Someone raised this issue the other day and I know it's been talked about a lot, with most opting for a manual gearbox, but it got me wondering if there is a benefit to having an auto box.
For my Starion, frankly greed got the better of me in choosing a larger motor - Kostov 11 inch - and running it with the only available speed controller at the time, the Curtis 1231C. It's not an ideal match; a larger motor coupled with a smaller controller could yield all sorts of induction field issues. I had always intended to upgrade to a Zilla controller and get a few more batteries on board but Zilla seem to have endless problems and may indeed die out like the dinosaurs.
So what's all this got to do with gearboxes?
Manual gearboxes are more efficient for an EV, particularly if you have refined gear-handling driving experience. End of story. Or is it? Perhaps keeping the clutch disc in a car with a larger motor would have allowed for quicker gear changes under varying circumstances (I am no racing expert who can shift quickly with no clutch). Most EV conversions do away with the disc and pedal and hence become 'semi-automatic' (and save a bit of weight in the process). However, it takes a couple of seconds longer to synchro-engage the gears. You have to 'feel' them meshing together and try not to force it. Smaller vehicles may only use two gears and reverse. Heavier vehicles need to shift more if the pack and controller are modest.
Under demanding situations (e.g., driving up a steep hill with an idiot on your six) a quick gear change is not an option, thus forcing the poor EV to struggle under the wrong gear, sucking more amps. Two or more seconds trying to engage the gear would be enough to have that idiot hit your tail. In heavier cars, poor gear changes could result in using more 'fuel', reducing range and harming the pack if the controller is not up to spec. It takes good practice to shift quickly but not every road circumstance will be friendly enough to allow it.
Reduced Range on Auto?
Most EVers say an auto box will reduce the range since it takes continuing energy to run the hydraulics for auto gear selection. In an EV, for a manual stopped at the lights, nothing happens. The motor is not turning; no energy is used. For an auto you have to up the rpm permanently for smooth stop and go. This is where you lose effective range. Swapping a gearbox would mean you'd also need the wiring loom and computer for the overdrive. Setting the torque conversion revs would be tricky.
But in a simple three speed auto electric vehicle, perhaps the accelerator can be adjusted to a persistent voltage above zero and a revs check done. Haven't seriously looked into this, but if enough revs so the synchros engage correctly from a stop position is all one needs, perhaps 300 revs or so could work. Some EV'ers with autos say they don't need to do this, but that depends on the weight being hauled and type of auto gearbox it is, from a CVT to basic no frills hydromatic (ah, the old days).
With auto transmission it's true; more energy is used. But for a larger car with a modest controller, perhaps an auto transmission is a better option. Correct gear selection every time does preserve the battery pack from excessive current demand (and your bus-bar battery interconnects stay cool) and the controller and pack are happy for it. The loss in range may actually be LESS than using a clunky manual gearbox, depending on the vehicle, gearbox (some autos do shift slower than others) and driving conditions.
Five speed gearboxes are probably not suitable for modest EV spec'd larger vehicles. Many have converted vehicles with auto transmissions and as mentioned above the expected flat spot (where there are no revs from going from zero) seems not to be an issue for most. It just takes a second or so from foot down to go from stop. Upping the revs a little for permanent motor turning at stops is the only way to do it for larger motors (and I believe the new Soliton 1 controller has a facility for fine-tuning with an auto transmission).
Abuse is no Excuse
Since the previous owner of my coupe clearly abused the vehicle (boys will be boys), third gear is a little noisy. I could swap to an identical four speed box. Something to ponder, perhaps. So in summary, when considering a conversion look at all your power options. This is of course dictated by cost, but a modest conversion may indeed benefit from an auto transmission - and it's easier to shift manually if you want.
No gearbox at all
The rate battery technology is improving, commercial electric vehicles will do away with gearboxes altogether. For my vehicle, I could increase the volts (the motor would handle it) and thin 4th gear every time. I'd have to replace the bus-bars of course, and the charger which has a pack voltage limit, so the extra cost could be considerable.
In a perfect world no gearbox at all, wheel-mounted AC induction regen motors, pack inverter and plenty of amps/range would be just the thing, but I'd need more green to go that green:-) so for most home builders on tight budgets, keeping the gearbox will get around numerous cost issues.
As battery and component technology improves and gets cheaper my next conversion may indeed gearbox at all.
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