# Automatic, pros and cons



## puddleglum (Oct 22, 2008)

Hey all,
I'm in search of a donor car and I'd like some feedback. I know that standards are the norm and easier from many respects, but it seems like autos might have their advantages as well. I know this has been discussed, but when I searched, I couldn't seem to find much. i know a few guys have done it successfully as well.
If I'm going to make this practical, I need to build something that is roomy for 4 adults, that my wife will drive (no shifting while moving, power steering and brakes), and still inexpensive to convert and drive as a short range, in town, commuter car. I have an 11" forklift motor so I need a little bigger car for it as well. I'm thinking of maybe a PT cruiser but standards are hard to find.

So what are the pros and cons? If I use an auto., 
pros: easy for anyone to drive.
If I idle the motor, I can use factory p/s, alt. and a/c. so less add ons and no expense for electric pump and dc-dc convertor.
Cons: Less efficient and heavier. Does anyone know how much less?
More fabrication to use factory accessories.
More complicated motor mounting to mount to a T/C, and harder to get the trans to shift properly. Is that really hard or no too bad?
I might need a bigger battery pack, to make up for efficiency losses, more expense. 

Can anyone expand on this? What am I missing If using an auto, will the expense (both in the build and the wasted electricity) and difficulty outweigh the advantages? Will performance be worse as well? Does anyone have some good links to how to make it work?
There is a low mileage car for sale right now, but it has an auto. It got me wondering if it would work.


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## mechman600 (Oct 16, 2010)

HEY! A fellow Ecomodder.
Check this thread out:
http://www.diyelectriccar.com/forum...s-why-automatic-gearboxes-work-evs-29101.html


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## steelneck (Apr 19, 2013)

As i see it there are two major benefits using an auto trans. in av EV with a DC motor. The first is that it feels quite natural to drive with a weak engine brake, or rather no engine break in this case. You can make it with the converter in place or making it direct coupled. In the latter case you need a separate small pressure activated pump to have hydraulic pressure to engage the gear from standing still. It just activates when ignition is on and pressure is a bit below the normal pressure produced by the transmissions normal pump when driving (edit: no electronics needed for this, just a pressure switch). To go without the converter will save quite a bit of energy, since the converter is the major heat producing part in an auto tranny, and the EM do not need it due to its high torque from zero and low RPMs. Without the converter an auto tranny. is just as efficient as a manual gearbox, and we get almost uninterrupted power through all shifts and the whole speed range. I also think it is possible to have an idling function only when it is in neutral or park, to drive things like AC and such when standing still.

The other benefit is the Park function. Many of us, especially in cold climates in the winter, do not use the parking brake when parking. We park in first gear with manual gearboxes, because it is not uncommon with parking brakes that freezes stuck. But with av EV the car would happily roll away if you park in first gear without the handbrake engaged, the park function of auto trans. solve this problem.


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## JRP3 (Mar 7, 2008)

Since I had both a manual and automatic in the same car I can speak to efficiency, at least in my case, it was not as good with the auto, even using direct drive. I would not recommend trying direct drive unless you are a good machinist or know one, getting the direct drive coupler right was a pain. You can't idle the motor if you go direct drive since it will bang into gear when you shift out of park or neutral, not to mention it will keep driving the car forward when you stop in gear. You can read the full details of what I did starting here:
http://www.diyelectriccar.com/forums/showthread.php/eviero-ac31-se100-fiero-build-41752p26.html


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## steelneck (Apr 19, 2013)

It would be possible to have idle without the TC, if there is a way to tell the controller to idle _only_ in neutral or park.

@JRP3: Did you ever trace where the energy went away when you got lower efficiency with the auto, or was it all about less optimal gear ratios? Going from 48 Ah to 60 (both 115v?) is almost 1,4 kWh. That is quite a lot of heat that must have dissipated somewhere.


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## JRP3 (Mar 7, 2008)

Some of it was heating of the automatic, it certainly gets hotter than the manual did, but I did find that if I used 3rd as much as possible my efficiency gets much closer to that of the manual.

As for idling only in park or neutral you'd need a way to stop the idling before you even shifted into gear or it would cause a bang. I experimented with trying to spin the motor to pressurize the transmission before I hooked up the 12V pump, and if the motor is spinning much at all when you shift out of P or N it's not good.


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## steelneck (Apr 19, 2013)

JRP3 said:


> Some of it was heating of the automatic, it certainly gets hotter than the manual did, but I did find that if I used 3rd as much as possible my efficiency gets much closer to that of the manual.


I guess you had 1:1 ratio in third? (most do) Manual gearboxes are also more efficient in the direct 1:1 gear where the layshaft do not have to transfer any power and just spins along. Though cold manual gearboxes can be very heavy to turn due to its more viscous quite thick oil compared to ATF (though many manual boxes runs quite happily with ATF too with light load).

Some auto transm. may have a quite large pump, large enough for the tranny to behave normally even with quite a lot of internal leakage. Big pumps like that need more power than necessary for a tight gearbox, heating oil.



> As for idling only in park or neutral you'd need a way to stop the idling before you even shifted into gear or it would cause a bang.


Yes, the function needed is a switch, possibly on the gearstick, that tells the controller to quit idling when you put it in gear. The controller of course must have some input for it too, i do not know if any has. The motor would still be spinning, without power, when the clutch for first gear engage, but this engagement can be adjusted to be quite smooth, actually really smooth if you want. There are different ways to do it on different transmissions, but it boils down to restrict the hydraulic flow to the clutch to control how fast it engages.


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## JRP3 (Mar 7, 2008)

Wouldn't weaker clutch engagement mean more slipping, and wear? As it is I find that my current 12V pump doesn't have enough pressure and/or flow to fully engage the clutch when the fluid has warmed up, which is particularly bad when stopped on a steep hill. It slips and shudders until the drive motor turns the internal pump enough to engage the clutch fully. I need a bigger pump. Currently I have a 60 PSI 2 GPM unit so I'm thinking at least 70 psi and more than 2 GPM, I'd like at least 2.5 GPM. This is pretty close at 70 PSI and 2.2 GPM which may be enough.
http://www.northerntool.com/shop/to..._sku=2682271&gclid=CLLuyKqZh7kCFRGg4AodPzgAlg


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## steelneck (Apr 19, 2013)

JRP3 said:


> Wouldn't weaker clutch engagement mean more slipping, and wear?


Not weaker, you need _slower_ engagement.

If you need 100psi for the clutch to not slip when fully engaged, but want to have smooth clutching, you need a small jet for the oil to slowly build that pressure in the clutch. Yes you will get smoother engagement by lowering the pressure to 50psi, but then you do not have enough pressure to avoid slip when you floor it, or even worse at low RPM if you have internal leakage.

Now a bit more on the topic of energy waste in auto gearboxes. The big thief is the converter (without lockup), and second is the internal hydraulic pump. The planetary gears them self is probably more efficient than a manual gearbox, due to less moving parts. The converter can an EV do without, but the internal pump is still sized to maintain enough pressure with the ICE at idle, even with some internal leakage. That means quite some energy waste at higher RPM. A good solution to this is some pressure controlled external pump drive, not using more energy than necessary to just deliver volume enough to maintain hydraulic pressure. But now we are talking a bit more engineering than just some motor-gearbox adapter...


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## puddleglum (Oct 22, 2008)

steelneck said:


> It would be possible to have idle without the TC, if there is a way to tell the controller to idle _only_ in neutral or park.


The biggest advantage of having an auto, that I can see, is idling the motor, and being able to use the factory PS system. Since a large percentage of my driving is idling through parking lots, idling the motor in neutral would be no help. it seems to me that it would make more sense to keep the TC, set it to lock up just above idle, idle the motor in gear and turn the idle off in neutral. Trying to work out the TCM control still seems like it would be the biggest challenge to using an auto. I know nothing about electronics, and that challenge seems almost insurmountable.


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## JRP3 (Mar 7, 2008)

If you are going to keep the TC and use the motor to run the PS then why bother to turn off idle when in neutral?


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## puddleglum (Oct 22, 2008)

My reasoning, which I admit could be faulty, is that I am going to sacrifice some efficiency to use an auto when driving anyway, so I might as well take advantage of being able to use the stock power steering and alternator and eliminate one extra modification. Cutting the idle function in neutral would offset a little of that loss when coasting to a stop or sitting at a traffic light. I don't need PS then. It'd be like going to neutral and killing the engine in an ICE/auto car. I already do that now to save gas.


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