streamlined than a cobra) and it won't affect the looks as long as you have good brakes Do you have a video camera? Would love to see some footage of it in action if you can, keep up the good work!

[ev99saturn]

Yup. I set it up in the tool. Here is a link to my settings.

http://ecomodder.com/forum/tool-aero-rolling-resistance.php?Weight=1928&WeightUnits=lbs&CRR=.02 &Cd=.5&FrontalArea=1.54&FrontalAreaUnits=m^2&FuelW h=33557&IceEfficiency=.8&DrivetrainEfficiency=.98& ParasiticOverhead=240&rho=1.225&FromToStep=5-200-5

It doesn't agree very well with the actual wh/mi I am getting, but obviously some of the entries are simply a swag. I made the tires higher rolling resistance than normal car tires, but maybe it's not high enough?

The calculator says I should be getting around 150 mpg equiv at 50 mph, which is in the vicinity I had initially hoped for. My Saturn EV was around 140 and weighed 1000 lbs more!

Originally Posted by MetroMPG

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.)

[allen_dodge]

Originally Posted by ev99saturn

I am not yet monitoring the exact temp of the motor, but do have 120F sensor watching for that mark.

120 is pretty warm, any temperature increase will still affect power and efficiency, so it might be worth the time to put in a gauge and connect a manual override switch to force the fan to operate at full speed. It's likely brushless so I would be surprised if it draws more than a few amps, and if there is a noticeable drop in current draw then it is probably worth leaving the fan on full blast, especially at the strip . I guess another option would be to somehow lower the temp that the fan kicks on at, but you will just have to measure the cost/benefit of running the cooling fan all the time. As long as your amp gauge includes the power used by the fan, you should be able to easily measure any gain in switching the fan on and off. Since heat is the enemy of all electronics, I would also put a sensor on the speed controller and consider extra cooling if possible, not sure about the one you're using but most DC controllers have at least a 10% efficiency loss or greater, similar to if you were using a transmission. Any reduction in temperature helps so you could even resort to liquid cooling on the electronics, similar to what they use in some high end computers. Also, I've even considered using a relay that only engages at WOT to bypass the speed control altogether but with too much voltage this could be catastrophic.

As far as transmissions, I would leave it exactly the way it is if you do mostly highway driving. I'm gonna go check out some of the vids now...

[ev99saturn]

The forced air blower motor that feeds the traction motor to cool it, is set to run at half speed normally. Mostly because of the noise that I don't want to hear. It only draws about 6 amps at full power, but is the noisiest part of my otherwise silent car.

A thermal switch (120F) monitors the air exiting the traction motor and will turn the blower up to full speed if needed.

The main traction motor has a safety warning switch inside that is switched when the motor reaches 150C as "an early warning of an overheating situation". The motor is rated for operation up to 180C temperature.

For the transmission, or lack thereof, I plan to leave it that way at this point. It's a lot of work to redesign and rebuild the drivetrain to insert a tranny.

I consider the car largely experimental at this point, so out of sheer curiosity want to see if it's really viable as a direct drive. So far in the test drives it is running cool and has no problem accelerating with a light throttle. It only takes tens of battery amps to start it rolling and maybe 250-300 for modest acceleration.

Part of the problem may be in educating my right foot in how to do 'modest acceleration'.

For now, the car can only make about 135 kw, so motor heating is not an issue. But once the rest of the batteries are build and installed that will increase to 345kw, then I will be monitoring temperatures more closely.

Originally Posted by allen_dodge

120 is pretty warm, any temperature increase will still affect power and efficiency, so it might be worth the time to put in a gauge and connect a manual override switch to force the fan to operate at full speed. It's likely brushless so I would be surprised if it draws more than a few amps, and if there is a noticeable drop in current draw then it is probably worth leaving the fan on full blast, especially at the strip . I guess another option would be to somehow lower the temp that the fan kicks on at, but you will just have to measure the cost/benefit of running the cooling fan all the time. As long as your amp gauge includes the power used by the fan, you should be able to easily measure any gain in switching the fan on and off.

As far as transmissions, I would leave it exactly the way it is if you do mostly highway driving. I'm gonna go check out some of the vids now...