Advice on Hybrid Kit Car

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timpootle
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Postby timpootle » Fri Oct 03, 2008 4:46 pm

Hi Malcolm

Can you explain why 60Ah of LiPO4 gives more range than 100Ah of PbA?

Is it purely a weight thing? Or real-life capacity versus data-plate figures?
Tim Crumpton

MalcolmB
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Postby MalcolmB » Fri Oct 03, 2008 7:14 pm

Hi Tim
I suspect you already know the answer but it's a good question so here goes :wink:
There are two main factors. The first is what's known as the Peukert effect, which effectively means that the faster you draw current from a battery the less real-life capacity you get from it. This effect is much more pronounced in lead acid batteries than it is in lithium ion. In practice it means that a lead acid battery that's rated at 100 Ah over a period of 20 hours will only deliver around 60 Ah when discharged over a period of one hour, which is the sort of timescale we're interested in for an EV. In contrast most lithium ion batteries will give over 90 per cent of their rated capacity when discharged in an hour.

The second factor is that the deeper you discharge a lead acid battery the shorter its useful life will be. If you actually take 60 Ah out of a 100 Ah battery regularly you're only likely to get a few hundred cycles out of it. This means that you need to give a good margin of spare capacity when sizing a lead acid pack, otherwise your batteries won't last very long.
You get a similar effect with lithium ion, but again it's much less pronounced, and the best cells are reported to give over 2000 cycles even when they're deeply discharged on a regular basis.

The overall effect is that you need a considerably smaller pack, in terms of Ah rating, to go a given distance with lithium than with lead, and that's without taking into account the extra weight of lead.

By the way, I don't have any shares in lithium, but I'm very tempted
:)

Chris King
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Postby Chris King » Fri Oct 03, 2008 7:57 pm

qdos/Malcolm(b)
Thanks for all this. You have given me some home work to do!!!

I am still set on the 4 wheeled Pembleton (Brooklands) as this has a space behind the seat (one builder has put in Dicky seat - albeit for children). The generator package, like the one seen in Malcolm's van includes - fuel tank, frame, exhaust system, control panel, battery, lots sound proofing, even more frame, lots of cooling vents and a wheeled carriage. If you take this lot away the actual diesel engine (see www.directequipmentsales.co.uk and follow the heading Diesel engines) and the generator will fit in the rear of the Brooklands.

I am going to see the maker of the frame next week to measure up and make sure. When I work out how you put photographs on this Forum site I will be able to show you in more detail. Thanks for your help.

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Jeremy
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Postby Jeremy » Sat Oct 04, 2008 9:30 am

Those small diesel engines are certainly fairly light and cheap, so I would think they'd work well.

One way of building a cheap and light on-board charger might be to just couple one of these engines to a big alternator from a commercial vehicle. It's fairly easy to modify an ordinary vehicle alternator to get voltages up to around 100 - 120V without rewinding any coils, all that's needed is to remove the built in regulator and rectifier and replace it with one that will work at a higher voltage. It's quite feasible to get a few kW from a vehicle alternator in this way, without overheating, as the stator coil current remains within the original spec.

The key to getting the speed you want will be to reduce the aerodynamic drag. Some cars can get the power required to do around 60mph down to about 10hp or less, but they have a much lower overall drag coefficient than this particular kit. My guess is that, despite it's low frontal area and low rolling resistance from the narrow tyres, this car is going to need around 15hp to 20hp to cruise at 60 - 70mph, so the 5kW (6.7hp) from the generator is only going to contribute about 30 to 40% of the power requirement at best, the rest will need to come from the battery pack.

Overall it's a good concept though, as the generator should be able to run at a fairly high load all the time (so increasing efficiency). Average power requirement can be calculated fairly easily for a conventional vehicle, if you know the fuel consumption. As an example, take a small diesel car that averages around 60mpg with a specific fuel consumption of around 200g/kWh (pretty typical for a modern diesel), and an average speed of 40mph (pretty typical for mixed road usage).

One hours driving would consume an average of 40/60 of a gallon, or about about 3 litres. Diesel has a density of about 0.85kg/l, so 3 litres weighs about 2.55kg. If the engine SFC was 200g/kWh, then the engine will have delivered an average power of 12.75kW, or about 17hp.

To make your small hybrid independent of any external charging requirement you are going to have to reduce the average power requirement a fair bit. For a 5kW average power requirement you will need to aim at making the car efficient enough to run at around 150mpg, in effect, for a 40mph average speed. As power requirement (and fuel consumption) is more or less directly proportional to the cube of speed, a higher average speed will very significantly increase the demand on the system.

Jeremy

Chris King
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Postby Chris King » Sat Oct 04, 2008 7:37 pm

Jeremy

Just when I though I had some idea of what I was going to do!!!!!!

Thanks for that - as I was wondering how the car would perform on just the 5kw generator alone. My homework this week will now have to be gone over again. The good news in this matter - is that I am not out for speed - but for a steady cruise with the ability to 70mph if necessary. My 'Cunning Baldrick Plan' is to see if the principle works in this - my first car build. If it works ok - then I will then be more confident to swap for better batteries and improved power etc.

As the Pembleton is made from bits and pieces - indeed the Pembleton Forum encourages this - I have a fiendish design to improve the aerodynamic shape - the top of an old aluminium Barbecue to improve the slab/brick shape. But not sure if your formula can be applied to this!

Best wishes - Chris

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Jeremy
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Postby Jeremy » Sat Oct 04, 2008 7:58 pm

When it comes to aerodynamics it's worth noting that the rear has more effect than the front. The three wheel version, with a tapered tail, would give a significant drag reduction and reduce rolling resistance by a fair amount.

I'm sure you can do the sums, but for a small diesel car that does 60mpg at an average speed of 30mph, the average power drops to 11.35kW, a worthwhile saving. I'd hazard a guess that a reasonably efficient three wheel car could get the average power requirement down to around 7 or 8kW and still retain "normal" small car performance.

Jeremy

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ChrisB
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Postby ChrisB » Sun Oct 05, 2008 10:41 am

Chris King wrote:The generator package, like the one seen in Malcolm's van includes - fuel tank, frame, exhaust system, control panel, battery, lots sound proofing, even more frame, lots of cooling vents and a wheeled carriage. If you take this lot away the actual diesel engine (see www.directequipmentsales.co.uk and follow the heading Diesel engines) and the generator will fit in the rear of the Brooklands.

I am going to see the maker of the frame next week to measure up and make sure. When I work out how you put photographs on this Forum site I will be able to show you in more detail. Thanks for your help.


That would be my van :wink:

Chris, have you actually run your engine up in the open frame :? they are really quite nasty and noisey units :cry: I only bear mine cos its in all that casing, yes it adds a fair few KG but it does mean that you can atleast stand at the back of the van and just about hold a conversation, the open frame ones are deafening.

directequipmentsales is exactly where I got mine from 8) nice chap and they do spares, although do be careful as the spares are not true spares as in the sence of brand new, generally they are items removed from blown up/dead units.
On that subject do remember what your buying, these generators are your normal standard Chinese copies of branded generators but of course 1/4 or less the price, so remember you can "sometimes" end up with something that is 1/4 of the quality.
Certainly I wouldnt want to run mine at its max output for long periods, also change the oil and clean the filter after 10hrs or less, you'll be surprised at how much metal it collects if its a new unit :shock:, mines had two oil changes now, one at 10hr and another @ 40hrs and the 2nd one was a lot cleaner, I think they just take less consideration when cleaning the blocks out from the milling and hence a lot of the time the engines dont last as long.

ChrisB
I reject reality and substitute my own !!!!!!

Chris King
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Postby Chris King » Sun Oct 05, 2008 3:26 pm

Chris(B) and Jeremy

Sorry Chris - I was getting confused and realised as soon as I sent it that it was your van. I got my diesel generator second hand for £25, took all the bits off and ran the skeleton in the frame to make sure it worked. I intend to place this in the rear in an enclosed sound proofed and sectioned off partion where the Dickey seat would be. The exhaust in the usual place. I would like to see your unit and how you have connected things up. If you are going to any venues within reasonable travel distance of Kingston, Surrey - please let me know by email.

Jeremy - I will let you know the views of the designer when I see him mid week and asked numerous questions (Even more now since you joined in!!!!)

Best wishes - Chris(K)

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qdos
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Postby qdos » Sun Oct 05, 2008 10:11 pm

Basically what we're saying is that to get 70mph you'll struggle with anyting less than 15 Kw or 20 Hp

As Jeremy and I have mentioned, with such low power on tap aerodynamics are a very big factor and anything you can do to improve this will be noticed on the top speed Trust me I've had a lot of fun playing round with it even just lowering my seat made a 5-10 kmh difference I'd also run with a tonneau over the passenger seat area as this will make a big difference to the drag I run my car all the time with the tonneau closed as much as I can I've even managed to zip myself in entirely with just my head outside though the legalities of this I would not stand and discuss with a Police officer as I expect it's a tad on the naughty side as I can't see the speedo etc though I've got a secondary set of instruments I'll fit on the other side of the tonneau which should address that

It's all good fun playing around with this kind of stuff though and if you're remotely inclined to be inquisitive then have a go :)

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