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Thread: The ULTIMATE 2-stroke bike engine

  1. #1
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    The ULTIMATE 2-stroke bike engine

    OK - this isn't intended as a thread about a bucket engine but "Buckets" seems a good place for it considering who regularly posts here.

    I've been thinking what the ultimate 2T engine would be unencumbered by any rules and restrictions. Huge marine diesels are very efficient but don't actually make much power considering their size. An RS125 makes lots of power but efficiency is down. There are direct injected engines in outboards, scooters and snowmobiles. But given a few very basic criteria like it has to fit in a bike and has to run on petrol, what would the engine be.

    My thoughts were possibly a twin, parallel, tandem, or probably V. About 500cc, forced induction, 4 valve heads solely for exhaust, fuel injected directly into the intake passages. The intake passages would be straight and of a tuned length, or not, directed into the cylinder at the optimum angle and fed from a pressurised annulus. The cylinders would be phased to fire 180deg apart to even out the load on the supercharger and the demands on the intake plenum.

    Transmission wise I don't see why more bikes don't use a CVT style transmission. Not a basic scooter version but something like that used in high performance snowmobiles. A 4-stroke style plain bearing bottom end could be used but then there would be issues with piston length and sealing intakes to crankcase.

    I'm just curious what other people think would be the engine configuration given very few rules. I'm trying to think of something for TZ to get into before he gets too old that would be interesting and push boundaries a bit.

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    Quote Originally Posted by speedpro View Post
    Transmission wise I don't see why more bikes don't use a CVT style transmission. Not a basic scooter version but something like that used in high performance snowmobiles. A 4-stroke style plain bearing bottom end could be used but then there would be issues with piston length and sealing intakes to crankcase.
    You would have to ride it very differently from a normal transmission. The breaks would have to be something special to ride it fast. I remember racing an auto scooter you had to use the rear break a lot to turn it in. I would go for a conventional transmission. Would be good to see a mad project on the go.

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    Quote Originally Posted by speedpro View Post
    OK - this isn't intended as a thread about a bucket engine but "Buckets" seems a good place for it considering who regularly posts here.

    I've been thinking what the ultimate 2T engine would be unencumbered by any rules and restrictions. Huge marine diesels are very efficient but don't actually make much power considering their size. An RS125 makes lots of power but efficiency is down. There are direct injected engines in outboards, scooters and snowmobiles. But given a few very basic criteria like it has to fit in a bike and has to run on petrol, what would the engine be.

    My thoughts were possibly a twin, parallel, tandem, or probably V. About 500cc, forced induction, 4 valve heads solely for exhaust, fuel injected directly into the intake passages. The intake passages would be straight and of a tuned length, or not, directed into the cylinder at the optimum angle and fed from a pressurised annulus. The cylinders would be phased to fire 180deg apart to even out the load on the supercharger and the demands on the intake plenum.

    Transmission wise I don't see why more bikes don't use a CVT style transmission. Not a basic scooter version but something like that used in high performance snowmobiles. A 4-stroke style plain bearing bottom end could be used but then there would be issues with piston length and sealing intakes to crankcase.

    I'm just curious what other people think would be the engine configuration given very few rules. I'm trying to think of something for TZ to get into before he gets too old that would be interesting and push boundaries a bit.
    sounds like a jetski or snowmobile


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    Quote Originally Posted by speedpro View Post
    An RS125 makes lots of power but efficiency is down.
    RS125 For the win! It depends what you mean by effciency, if you mean power to displacement efficiency then the RS is great!
    If you mean fuel efficiency, your'll be surpised. My RS gets roughly 10kms/litre not actually that bad. (Based on 300ml's per lap at manfield)

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    Quote Originally Posted by quallman1234 View Post
    RS125 For the win! It depends what you mean by effciency, if you mean power to displacement efficiency then the RS is great!
    If you mean fuel efficiency, your'll be surpised. My RS gets roughly 10kms/litre not actually that bad. (Based on 300ml's per lap at manfield)
    Nah bro - you'd spend all race day working on it
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    CVT - sorry but you need every gear you have on a 2 stroke. Its to do with the fact that all the good ones have the most nonlinear power curves on the planet. The only way I can see CVT working is on a track like NASCAR.

    As for the engine. I really like a lumpy single smoker. Something special about that sound and the way you ride it. Feels more real, like if you get it wrong the whole engine stops.
    But one day I want to chuck a Yamaha 2t outboard in a bike frame just for fun.
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    Quote Originally Posted by richban View Post
    Would be good to see a mad project on the go.
    That's what I'm thinking, not putting a blower on an existing engine and just making it "more". Jetskis and snowmobiles are conventional 2-strokes and all very normal though they certainly have the goods. I'm asking if anyone knows if a supercharged 2-stroke built solely as a supercharged engine, not a modified normally aspirated engine, would be better than what are admittedly good modern conventional designs. Development of supercharged 2T engines in racing was killed off before they really got sorted. With modern designs and materials would/could they be better in any way. Turbo or supercharged 4T engines have advantages and I'm thinking a purpose built 2T forced induction engine would also have advantages. But what form would it take to most benefit from the supercharging? If you could head off in the efficiency direction of the big marine diesels but in a smaller high revving package you'd have to be on a winner.

    As for CVTs, how many gears do you need? A CVT has infinitely variable ratios. With careful tuning of weights and ramps they will match most engines nicely.

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    Quote Originally Posted by avgas View Post
    CVT - sorry but you need every gear you have on a 2 stroke. Its to do with the fact that all the good ones have the most nonlinear power curves on the planet. The only way I can see CVT working is on a track like NASCAR.
    Snow mobiles do very nicely with CVT transmissions.

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    Such a machine may be eligible for land speed records.

    I've also had much thoughts about the 4 valve head used for exhaust and the annular entry inlet ports. Maybe one could use a massive turbo housing wrapped around a barrel. (on a single or maybe a V-twin.)
    I believe such designs have been used and are very efficient in their application but I'm not sure how they would work at the revs that would probably be needed to make high power levels per cc. (if that's the goal)
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    Quote Originally Posted by speedpro View Post
    Development of supercharged 2T engines in racing was killed off before they really got sorted. With modern designs and materials ...... I'm thinking a purpose built 2T forced induction engine would also have advantages. As for CVTs, how many gears do you need? A CVT has infinitely variable ratios. With careful tuning of weights and ramps they will match most engines nicely.
    Supercharging, CVT, and the Orbital fuel injection and engine management system could be would be worth having.

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    A large turbo compressor housing fitted around a barrel is an idea but the airflow "around" the cylinder would probably continue through the ports to inside the cylinder. That might actually be a good thing depending on how the scavenge flow worked. I was thinking more of the ports and ducts being like a conventional setup but the ducts would be straight, possibly of a tuned length, and fed from a plenum which extended around the cylinder. The plenum being pressurised. Fuel would be injected only after the exhaust valves closed. Looking from above the cylinder the port ducts would resemble fat spokes of a wheel and the annular plenum would be the tyre.

    If a 4T type bottom end was used an oil control ring would be needed at the base of the piston. With either bottom end, lubrication of the piston would be a problem with the mixture entering the cylinder directly through the port ducts. I must have a look at an OCP engine.

    I type too slow

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    The OCP has a total loss lubrication system and looks to inject oil immediately before the reed valves which feed the crankcase. The crankshaft seems to have been drawn with a boltup big end on the rod. Apart from the fuel injection and the very good oil injection system the OCP engine looks remarkably "normal".

    Maybe just a small amount of regular 2T oil injected into the mains and left to lubricate the cylinder like normal would be OK. There wouldn't be the flow of air through the crankcase on my idea of a forced induction 2T engine so it wouldn't need the quantities of oil like a standard high performance engine. With a common crankcase for a twin firing every 180degrees any volume displaced by the downward travelling piston would be made available by the other piston which would be travelling up so there would be little crankcase pressure fluctuation, only air and oil mist moving back and forth. A flat twin with a single crankpin would be simple, apart from trying to smooth it out.

    Maybe a twin crank, 90degree v-twin, with a common crankcase volume. My head hurts even trying to visualise the forces to balance.

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    Quote Originally Posted by speedpro View Post
    A large turbo compressor housing fitted around a barrel is an idea but the airflow "around" the cylinder would probably continue through the ports to inside the cylinder. That might actually be a good thing depending on how the scavenge flow worked. I was thinking more of the ports and ducts being like a conventional setup but the ducts would be straight, possibly of a tuned length, and fed from a plenum which extended around the cylinder. The plenum being pressurised. Fuel would be injected only after the exhaust valves closed. Looking from above the cylinder the port ducts would resemble fat spokes of a wheel and the annular plenum would be the tyre.

    If a 4T type bottom end was used an oil control ring would be needed at the base of the piston. With either bottom end, lubrication of the piston would be a problem with the mixture entering the cylinder directly through the port ducts. I must have a look at an OCP engine.

    I type too slow
    Sounds very similar in concept to a Detroit Diesel engine as produced by the yanks.
    Supercharger feeding the crankcase around the liners which have a spiral port arrangement about halfway along their length ( somewhere near the bottom of the stroke if memory serves )
    Flat head with 4 Ex valves and injector in the centre of the valves .Combustion chamber in the piston crown.

    Yes it is a 2T and they sound like they are screaming at 2500 rpm -- beautiful !!

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    You are describing GM (Detroit), Nissan UD diesels, among others. Commer TS-3 and Napier Deltic use (used) some of your ideas but are more interesting due to opposed piston design (Commer TS-3 sound is beautiful)
    it's not a bad thing till you throw a KLR into the mix.
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    Quote Originally Posted by jasonu View Post
    Snow mobiles do very nicely with CVT transmissions.
    Yeah they do. But when you snap the throttle off they slow down fast due to heaps of drag on the show. Bikes will keep on rolling. And like I said you would need some very modular rear breaking to turn it in like a conventional gear boxed bike. Especially if it was a 200hp 500. Rolling into a corner at 260 with just that rear break to get it turned would be interesting.

    CVT would be great if building a drag bike.

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