What does a 2 stroke power valve do?

[mikesaa309]

My mate has an RS 125. I'm not a 2 stroke person and nor is he in all fairness he's just one of these people who buy bikes because it's for one cheap and two thinks it's "cool". But he hardly goes anywhere except round town.

We some how got on to the conversation of a power valve and as I haven't a clue about 2 stroke engines other than it's stroke cycle and not having valves like a 4 stroke does as the piston is the valve, I looked up what a power valve does.

I found a video that wasn't overly informative that said that the power valve opens up the exhaust port. But then I thought well if it just opens up the port to allow more exhaust gas to get out of the cylinder then why not just remove it to have it constantly open?

Then I come to the conclusion that it's because when it's closed it allows for more power at lower RPM's then opens up to allow more power at the higher revs. I'm I correct in saying that or have I got the wrong end of the stick?

[Kickstart]

Hi

Simple terms it is variable valve timing for the exhaust side on a 2 stroke.

All the best

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

[Nobby the Bastard]

A two-stroke exhaust is optimised for a particular engine speed. You can either optimise it to make decent power at low engine revs or optimise it for high engine revs.

This is done by altering the volume of the exhaust and thus the frequency of the exhaust pulses that scavange the exhaust and help the gases to be expelled from the exhuast.

A power valve enables you to have two volumes, thus allowing optimisation at two eengine speeds.
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Triumph Sprint ST 1050

[pits]

If you get the electronic programmable ones then you can set how much extra bhp to put in as the power band kicks in, as it generates its own bhp. If you buy a red Viper 74C80N you can put up to 20 extra bhp in.
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[G]

[stevo as b4]

Yeah just think along the lines of four stroke variable cam timing, cam phasing, or variable valve lift, and you won't be too far out.

The 2stroke powervalve can be of many different designs, and not all of them increase exhaust port height and or time area, but alter exhaust system volume or a combination of 2 or all three in some cases.

I suppose you could liken it in a crude way to having variable diameter exhaust header pipes on a 4stroke engine, or an EXUP system even.

[Pete.]

This is how I understand it.

Your 2 stroke exhaust is designed to be most efficient at high revs. There's a trade-off involved in that to achieve this it's made of such a shape that the reflection from each power pulse arrives back at the exhaust port just as the next pulse is generated by the combustion of the next firing cycle. Now because each of these pulses travel at more or less the same speed regardless of revs but the frequency of combustion is a function of engine rpm, it stands to reason that there is going to be a small window of revs where it works great - the 'power band' but either side of this is a decreased efficiency as the time of the reflection is departed further from the generation of the next pulse. In simple terms the reflection has passed before the combustion happens.
A power valve is basically a restriction across the exhaust port that slows down the gases somewhat which broadens the width of the pulse to delay the arrival of part of the reflection so that it's better timed to the next pulse. At low revs the port is mostly closed but still ample for the small power demand, and as revs increase so the power valve opens more to adjust the timing of the reflected pulse to match the engine rpm, thus giving the best efficiency across a broad range of revs and increasing low and mid-range torque.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Pete.]

This is how I understand it.

Your 2 stroke exhaust is designed to be most efficient at high revs. There's a trade-off involved in that to achieve this it's made of such a shape that the reflection from each power pulse arrives back at the exhaust port just as the next pulse is generated by the combustion of the next firing cycle. Now because each of these pulses travel at more or less the same speed regardless of revs but the frequency of combustion is a function of engine rpm, it stands to reason that there is going to be a small window of revs where it works great - the 'power band' but either side of this is a decreased efficiency as the time of the reflection is departed further from the generation of the next pulse. In simple terms the reflection has passed before the combustion happens.
A power valve is basically a restriction across the exhaust port that slows down the gases somewhat which broadens the width of the pulse to delay the arrival of part of the reflection so that it's better timed to the next pulse. At low revs the port is mostly closed but still ample for the small power demand, and as revs increase so the power valve opens more to adjust the timing of the reflected pulse to match the engine rpm, thus giving the best efficiency across a broad range of revs and increasing low and mid-range torque.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Teflon-Mike]

https://upload.wikimedia.org/wikipedia/commons/thumb/5/52/Arbeitsweise_Zweitakt.gif/550px-Arbeitsweise_Zweitakt.gif
You have to understand about a thing called Kaaden effects, discovered by father of the Hi-Po 2T Walter Kaaden, chief engineer at MZ in the 1950's.
BASICALLY he 'invented' the expansion chamber exhaust.
Two strokes are 'ported' that is they dont have valves, flow of charge in and out of the cylinder is controlled by holes or 'ports' in the cyclinder wall.
In theory, a purely piston ported two-stroke shouldn't work.... it sucks charge into the crabk case when the piston rises, and SHOULD push it all back out, the same way, when it falls. Fact that it doesn't is down to reverse flow restriction, bigger transfer ports then inlet ports, making it easier for it to squash gas into the cylinder than back out the intake, and then a whole shed load of shit in the margins of physics; gas-flow-inertia and resonance effects being significant ones...
anyway, by some dint of curiouse and inconsistant anomolies, piston ported two-strokes DO manage to get charge into the crank case, and thence into the cyclinder... AND getting there at a time when a ruddy great hole is open in the side of the pot to let the exhaust gasses out, reasonably it all ought to just bigger off out the exhaust with it, and not stay in the pot, especially as the exhaust port has got to be open longer than the transfers, so as the piston starts coming up, its as likely to squash new charge out the exhaust port as it is to compress it into the combustion chamber, and even if it does trap anything in the pot... well, chances are its only going to be a part pot full, and as much old exhaust gas as it is fresh charge....
enter Walter Kaaden's genius... he looked at these problems in intricate and scientific detail, and what he discovered, long before the expansion chamber, was that you could grossely increase the two-strokes efficiency by 'valving' the intake, so it didn't rely on the symetrical port timing of the piston passing ports. By using a disc with a slot in it on the end of teh crank-shaft, he could open the inlet port allowing charge to be sucked into the engine, when the piston went up, but closed the port to keep it all there when the piston came back down again.... first problem solved.
Next; having got a chunk load more charge into the crank-case, he had to get it into the cylinder and stop it buggering off down the exhaust port. His solution to this was 'fountain porting'; basically using a flat or almost flat topped piston, and having two transfer ports arranged to blow almost directly at each other accross the top of the piston, you get two jets of fresh charge, that in the middle of the piston hit each other head on, like two crashing cars, and in teh words of Jazz and the plastic generation; 'the only way to go is up', you get a 'fountain' of fresh charge, jetting up the middle of the cylinder, avoiding too much mixing with the old charge; better still; if you have a hemispherical combustion chamber at the top, when the fountain reaches it, it hits that dome, spreads out and washes down the walls of the cylinder, pushing the old exhaust gas ahead of it, and washing it out the exhaust port, all with minimal turbulance and mixing contaminating old and new charge.
Chap was a genius.
Anyway; fresh charge could still bugger off down the exhaust port, AND better he got at getting gas into the cylinder, using more and more opposed ports, more charge he started loosing down the exhaust ports; especially the more 'advance' he put on the port timing to get the exhaust out easier to make way for the extra charge he was getting in.
Enter the expansion chamber; by designing the resonance volume of the expansion pipe, just right, at certain engine revs, or over a reletively narrow range of engine revs, he could get a 'reverse shock wave' timed to hit the exhaust port just before it closed.
Having let the fresh charge 'scavenge' the cylinder to get all the old exhaust out, this reverse shock wave can 'secondary scavenge' the exhaust and shove some of that 'lost' charge back into the pot before the port closes, hence get it back and doing something more useful, like make power.
All well and good, BUT, the culmination of the excersise was ever more radically 'ported' reed or disc valve two-strokes, making ever more power over an ever decreasing range of engine rpm, ever higher up the rev range.
Read up on some of the 1960's GP bikes, and you get five cylinder 125's (Particularly From Suzuki, who 'bought' all of Kaaden's know-how, getting his assistant and development rider, Ernst Degnar to defect to the West and work for them) with like 20 gears, that are almost 'unrideable', having essentially a fixed speed engine, that wont even idle beneath some stupidly high rpm, and makes power over perhaps 1000rpm rev range, leaving the rider to modulate power delivery almost entirely on the clutch and through teh gear-box.
OK... Yamaha... inovated the reed-valve independently of Kaaden's rotary disc. Same idea, but basically a simple flap valve or 'atmospheric inlet' between crank-case and carb. When piston rises, creating vacuum in the crank case, sucks the flap open; when piston starts to fall, creating pressure in the crank-case; flap is forced shut. simple, reliable and effective; its the same sort of valving early four-stroke engines often had... BUT suffers similar problems, mainly that its response lags behind the pressure, and is a restriction to flow until fully open; which is why fully mechanical cam operated valves, that like the disc valve, could be opened or closed in advance of when pressure changes happened, ready to start flowing charge, very quickly became the usual valving arrangement on four-strokes even in the pioneering era. However, the reed-valve, on the two-stroke DID have the advantage, that it tended not to be SO susceptable to making for such a peaky engine, and Yamaha realised that they were winning races with less power, as they had more tractable, more fogiving power delivery, than the Suzuki's.
This lead them, to stop chasing power and pushing Kaaden effects so far, and try and make more flexible engines.....

OK... the legend of the Elcie Boys... to get more power from a two-stroke... raise the ports... slap in an extra base gasket or two, and skim the shit out of the cylinder head to get the compression ratio back to something useful.

Lifting the barel a mm or 2 raises where the ports are in relation to the piston; and effectively advances all of the timing... its like a wilder cam in a four stroke. BUT, further you lift the ports; more 'peaky' the engine becomes.

Here lies the VTEC analogy. To make a more flexible engine, what you would like is the timing of a low exhaust port at low revs, but the timing of a high one at high ones.

And Yamaha discovered that they didn't actually need to physically change the hight of the port; using thier rotary drum with a slot in it, placed just near the port, they discovered it could mask enough of the port that it fooled the gas into behaving like a lower exhaust port.

Problem solved.... or at least for Yamaha..... they had a patent on it.

Consequently, when the Power-Valve Yammies were making big noises, the competition needed some way to do the same thing, without infringing on Yamaha's patents.

In paralell, Honda developed, I think it was thier 'ATAC' system, Kawasaki thier KIPS system, and Suzuki something even more convoluted that I seem to recall used three guilatine blaldes and a couple of drums! HOWEVER, essence of the challengers was that I think it was honda discovered that they could get a similer effect to a YPVS drum, through introducing low rpm 'turbulance' in the exhaust port mouth, opening and closing chambers off the side of the cylinder. That was the basis of the KIPS system as well, while Suzuki's wonderful system seemed to combine YPVS KIPS ATAC and that guilatine to actually change the exhaust port height.

Yamaha's system, though, is probably the most elegant and simple to effect, and in later evolutions, I believe was made to ustilise side chamber resonance as well.

BUT, its doing two things. First it is providing a pseudo variable valve timing mechanism to the exhaust port, but ALSO its changing the resonance of the exhaust port and expansion chamber, so that you dont loose that shock wave secondary scavenging effect.

Clever stuff, hugh?
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[Teflon-Mike]

Double-Post
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[pits]

[ws4936]

[stevo as b4]

Fook me Mike. The poor guy didn't ask to know the history of the modern and performance 2stroke engine, just what is a 2stroke power valve and how do they work.

You are aware I presume that Honda used at least 3 designs, Kawasaki have used way more than that and so did Suzuki. The Yamaha System is very simplified in it's first generation, compared to later systems.

You will find that all modern MX engines will use:

1, Variable height main exhaust port. ( also changes the compression ratio) low speed high compression and high rev's a lower compression ratio)

2, Additional auxiliary or boost ports with often different timings,

3, Resonance chamber's to add or reduce exhaust system volume.

Then there are some racing engines like the Honda NSR500 where they use water injection and power valves to manage and manipulate the heat of the exhaust gasses to make the exhaust system work as they desire at different engine speeds.

[expat200]

Ah but there's nothing quite so addictive as a HIGHLY tuned pre-powervalve two stroke coming on song.

Our old race spec X7's: usuable power from 8500 to 10500 and nothing elsewhere, wouldn't even tickover at less than 3000.
Bit of a laugh in the wet if you hit the powerband when you peeled in for a bend. Combined with hinged frame, bendy forks and skinny tyres.
Always riding with two fingers on the clutch ready for the inevitable seize when the rings/piston/little end/big end/main bearings/gearbox let go.

I saw one on the dyno with 47rwhp, more than double the original power.

...and dont even start on kwak triples...

[Fladdem]

Teflon Mike never fails to impress me with his posts.

I understand a bit about two strokes, but designing an expansion chamber,to me, is an art form. The chap from MZ was a genius, I remember writing an assignment about two stroke engines at college, and then being told off because we were meant to write about four strokes, despite the assignment brief saying to pick any engine type, I could have wrote about a Stirling engine if I'd wanted to.

I think pre powervalve 2 strokes are the most fun as well, my dads old X5 was a hoot, and he said his GT380,the flexi flyer, was really vicious after his mate played with the ports and he got a set of pipes on it. I wish I was around then, he paid £50 for that bike.
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Current:1991 Honda MT50 (Soon to be a H100/MTX/MT5 hybrid), 1976 Honda Cub C70, 2005 Honda Varadero 125, 1993 Yamaha TTR250 Open Enduro , 2010 Road Legal Stomp YX140, 1994 Honda CRM 250 MK III, 1999 Cagiva Mito 125, 1992 Honda CB400 Super Four, Stomp T4 230, 1984 Honda H100s, 2009 Sym XS125K
Past:2003 Aprilia RS125, 1982 Kawasaki GPZ550(FREE BIKE!)
I'm having more fun than a well-oiled midget.

[_Iain_]

From what I read about two stroke porting, when you raise the exhaust port it shifts the peak power higher up in the rev range.

Powervalve means you can raise the height of the exhaust port depending on engine speed.

If it ran with the powervalve wide open all the time it'd be very peaky and lack midrange power.

If it was shut, the midrange would be peachy but the engine would trail off way before the redline & not make as much power.

This way, when you've got out of the midrange and up into the top end, you don't need the port optimised for midrange grunt, so the valve opens and optimises the port for top end peak power.

That said, i'm a potato.
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Please be aware that the above post may be full of complete nonsense.
Riding: '07 KTM Duke II, Baotian BT49QT-20 Driving: '88 Volvo 340

[CG Sam]

[Kickstart]

[Enduro Numpty]

As a lifelong lover of 2 strokes it's great to read so many well informed replies

[m3-paul]

I loved riding the pre power valve 2Ts but I do enjoy the power valve versions more. They are just that bit more rideable.

Of course they are not blessed with loads of torque but it is nice to not have to wring the neck of the thing to get the performance. The YPVS on my tzr250 just makes the bike that bit more rideable.

You can really feel the power valve working as the exhaust port height changes as the revs rise. The best way to describe it to someone that has never ridden a power valve equipped 2T is like an old school turbo engined car. You get a bit of power at first but there is the lag, then it speeds up a bit as the turbo starts to spool, then as the turbo fully spools up you get the sudden shove and the engine revs out. Bloody addictive.

It is also interesting how different pipes change the way the power comes in.

My bike ran its original pipes which are by design decent expansion chambers and are regarded as great performers for standard factory items. The power came in at around 6500 and you really needed to change up at 9500 as there was little point trying to get any more out of it.

With my aftermarket expansion chambers (OX Racing) the power comes in at 7000 but will go all the way to 10500/11000 with ease if you want to. It also feels more punchy, definitely is more eager to rev and sounds bloody epic.

My bike runs the standard airbox and I have gone from the standard 230 mains to 250 as a precaution.
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Yamaha TZR250 - love the smell of TTS!!!!
CBR900RR Fireblade - so much fun

[Pete.]

[stevo as b4]

[Blau Zedong]

I prefer 2T's without riding aids

One of the things I enjoy on a 2T is wheelies, and I find a powervalve setup smooths the power out too much, whereas on something like my YR5 I can use the violent step in power to bounce the front end into the air easily.
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[stevo as b4]

I don't think anyone with a pulse would honestly say they don't like old school 2stroke power delivery more. I agree with Blue as I remember riding DT175's TY175's and old non powervalved Yamaha YZ125's, all were big fun!

I suppose the only attraction of a well set up modern 2stroke sports bike is that they can sound quiet like a std scooter, docile and smooth when ridden gently around town, but at higher rpm you still get the shrill rasp of a high performance 2stroke bike and the rush of power. I suppose an example would be something like an Aprilia RS125?

Then again, you miss out on the coughing, crackling and 4stroking of old school tuned stroker's as they hunt and lurch around at the point of just before coming onto the main jet.

Funny thing is that despite later stroker's arguably being easier to ride and faster, people want that early raw style machine more, and that's probably why RD350LC prices are and will always be far higher than RD350F1/F2's? etc

[stinkwheel]