Power/weight ratios

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Power/weight ratios

Postby hyve » Wed Aug 20, 2008 10:06 am

Since the "to clutch or not" thread ended up as a highly technical debate between a couple of experts, I thought I'd restate my question here. For those who've not followed the "clutch or not" thread, my query was: what level of motor output/ vehicle all up weight ratio is needed to permit one-speed, no clutch operation ?

Having been involved with vehicles for 50 years, with endless study and thought included, I'm not ignorant of the dynamics, just these electric vehicle questions.
So thanks for the lesson, Paul, but it doesn't really answer my question, which was posed as simply as possible for brevity. I could readily come up with an answer for an ICE, but electric motors are a very different animal. Full torque being available at zero revs means acceleration has to look quite unlike what I'd expect using an engine which may peak at 3000rpm, for torque.
Asking for an approximate kw figure assumed a fairly consistent relationship between kw and Nm, whatever the motor configuration, and also motors are usually described in terms of kw output, not Nm. I note, though the American tendency to describe motors in terms of diameter as a way of rating their usefulness ! This seems a bit vague to me unless using just one make.

If asked to build an ICE powered car of reasonable performance I'd suggest torque of at least 1Nm for every 10kg all up weight. Since electric motors behave so differently, to restate the question, what sort of figure would that translate to in their case ? Expressed in units which normally appear in their description, please.
Last edited by hyve on Wed Aug 20, 2008 1:18 pm, edited 1 time in total.
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Postby MalcolmB » Wed Aug 20, 2008 12:07 pm

The way I've done this is to look at the 'worst case' for my conversion. You originally asked about a single-ratio transmission, which is what I'm planning.

For a fixed ratio the worst case is pulling away from a standstill on a steep hill. This as you know also reflects the ability to accelerate.

According to the specs for my car (a mini) the smallest engine produces 45 ftlbs (60 Nm) of torque at around 2500 rpm. Obviously you would select first gear to set off when using the original engine, which gives an overall gear ratio of around 12:1 at the wheels. So maximum takeoff torque at the wheels would be 720 Nm, assuming that you gun the engine and slip the clutch.

To get the same sort of takeoff performance as a low-powered mini driven by a nutter I therefore need to gear my motor(s) to deliver similar torque at the wheels :D

Like I said, this is worst case. You also need batteries and a controller that are capable of delivering sufficient current to generate that torque, even if it is only for a few seconds. If you've chosen motor, controller and batteries on this basis then you should have ample power up to something like 75% of your maximum rpm, when torque starts dropping below a useful level.

I think...

PS. The link that Greg posted to Cedric's site is also useful, but you need to compare the torque figures of the Agni motor with whatever motor you're considering: http://www.agnimotors.com/home/index.ph ... &Itemid=43 See "Is the motor suitable for my vehicle"

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Postby qdos » Wed Aug 20, 2008 4:43 pm

Sorry for Paul's hijaking of the thread I was about to split it so that it could be put back on course to get your question back into the spotlight again but you seem to have done a good job of doing that yourself here :)

I think that a good thing to bear in mind is what sort of traffic and journeys is the EV being put to. If it's a town commuter then it may be that you'll only really need one gear. I think from talking to a number of people who have used EV conversions quite a lot that they tend to just use 2 gears and that again it depends on the journey as to which they use. if it's a town drive they use the lower gear and if it's a long journey they use the higher gear. It's not all that essential that you change gear in the way you would in an ICE vehicle.

Without a clutch it is likely that the life of the gearbox will be reduced simply due to the fact that it is more difficult to get a clean gear change. I do ocassionally make clutchless gear changes in my ICE car but they are seldom perfect and really it's not good for the car. If we were building the car right from the offset to be powered by an electric motor we'd use a completely different set up but this is rartely the case and what we have is a compromise. Of course you can also think about a different differential ratio too which we did with a sprint car whichwas good for 0 to 100mph all with no gear box. But then it's not supposed to be a commuter car

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Postby geekygrilli » Wed Aug 20, 2008 5:53 pm

Hi Hyve

My car will be fine with one gear - if the ratio was slightly higher than my current 2nd. It will accelerate well and top out at 40mph.
As it is it will do 32mph and its nippy off the mark.

Car weighs about 850 - 900kg, maybe more. Don't know what the motor output is... I think its 9kW at 48v, and I'm running at 96v, so does that make it 18kW? (Series wound)

If you are down this way you are more than welcome to a test drive (so long as I get a ride in your monster panda when its finished :wink: )
Last edited by geekygrilli on Wed Aug 20, 2008 7:09 pm, edited 1 time in total.

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Postby NickJ » Wed Aug 20, 2008 6:50 pm

It also depends on the type of motor, AC motors give better speed/torque results. Series wound DC give huge torque at low reves which tails off. Separately excited DC (as per berlingo) give a more controlable relationship. AC motors (within parameters) can give constant torque across a wide rev range. However AC motors and associated drive tend to be more expensive. The berlingo has only one speed, a 16kw ish motor (peaks at about 27kw I think) which is sep ex DC and can do about 60 mph You notice the non linear torque as it is very "nippy" up to 20-25mph but takes an age above 40. So to give a short empirical answer: Berlingo weighs 1500kg ish with the batteries, uses a 16kw nominal motor and can do 60 with a single gear. I dont know the torque of the berlingo motor off hand, but as said above DC motors at low revs will give loads of torque and as has been said elswhere in the thread it is usually the controllers/batteries which linit current and hence torque at stall.

I hope this is helpful!

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Postby MalcolmB » Thu Aug 21, 2008 8:17 am

Just hauled a few grey cells back from the brink of oblivion.

The simplest way to work out starting torque required at the wheels (in Nm):

M * g * r * sin a

M = mass of vehicle (kg)
g = acceleration due to gravity (9.8 m/s2)
r = radius of the driving wheel (m)
a = the slope of the road surface in degrees (to convert from a gradient, for example 1 in 10, calculate arctan of 0.1 = 5.7 degrees)

So the torque needed for a small car weighing 750 kg to pull away up a 1 in 3 hill (18.2 degrees) would be:
750 * 9.8 * 0.25 * sin 18 = 575 Nm (or 425 ft lbs)

1 in 3 is a pretty steep hill, but as Paul pointed out elsewhere, there are times such as climbing kerbs or ramps when you need that sort of torque, if only for a short time.

Using the torque at the wheel and knowing the maximum torque your motor produces you can calculate the maximum gear reduction you need. If you can also achieve your desired top speed with this same gear ratio then you should be fine with a single gear. If you need a different gear ratio to achieve your desired top speed then you obviously need more than one gear. Apologies if I'm teaching anyone to suck eggs here, but I'm just trying to get things straight for myself. :)

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Postby hyve » Thu Aug 21, 2008 9:18 am

Thanks for all the input, guys.
I think I've not quite made clear what I'm looking for here: a simple power:weight ratio figure which can be applied quickly as a rough guide.
For example, my Fiat VIN plate says 1150kg GVW and any conversion should bear GVW in mind. If I used a figure of 48kg per kw, which is what I derived from the Agni motor website suggested by Malcolm above, this results in a motor of just 24kw.
It doesn't matter what configuration the motor has, any more than it would using various ICE's. A diesel calls for taller gearing than a petrol; a 2-stroke more gears, perhaps. These are seperate factors to be decided next. The object here is just to get a ball-park figure to look for in the motor description.

In my example above I know that 24kw will not give what I call a sensible top speed. The car's 39kw petrol engine permits 80 mph, and as Paul has pointed out elsewhere, you need some kind of surplus over the actual requirement or acceleration becomes snail like as you approach your required cruise speed. Any head wind or gradient adds to the problem.
So I'm inclined to consign Agni motor's table to the "what you can (just) get away with" bin, even though it would appear from their figures that a 24 kw motor would pull my 1150kg away from rest with just one gear. The top speed of around 60mph which it would have to be geared to for their figures to work is not adequate, however. It may sound plenty until you remember that it will take a mile long flat with no head wind to get there from 50mph. Like most of the country, I'm not commuting from Chiswick into London. I need 60mph readily.

I expect I should do battle with the maths supplied by Malcolm, again and work it out. I'm suprised, though that nobody has ever done these sums already, since a quick guide to provide some sort of starting point has to be useful. I use little formulae like this all the time for similar reasons. Mild steel with a yield of around 200N/mm2, for example; letting me quickly estimate sizes of material to start with. It's not to get absolute, rock solid info, just a starting point. Design inevitably calls for compromises, juggling of parameters. Sums will need to be done over and over before seriously accurate figures are needed for final confirmation.
Here, we're only talking about a performance level which will vary according to personal preference in any case, so even less is a high degree of precision necessary.
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Postby hyve » Thu Aug 21, 2008 9:26 am

Christopher, you're very welcome to a ride here; the machine is now a runner, though I've yet to drive it any distance myself. Waiting for a fine sat. afternoon.
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Postby MalcolmB » Thu Aug 21, 2008 11:03 am

I think you need to be a bit more specific Peter.

Torque is what gets you up hills and gives you acceleration. Power simply determines top speed, i.e when you run out of torque. So what top speed do you want? If it's the same as for the original petrol version of your car then you'll need roughly the same power as the petrol engine could deliver. You just have to remember that electric motors are usually rated at a continuous power level, while petrol engines are rated at their peak. The peak power output of a series motor can be 5-10 times its continuous power rating.

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Postby GregsGarage » Thu Aug 21, 2008 11:15 am

qdos wrote:Without a clutch it is likely that the life of the gearbox will be reduced simply due to the fact that it is more difficult to get a clean gear change.

I don't completely agree. I do agree that each individual gearchange puts more wear on the gearbox syncros when changing gear without a clutch, but when my 11 mile drive only involves 1 or 2 gearchanges compared to probably 10 or 20 or more gearchanges in an ICE along with the wear on the clutch for the same journey I think it balances out pretty well. I would recommend using a high quality gearbox oil, probably synthetic based and think twice before going with a lighter grade than specified. A electric motor can deliver much more torque at slow speeds than an ICE and the gearboxes weren't designed with that in mind.
Greg Fordyce

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