4stroke/2stroke?

[Courageous T]

whats the difference?
50cc's are two stroke but arn't 2-stroke bigger bikes better?

[Gazdaman]

2 strokes aren't really used much in bigger capacity bikes anymore.

They used to be, the 750cc liquid cooled 'kettle'.

I think they're horribly polluting, expensive on oil, just there's more demand for large capacity 4 strokes.

Gaz

[haGGard]

I heard that after roughly 250cc, the stroke of a 2t will become to large and will become less efficient as they tend rev high
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[Smoto Bob]

LOL, its more that after 250cc (welll after 125 really) they tend to eat petrol and 2 smoke oil

Never worked out the mpg of my kx500 it was easier just to measure in seconds
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[Kickstart]

Hi

A 2 stroke produces power on every downward stroke of the piston, a 4 stroke only produces power on every other downward stroke of the engine.

Trouble is that a 2 stroke draws fuel and air into the crankcases then as the piston comes down the exhaust port opens and the burnt gases escape, followed by the transfer ports opening. As the exhaust port is still open (these ports are just holes in the side of the bore) some of the fresh air complete with unburnt fuel disappears straight out of the exhaust. For this reason they are have very dirty exhaust emissions.

There is a solution to this, using direct fuel injection. Basically draw the air in as normal but squirt the petrol in at high pressure once the exhaust port is closed again. The problem with this is injecting the fuel quickly enough for a high performance 2 stroke engine is not easy (there are 4 stroke direct injection engines, but then they only need to inject fuel half as often, plus being lower revving car engines have almost twice as long to inject it). Added to which lubricating such high pressure components is not easy (most diesels are direct injection, but diesel fuel is a reasonable lubricant while petrol is quite the opposite).

There have been a few direct injection 2 stroke. Aprilia make loads of direct injection 2 stroke scooters, but these are hardly high performance. Very clean running, very fuel efficient and use next to no 2 stroke oil. Bimota made the Vdue 500cc V twin 2 stroke with fuel injection but the system they developed was badly flawed (the bike landed up with a very sharp power band). An Australian company called Orbital hold loads of patents for direct injection 2 stroke engines and there was loads of interest from the car industry. However they were only getting engine lives of around 50000 miles which is fine for bikes but nowhere near long enough to be acceptable for the car industry. Honda have raced a direct injection 2 stroke off road bike, but they only touch 2 strokes when forced to so have not developed the idea further.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[r0b]

yeah the v-due was so bad bimota re-released it with cv carbs

and im guessing for a high performance 2t the optimum cylinder displacement is 125cc since all 2t gp bikes are (although this may be just a restriction of the rules?)


this might be usefull for people who dont understand, theres also some really cool stuff about rotary engines (wankel norton anyone ? )

https://science.howstuffworks.com/two-stroke.htm
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[Kickstart]

Hi

They rereleased them with normal Delloto slide carbs.

Theory as to why they had such a nasty power band was that they relied on the piston coming up to impact with the fuel being injected to attomise the fuel. Nice idea in a way, but only worked well when the piston happened to be in the correct place for this. With the timing of the injection this only happened over a fairly narrow rev range and the power tended to come in with one hell of a bang.

When they fitted carbs they lost 10hp and also landed up with a far dirtier running engine (the injected bike had no problems passing emissions rules round the world even without cats) which could not be sold in most markets.

The GP rules limited the number of cylinders in each class, which resulted in them all having 125cc cylinders. Up to the late 1980s the 125 class was limited to a maximun of 2 cylinders and the 125 twins were a hell of a lot quicker than the singles. The rule changes killed off a load of the smaller 125 race bike makers and pretty much made it a Honda only series for a few years until Aprilia got serious. The advantage of the single cylinder rules was that the bikes were cheaper, plus they pretty much only had to design one barrel for all 3 major classes.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Beef]

you can get alot more power out of a 2 stroke engine, iv got a RGV250 (v-twin) and its putting down 68bhp at the back wheel, they are alot easier to tune than a four stoke

[stevo as b4]

True i think 125cc is probably the optimum size for a 2 stroke cylinder. Its the cylinder capacity that has had the most development of all 2stroke's, and it seems to be capable of far higher power per litre figures than the larger capacity 2stroke cylinders can acheive. A 250cc MX engine works fairly well, but has a better midrange and smoother build up into the powerband than a 125, but its rpm and usable rev range is lower and it cannot make up for this with the better/nicely spread power delivery, so ultimate power suffers. The big capacity 2stroke engines, especially big singles have probelms with cylinder filling at higher rpms, and there is not enough time to fill a big cylinder with mixture at high speeds, so they cannot produce the same amount of power per litre as smaller engines can, but despite a comparitively narrow usable rev range, they have huge torque and midrange, much more on a 500cc MX bike than any 600cc or sometimes bigger 4stroke 4cylinder engine would acheive!

[stinkwheel]

Everyone seems to have a downer on 2-strokes these days. I like the way that they are very simple in terms of moving parts yet hugely complex in terms of design.

One of the main reasons they are dying a death is because the racing authorities have brought in big descrepancies in the allowable weights of racing 2-strokes as compared to the 4-strokes. This is deliberate because the companies want to concentrate their engine development on something that will pass the emissions regulations when they translate the technology into a road machine.

A 2-stroke moto GP bike would still be faster than a 4-stroke if they hadn't been activley descriminated against
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[Kickstart]

Hi

The shorter the stroke the higher you can rev it. The more cylinders you have then the smaller bores you can have for the same stroke.

Yes a 125cc cylinder works well, but nothing like as well as 2 62.5cc cylinders.

Check this thread where is is expecting a 200kmh (125mph) top speed from a 2005 Honda RS125. Back in 1987 PB had a 125 Frency and a GP 125 twin (think it was an MBA) managed 142mph. OK the RS is running lower gearing, but I wonder if it really could manage the extra 17mph, and that is with 18 years more development behind it.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Frost]

You can't run 62.5cc cylinders on a GP500 though, and thats what the maufacturers want to focus on. 4 cylinders is about optimum for a 500 so it makes sense to compromise the design of the smaller bike's for the benefit of development of the larger bikes.

Unfortunatly all the road going 2 strokes we have now are commuters, with the sports bikes being detuned versions of compromised race bikes.

The potential for 2 strokes is huge. If only a manufacturer would develop a modern 4 cylinder injection 250 then it would be a hell of a machine capable of embarrasing everything round a track. It could very well spark all the manufacturers to develop similar bikes in an effort to keep up.

Such a shame it'll never happen

Any more news on that RSV450? last i heard they canned it. Sad as it's about as close to a modern 2 stroke race bike we shall ever get, even if it is a V twin 4 stroke

[Kickstart]

[mchaggis]

Why can't we have normal carburetted fuel introduction, directly through the inlet side port without pissing about taking it through the crankcase, and have a wet sump and oil pump to feed the bearings? Just have some kind of plenum, supercharged or otherwise, and run it like a normal two stroke? You don't solve the unburnt HC emissions without conventional 4 stroke exhaust valves of course.

Feel free to shoot my idea full of holes, it's not really something I've given a lot of thought to.
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[Frost]

[Acid]

[simon1221]

More around the 30bhp mark, and thats after mods (exhaust, vforce3 etc etc)

I think the standard RS is about 28bhp, Mito is abou 28bhp, NSR 26-27bhp, TZR + 2T XTC + DT 22bhp

I think the RG is in the middle somewhere.

However that is before tuning and port polishing and god knows what else.
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DNA50 (sorry) ->> TZR125l >> GPZ500S

[Josh|RD125LC]

You've just got to think of the power of the Suzuki RG500, Yamaha RD500LC, Honda NS400, to believe that large capacity two strokes rule
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[binge]

[mchaggis]

Well, it needn't be a wet sump, just a pressurised oil feed for the main crank bearings and conrod bearings in the same fashion as four stroke. Two strokes are more powerful simply because they fire on every second stroke rather than on every fourth. That's mainly a function of the fuel/ air flow, in that they induct new charge in the same stroke as expelling spent charge.

In basic terms, one stroke does expansion, exhaust and induction, and the other compression. I'm just wondering if having the volume decrease under each piston is necessary to help push the gas into the cylinder above in co-ordination with the volume increase in the cylinder.

Was the V-Due directly injected, rather than injected before the inlet opening? I don't see why they can't inject it before the inlet or use carburettors. Of course, they'd lose some of the control you can have over an engine like that, some of the marketing spiel, but perhaps also a lot of cost and problems. A complete loss lubrication system always did seem a bit silly to me.
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[Courageous T]

wow, of all the essays

[Kickstart]

[mchaggis]

I thought those things might be problems. How does the V-Due get the charge into the cylinder? Is it still through the crankcase with total loss lubrication as normal, just with direct injection to avoid unburnt HC emissions?

Giant two strokes with proper valvetrains are really quite clever though.
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[Kickstart]