Power/weight ratios

Have you made or bought a converted vehicle if so this is for you
User avatar
Jeremy
Posts: 472
Joined: Thu Jul 31, 2008 6:35 pm
Location: Salisbury

Postby Jeremy » Sat Aug 23, 2008 10:59 am

hyve wrote:Unfortunately with one motor only a car still needs a differential and as Paul says, probably a 5:1 step down. This section of the modern fwd transmission is a large chunk of it's mass, so there is not so much to be saved there after all. Adding a clutch puts it back up of course, but I'm not setting off down that road again !!!


That's one very good reason for looking at a two motor solution for FWD. It's not that mechanically complex to make up a housing to hold bearings that would support the inboard drive shaft ends. At a pinch it could possibly be done with off-the-shelf housed bearings that just bolt to mounting plates. Add a pair of belt or chain drives to two motors and you have the potential for a pretty light weight and simple system.

Jeremy

MalcolmB
Posts: 423
Joined: Sat Jun 23, 2007 8:07 pm

Postby MalcolmB » Sat Aug 23, 2008 11:38 am

I didn't mean to sound as if I was questioning your rationale, just to point out that a single ratio transmission is a special case and involves certain compromises. I totally agree with your aim of eliminating as much mechanical complexity as possible. It's one of the main attractions of electric systems for me too. Good luck with your project!

User avatar
EVguru
Posts: 305
Joined: Fri May 25, 2007 5:17 pm
Location: Luton
Contact:

Postby EVguru » Sat Aug 23, 2008 11:47 am

You do have to consider what would happen in the event of one motor failing. I saw this happen in EV racing, though more accurately it would have been called an 'energy run'. The FWD car speared off into the track wall coming onto the straight. This was the result of a poorly thought out chain drive and the motor was also spectacularly destroyed. If you have two motors connected in series and the load on one is removed most of the voltage will appear across the other motor, possibly overspeeding it. If the motors are conected in parallel, but the motors are series wound, the same think can ocour.

The Aerovironment prototype Impact for GM used two motors, but GM went with a single motor and single ratio transaxle for the EV1. Their assesment was that this was safer and just as efficient. One reduction drive rather than two. It also gave them somewhere to incorporate a transmission lock.

It's very tempring to think that getting rid of the standard gearbox MUST be more efficient, but that's not always true. Efficiencies multiply and average efficiency is often more important than peak efficiency. An AC drive system can have a very wide speed/torque range and get away with a single ratio. A seperately excited DC brush motor can also do quite a good job (Citroen Berlingo).
Paul

http://www.compton.vispa.com/scirocco/
http://www.morini-mania.co.uk
http://www.compton.vispa.com/the_named

User avatar
Jeremy
Posts: 472
Joined: Thu Jul 31, 2008 6:35 pm
Location: Salisbury

Postby Jeremy » Sat Aug 23, 2008 12:18 pm

There's a world of difference between a racing vehicle and a road going one, though. If some simple safety features were built in, like driving both motors from a common controller, or arranging for a dual controller system to shut down in the event of a single controller failure, then the risk would seem to be perfectly acceptable. IMHO, the controller is more likely to suffer a problem that the motor itself, hence the above suggestions.

Anyway, many modern (and not so modern) FWD cars have a steering geometry that is highly resistant to asymmetric drive conditions. I had an inboard drive shaft failure on a Citroen GS about 25 years ago. The shaft coupling to the inboard disk brake sheared completely at speed (made a hell of a noise!). Despite only having brakes on one front wheel the car pulled up in a straight line, with only the very slightest tendency to pull to the right. Apparently, Citroen built in a great deal of self-centring into the design, to deal with the high-speed blow-out case. I suspect that many other cars behave similarly.

My opinion remains that such a drive system has the potential to be lighter and more efficient than a gearbox and diff, particularly for a light car conversion with a modest speed range.

Jeremy

MalcolmB
Posts: 423
Joined: Sat Jun 23, 2007 8:07 pm

Postby MalcolmB » Sat Aug 23, 2008 1:12 pm

So if you have series motors connected in parallel and there is a failure in the drive on one side, would the unloaded motor be likely to overspeed? Can you suggest a simple safety mechanism?

User avatar
Jeremy
Posts: 472
Joined: Thu Jul 31, 2008 6:35 pm
Location: Salisbury

Postby Jeremy » Sat Aug 23, 2008 1:55 pm

If the motors are parallel connected and driven by a conventional controller that doesn't motor-current control then one motor won't realise that the other has failed, unless it shorts the supply as it does so. If the failed motor simply goes open circuit, and stops working, then the other motor will continue to run as before.

If one drive system failed, then there is a chance that a motor could run away, but this is a chance with just a single motor as well. Without getting into the clutch debate, an argument could be made that says that you shouldn't use a clutch with a series motor as if the clutch pedal is depressed at the same time as the throttle then there is a chance that the motor could over speed. Of course, the same risk exists with a normal ICE vehicle with manual transmission, but we don't worry too much about it - we consider it to be an acceptable risk.

Whilst I'm all for promoting safe practice, it's worth just stepping back and looking at the risks we accept with existing vehicles from time to time. I found (to my considerable cost) that it's perfectly possible to get an asymmetric drive failure on a "normal" car. The risk of drive shaft failure is always there, and is probably no less than the risk of a catastrophic electric motor failure.

If we don't push the boundaries a bit we'll never improve anything, so a bit of extra risk is probably inevitable in what we're doing, anyway.

Jeremy

hyve
Posts: 60
Joined: Sat Oct 06, 2007 1:35 pm
Location: Chesterfield, Derbys.

Postby hyve » Wed Aug 27, 2008 7:54 am

EVguru wrote:It's very tempring to think that getting rid of the standard gearbox MUST be more efficient, but that's not always true.


To be honest, Paul, I cannot assess the relative efficiencies of the various systems so have to take your word for this. I note what you say about GM and their single motor decision, also.
My approach is simply to try to avoid any complexities which can be eliminated; the approach of the one-off builder rather than large manufacturer. I know we've been talking generally and probably looking for optimum solutions, so I accept that GM's solution may well be the best way to go.
Originally I asked about power/weight ratios with the objective of creating a single speed clutchless design, based on my approach to building; ie, as simple and light as possible. For an EV with it's need for a large load of batteries I believe this is quite relevant.

I feel this is worth exploring as an idea because others attempting one-off conversions might also find it useful. Malcolm, for example; right now working on a twin motor set-up. Losing the clutch, gearbox AND differential and avoiding the expense of a large, new motor seems very good sense in simplification terms, even if electrically it may not prove so efficient.
Finding a final drive and differential unit of appropriate weight/strength and with exactly the right ratio, to avoid the losses, complexities and extra weight of a two-stage reduction will be no problem for GM, but for us is not that easy.

Efficient design is, as you've said about OVERALL efficiency and for any designer this also includes system cost, in both time and money.
Peter Ph

User avatar
EVguru
Posts: 305
Joined: Fri May 25, 2007 5:17 pm
Location: Luton
Contact:

Postby EVguru » Wed Aug 27, 2008 8:48 am

My approach is simply to try to avoid any complexities which can be eliminated; the approach of the one-off builder rather than large manufacturer.


In which case; Keep the gearbox!

You get most of your engineering done for you, driveshafts, bearings, mountings, etc. All you have to do is couple your motor to the gearbox and pick up on the engine mountings. You get a 'free' reverse gear and the choice of ratios gives you a better chance of getting away with a smaller motor.

With a twin motor direct drive setup, you've got to engineer bearing mounts, reduction drives and motor mountings. You're going to need a pair of matched motors (one set up for reverse rotation) and a reversing contactor set per motor. You're either going to need a controller per motor or another contactor set and series/parallel switch the motors. That in itself requires an extra control system to prevent you from blowing the controller.

Malcolm, for example; right now working on a twin motor set-up.


Malcolm is in a minority position, he's converting a Mini. The transmission is pretty awkward to couple a motor to. That swings the balance towards an alternative.
Paul

http://www.compton.vispa.com/scirocco/
http://www.morini-mania.co.uk
http://www.compton.vispa.com/the_named

hyve
Posts: 60
Joined: Sat Oct 06, 2007 1:35 pm
Location: Chesterfield, Derbys.

Postby hyve » Thu Aug 28, 2008 9:26 am

I should have said, complexities in the design. I can see that for many people your approach will be simplest in terms of work, but for me it's the finished job which matters.
A clutch, plus gearbox which is well over strength and has an excess of gears is extra mass and complexity which an EV can do without. If there were a neat, light 2 speed converterless auto ( needing no seperate clutch ) and final drive unit designed for maybe 25kw, then I might be persuaded to choose that option. So far as I'm aware there isn't.
I do try to think in terms of repeatability, when designing. Working around parts which are far from suitable is to be avoided, in my experience.
Peter Ph

User avatar
qdos
Posts: 2089
Joined: Fri Apr 27, 2007 10:26 pm
Location: Dorset
Contact:

Postby qdos » Thu Aug 28, 2008 9:53 am

I know it's not very efficient but what about a CVT ? works amazingly well on quadricycles and has reverse


Return to “Conversions”

Who is online

Users browsing this forum: No registered users and 16 guests