direct drive vs transmittion effeciency

8228 Views 13 Replies 11 Participants Last post by Jozzer, Jan 30, 2011

OMT

Discussion Starter · Jan 27, 2011

so im just finishing up my current conversion, its a small 1999 hyundai accent, 72v 230ah, or around 16kwh and i was thinking of what i would do different in my next car and it brought me thinking.

is direct drive with an 11" motor more efficient than a 9" on a transmission? it would make sense that the 11" direct would be more efficcient, because it doesnt need to move gear and crap around, but then your top speed will be very low if you have 144v or less?

i have no experience with direct drive so anyone who has done a direct drive conversion, is it just a better drivetrain for raacing/drag? or can any normal car with 144-156 go direct drive and getmore range?

thanks

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dtbaker

· #2 · Jan 28, 2011

its going to be a function of whether your desired speed can be achieved within the rpm limits of the motor, and whether the torque at the given ratio can give you the acceleration you want.

An 11" motor is BIG and HEAVY and has a lower rpm limit usually.... you may well be better off with all-around performance with a smaller motor and a transmission. just my .02

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OMT

Discussion Starter · #3 · Jan 28, 2011

mhmm. well as im converting my car ( i already have several buyers interested :l) when i got my 9" motor from a forklift i shortly then got an 11" GE, about 15" long if i can remember right. has lots of bars, brushes identical to a kostov. and i was considering doing a DD direct drive conversion with a small car such as an rx7 or 8...ect. if i am using lead, would one assume 150-180v be sufficient to get to speeds of 100-160. i know of course the diff. ratio is what affects the top speed, but would there be a range benefit of going direct drive or none/very minimal?

btw i havent weighed the motor, but my 9" weighed 120lbs, and i bet the 11" weight is around 150-170

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dreamer

· #4 · Jan 28, 2011

The biggest problem, I think, would be the effect on acceleration. A transmission is going to provide a torque multiplier of 3x or 4x compared to direct drive to just the differential. The torque available from an 11" motor is not 3x or 4x the torque of a 9" motor, so it will not compensate for the lack of a transmission. Of course, running huge amps to the motor could compensate, but otherwise you are talking about worse acceleration. For an around town car, or where range is more important than performance, direct drive might be fine. Or if the vehicle is very lightweight like a motorcycle, trike, etc. where the torque required at the wheels to get it moving is less.

The other obvious problem with direct drive is that you need to use double contactors to reverse the motor rather than use the transmission's reverse gear.

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dtbaker

· #5 · Jan 28, 2011

the average auto transmission is above 90% efficient helical gears. Once spinning you have little to gain by losing the mass, and a lot of performance sacrifice either in acceleration or top end without a tranny....

To gain the extra torque with an 11" costs you probably an extra 150 pounds that you have to accelerate along with high voltage and amps to feed it, and fixing the gearing puts a hard limit on top end based on the rpm of the motor...
my .02

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toddshotrods

· #6 · Jan 28, 2011

I'm probably one of the biggest proponents of direct drive around here, but for the setup you described there's no question (IMO) - you would be much better off with a transmission. Even if you use a manual more as a mode selector (city 2nd gear/highway 4th...) I think your overall efficiency will be greater with selectable gearing.

For direct drive, you have to gear the differential for the top speed you want, at the expense of lower end acceleration. You have to run high enough voltage to turn the motor faster than you may have had to with a transmission, and you have to pull a lot more power from your pack each time you accelerate from a stop, or slow roll.

If you bump the specs up to say 200+volts and a high enough amp-hr/c-rate combination to pull north of 500amps easily, you might be getting into direct drive territory. I also think some of the more advanced lithium composition batteries would be better because you would get more for less weight, making the motor's work easier. In other words, it's not really the best option to build a "budget" EV.

Ideally, I think it's the ultimate EV powertrain. The right setup of motor/controller/batteries makes full use of all the attributes of the electric motor - like Tesla Roadsters. It's just smooth, unrelenting, acceleration, with no vibration, no clicking, no clacking, no fuss...

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ecc3189

· #7 · Jan 28, 2011

dtbaker said:
the average auto transmission is above 90% efficient helical gears.

I might be mistaken but isn't the torque converter highly inefficient due to fluid turbulence both in shifting and acceleration?

But a proper controller supplying 144v and a healthy amount of amperage would give a 9" motor plenty of power. The battery would be limiting factor.

I'm still going for it though, one less thing to make noise and use oil...

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OMT

Discussion Starter · #8 · Jan 28, 2011

ok thanks, i totally agree with you by the way if i was building a high performence ev it would no question be direct drive. i think for my next car ( i have no clue what, an old porche is my dream) im going to remove first and fifth gear from the trans, so it will be 3 speed forward, and reverse( might only leave 2 gears) so i can remove some weight from the trans that the motor has to spin even when it isnt in that gear.\

thanks for the responses my curiosity is satisfied

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FB Performance Trans

· #9 · Jan 28, 2011

OMT said:
so im just finishing up my current conversion, its a small 1999 hyundai accent, 72v 230ah, or around 16kwh and i was thinking of what i would do different in my next car and it brought me thinking.

is direct drive with an 11" motor more efficient than a 9" on a transmission? it would make sense that the 11" direct would be more efficcient, because it doesnt need to move gear and crap around, but then your top speed will be very low if you have 144v or less?

i have no experience with direct drive so anyone who has done a direct drive conversion, is it just a better drivetrain for raacing/drag? or can any normal car with 144-156 go direct drive and getmore range?

thanks

I think most would agree that for normal street use, an EV equipped with a transmission is more efficient then using a direct drive set up.

I recently responded to another post which also might be of interest. It was also related to comparing direct drive versus using a transmission in a drag racing application. See Post #29 on the following link:
http://www.diyelectriccar.com/forums/showthread.php/automatic-solution-ev-conversions-53052p2.html

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Woodsmith

· #10 · Jan 28, 2011

I'm working on direct drive for my reverse trike.
The only reason I think I can get away with it is because I am planning a maximum weight of 600kg and I am using an 11" motor.

The 'thinking' is that the 11" should have enough torque to spin the rear wheel with the ratio I need to achieve my top speed.
This does mean that I am working to achieving 80mph at 3500rpm and I will be doing my, limited, urban driving at less then 1000rpm.
A two speed would be good but I've not found anything suitable yet.

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nimblemotors

· #11 · Jan 28, 2011

Good info FB.
I used a 5:1 direct drive in my EV-hybrid, and it was piss-slow using my DIY 120v 500amp controller. I replaced it with a 3.7:1 gear, and added a snowmobile belt CVT, which gave it roughly a 14:1 to 4:1 range, and this worked very well. I tried using the CVT direct-drive and it did not work.

If you have a light enough car, the CVT will work and weighs almost nothing! 10lbs maybe for a HUGE increase in performance. Oh sure the belt has some slippage, if you want 99.9999 efficiency get a trans, the heat loss in overloading a motor and controller with 1000amps is also loss.

FB Performance Trans said:
I think most would agree that for normal street use, an EV equipped with a transmission is more efficient then using a direct drive set up.

I recently responded to another post which also might be of interest. It was also related to comparing direct drive versus using a transmission in a drag racing application. See Post #29 on the following link:
http://www.diyelectriccar.com/forums/showthread.php/automatic-solution-ev-conversions-53052p2.html

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Coley

· #12 · Jan 29, 2011

OMT said:
ok thanks, i totally agree with you by the way if i was building a high performence ev it would no question be direct drive. i think for my next car ( i have no clue what, an old porche is my dream) im going to remove first and fifth gear from the trans, so it will be 3 speed forward, and reverse( might only leave 2 gears) so i can remove some weight from the trans that the motor has to spin even when it isnt in that gear.\

thanks for the responses my curiosity is satisfied

To take a gear or two out of a stick trans, you need to machine spacers to take their place.

A big waste of time for a little weight saving.

Your motor wouldn't even notice the change.........

With a stick, you may need all the gears at different times.

I have a 5 speed and use them all, in different circumstances.

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trej

· #13 · Jan 29, 2011

I was wondering whether anyone has tried using the gear system from a push bicycle of some sorts? I know that the peak power of your average human (on a bike) is only about 1kw, but I'm not sure what power the gears are designed to handle. Maybe a <10kw motor connected to a bicylce gear box and a light weight trike frame (<400 lbs) would work.

I'm interested to hear any feedback on this.

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Jozzer

· #14 · Jan 30, 2011

trej said:
I was wondering whether anyone has tried using the gear system from a push bicycle of some sorts? I know that the peak power of your average human (on a bike) is only about 1kw, but I'm not sure what power the gears are designed to handle. Maybe a <10kw motor connected to a bicylce gear box and a light weight trike frame (<400 lbs) would work.

I'm interested to hear any feedback on this.

6kw on an 80lb bike can tear the gears up before you get the end of the street in my experience.Top quality components help, but not for long...

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