Two-stroke performance tuning?

[Skunk]

The printing industry?

Figures.

I believe I have put a good case forward that an expansion chamber does NOT "supercharge" a cylinder.

An insult and more non information. All you chart shows is the effect of the scavenge working WITH the chamber and AGAINST the chamber.

Hmmmmm,

So, back to definitions again.

You're the one defining supercharging as over 100%VE. I only asked if that was the standard as all MY research doesn't show that (being in the printing trade - 'figures' eh?) You being in the trade may have more info but you seem to prefer to insult than share. How does that relate to your post as quoted by inlinefour?

[SS90]

An insult and more non information. All you chart shows is the effect of the scavenge working WITH the chamber and AGAINST the chamber.

You're the one defining supercharging as over 100%VE. I only asked if that was the standard as all MY research doesn't show that (being in the printing trade - 'figures' eh?) You being in the trade may have more info but you seem to prefer to insult than share. How does that relate to your post as quoted by inlinefour?

Care to share your "research" Skunk?

I would like to read a credible source that shows that "supercharging" is ANYTHING that simply increases VE (be it over 100% or not)

That's quite an interesting point, because we all seem to be stuck on definitions here.

Although, this is getting a little tiresome.......

Can anyone quote a source that defines "supercharging" (and in particular, it's relation to VE)

Anyway, if a Two Stroke chamber really "supercharged" a cylinder, would that not make them ILLEGAL in GP racing?

And, just to make a point,perhaps you should consider yourself when it comes to insults.........

[Skunk]

Care to share your "research" Skunk?

I would like to read a credible source that shows that "supercharging" is ANYTHING that simply increases VE (be it over 100% or not)

A supercharger is an air compressor used for forced induction of an internal combustion engine. The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally-aspirated engine, which allows more fuel to be provided and more work to be done per cycle, increasing the power output of the engine.

The term supercharging refers to any pump that forces air into an engine, but, in common usage, it refers to pumps that are driven directly by the engine, as opposed to turbochargers that are driven by the pressure of the exhaust gases.

Sorry this is from the Internet. But it got there from a book. And the book was written in a University to teach students. Is that close enough to credible?

As all this from a comment I made to show that air/fuel mix IS pushed back into the cylinder by the chambers pressure wave (why else have one?) and I only used the term supercharge as a colloquial term to explain lots in a few words.

[SS90]

A supercharger is an air compressor used for forced induction of an internal combustion engine. The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally-aspirated engine, which allows more fuel to be provided and more work to be done per cycle, increasing the power output of the engine.

The term supercharging refers to any pump that forces air into an engine, but, in common usage, it refers to pumps that are driven directly by the engine, as opposed to turbochargers that are driven by the pressure of the exhaust gases.

Sorry this is from the Internet. But it got there from a book. And the book was written in a University to teach students. Is that close enough to credible?

Nope.

Sorry. Skunk.

That is a SUPERCHARGER (a mechanical one)

The terminolgy I am looking for is

"What constitutes something that is "SUPERCHARGED" (and in particular, at what VE percentage constitutes something that has BEEN "supercharged" (in relation to an internal combustion engine.

Not "what is a mechanical superchager"

Sorry for the confusion.

Because if we use that definition, "supercharging" is ANYTHING that forces air into an engine, and CLEARLY an expansion chamber does not do that.

[Skunk]

Because if we use that definition, "supercharging" is ANYTHING that forces air into an engine, and CLEARLY an expansion chamber does not do that.

What does the pressure wave do then?

[bucketracer]

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Give it up Skunk. SS90 hasn't got anything, he's just wasting your time.

.

[Buckets4Me]

I am quite confident that I have more experience with scavenge patterns than you.

In any event.

To summate your post.

Scavenge patterns essentially stop the incoming fresh charge from heading out the open exhaust port.

The end.

then why do we have a chamber and not a straight pipe ??????? like a 4 stroke ????????

[Buckets4Me]

What does the pressure wave do then?

it stops the scroll diods from working

[Buckets4Me]

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Give it up Skunk. SS90 hasn't got anything, he's just wasting your time.

.

yes BUT he is funny

I wouldn't mind if he could tell me what the chamber dose as I have been misinformed and would like to be better educated.

I always thought the chamber forced more good air and fuel back into the cylinder but it looks like it's only there for looks or to stop the new charge coming out ?????

[Fooman]

What we need is something like the attached graph, but normalising the pressures to take into account the dV from piston movement and combustion - any increase in cylinder pressure outside that from compression and combustion would be due to a supercharging effect.

It is an interesting point as to what does supercharging mean - I am very happy with the definition of VE being greater than 100% - more air goes into the cylinder than when at ambient pressure. Ambient is very important - a supercharged cylinder could be running at less that 1 bar absolute, if it is at altitude. But can one consider supercharging to be a simple increase in VE?

A quick scan of an old thermodynamics text (Karlekar, 1983) suggests that for any "real" reciprocating compressor (i.e. a crank/piston/cylinder arrangement with intake and exhaust with residual fluid, heat transfer), the volumetric efficiency can be written as (trying to translate into ASCII):

VE = [ 1 + CF - CF(pH/pL)^(1/n)](vS/v2)

where CF = clearance factors, v = specific volumes, p = pressures, n = polytropic exponent (~ how ideal the fluid is).

Not pretending to understand all that, the upshot of it is that for a certain set of pressure ratios and finite clearance volume, the VE will be less than unity , or less than 100%.

So perhaps supercharging is an increase in VE (and hence availability of extra fuel/air mixture to burn), above the theoretical limit set by the geometry of thermodynamic system under investigation.

I am sure that for a two stroke motor, the above equation will be slightly, or even rather, different, but if it does not contain a term for returning intake air, or even pressure from the expansion chamber, then if there is any effect from the chamber, it could be considered dynamic supercharging, although this would only be if the pipe is on-song as it were.

Cheers,
FM

[Skunk]

I am sure that for a two stroke motor, the above equation will be slightly, or even rather, different, but if it does not contain a term for returning intake air, or even pressure from the expansion chamber, then if there is any effect from the chamber, it could be considered dynamic supercharging, although this would only be if the pipe is on-song as it were.

Sorry Fooman - SS90 says there is no return from the pipe.

And that graph is Gold!

[Fooman]

Sorry Fooman - SS90 says there is no return from the pipe.

And that graph is Gold!

He may be right: There may be no return from the pipe, but what about the exhaust port?

Cheers,
FM

[F5 Dave]

I hit the unsubscribe button a while back, but stumbled back.

That graph is quite a nice representation. To put it in context one presumes it is a piston port engine with ~ 188 degrees duration & these measurements were presumably taken while running at tuned frequency, maybe 10,000rpm. Look at the steep curve pressure increase just a whisker before ex closing. Doesn't show transfer IO so can't see blowdown, but I assume this is a cooking engine rather than a race one.

Hi IL4, long time since seen you here, hope all is well.

[Skunk]

He may be right: There may be no return from the pipe, but what about the exhaust port?

Cheers,
FM

Are you saying the return is mainly from the port to flange area and not so much from the header area?
My understanding is the fresh charge will exit the exhaust port by 100 or more mm (generalisation) before being pushed back.
I also understood that scavenging was to clean out the cylinder of old charge as completely as possible (even if it ends up in the pipe).

Am I correct in any part?

[F5 Dave]

Pipe wrapping has fallen out of favor as it heats the returning charge that is expelled into the header. Well. . . the scavenging aids cylinder filling, the clean separation of gases (or grubby attempt to do so) is where the rest of the engine design comes into play. But this is easier with some vacuum at the appropriate time.

Of course when pipe out of tuned freq the negative & positive waves arrive at just the wrong time to perform disruptive work.