# Direct drive with a Torque Converter only?



## dougingraham (Jul 26, 2011)

You don't need the torque converter. All they do with an auto transmission is let the motor spin without the vehicle moving. Since an electric motor has essentially full torque at zero RPM there is no point. The only thing the torque converter will do for you is reduce your efficiency.

What kind of project are you considering?


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## pgt400 (Jul 12, 2008)

I guess I was thinking it behaved more like a mechanical TC (ie snowmobile where pulleys change ratios). No project right now.


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## Agust Sigurdsson (Oct 25, 2009)

dougingraham said:


> You don't need the torque converter. All they do with an auto transmission is let the motor spin without the vehicle moving. Since an electric motor has essentially full torque at zero RPM there is no point. The only thing the torque converter will do for you is reduce your efficiency.


I find the statement above on torque converters somewhat misleading and want to comment on it.

At standstill and low speeds a torque converter actually increases the torque which is available for acceleration by a factor up to 2 or 3 at standstill. As soon as the vehicle starts moving the rotational speed difference across the converter declines and so does the torque multiplication effect.
Doubling the torque is a blessing for an IC engine. Allowing it to idle when Drive is engaged is of course another benefit but not the sole benefit as implied above.
However this comes at a cost. The internal losses of the torque converters become significant as speed increases. But modern transmissions have a built-in remedy: The "lock-up clutch" which bypasses the converter at cruising speed eliminates this loss.
This clutch is usually controlled by the Transmission control computer via a 12 Volt electric solenoid which in most car models is easily accessible. 

Most of the DIYers at this forum seem to prefer cars with manual transmissions for their conversion projects. I believe that the potential of converting automatics is underestimated and often dismissed without reason.

If you select a motor for your conversion with sufficient torque you will obviously not need a torque converter, but if it comes for free with the donor car and does not hurt performance - why then dismiss it without further thought ?
There are some freebies with it such as Park and of course you can run the motor at idle if you want to run the legacy hydraulic power steering pump from it. Finally if you are experienced in electronics you may want to design a controller for the transmission to your own requirements.

Availability of the torque multiplication might even relax some design criteria for selection of motors for your next design.

For a while I have been playing with a salvaged Opel Astra in my backyard. It has automatic transmission and I have replaced the transmission control computer with my own mock-up switches - learning to control the transmission electrically. The testing has mostly been with the wheels off the ground but so far successful. I can not foresee any major obstacles if I would decide and proceed to a full EV conversion project.


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## pgt400 (Jul 12, 2008)

That was my original thought. Take advantage of the torque multiplication of the TC and use direct drive. No transmission.


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## Agust Sigurdsson (Oct 25, 2009)

pgt400 said:


> That was my original thought. Take advantage of the torque multiplication of the TC and use direct drive. No transmission.


Well that is my thought too.
However if (or when) I make final plans of my conversion project I would be tempted to take advantage of a gearbox with 4 selectable speed ratios plus a torque multiplication which I can switch on and off whenever I want. 

If some of the regulars of this forum would be so kind to add facts to this thread I would be pleased to read them. If there is some evidence that Automatic transmissions are lossy and not suitable for electric driven vehicles, please reply, give links and any kind of reference available to prove the point. Until then I will continue to believe that manually controlled gearboxes are fine for conversion projects, but "automatc" gearboxes offer the DIYer a lot more of opportunities to play with. 

Agust


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