Flywheels For Short-Term Power

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andylaurence
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Location: Bristol
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Postby andylaurence » Wed May 20, 2009 9:38 am

Jeremy wrote:Doubling the power of your Smart Roadster would increase the top speed by about 28mph, or about a 26% increase in top speed.

If that power increase was obtained by increasing the engine max rpm, without changing the rpm/torque characteristic, then the acceleration times would remain similar to those with the 80hp engine, as acceleration is only dependent on the usable torque available at the wheels.

In practice, this means that performance at lower than maximum speed wouldn't improve much, unless the gearing was also changed to increase the torque available.


You know what I mean! It's highly unlikely that a 160bhp Smart Roadster (if it did exist) would rev to 13000rpm (aside from those bike engined ones). Besides, if it did, I could do double the speed in each gear, so I'd keep in a lower gear, resulting in fewer gear changes and more wheel torque in each gear.

Jeremy wrote:The big advantages that electric power provides for your application is massive low speed torque, which will reduce your times by enhancing acceleration, and the ability to accurately control the torque available at the wheel, to avoid wheelspin and get maximum traction. The simple way to do this is to mimic the Prius controller (not it's traction control system). The Prius controller monitors the motor phase current in order to control the motor torque, which is an ideal way for you to get maximum traction without being accused of using a traction control system.


That is a good solution. Luckily, I don't have to worry about the legality traction control systems as they're not banned. Having said that, the complexity of making one would stop me. I remember from my days racing radio controlled cars as a kid that I used to have a clutch that slipped at a certain torque, minimising wheelspin. It became redundant when current-limiting electronic speed controllers became available. It worked very well, although you had to set the current limits each time you went on track to match the conditions, which obviously had an effect on battery life.

Jeremy wrote:Lithium batteries have a high energy density, but lead acid batteries are often better in terms of power density. Even cheap lead acid batteries are capable of delivery extremely high power levels for short periods, just the characteristic that you are looking for. I haven't looked closely at the current state of the art with lithium, but last time we debated this here (in the context of a hill climb competition car) we concluded that lead acid still had the edge over lithium.


I'll need to re-assess, I think. I've compiled a spreadsheet of about 30-40 batteries, but it's based on the continuous rating of the cells, which I suspect is biasing the lithium cells. Perhaps I should re-do the spredsheet using the 60s values.

Jeremy wrote:A bit of work to estimate the sort of maximum torque your chosen wheel/tyre combination will take before breaking loose, together with the sort of speed profiles that you're likely to be running, should allow the optimisation of the design to be as quick as is physically possible, apart from the vagaries of conditions and the drivers skill! In many ways, an electric drive system is ideal for this type of competition, because it allows so much control, relatively simply.


The reason I've gone down this route is because it seems so ideal for the events. I have done some planning on the power/torque required but it varies so much depending on the weight of the car. I'm hoping that the rolling chassis can get down to below 500kg. The Haynes Manual says the standard car isn't much over that but I think that weight is best described as optimistic! Still, I've removed 30kg with the fuel tank, I'd assume more than 100kg with the engine/gearbox and the exhaust weighed a few kilos too. Sadly, I didn't have my scales when I removed the exhaust and the engine/'box were too heavy to weigh. I guess I'll be in a better position to make calculations when the rolling chassis is ready and I can find some scales to put it on.

Cheers,
Andy
Image
Above figures include track days and the odd competition.

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Jeremy
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Postby Jeremy » Wed May 20, 2009 12:15 pm

You can get a lot of weight out of a Mini shell, but not without causing a few other problems. I removed all the glass, except the windscreen, and replaced it with Perspex, cut the centre of the door skins, roof panel and rear bulkhead out and replaced them with pop riveted in alloy sheet, replaced the bonnet and boot lid with fibreglass and stripped all the stuff like unused seats, carpets and masses of soundproofing guff out of the shell. The fuel tank was replaced with a small alloy one on a light frame where the boot floor used to be (I cut that out as well). The rear subframe was scrapped and replaced with a lightweight beam axle, bolted directly to the shell and the rear roll cage pickup points. I also went to the extreme of scraping off all the heavy underseal.............

The net result was a Cooper S shell that, even with the mandatory roll cage, was probably about 1/2 to 2/3rds of the weight of the original.

The only snag was getting the suspension rates to match the lighter weight, as the natural rising rate doughnuts were then far too stiff. The fix for this (after loads of playing about with Hi-Lo's etc) was to drill big holes in the rubber with an auger to soften them up a bit, a neat trick that I think Dave Vizard first came up with.

Jeremy

andylaurence
Posts: 108
Joined: Wed Apr 25, 2007 6:45 pm
Location: Bristol
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Postby andylaurence » Wed May 20, 2009 12:34 pm

Jeremy wrote:You can get a lot of weight out of a Mini shell, but not without causing a few other problems. I removed all the glass, except the windscreen, and replaced it with Perspex, cut the centre of the door skins, roof panel and rear bulkhead out and replaced them with pop riveted in alloy sheet, replaced the bonnet and boot lid with fibreglass and stripped all the stuff like unused seats, carpets and masses of soundproofing guff out of the shell. The fuel tank was replaced with a small alloy one on a light frame where the boot floor used to be (I cut that out as well), I also went to the extreme of scraping off all the heavy underseal.............

The net result was a Cooper S shell that, even with the mandatory roll cage, was probably about 1/2 to 2/3rds of the weight of the original.

The only snag was getting the suspension rates to match the lighter weight, as the natural rising rate doughnuts were then far too stiff. The fix for this (after loads of playing about with Hi-Lo's etc) was to drill big holes in the rubber with an auger to soften them up a bit, a neat trick that I think Dave Vizard first came up with.

Jeremy


Fantastic! So far, I've scraped the sound deadening off, remved all the interior, binned the bootlid and front and for fibreglass items and removed all the ICE-related stuff. I've also removed most of the loom. I've not touched the doors as I think they'll probably be replaced with a composite material at some point. The Perspex is also work in progress. As for the spring rates, I've seen actual springs that can replace the rubbers. I don't know whether you can get them in varying spring rates or not though.

Moving onto power sources, I've had a quick look at lead-acid and lithium packs.

The Odyssey PC1700 has a 5 second peak rating of 1700A. At 12v, that's 20.4Kw and with the 27.6kg it weighs, a specific power density of ~739w/kg. They're ~£13/Kw at £266.66.

The Elite 4800 has a peak rating of 168A. At 3.7v, that's 621w and with the tiny 140g weight, a specific power density of ~4440w/kg. They're ~$52/Kw at that price.

To me, that says the lithium cells have a significant power to weight advantage, but are over twice the price for a specific peak power. The negativity towards them from those of you far more experienced than I suggests that the figures may be misleading. I'm now very, very confused!

Cheers,
Andy
Image

Above figures include track days and the odd competition.

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

Postby MalcolmB » Wed May 20, 2009 1:26 pm

Interesting discussion! Just for information, the consensus on the Mini forum seems to be that the mini engine and gearbox with ancillaries and fluids weigh around 140kg.


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