Two-stroke performance tuning?

[gav]

With all that braking going on, maybe time to develop a KERS system for the bucket?

[Ooky]

Note havn't read last 3 pages turned in to a cock size comp so i skipped ahead

The exhaust uses the pressure waves created by the gasses exiting from the combustion chamber into the exhaust port. These waves have the caratteristics of reflecting as a negative wave (suction) if they meet an open end of a tube. On the other way if they encounter an closed end of a tube they reflect as a positive wave (stuffing). The waves move indipendently from the movement of the gas in which they travel. In comparsion to the gas flowing in the exhaust, they have a much higher speed. Similar to waves on the water; the water is still but the waves travel with an certain speed.

The main parts of an exhaust are; the header, the diffuser, the belly, the baffle cone, the tailpipe and the silencer. The diffuser is the “open end” of the pipe; it creates the suction wave. The baffle cone is the “closed end” of the pipe; it reflects the pressure wave, which does supercharge the cylinder. The steepness of the cones defines the strenght and duration of the reflected waves. The header and the belly section act as distancers, this way the needed timing of the waves can be tuned.

The next important thing is the tailpipe. The purpose of the tail pipe is to create and maintain the proper pressure and temperature inside the exhaust. It is very important to select the right diameter of the tailpipe; to big will result in poor performance, to small will result in a melted piston.

The silencer, as his name says, is needed to silence the sound. The most common type of silencer used has a perforated tube in the middle with glasswool around. The good side of this kind of silencer is that it efficiently reduces the sound level while creating little obstruction for the escaping gasses.

The phases
(Note; arrows show the gasses and their direction, the arcs show the pressure waves)


After the ignition the gasses in the combustion chamber begin to expand and move the piston towards the BDC.


The piston uncovers the exhaust port. The burnt gasses begin to escape from the combustion chamber at a high speed, this generates a pressure wave, which begin to travel into the header.


At this point the phase of scavenging in the cylinder with fresh mixture begins. The pressure wave reaches the diffuser where it reflects as a suction wave.


The suction wave has reached the cylinder where it helps to pull out spent gasses and fill the chamber with fresh mixture. A part of the fresh mixture can escape into the exhaust. At this time the rest of the pressure wave that was not reflected as a suction wave reaches the baffle cone, where it reflects as a pressure wave


At this point the piston has closed the transfers, but the exhaust port is still open. The pressure wave reflected by the baffle cone reaches the cylinder and stuffs back the fresh mixture escaped into the exhaust. This is the way that a slight supercharge is created in twostroke engines.


Unluckyly the twostroke expansion chamber works only in a limited rpm range. The engine produces the maximum torque when the exhaust pulses enter in resonance with the opening of the cylinder ports. When the rpm are under the resonance of the exhaust, the suction wave reaches the cylinder too early, the transfers aren’t open so the suction wave can’t help suck out spent gasses and fill fresh mixture into the cylinder. On the other end when the rpm are too high the transfers are already closed by the piston moving upwards, so the suction wave is too late.

The rpm at which the pipe will be in resonance depend on the length of it and the temperature of the gasses. With a temperature change also changes the speed at which the waves travel. The higher the temperature, the higher is the speed at which the waves diffund. The higher is the wave speed, the higher are the rpm at which the engine will make power. We can see sometimes at races when the climate is cold, pipes are stripped with insulation that provide the optimal temperature.

Characteristics

The geometry of the pipe dictates the “character”. The length dictates the rpm at which the pipe enters in resonance. In a short pipe, in comparsion to a long one, the waves have to travel a shorter distance, this means that a short pipe will enter in resonance with the piston movement at a higher crank rotation speed. In short words; a long pipe will make power at low rpm while an short one will make power at high rpm.

The conicity of the diffuser and baffle cone determinate the length and amplitude of single waves reflected back to the cylinder. The higher the cone angle is, the shorter but with an greater amplitude the reflected wave will be. So an pipe with higher angle of the cones will make higher power, but in a narrower rpm range. Compared to a pipe with lower cone angles, an bike with such pipe will be more “nervous” to ride, the transmittion will be harder to tune up. It is in base to the type of engine and riding that the pipe has to be designed for. On a scooter with an engine with automatic variable transmission, the rpm range needed is narrower than the one for a bike with manualshift transmission, so pipes with higher cone angles can be used.
This graph shows the difference of shape of the power band on pipes with different angle of the cones.

[Skunk]

< snippet >The main parts of an exhaust are; the header, the diffuser, the belly, the baffle cone, the tailpipe and the silencer. The diffuser is the “open end” of the pipe; it creates the suction wave. The baffle cone is the “closed end” of the pipe; it reflects the pressure wave, which does supercharge the cylinder. The steepness of the cones defines the strenght and duration of the reflected waves. The header and the belly section act as distancers, this way the needed timing of the waves can be tuned.

Very informational post. Thank you.

Original web page is here

[Buckets4Me]

thanks for that

explains what i thought was happening

and as a side note

when I had my bike up on the dyno
the hotter it got the more HP it made
measured the pipe at around 360 oC at the belly?? middle between cones

19hp not to bad I thought
time to get it back and tune it up a bit as have only played with timing and jetting at the track
also I have a few spare pipes to try

and I may just take a streight piece with me to see what difference a chamber and a non chamber make :P

[koba]

has any one supercharged a 2 stroke?

Wow! Look at what you started!

Does that deserve a KB reward for 'Most inflammatory innocent question'?

[bucketracer]

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The fact is that Teezee didn't post anything about scavenge patterns, because in the ESE thread, he was unaware of the concept.

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The fact is that SS90 didn't post anything about other expansion chamber functions, before in the 2-stroke tuning thread, because SS was unaware of them.

Untill TZ350 pointed it out to him. Post 339 and 426

Anything you read on here now from SS90 about how an expansion chamber is more than a supercharger is because of TZ's tip to him.

So SS90, its easy to drop a vague hint and then try and claim all the credit, any one could do it.

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[koba]

Well, Teezee, perhaps you could "gently encourage" your "entourage" not to make such personal attacks on me, and, have open discussion on such things, rather than "your wrong" (and then not proving me wrong)

I'm not part of anyones "entourage" but I still agree with Sketchys naughty post. That judgement of mine is based on your posts, not anyone elses.

[Skunk]

I simply give as good as I get Teezee.

The first stones where thrown from your "camp".

I'm in my "camp". Your "informational" posts make me respond like I do. I have only met TZ350 twice so I can hardly be part of his "entourage".

[Buckets4Me]

Wow! Look at what you started!

Does that deserve a KB reward for 'Most inflammatory innocent question'?

and can we get one for the person that writes the most words without saying anything usefull ?

[Skunk]

Here's something useful: http://www.bimotion.se/Theory/Theory.html

[bucketracer]

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Mine's supercharged - it has a chamber.
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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.
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A "supercharger" is forced induction.

Ergo, an expansion chamber, cannot "force induce" an engine.
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Because if we use that definition, "supercharging" is ANYTHING that forces air into an engine, and CLEARLY an expansion chamber does not do that.
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all the attention would be on increasing the (I believe non-exsistent) "supercharging" effect of the chamber.
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I have already conceded there is a "supercharging effect" caused by the expansion chamber (actually I have never claimed there wasn't)
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SS90 you say you never claimed there wasn't a "supercharging effect", well here is a bit of history from the supercharging dance.

And SS90, Skunk didn't claim his expansion chamber was a supercharger either.

So how ever did you manage to make such a drama out of his reference to the "supercharging effect".

You could have just left it at my earlier post.

I am sorry your wrong again. Supercharging is not just forced induction at the inlet. Its forced induction per see.

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.

And from the Oxford Dictionary

supercharger
• noun a device that increases the pressure of the fuel-air mixture in an internal-combustion engine, thereby giving greater efficiency.

http://www.askoxford.com/concise_oed...harger?view=uk

The expansion chamber most certainly can supercharge a 2-Stroke.

Please back your opinion with references otherwise its just your opinion.

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So we got past semantics and have established the supercharging effect of the expansion chamber.

Even better would be a mechanical supercharger at the inlet side and direct fuel injection. Then we could have a real fuel efficent supercharged 2-Stroke. I guess with the developments in DI its looking possible.

The problem with early attempts at supercharging 2-Strokes was their thirst as the Schnurle loop scavinging system was unable to stop a lot of the fuel mixture being lost out the exhaust and expansion chambers as we know them, with their plugging effect had not yet been developed.

Early Supercharged DKW http://www.motorbase.com/auctionlot/by-id/1706703174/

Now, thats what I call a "real" supercharged 2-Stroke and I suspect so does SS90 and with some modern thinking and maybe with a turbo as the supercharger you could have a real weapon.
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[TZ350]

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Bucketracer, I know a sideshow easily distracts SS90 and I could quote him and try and embarrass him too, but if you’re interested in what he has to say, don’t.

SS90

My understanding is that the loop scavenge system was first patented in 1924 by Dr E Schnurle and the original patent drawings clearly show the transfer/scavenge streams flowing in, up, over, and out the exhaust port. Carefully aimed transfer ports create this scavenging flow pattern.

But I am not sure where the term "closed loop" you use comes from.

My understanding is:-

The scavenging flow pattern serves three purposes. 1, to drive the remnants of the exhaust gas from the cylinder. 2, to minimize mixing of the incoming fuel mixture with the outgoing exhaust gas as this mixing dilutes the incoming fresh charge. And 3, preventing as much as possible, short circuiting of the incoming fresh charge directly out the exhaust port.

I think the expansion chamber serves three major functions. 1, on its negative pressure suction pulse, an extraction effect of the exhaust from the cylinder and assisting the transfer of the fresh mixture. This suction pulse can travel from the exhaust port to the carburetor mouth.

2, on its positive pulse there is a slight supercharging effect but its main purpose is a plugging effect that assists the scavenge pattern trap as much fresh charge as possible in the cylinder.

And 3, there is an elevated pressure within the expansion chamber set by the bleed down stinger. This internal working pressure and the slight supercharging effect ensure the final pressure of the fresh mixture trapped within the cylinder as the exhaust port just closes is higher than it would have been.

So we have Scavenge Pattern, Suck to Extract and assist Transfer, Blow to Block, Working Pressure, Slight Supercharging Effect to create higher cylinder pressure.

Brought about by transfer port shapes and angles, port time areas, and expansion chamber tuned length cone lengths and angles, with each playing their own part.

I am not married to these ideas or my understanding of them and am interested in anything you can tell us about the function and subtitles of the various elements that make up the 2-Stroke system.

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[SS90]

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My understanding is:-

The scavenging flow pattern serves three purposes. 1, to drive the remnants of the exhaust gas from the cylinder. 2, to minimize mixing of the incoming fuel mixture with the outgoing exhaust gas as this mixing dilutes the incoming fresh charge. And 3, preventing as much as possible, short circuiting of the incoming fresh charge directly out the exhaust port.

I think the expansion chamber serves three major functions. 1, on its negative pressure suction pulse, an extraction effect of the exhaust from the cylinder and assisting the transfer of the fresh mixture. This suction pulse can travel from the exhaust port to the carburetor mouth.

2, on its positive pulse there is a slight supercharging effect but its main purpose is a plugging effect that assists the scavenge pattern trap as much fresh charge as possible in the cylinder.

And 3, there is an elevated pressure within the expansion chamber set by the bleed down stinger. This internal working pressure and the slight supercharging effect ensure the final pressure of the fresh mixture trapped within the cylinder as the exhaust port just closes is higher than it would have been.

So we have Scavenge Pattern, Suck to Extract and assist Transfer, Blow to Block, Working Pressure, Slight Supercharging Effect to create higher cylinder pressure.

Brought about by transfer port shapes and angles, port time areas, and expansion chamber tuned length cone lengths and angles, with each playing their own part.

I am not married to these ideas or my understanding of them and am interested in anything you can tell us about the function and subtitles of the various elements that make up the 2-Stroke system.

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I guess, that would about cover the basic aspects.

As you say there is a "Supercharging effect" (not meaning a supercharged cylinder) and yes, it operates as such.

Dr Schnürle (yes, I have read about 2/3rds of his work) described (in the later tests) what would translate as "Turning back flow" when observing the air fuel mixture in relation to the "swirling pattern" formed inside the cylinder, and the fresh charge.

I.E, he noticed that there was a vast improvement in the amount of good air/fuel mixture that stayed in the cylinder, rather then following the exhaust gasses out.

This is why I consider that the EASIEST way to describe the Schnürle loop system is "keeping as much of the air/fuel mixture in the cylinder"

How it actually does that, (the separation of burnt gasses etc) is a little more complicated.

The BEST way to see this is by building a test rig as described by Gordon Jennings.

To actually see (and describe what you are seeing) makes a big difference to your understanding and comprehension of what aspects to pay attention to.

Something else he noted (again, in later work) is that while the system "reduces short circuiting", there is no real way to 100% stop the "short circuiting" completely........ and, the air/fuel mixture that FIRST enters the cylinder is the MOST LIKELY to short circuit.

This is VERY VERY important to understand.

He surmised that if you could perfect the "schnürle loop", (i.e, no short circuiting at all) you would essentially have a "closed loop system" (as far as the fresh air/fuel mixture was concerned.)

This finding was (in my opinion) the most important thing (other than the actual "schnürle loop" it'self).

Bearing in mind, that the original system comprised of only 2 transfers,(and obviously no "expansion chamber" this would indicate (and subsequent recreated tests of my own backed it up) that any improvements on the "schnürle loop" system come from REDUCING THE LIKELYHOOD of "short circuiting" earlier in the cycle.

If you can achieve this, then combined with an expansion chambers ability to return the (now reduced quantity of combustible air/fuel) you are starting to make more power, and as such, your expansion chamber design needs to alter in order to take advantage of this.

This is the primary reason why (like F5dave said) "a new pipe does not really work on an old cylinder design"

In the 40's many many companies (particularly German,but a few French companies where on the ball as well) worked very hard on this very thing (in fact, in 1941 a solution was found) quiet simple, and essentially the "orbital" engines (first designed in the 70's) are simply a development of this concept.

They operate (remember 1941) simply by "sacrificing" externally introduced compressed air early in the cycle, so that, as the FIRST air to enter the cylinder through the transfers is THE MOST LIKELY to "short circuit", this "sacrificial air" heads out the exhaust port, rather than the fresh air/fuel mixture.

This system works really well (you can already get "orbital" equipped engines, which, as I say are simply extensions of this early work.

There are a few examples of these engines here in Europe, and all from the late 30's and early 40's.

It is a little "clumsy" simply "masking" a scavenge pattern fault, but particularly in the case of single transfers, it is the best solution.

Forgoing the complications of such setups, you will notice that no manufacturer has (mass-produced that is, well YET.......next year MAYBE) such an engine.

The reason for this is with further understanding of multiple transfers, it has become clear that you can dramatically reduce the "early short circuiting" (because again, THE AIR FIRST ENTERING THE CYLINDER THROUGH THE TRANSFERS IS THE MOST LIKELY TO SHORTCIRCUIT) with "well designed scavenge patterns" (but, only with multiple transfers), but even with such designs, there still is SUBSTANTUAL short circuiting.

Of course, such designs reduce effective transfer area, but it is an acceptable trade off.

Have a look back at the ESE thread where I mention a concept for your GP125 scavenge patterns (in relation to the likely shortcircuiting from the somewhat strange secondary transfer placement compared to the rear boost port), combined with using the dished piston to your advantange.

That concept comes from my experience with different transfer angles.

Also, much earlier in this thread I used the Quattrini 125 cylinder as an example.

I mentioned that I had spoken to a few customers regarding the (I think) meagre "plug and play" power results (17.1PS) compared to the customers mentioning "it uses alot of fuel" if it was pushinng out 28PS, and "using alot of fuel", then I would not be concerned....but this indicates there is short circuiting (wasted fuel)...... I posted the original port angles, and suggested I would change the boost port angle to remedy this.

What's your opinion on an angle that would improve this Teezee?

Another thing I recommend is to (if you have the resources) test all your initial "concepts" out on an engine with no expansion chamber...... if you can make more power with a cylinder (in regards to keeping more fuel/air mixture in the cylinder) that does not have the advantage of an expansion chamber returning "overscavenged" mixture, you can better gauge your gains.

Introducing the expansion chamber after such tests will only increase your power, and take away confusion over pipe design VS scavenge pattern design.

So, basically, because of the amount of gain (even today there is more to come) you make from more efficient scavenge patterns alone, this is an example of how, despite the "supercharge effect" of an expansion chamber I don't consider a two stroke cylinder to be "supercharged"

If it was, like I say, we would concentrate our efforts on increasing this (I believe non exsistant "supercharger"...even though I agree you could consider there is a "dynamic supercharging EFFECT") rather than keeping the fuel in the cylinder.

I don't feel ANYONE has to agree with me on that, by the way, everyone is entitled to their own opinion,be it experienced or not!
But expansion chamber design is not only influenced by cylinder design alone.

already covered (but not in a comprehendable fashion) has been some aspects of crankcase volumes.

When you have "modern porting" (to coin a phrase) combined with "a modern exhaust", the requirements of your crankcase volumes change dramatically......(as I have previously mentioned)

I have previously referred to Japanese research from the 60's which I pointed out was only applicable in certain situations (chamber and cylinder design for example) there is plenty of information available on what is considered "the new general crankcase volume"

My personal research has only confirmed this to be true. (the last time I posted a picture of a crank I cut, it was 1.4:1, I know go lower, with substantual gains)

I assure you that anything (with a "modern set up") With a primary compression ratio OVER 1.3:1 is only detrimental.

In the case of (for example) Team ESE GP125 (with the old porting/pipe) then closer to 1.5:1 is correct.

I find the correct crankcase volume (for a given situation) is (mostly) dependent on expansion chamber design.

And then, the inverse must be true, in that, Crankcase volume must be a part equation in expansion chamber design (among other factors)

A few years ago, (due to expansion chamber design) I had (I thought) proved to my self that around 1.5:1 was finite, (and, like I had previously posted I always kept them around 1.48:1 however over the last year I have since learned (and proved to myself) that it is omnipotent to build an engine as a complete system, rather than the old "erm, now try this pipe" approach of old.


This is why I stand behind my statement that "a crankcase is more than simply a place to store the air/fuel mixture until it is sucked up the transfers" (sorry Speedpro)

[TZ350]

This is why I consider that the EASIEST way to describe the Schnürle loop system is "keeping as much of the air/fuel mixture in the cylinder"

How it actually does that, (the separation of burnt gasses etc) is a little more complicated.

The BEST way to see this is by building a test rig as described by Gordon Jennings.

To actually see (and describe what you are seeing) makes a big difference to your understanding and comprehension of what aspects to pay attention to.

Interesting Post, Thanks

For anyone interested in making a flow rig described by Jennings. Page 122 http://www.vintagesleds.com/library/...20Handbook.pdf

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[F5 Dave]

Perhaps now your terminology 'short cct' is not what I would use.

The first gas entering the cylinder (esp in single transfer designs) bounces off the rear or head & escapes out the exhaust. I'd tend to call that escaping gas rather than short ccting.

Short ccting I'd say is the gas that come out of the front transfers and turns the corner hugging the cylinder between the trsf & the exhausts (hence short circuit). This tends to happen worst when the transfers are close to the exhaust. A modern setup often has a thin strip where a bridged port encroaches into the transfer clear area. (look at an RGV, RS or whatever).

Now if we consider that a crankcase as a pump, it is a pretty lousy one at low revs, worse with lower primary compression. Add to this the effect of the pipe scavenging being rev dependent; it is easy to imagine a bunch of gas short ccting when the pipe is out of tune and sucking at an out of freq range when the primary pressure is low.

I'd be surprised if the first gases would short cct as they would be spat out when the transfers open with most pressure so would be more likely to complete the loop (& then maybe escape).

But as the case pressure reduced, the velocity would reduce, making a short cct path the one of least resistance esp if the pipe was scavenging on 2nd order.