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

[allen_dodge]

Originally Posted by ev99saturn

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.

yeah, its true they say driving habits usually make more of a difference than modifications, and with a car like that it's just hard not to drive it fast. However, even very small changes can add up, like gear oil viscosity especially at low temps. And even if efficiency isn't your main goal, you also increase peak performance and more sustained output with any of these modifications so it's really a win-win. I'm sure motor heating isn't an issue at this point, but the sensor is there as a safety feature and ANY heat generated is a direct indication of efficiency, and that goes for all the electronics, not just the motor. Accurate temperature gauges on all these things will give you some feedback on your driving habits, etc. as well as one more ruler to measure the effectiveness of any other modifications.

[fb_bf]

Before you start driving more conservatively, I wanted to point out that the energy lost in accelerating a mass to a given velocity is the same whether you do it slowly or quickly. The POWER required to accelerate faster is the difference. Your batteries store energy, so as long as your accelerating to the speed you want, (not going faster, then slowing down) it shouldn't matter how fast you accelerate. I've been waiting to bring this up in a forum to see the reaction of the people responding. For an electric car the extra losses from accelerating quickly would be related to additional I^2R losses since you driving more current to get that faster acceleration. I don't know how much that is in these motors. It might be that there isn't that much range penalty for having fun with an electric car. You really need to get an accurate assessment of the wh/m number at 50 mph. You mentioned you where doing the measurement on a road with a slight incline. Even slight inclines make a big difference. I do my measurements going both ways on a road, then average the results. My controller (soliton JR.) can log this information while I drive. That is one way to measure it. You can check out my site to see how to measure your coefficient of drag doing coast down tests. I have a spread sheet there to help. If it is confusing just contact me. I'd be glad to help. Those wh/m numbers seem too large to me.

[k-vette]

Just coming from electric RC cars, they all say this is true. That's one of the coolest things about an electric car, you can step on it and not feel bad for it!

[redpoint5]

Originally Posted by ev99saturn

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

From what I've learned on this site, you want to do one or the other; not both. The belly pan being most efficient, and a front air dam if you aren't running a belly pan.

[Frank]

Hi Wayne,

(Glad you found this site.) Pay no attention to the tranny-lovers behind the curtain! You're fine with your choice. The Gear Vendor will only help if you start revving too high when racing. Metric picked up some performance with his Fiero when he added his. All the aero suggestions are good. The only thing I don't think anyone has raised is perhaps some way to smooth airflow tumbling over the top of the windshield. I don't know what the OEM's do but there must be some tricks to make improvements in this area.

I'd still like to get down your way sometime this fall to see your car in person.

regards,
Frank

[Frank]

I forgot to add re the explanation about energy required to accelerate is the same (fast or slow): I agree. I think the concern about rapid acceleration was mostly due to PbA batteries where you have to worry about Peukert and not over-stressing them too much. There would be some marginal I2R losses but I would think very hard to measure. The other impact is that your average speed over a certain distance would increase slightly but unless you're constantly speeding up and slowing down it'd be hard to measure.

Relieve some more guilt: add PV to your roof and drive on solar!

[fb_bf]

Frank,

Your right about the Peukert effect, but with li-on batteries it is small. I'm glad to hear two people so far agree with my thoughts an acceleration and electric cars. On my midget I was able to measure a 10% increase in drag at 50 mph with the top down, so getting a soft top or hard top will help, but what fun would that be. I can't picture a cobra with a top.

[ev99saturn]

Interesting point. The air dam would reduce the turbulence under the car so would diminish the belly pan's benefit. But would there still be some anyway?

Another thought is that the belly pan, while helping under the vehicle, still won't do anything for the wide tires. By using an air dam, it will reduce the airflow hitting the tires. It won't do anything for the stickiness of the tires.

If there is a gain doing both air dam and belly pan versus only one or the other, it's worth trying. As bad as the aerodynamics on this car is, I'll take any gain I can!

Originally Posted by redpoint5

From what I've learned on this site, you want to do one or the other; not both. The belly pan being most efficient, and a front air dam if you aren't running a belly pan.

[ev99saturn]

Hi Frank,

They do make both a rag top and hard top option for the car, and one will probably get added at some point after everything else is done on the car.

For now, I want to get lots of test data for baseline and to measure the changes.

One of the challenges now, is the weather cooling off. Lithium is not as affected by the cold as lead-acid, but still does have some reduction in capacity. Not sure if there is much at 40F compared to 70F, but any amount will offset the test results.... perhaps as much as adding one or more of the aero-improvements I am now planning?

If/when you plan to get to my area, please do let me know and we will hook up. I would like to speak with you more about the Loring Timing Association. It keeps rolling around in my mind....

Cheers,
Wayne

Originally Posted by Frank

Hi Wayne,

(Glad you found this site.) Pay no attention to the tranny-lovers behind the curtain! You're fine with your choice. The Gear Vendor will only help if you start revving too high when racing. Metric picked up some performance with his Fiero when he added his. All the aero suggestions are good. The only thing I don't think anyone has raised is perhaps some way to smooth airflow tumbling over the top of the windshield. I don't know what the OEM's do but there must be some tricks to make improvements in this area.

I'd still like to get down your way sometime this fall to see your car in person.

regards,
Frank

[ev99saturn]

Yes the wh/mi hours are painfully high. The good news is that there is plenty of room for improvement!

Your thoughts on acceleration are interesting, have you gathered any good test data on it?

At some point in my testing, once I have a good baseline, I will do a run or two accelerating at WOT up to the cruising speed on the test route.

Marginal changes in acceleration might not show as much measurable difference given all of the other variables... wind, temperature, etc.

Here are a few examples where light or heavy acceleration would change the efficiency. 1) recommendations on this site; to use a light throttle to help increase efficiency. 2) a top fuel dragster briefly makes as much power as a 747 and uses a lot of fuel to go 1/4 mile. If it were to drive the 1/4 at suburban street speeds, it would probably use less fuel.

Of course both of these examples are gasoline based, not electric. Is there a difference? Testing should provide some good clues....

Wayne

Originally Posted by fb_bf

Before you start driving more conservatively, I wanted to point out that the energy lost in accelerating a mass to a given velocity is the same whether you do it slowly or quickly. The POWER required to accelerate faster is the difference. Your batteries store energy, so as long as your accelerating to the speed you want, (not going faster, then slowing down) it shouldn't matter how fast you accelerate. I've been waiting to bring this up in a forum to see the reaction of the people responding. For an electric car the extra losses from accelerating quickly would be related to additional I^2R losses since you driving more current to get that faster acceleration. I don't know how much that is in these motors. It might be that there isn't that much range penalty for having fun with an electric car. You really need to get an accurate assessment of the wh/m number at 50 mph. You mentioned you where doing the measurement on a road with a slight incline. Even slight inclines make a big difference. I do my measurements going both ways on a road, then average the results. My controller (soliton JR.) can log this information while I drive. That is one way to measure it. You can check out my site to see how to measure your coefficient of drag doing coast down tests. I have a spread sheet there to help. If it is confusing just contact me. I'd be glad to help. Those wh/m numbers seem too large to me.