Thoughts/help for a new boy....

Have you made or bought a converted vehicle if so this is for you
AndyDawson
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Thoughts/help for a new boy....

Postby AndyDawson » Wed Sep 10, 2008 8:10 am

I'm newly aboard, considering an EV project.

basically aiming at producing a short(ish) range comuting vehicle - up to c. 40 mile round trips, but has to be capable of mixing in mainstream traffic - which pushes up the top speed requirement.

All of which is leading me to consider something along the lines below:

1 - bodyshell ideally sourced from something like a "GTM" kit car - mini based, GRP monocoque (hence light - c 120KG for the shell).

Apart from anything else, the monocoque has lots of spaces for batteries!

2 - parallel hybrid power - electric power up to c. 30-40hp (one or two motors), battery pack range for perhaps 15-20 miles, alongside smallish motorcycle/large scooter engine (400-500cc).

Generally, the design is hanging together fairly well - I'd be thinking of powering the front wheels electrically (it uses a mini front subframe, so integrating driveshafts isn't a challenge - I'm not convinced about hub motors, on the grounds of waterproofing, and unsprung weight), and using the bike unit at the rear, driving through a Fiesta diff as done in another kit, the Sylva Riot).

However, the big challenge looks like being integrating the bike engine and electric drive. first thoughts would have been to arrange matters so that up to some reasonably low speed (10-15 mph), the electric motor would provide the majority, if not all the drive, progressively reducing above that until at over (say) 40, the electric drive would be basically idling, and the bike unit providing the drive.

Ideally, I'd also like a switchable option to run it in "pure" electric mode, hybrid or pure petrol. Things like regenerative braking would be nice to have, but not an absolute necessary.

At the moment, I'm struggling with the control system design - does anyone know of any controllers that incorporate a speed-sensitive input, in addition to "throttle position"? Alternatively, I'd have to incorporate some form of electromagnetic clutch - anyone know of any?

There'd also be some mechanical trickery necessary to manage the petrol engine's output - but I think that's fairly easily soluble by playing games with the relationship between the throttle pedal and the actual throttle.

Any comments/thoughts welcome - even, am I smoking dope!
Last edited by AndyDawson on Wed Sep 10, 2008 8:19 am, edited 1 time in total.

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qdos
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Postby qdos » Wed Sep 10, 2008 8:15 am

Nice idea for a project not sure about the room for everything though as I suspect it's going to be a bit of a tight fit. Have you got a budget ?

AndyDawson
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budget

Postby AndyDawson » Wed Sep 10, 2008 1:25 pm

rough numbers....

picking up a car (in fairly rough condition - c £1K

donor bike - £300-500

Assorted electrical components - £1500

Sundries - £500.

I can weld & spray paint passably well, do should be able to do 90% of the work myself.

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qdos
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Postby qdos » Wed Sep 10, 2008 4:56 pm

Think your assorted electrical components is rather light I'm afraid £3k might be more realistic

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Jeremy
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Postby Jeremy » Wed Sep 10, 2008 5:13 pm

One thing to look closely at would be the trade off between the weight, volume, cost and complexity of having an ICE engine, transmission, fuel and exhaust systems, against having a pure EV.

Including an ICE may well push you into higher costs, both in terms of initial capital outlay and also running costs (adding an ICE means road tax, emissions testing etc). Bear in mind that including an ICE will increase running costs by £170 a year in road tax alone, as it's likely that the finished car will fall into the older car tax bracket. A pure EV would be free of road tax.

As your range requirement is reasonably modest and your choice of donor is light, reasonably aerodynamic and has a pretty good internal volume for its size, you may well be better putting the money set aside for the ICE etc into more batteries.

Jeremy

AndyDawson
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food for thought..

Postby AndyDawson » Wed Sep 10, 2008 5:31 pm

Qdos, Jeremy,

thanks for the responses.....food for thought

Qdos - what sort of breakdown of the £3K would you be thinking of? I'm trying to avoid Lion batteries - indeed, I thought the hybrid option would be likely to allow using lead-acids, due to a gentler charge/discharge cycle?

Jeremy,

I'm open to suggestions - what sort of horse power would I need in order to have a (say) 75 mph maximum speed - and hence thoughts re battery capacity?

(btw - are you the chap who masterminded the electric bike on Scrapheap challenge?)

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Jeremy
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Re: food for thought..

Postby Jeremy » Wed Sep 10, 2008 6:01 pm

AndyDawson wrote:I'm open to suggestions - what sort of horse power would I need in order to have a (say) 75 mph maximum speed - and hence thoughts re battery capacity?

(btw - are you the chap who masterminded the electric bike on Scrapheap challenge?)


75mph is pushing it a bit for a budget, direct drive, system, but 60mph + is probably a reasonably practical objective. The big current draw comes from acceleration and hill climbing, average current drawn from a pack will be much lower, even when cruising at road speeds. Generally speaking, there can easily be a 5:1 (or more) ratio between peak and average power demand for an ICE powered road car.

My guess is that a GTM at around normal weight would only need around 10 to 15hp to sustain 60mph on level ground with little wind. For comparison, the very aerodynamically clean, but much heavier, Toyota Prius only needs about 8hp to cruise at about 60mph. As electric motors are able to provide much greater torque at low speeds than an ICE, the ratio of maximum to average power can be reduced, 2:1 makes for a reasonable performance. There are lots of bigger, heavier, EVs driving around quite well on just 20 to 30hp.

High current alone doesn't kill lead acid batteries, they are pretty good at chucking out very high currents for short periods. The problem with lead acid is relatively short cycle life when continuously deep discharged, plus the loss of useful capacity that results from drawing high average current from them.

Jeremy

PS: I didn't help with the series 10 electric bikes show, but appeared in series 9 in the airboats show, was shortlisted for the US/UK/France powered flight centenary team and advised on the human powered flight show. My expertise is really just as a scientist and aircraft designer, rather than with EVs. Paul Compton (the other Scrapheap expert on here) is the EV guru.

AndyDawson
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Postby AndyDawson » Wed Sep 10, 2008 6:50 pm

"75mph is pushing it a bit for a budget, direct drive, system, but 60mph + is probably a reasonably practical objective."

you mention "direct drive" - would I be right to guess that the inclusion of a gearbox between the electric drive and the final drive would give a greater viable speed range? (since I've been assuming a directly linear relationship between supply voltage and motor speed).

At the sorts of torque/power rating we're discussing I'd have no qualms about utilising a motorbike gearbox. Accepting, of course that it adds complexity!

"My guess is that a GTM at around normal weight would only need around

10 to 15hp to sustain 60mph on level ground with little wind"

Thinking about the throttle openings needed, I'd concur.

"For comparison, the very aerodynamically clean, but much heavier, Toyota Prius only needs about 8hp to cruise at about 60mph"

And, now you mention it, although clean, the Prius has a rather larger frontal area.....I assume the 20-30 hp mentioned is nominal not peak?

"The problem with lead acid is relatively short cycle life when continuously deep discharged, plus the loss of useful capacity that results from drawing high average current from them."

Which implies that a reasonably conservative approach to sizing the batteries should ammeliorate that........albeit at the cost of some weight penalty. Perhaps the sensible approach would be to prototype on lead-acid, and invest in lithium ion when the basics are proven!

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EVguru
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Postby EVguru » Thu Sep 11, 2008 1:16 pm

AndyDawson wrote:
you mention "direct drive" - would I be right to guess that the inclusion of a gearbox between the electric drive and the final drive would give a greater viable speed range? (since I've been assuming a directly linear relationship between supply voltage and motor speed).


To put it simply, yes. Providing that is you have sufficient power (and some to spare) for your desired cruising speed.

With a permenant magnet brushed DC motor there is a more or less linear relationship between voltage and rpm with a variation between full and no load. With the common budget option of a series wound motor the no load rpm

At the sorts of torque/power rating we're discussing I'd have no qualms about utilising a motorbike gearbox. Accepting, of course that it adds complexity!


A motorcycle gearbox will be straight cut and constant mesh (dog selection) and really rather noisy with little other noise to mask it.

"The problem with lead acid is relatively short cycle life when continuously deep discharged, plus the loss of useful capacity that results from drawing high average current from them."

Which implies that a reasonably conservative approach to sizing the batteries should ammeliorate that........albeit at the cost of some weight penalty. Perhaps the sensible approach would be to prototype on lead-acid, and invest in lithium ion when the basics are proven!


You should aim to discharge lead acid not more than 50% on a regular basis.
Last edited by EVguru on Thu Sep 11, 2008 3:04 pm, edited 1 time in total.
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Jeremy
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Postby Jeremy » Thu Sep 11, 2008 3:02 pm

I agree with Paul about the bike box, they are pretty noisy and difficult to interface with as well.

Ideally a simple two speed gearbox would do the job, as few road going EVs need more than two gears, often quite widely spaced. I've wondered about using an external overdrive unit instead of a gearbox. The main snag with some of these would be whether the internal clutch could take the torque. I managed to break a couple of Laycock overdrive units years ago (fitted to a 3 litre Scimitar), in both the cone engagement clutch failed.

The nice thing about a Laycock type overdrive unit is that is quite compact and is often electrically operated, although it needs oil pressure to work. I suspect that the manual change Fairey unit (as fitted to some Landrovers) might be the most robust one readily available, although I don't know much about them.

Jeremy


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