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Discussion Starter · #1 ·
First off, this is just planning for the long term since I won't have money or time for at least 8-9 months. I have been doing lots of research and I decided to post because I can't seem to find direct answers for some questions I have.

The idea is to make a college project out of my e30 once the motor kicks the bucket. I love my car and my goal is to convert it to electric drive while maintaining all of its current performance. This means top speed of 100 mph or so, 0-60 in 9ish seconds and curb weight of 2700 or better.

Right now my planning and research is in the general phase and I don't have any details in mind but I have a few questions about electric conversions.

First, what power do I need out of a motor to equate to about 120 hp and 170 ft-lbs that I have in my current ICE? Ive seen people using 24ish hp motors and I've seen racing guys using dual 150kw motors so where is the in between? I plan to take this car on track so I want to maintain the performance it has right now but I can't afford some monster motor either.

Second, Is it unreasonable to use the motor in a direct drive or is it cheaper to have a simpler controller and just use the tranny? I would love to do away with as many components as possible to make the car more efficient. I also want to fit the motor where the tranny is now to give the car better cg height and weight distribution.

Any suggestions and or links are appreciated. I have been having trouble finding good resources on understanding EV performance so if you guys wouldn't mind posting some information for me then I can better grasp what it will take to get this project rolling. Thanks!!
 

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Howdy-

You have a very ambitious plan. It is certainly possible to build a car with those parameters but you will need significant budget and serious engineering and building skills.

As for sizing motors (and a lot of other things to consider when planning a conversion) look at the wiki. You are comparing apples and oranges in continuous and peak ratings. You also need to know the relationship of Kilowatts to horsepower (10 horsepower is about 7.5 kilowatts).

Direct drive is certainly doable with the right motor and controller combination.

You didn't state your range goals but by the time you have pulled all the ICE stuff and related support components, then installed the motor, controller, charger, etc you may only be down a couple of hundred pounds from stock. To maintain stock weight or close to it the battery is going to have to be light. With the right type of lithium cells you can still get tons of power out of such a battery, but the tradeoff of a small, light battery is modest range.

Good luck.
 

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Discussion Starter · #3 ·
Howdy-

You have a very ambitious plan. It is certainly possible to build a car with those parameters but you will need significant budget and serious engineering and building skills.

As for sizing motors (and a lot of other things to consider when planning a conversion) look at the wiki. You are comparing apples and oranges in continuous and peak ratings. You also need to know the relationship of Kilowatts to horsepower (10 horsepower is about 7.5 kilowatts).

Direct drive is certainly doable with the right motor and controller combination.

You didn't state your range goals but by the time you have pulled all the ICE stuff and related support components, then installed the motor, controller, charger, etc you may only be down a couple of hundred pounds from stock. To maintain stock weight or close to it the battery is going to have to be light. With the right type of lithium cells you can still get tons of power out of such a battery, but the tradeoff of a small, light battery is modest range.

Good luck.
Thanks for the reply! I have looked through the wiki but I am still having trouble understanding the ratings that manufacturers are putting on their sites. For example, the 9" Advanced motor says that at 144V it produces 28.5 hp continuous and 100 hp peak. I understand that continuous can't be compared to an ICE (since they have different power profiles) but 100 hp peak is still not that much and how long can the motor be run at peak?

For range I plan on fitting an efficient ICE/generator combination (EPA certified and with a whole bunch of experimental add-ons to catch waste heat). I have been interested in designing a super efficient drive system since hearing that gas engines have the efficiency of a dead cat and that is kinda why I got into engineering.

Any other advice on what kinds of motor/ controllers would be cheap or best? I know this forum has lots of knowledgeable people so any light shedding would be appreciated!
 

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Discussion Starter · #4 ·
I've done some math work and can see that 100 hp from an electric motor would be more than enough but I still don't understand what a manufacturer means by peak. Does that mean only for 1 min? 5 mins? no way to tell?

And a lot of the EV builds I have seen have been using higher voltages than the motor manufacture quotes, how do you know what a motor can handle? I guess just balance voltage and amperage to stay close to rated power output?
 

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Theorically, you can have the peak hp from a motor until one of the internal component reach the max temperature of the insulating varnish(180°c in most case, class H).

So the main problem is to cooled down the motor. Some motor are watercooled, but all popular serie DC motor are cooled by air. If you increase the air flow, you can increase the time you can push the peak hp from the motor.

If you think go with DC motor (cheapest way), Kostov motor and Warp can be great choise.
For exemple, a 9" diameter motor, 156lbs, rated 32 hp(Warp 9) can produce around 169 hp and 209 lbs-ft of torque at 144v and 1000A. I said "can produce" because this high power will be only reachable with big air flow and some modification to the motor.
The max torque is availiable from 0 to 3500-4000 rpm depending of your voltage. This motor can give 170 lbs-ft at 800-850Amps

I think use the original transmission is a good thing especially because you need to maintain the top speed of 100mph.
If no, you will need a bigger motor. Something like Warp11 Hv or Kostov 11" 250v and change your differencial ratio to put out the motor RPM range.


And after all this, don't forget, the battery is the most important part to have decent performance and range.
 

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Discussion Starter · #6 ·
So is there no place to check what a motor can do and what air flow is needed for that performance? Seems kinda dumb to use trial and error to find out how much cooling a motor needs under a certain load.

I also have a question for (if) there are any road racers out there, since you spend a good portion of your time at full throttle, how do you stop you motor from staying at peak amps/voltage? Do you limit the motor with the controller so you aren't getting peak power all the time?

Thanks again for your guys help in understanding EV dynamics
 

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You can compare with other similar conversion: http://www.evalbum.com/type/BMW/16

And yes, many today controller can be adjusted to regulate how many amps and volts the motor take. See some controller specs (Soliton1, Warp drive or Kelly).

You can have a battery pack of 300v able to push 1500A peak and only push 120v and 500A through the motor....... but in reality it's often the oposite!.. the battery is the weakest link!
 

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Discussion Starter · #8 ·
well its a good thing I'm starting to plan this early! looking on EValbum (have looked before but it was a different set of bmw projects) shows how many different approaches there are. I guess the basic idea is the a warp or kostov 9" will be able to handle the power I need so now I need to focus on power storage and control.

Does it seem reasonable that I will be able to get around 30 kW continuous and 100 kW peak from a 9" at 144v without too much modification?
 

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Discussion Starter · #9 ·
Does my math appear right?

I calculated that my e30 takes about 21 kW to go 70 mph so if I want a 100 mile range that works out to 30 kwh. For a 144V system this equates to 208 Ah or 260 Ah using 80% DoD. This works out to $650 for a set of batteries that match this (from this site: http://www.manzanitamicro.com/index...n=com_virtuemart&Itemid=64&vmcchk=1&Itemid=64)

If however I used a 7.5 kW generator (epa approved of course) the kWhs drop to 19.2 which means 166.4 Ah (80% DoD). This works out to $416 for the battery pack. Since the generator is $800 this doesn't sound good finacially but since I can refill the gas tank that gives me a much better range (and unlimited range if I only go 44 mph...)

Just thinking out loud but does this all sound right?
 

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Humm... your math isn't good!

A 30Kw/h lithium battery pack cost around 12000$...

And a 30Kw/h battery pack with battery of your link (headway 8ah) the cost will be 20000$!!
Each cell give 3.2v and 8Ah, so 25.6w/h....... 30000w/h / 25.6 = 1172 cells x 17$ = 19924$
 

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Discussion Starter · #12 ·
Oh yeah, that was dumb...I based the number of batteries off of required Ah and not energy. Oops. Yeah that makes more sense and gives me even more reason to figure out a good gen-set...
 

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Hi ecc3189

more reason to figure out a good gen-set...

I thought you were going electric for the higher efficiency - a gen set adds another two power conversion stages with all their inefficiencies to your already inefficient gas engine!

Additionally a small gen-set engine will be less efficient than your original engine
 

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Discussion Starter · #14 ·
well yes it is less efficient that way but the overall goal is to end up with an ultra efficient ICE engine that doesn't have any of the compromises involved with having to operate over a range of speeds. A series hybrid seems like the only way to make the ICE as efficient as possible.

Won't be perfect for my budget but its a start. And the engine I use won't be just any inefficient generator. I will only buy an epa and carb approved motor that is as small as possible
 

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Hi ecc3189

I will only buy an epa and carb approved motor that is as small as possible

I may be wrong but I think EPA and CARB standards are about emissions not efficiency,

it is difficult to make a small engine as efficient as a larger one,

a lot of the losses follow a cube/square relationship - where if you half the size you get a quarter the losses but an eighth the power

Think about the cylinder head and combustion chamber - a smaller unit has a higher area to volume relationship and corresponding heat losses

You can normally make a smaller engine with a higher specific power output but you end up with a lower specific efficiency,

The Prius does use a more efficient engine but I don't think you can get an Atkins cycle gen set!
 

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Discussion Starter · #16 ·
Hi ecc3189

it is difficult to make a small engine as efficient as a larger one,
That doesn't make sense to me since every car makers 'efficient' models involve a downsized engine. Which I can understand will have less power and therefore less fuel usage.

a lot of the losses follow a cube/square relationship - where if you half the size you get a quarter the losses but an eighth the power

You can normally make a smaller engine with a higher specific power output but you end up with a lower specific efficiency,
I'm just starting my research so I don't know anything about that kind of relationship. It makes sense and explaines why powerplants are big and efficient but I still maintain that there has to be an efficient solution to gas ICE's out there.

Think about the cylinder head and combustion chamber - a smaller unit has a higher area to volume relationship and corresponding heat losses
Wouldn't the area and volume change relative to each other? seems like you could design an engine with a similar area/volume ratio that is smaller.

And my overall goal (probably when I'm 70 and retired) is to have a series hybrid with an engine designed from the ground up to be efficient so maybe it will be a constant speed Atkins cycle. For now though I think i have to move to less efficient (and slower) to get experience with this stuff on the budget I have :(

Any engineers out there with suggestions on good books or papers to read about engine efficiency in transportation?
 
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