OzVMX Forum
Clubroom => Tech Talk => Topic started by: Husky500evo on December 04, 2008, 03:43:17 pm
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(http://img.villagephotos.com/p/2007-4/1252864/SuperchargedXR80.jpg)
I scanned an article from an old Dirtbike magazine which features an XR80 top end on a KX80 bottom end & uses the idea of a crankcase supercharger setup. The crankcase supercharger is basically the same idea that was used on a Maico 4 stroke prototype a few years earlier . I have posted an article on the Maico prototype in the Maico section . I don't really know why this idea never took off , but I read that the Maico had problems because the inlet tract was too long & that it got hot very quickly & lost power .
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I think the problem with this idea is that the crankcase volume is the same as the volume above the piston so it's not really supercharging it with any extra.
It may have some scavenging benefits by having some pressure buildup for when the valve opens, but they would be minimal.
I recall this article & seem to remember this project gathering dust in a corner of his workshop when I visited Dave's workshop in 85
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It never took off because it won't fly. Where exactly is the 'supercharging' coming from??? A decending piston creates very little in the way of positive pressure in the cranckcase. A falicy which seems to be what that all seems to be based on. Can't enlarge the text either anyway of doing that
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Thanks Lozza and GMC that is exactly what I was pondering on the above. It just does not actually do anything.
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It might work as a multi cylinder.
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The Norton Wolf two stroke tried something similar didn't it. It had a piston with two bores, the lower larger bore was to act as additional pumping.
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Same as the PUCH 125 ?
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I wonder if the Date on that article is April 1 ?
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(http://)
I am still learning to drive my new scanner . I have tried a different way of posting the article , but it appears a bit blurred when magnified . I also have to scan the last page . Any tips on making it more clear .
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to get the words one would need a scanner with OCR
Optical Character Recognition.
many have that these days .
It transforms what we see as a picture of words or even hand writing into actual text.
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From memory, CCM tried something similar back in the 70's. They called it the "Five Stroke" project. I think it used the decending piston to create more pressure fed back up through transfer ports similar to a 2 smoke. It retained the inlet and exhaust valves.
Must dig out the book.
It is all a bit acedemic now as modern fuel injection and electronics have made it obsolete.
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Crank speed may be doing all the work rather than piston .
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Crankcase supercharging works, there is a group from Melbourne that have the patent on a type of this design and have engines up and running and are currently marketing it to manufactures. It is mainly to be used where traditional 2 strokes would be used as it has a lot less emissions but similar power/weight of a 2 stroke www.hhed.com.au
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;) You may have something there, oldfart. In the late sixties Smokey Yunick sealed the bellhousing on his NASCAR and ran plumbing up to the airbox over the carby. At top speed the flywheel gave about 2 psi of Supercharge.
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keeping constant pressure i imagine is the biggest drama, you have the crankcase displacement being realtively linear to acceleration but the engines air requirement is not a constant....it all works much better if you hold things at a constant speed ::).
I work for turbocharger manufacturer, we are currently up to almost 5 atmospheres, so 70 odd psi of charge air pressure on large engines at relatively constant pressure.
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About 10-15 years ago a bike and bloke from the Gippsland area was featured in ADB with outragous claims of improved performance of the order of 50% from his KTM 250 two stroke. There were no great details given but I remember guessing that he may have put vanes on the flywheel to increase the bottom end pressure.
Never heard anything further ::) :P.
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Well guess there is a seized KTM250 somewhere in Gippsland waiting to be discovered ;D.
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Yes Mark I have that Dirt Bike article too. The poms were experimenting w this before CCM. Somebody was doing it on an old 500 Ariel before that.
Seems to me you could potentially get some supercharging effect cos you get 2 bites of the cherry - 2 pumps of the crankcase to one inlet stroke in the top-end. In theory at least.
But the friction/throttling losses thro'out the system would kill it at any reasonable rpm on a single cylinder design I would think. In a spark ign engine, throttling losses are one of the major causes of inefficiency (compared to compression ign engines), & in this engine, you not only have a long inlet tract (on an OHV/OHC engine), but the inlet charge has to pass thro 2 reed valves, a crancase & a poppet valve.
Volumetric efficiency wouldn't be that great in the end I wouldn't think. My guess is that whatever supercharging effect you achieved would be negated in losses thro the inlet system at higher rpm. There maybe some gains at low rpm. (Perhaps it might suit a trials bike application) I'd think there'd be noticeable lag on throttle response w that loong inlet too.
But the engine on the hhed website (mentioned above) is quite ingenious. Running as an opposed twin w 180deg crank overcomes some of the probs (eg less reedvalves needed) & the side-valve configuration makes a much more compact design w shorter inlet tract. But sidevalve combustion chambers are inherently thermodynamically & volumetrically inefficient (compared to OHV/OVC ones) & I'm more than a little skeptical of their claim to 37% thermal efficiency. (There is an asterisk beside that claim but I don't see any footnote explaining the asterisk). Not sure the supercharging effect would be that great either in that design but there should be some. No question over the quality of engineers behind the project tho.
The Norton Wasp engine mentioned above was different altogether. It was a stepped piston 2 stroke design which doesn't use the crancase, & you can get almost as much supercharging as you want w that design. The poms were quite hopeful of that design & as I recall it was Bernard Hooper developing it - an engineer w lots of runs on the board in Brit m'cycle design/industry. However it seems to me the pumping losses in that design (esp in single cyl variants) would almost kill it's potential benefits too, tho someone may yet prove me wrong on that.
I remember an article in one of the engineering journals back in the 70s about a radial 4cyl stepped piston engine of about 1600cc being developed in Melbourne for use in cars. Never heard anything more about it since. Obviously it died too.