Getting the power to the road

Having thoroughly discussed wheelmotors on here, I think it’s time to change subject completely and talk about wheelmotors.

Yes, you did read that right, and no I haven’t lost it.

Several home-built EVs, especially ones done for-cheap (For instance, Forkenswift), they’re done with DC motors taken from old forklift trucks. Usually the “drive” motor is used, and all is well. But flicking through the Princess Auto catalog a little while ago (What else do you do at 2:30am when you can’t sleep?), I came across a few interesting things, not least were some small, but reasonably spec’d Hydraulic motors. (I can’t link to them, their website is stupid – If products have the same name, you can only see one of them!) A little more digging, and I find a set of things called “Inline Axial-Piston” motors, which have great characteristics (Torque up to 83 ft-lbs (each!), top speeds of 3-4000 RPM). So my thinking is that, perhaps, one could take the “Pump” motor from a forklift, hook up some high-pressure hoses, and have the basis for a very space-efficient vehicle. With no need for drive shafts at all, it gives all kinds of space for batteries, and if you were custom-designing your vehicle, you could have a flat floor with the batteries inside (Using thin cells like LiFePO4 laid on their sides, for example), a small “Engine” compartment with the motor, controller and hydraulic splitter, and just some high-pressure hoses going from there to the wheels. Driving the motors in series/parallel would give reliable performance with behaviour like a limited-slip differential, and the difference in displacement between the pump and motors will act as the gearbox. And the units themselves are reasonably small and light, which completely throws away the whole “messes up the handling” problem.

But I’m no hydraulics expert (Heck, I wouldn’t know where to begin, to be honest), so ideally what I’d really like is for some hydraulics guru to take a look at this and tell me if I’m nuts for considering it. I know hydraulic drive is used in some low-speed vehicles (dump trucks, forklifts, that kind of thing). So why not high(er)-speed situations like this? We don’t need all 3000 RPM (heck, with 15” wheels and average tyres, you reach 120KPH (75MPH) at 1185RPM! 1500RPM would suit perfectly (give a little leeway for the racers out there :) )), but we do need the torque and power. Ideally, this system should also drive backwards. Not only for reversing the vehicle, but to be able to drive the motor ‘round, giving the option of regenerative braking too.

So, someone who has an idea what to do with these pipes and things, drop me a line and let me know if this’ll work or not.

Hybrids, in series

It’s been a little while since I’ve posted anything here, but there is good reason – I’ve been haunting the forums of DIYElectricCar.com. I do still have some thoughts to share here (And which may be posted on their forums also).

“Series” Hybrids. These differ from “Parallel” hybrids like the Prius in that you have a generator hooked to the batteries, which is then hooked to the drive wheels, rather than (as the Prius has) both motors (ICE and electric) hooked to the wheels with some kind of fancy balancing transmission system.

Sounds easy, right? Grab an electric-start genny, throw it in the trunk of the car, make a quick-and-dirty bridge rectifier, and when the batteries get low just fire up the genny to recharge them as you go. Unfortunately, it’s not quite that easy (When is it ever that easy?). You have to get the voltage requirements and power load worked out properly too. For instance, the above won’t work if you’re not running a 120v battery pack. Your rectifier will give you a 120v (or 240v) feed, which will be too high/too low for your batteries and damage them, or not help in any meaningful way.

Of course, the other potential problem is that to recharge a cell you have to supply it more power than it runs at. So, to recharge a 12v lead-acid cell, you have to pump in 13.8v (Or there abouts). If you’re supplying higher voltages to your batteries, your drive motor is more likely to run directly from your generator, as there is more power available from there, and the batteries won’t see (much) charge. Some might consider this not an issue. If the motor needs the power, it would be draining it from the batteries anyway, and during the “lean” load periods the batteries will still be getting their charge.

A solution would be to run two battery packs in parallel, and when the time comes to start the charging system, switch one of the packs from “Run” to “Charge”, reducing the batteries driving the motor to one pack (And so halving the runtime) and charging the other, and switching between the two as needed. So, for instance, when the pair of packs reach 75% DoD, disconnect one pack and start charging. When the remaining pack reaches 70% (The danger zone for most battery packs), or the charging pack reaches 100%, switch packs so the freshly-charged pack drives the motor while the depleted pack now gets charged from the genny. The issue here would then be balancing the packs so they can be used in parallel once again. Of course, you could always just hook up the packs in parallel again and let them balance each other out, but that’s something that’s not so good for the batteries.

Any thoughts? Suggestions? Comments?