Two-stroke performance tuning?

[bucketracer]

I'm not too sure that I would subscribe to an expansion chamber being a "supercharger"........

A supercharger is essentially a compressor that forces air into an engine......An expansion chamber doesn't do this.

Are you referring to the expansion chamber "evacuating" the cylinder to below atmospheric pressure, assisting the positive pressure from the crankcase below, thereby increasing the delivery ratio?

Because that's not a supercharger.

An expansion chamber which has been properly matched to an engine will effectively supercharge it.

This is done by pressing extra mixture back in through the exhaust port with the force of a returning pressure wave in a resonant exhaust system.

How this happens is explained very well by:- http://scooter.wikia.com/wiki/Expansion_Chamber

Pic-1 The positive pressure pulse leaves the exhaust port As the mixture is combusted inside the cylinder, the piston is forced downward and opens the exhaust port. The force of the exhaust pressure leaving the cylinder creates a positive pressure pulse as it moves out into the expansion chamber.

The time between exhaust port opening and and transfer port opening is called "exhaust lead". Exhaust lead allows the high-pressure exhaust gases in the cylinder to blow-down (reducing pressure) before the transfer port opens. If this doesn't happen, exhaust particles may back-flow into the crankcase and contaminate and heat the incoming fuel/air charge, thereby robbing the engine of power.

Pic-2 Negative pressure waves The inertia of the out flowing exhaust particles out into the divergent code creates negative pressure waves and a strong partial vacuum (about minus 7psi) near the still opening exhaust port shortly after the transfer port has opened. In addition to the sucking out tail-end exhaust gases, these suctions cause fresh air/fuel mixture to be sucked through the transfer port into the combustion chamber.

Pic-3 Positive pressure waves The remaining energy in the negative pressure wave continues through the pipe and is reflected off the convergent cone at the rear of the unit and returns to the engines exhaust port. During this point, the negative pressure will actually end up pulling extra fresh gas and air into the header of the expansion chamber.

Pic-4 Supercharging through the exhaust port Finally just as the transfer ports inside the cylinder are closing, the returning positive pressure wave compresses the extra fuel and air mixture back into the cylinder through the exhaust port. The cylinder now contains the extra mixture for the next combustion--effectively supercharging the engine and increasing efficiency and performance.

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[SS90]

I thought that the chamber also helped by sucking fresh charge into the chamber then forcing it back into the cyliner using the presure wave from the exaust bouncing of the rear cone of the chamber ???

well I guess I was wrong

Hmmm,

Sort of.

from what you write, I am taking it that you see that the expansion chamber "deliberatly" draws fuel in from the combustion chamber into the chamber, them "pushes it back in" when it is needed.

That's how I interpret your post anyway.

Because the fuel sucked into the chamber is not a deliberate thing at all........ It's incredibly wasteful, and engineers spend millions of dollars trying to stop that very problem.

If it was beneficial, fuel injected two strokes would have been on the market 20 years ago.

The fact is, because the fuel gets into the chamber (and only a percentage manages to get forced back in) is the reason for such poor emissions in the first place.
While it is quite easy to get lost in terminology, in a simplistic form (which is better) you sort of grasp the concept.

But, that's not a supercharger either.

This particular "effect" you can take advantage of, certainly does not "super charge" a two stroke engine.

Essentially, (trying not to "overcomplicate" things) The negative pulse waves (caused by the expansion chambers diffuser, the length and angle of which dictate the amount of pressure),along with the inertia of the exhaust gasses, "excavate" (suck I guess would be the best description) the cylinder (which is obviously something closer to atmospheric) to assist the positive pressure in the crankcase to get the air/fuel mixture into the cylinder.

Basically, the design of the diffuser (if it is designed correctly that is) significantly assists the pressurised crankcase in getting well compressed fuel into the combustion chamber.

Obviously, the exhaust port is open, and fresh good air/fuel mixture makes it's way into the expansion chamber (sometimes called "over scavenging")

As this good air/fuel mixture (importantly, along with the energy is traveling towards the pipe outlet, it must past through the convergent cone, causing a positive wave to return to the cylinder, pushing (some) of the "over scavenged" fuel back into the cylinder.

I too have seen people refer to this as "supercharging", but like I say I don't subscribe to that....... I see it as simply returning "overscavenged" burnable fuel to the cylinder.

It is for this very reason that companies like "Orbital" (just as one example) spend so much money and time developing systems that clear the cylinder of all burned gasses (injecting oxygen from the atmosphere is one way), BEFORE injecting the air/fuel mixture (AFTER the exhaust port is closed............... There by negating the effect of the returning pressure wave............

[bucketracer]

This particular "effect" you can take advantage of, certainly does not "super charge" a two stroke engine.

the convergent cone, causing a positive wave to return to the cylinder, pushing (some) of the "over scavenged" fuel back into the cylinder.

I too have seen people refer to this as "supercharging", but like I say I don't subscribe to that....... I see it as simply returning "overscavenged" burnable fuel to the cylinder.

However you squeeze extra air or air/fuel mixture into an engine above what it can normally aspirate for its self is supercharging.

You might not call this supercharging but the rest of the world does.

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[SS90]

[QUOTE=bucketracer;1129314085].


An expansion chamber which has been properly matched to an engine will effectively supercharge it.

This is done by pressing extra mixture back in through the exhaust port with the force of a returning pressure wave in a resonant exhaust system.

How this happens is explained very well by:- http://scooter.wikia.com/wiki/Expansion_Chamber
QUOTE]
Hmmm,

Well, in my experience, not everything written on a single web page can be considered correct.

You are of course entitled to your opinion.

Like I say above, if it indeed an expansion chamber "super charges", a cylinder, then why has so much effort been expended to negate the need for the over scavenged fuel to be returned to the cylinder? (the "super charge" effect)............... the only reason expansion chambers are designed the way they are is to best compromise on the unique requirements of the intake/exhaust cycle.

Basically, the fuel that is returned to the cylinder (you say "supercharged") via the returning pressure wave is only there in the first place because it simply flows out the open exhaust port..........

With a system like Orbital's designs, they inject the fuel AFTER the exhaust is closed.

The expansion chamber design is somewhat different to what you would have for a carbed bike (though not dramatically, like I first expected)......... because you still want to take SOME advantage of the positive pressure.....but it is at a different time, and for a different duration than "normal"......because there is only compressed air to "escape" out the open exhaust port.

Personally, I don't agree with the writer of that particular "page" when it comes to the description of "supercharging".

I feel it is an important thing to note.

[SS90]

You might not call this supercharging but the rest of the world does. I guess they are wrong.

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The "rest of the world" does not consist of a web page by an unknown author.

[bucketracer]

from what you write, I am taking it that you see that the expansion chamber "deliberatly" draws fuel in from the combustion chamber into the chamber, them "pushes it back in" when it is needed.

That's how I interpret your post anyway.

Because the fuel sucked into the chamber is not a deliberate thing at all........ It's incredibly wasteful, and engineers spend millions of dollars trying to stop that very problem.

Sucking extra air/fuel mixture into the exhaust and then ramming as much of it back as you can is most certainly a deliberat thing, its where the 2-stroke gets its power.

The pity is that it all does not get pushed back and this is where the 2-stroke gets its dirty reputation.

This is where DI Direct fuel Injection helps, as the fuel is injected into the cylinder after the supercharging effect has taken place. With DI no fuel is lost as its injected after the exhaust port has closed.

DI is a big advance, TerraRoot posted something about it on the ESE thread a couple of days ago.

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[bucketracer]

The "rest of the world" does not consist of a web page by an unknown author.

Well, in my experience, not everything written on a single web page can be considered correct.

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You are right about that.

A quick Google will turn up many descriptions of the 2-stroke/expansion chamber supercharging effect.

I do have more respect for opinion that is backed up by references, don't you.

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[SS90]

Sucking extra air/fuel mixture into the exhaust and then ramming as much of it back as you can is most certainly a deliberat thing, its where the 2-stroke gets its power.



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No it is not.

The fact the fuel makes it into the chamber is certainly NOT deliberate.

Why do you think that so much effort has been spend on Scavenge patterns?

The whole "scavenge Pattern" concept is to reduce the amount of fuel that gets out the exhaust port.

That's why a two stroke is called a "schürle loop" (closed loop).........

The whole idea is to keep as much fuel in the cylinder as possible, it looses alot of it's charge density when it travels from the crankcase, to the cylinder, out the exhaust port, into the chamber, then back into the cylinder..........

I don't agree that a two stroke "get's it's power" by returning overscavenged fuel into the cylinder.........

A two stroke that loses less fuel out the exhaust makes more power than one that looses more.

Regardless of how much of that overscavenged fuel is returned to the cylinder.

Like I say, Supercharging is something quite different.

[B.Crump]

i have put a expantion chamber on my one and iv just done a top end rebuld and it goes very well port matched .i couldnt get that far in to smooth all the surfaces is there a spectial tool i can get to get right in the motor?

but does ant one know if anyone has supercharged a two stroke bike? i kow people have turbo charged two strokes i have a book on it (old one)

[bucketracer]

No it is not.

The fact the fuel makes it into the chamber is certainly NOT deliberate.

When you quote me, please be accurate. I said air/fuel, you changed it to fuel. air/fuel mixture and fuel is two separate things.

Old School which is where we are now, is to suck air/fuel into the exhaust and ram it back.

New School DI is to suck air only into the exhaust and ram it back and inject the fuel after the exhaust port closes thereby saving fuel and cleaning up the exhaust.

Both still use the resonant forces in the exhaust system to supercharge the motor.

As I said before, however you squeeze extra air or air/fuel mixture into an engine above what it can normally aspirate for its self is supercharging and is most certainly deliberate in a 2-Stroke.

If you don't think so please backup you opinion with some authoritative references.

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[F5 Dave]

Kaaden would be turning in his grave. Why don't you continue to use a megaphone pipe if the divergent systems are all important? The reason 2 strokes are particularly dirty is that the pipe is tuned over a very short rev range, the rest of the time there is mondo waste. An efficient variable gearbox bound to a constant speed engine may be a worthwhile improvement.

As said before, even though it isn't mechanically driven & working on the intake; if it is pressurising the engine more than what it could normally aspirate it is by definition supercharging it.

Irrespective to how much effort has gone into 'preventing' it, for a start the Schnurle(sp) loop engine was invented before the chamber, but still served & now serves a purpose. Better purity & less loss esp when pipe is out of tuned range. It delays the mixture in the cylinder so that while more is being sucked in (& rammed in with inertia) then in the blowdown period as much as poss is crammed into the cylinder & in fact what couldn't be contained is then pushed back in at the last mo. (vision of Monty Python's Mr Creosote )

By the same token air intakes pressurise a sealed airbox, that is supercharging. Both albeit to a minor level compared to a blower. & often still showing a negative pressure compared to suction pressure.

[SS90]

When you quote me, please be accurate. I said air/fuel, you changed it to fuel. air/fuel mixture and fuel is two seperate things.

Old School which is where we are now, is to suck air/fuel into the exhaust and ram it back.

New School DI is to suck air only into the exhaust and ram it back and inject the fuel after the exhaust port closes therby saving fuel and cleaning up the exhaust.

Both still use the resonant forces in the exhaust system to supercharge the motor.

As I said before, however you squeeze extra air or air/fuel mixture into an engine above what it can normally aspirate for its self is supercharging.

A "supercharger" is forced induction.

Ergo, an expansion chamber, cannot "force induce" an engine.

It can, however, increase the delivery ratio.

That is an important difference.

I'm not arguing over semantics here.

An expansion chamber helps to return over scavenged air/fuel into the cylinder.

That's not supercharging.

Maybe you could "Wikipedia" supercharging.

[F5 Dave]

Now you are quoting the internet. Just because it is most common doesn't mean it is the only way to 'overfill' the cylinder.

[SS90]

Kaaden would be turning in his grave.

As said before, even though it isn't mechanically driven & working on the intake; if it is pressurising the engine more than what it could normally aspirate it is by definition supercharging it.

Irrespective to how much effort has gone into 'preventing' it, for a start the Schnurle(sp) loop engine was invented before the chamber, but still served & now serves a purpose. Better purity & less loss esp when pipe is out of tuned range. It delays the mixture in the cylinder so that while more is being sucked in (& rammed in with inertia) then in the blowdown period as much as poss is crammed into the cylinder & in fact what couldn't be contained is then pushed back in at the last mo. (vision of Monty Python's Mr Creosote )

By the same token air intakes pressurise a sealed airbox, that is supercharging. Both albeit to a minor level compared to a blower. & often still showing a negative pressure compared to suction pressure.

Again, it is easy to get lost in terminology, but for example, the crankcase (at a certain point) is indeed a pump.

However, when the extraction effect of the chamber comes into play, the crankcase is no longer a pump.

But, when the chamber is not assisting the delivery of air/fuel to the cylinder, the crankcase is a pump.

So, is the crankcase a pump, or not?

I say YES.

(EDIT)
But it is only a pump when the expansion chamber is not providing "extraction" effect.

SOME people say that the expansion chamber "supercharges".

HOW?

The only reason the crankcase delivery ratio increases, is because the negative pressure (vacume) goes through the transfer ports, and is trapped in the cases when the transfers close.

I don't see that as "supercharging"

[SS90]

Now you are quoting the internet. Just because it is most common doesn't mean it is the only way to 'overfill' the cylinder.

Not quite. I am trying to demonstrate the importance of separating the different phases of a two stroke engine, so we can all understand expansion chamber design.

As you well know, it's not "black magic"

I don't see how increasing the delivery ratio of a two stroke is "supercharging", when you take the crankcase into account.