Phenomena-Plus

So , Derek ,
Here`s a little nudge to get " help " moving along !
Take a look at the recently posted ex. pressure traces , note where the line crosses atm. going both negative and positive ,
what we are looking for is the lowest value and broadest span of these lines . Equate these points against header and diffuser
dimentions of length and diameter , from the piston face , if you`re right in the ball park you`ll suck masses of clean ,fresh ,cool ,
combustable gas into the cylinder !

Point two , get hold of some Vinamold from Amazon , only about £15 or so . take mould plugs of all your ducts and you will
be able to see in three dimentions how they shape up , and , if they are found wanting , put them right !

I must say i`m supprised this technique has`nt been mentioned before ?

Stay in tune all of you , Trevor

Scuse me Derek & Trevor for butting-in but are we talking about the same thing...

My Dr Joe dimensioned pipe was -- from the exhaust port -- a short length of parallel, then the diffuser, another fat (major diameter`) PARALLEL portion, the nozzle and a thin tail pipe ... All dimensioned....

What I have seen on here -- and on 2-stroked-scooters in the street -- is the diffuser connecting direct to the nozzle...

Obviously the mouldings you speak of are on the outside. If you had the fat parallel portion telecsoping (for experimenting, as I mentioned earlier) you could mould the inner shapes and also see the rugosities (or weld lumps) I mentioned before....

...Hmmm?

Humble apologies Derek, for butting in -- but if I didn´t I´d be sure to forget!

Cheers!

thank you Trevor

not wanting to appear thick, but I havent a clue what your explaining on the crossing points "could you please elaborate a little more on this subject, also could you please explain what the diameters and cone to do with this wave, I thought the wave was caused by the exhaust opening rate / timing and the shape of the port window/duct, but I have asked for some vinmold to be ordered we will have it when Iget back home.

John please dont appologise for making comments, you seem to be the only other willing/capable (not sure which is correct wording for this) of contributing to Trevors "snip bits", I just wish there was more and not so much missing.

I'm sure we all inteligent enough to know once you have a good base engine, its only 40% of the package the other is carburation timing / gearing / riding & cornering/ tyres, and your head and the "weight of the rider"!!!!!!! to pull it all together etc etc.

while I understand we never like giving away so much, Im sure these contributions are enjoyed by many, and so helpful when trying to understand the physics of what is happening inside the expansion chamber,
in order to understand what to change to make an improvment.

I think Johns Idea to make a slip joint for the centre section to test is a great idea for the dyno!

but when there is so much missing, it can make it more frustrating when trying to understand what is being taught, while I'm sure there are many others who could make meaningfull comments here, they chose not to get involved, this is a shame as any comment good and bad or indifferent, its always helpfull to someone.

So please keep commenting john and all dont stop.

regards Derek

Yeah well, thanks Derek. The pipes I see now do NOT have a parallel major dia after the diffuser*** which came after a parallel piece of pipe from the exhaust port on my Bantams.

I did ask the question about which is the optimum -- in terms of the diffuser being direct from the exhaust port (á la George´s sketch) acting as a diffuser -- or as the above according to Dr Joe... and still await a raspberry retort to the effect of my being out of touch!!

In my crappy way of doing it (my excuse is that I took on too much...??) the telescoping major dia portion of the pipe certainly facilitated obtaining the `optimum´ length out on the track. It would be a big help on the dyno and maybe someone already knows why it would not be worth the bother. If the two different designs -- as spoken of here -- could be dyno tested we´d get to know ....

Too late for me though but there´s just this niggling thought that I got it all wrong anyway -- certainly looks so when I look at George´s pipe which is diffuser -nozzle and tail pipe....

*** an elitist Thermodynamics writer calls a diffuser a "Negative Nozzle". Just another piece of useless information...

Cheers!

What happens when the piston uncovers the exhaust port and then a little while later the transfer ports? That question has been with me for a very long time and sometimes I think I know...

The exhaust gas pressure -- at a fairly high pressure, that´s already decaying, has the crankcase pressurised charge -- at probably above 2 atmospheres pressure .--pushing its way in where the combusted gases were and then its pressure also decaying... The experts call this `Loop Scavenging´, which you know already, anyway...

The exhaust port and transfer ports are open together whilst the piston goes down to BDC. If there is a back-pressure wave during this time -- won´t exhaust gas be forced in with the fresh fuel-air charge? Then the piston rises to close off the transfers and block off the incoming charge whilst the exhaust port is still open -- (I think we used to call that period of tansfer to exhaust the "Blow Down period"???)...

So, there we have the piston rising, trying to compress the charge instead of which the exhaust port being open is allowing some of teh fresh charge to be lost with the remnants of exhaust gas flow. RIGHT! now isn´t it just now -- as the exhaust port is closing that we want that returning pressure wave to punch that charge back in??

I think it is. So why bother having the reflecting pressure wave coming at exhaust port opening time? Have it coming exactly in that period of Transfer-port-closing-and-exhaust-closing...??

Or have I got it wrong? Is it a matter of mass and speed of gas flow during the whole while of exhaust port being open which makes the boosting of pressure during Blow Down of exhaust most efficient. I´d like to know.

I guess you users of simulation programmes can say what the exhaust gas pressure decay looks like when matched against degrees of crank-angle -- but I´d like to see it on a PV diagram taken off a dyno test. I´ve mentioned this to Slick in the IoM but he has a lot of race bike prep to do which is mostly 4-stroke engines anyway.

Major point at the moment -- on here, I mean -- is a full understanding what the resonant pipe really does.

Like, I said above -- I THINK I know....

Cheers!

No reply to my query -- but I ain´t giving up!

OK! now I think I have it -- the returning shock wave causes a high rise of exhaust pressure (adjacent the exhaust port) which rapidly drops when the exhaust port opens, to then remain low until the port closes at 135° crank later when it repeats its previous high pressure.

This is where I got my "things" in a twist. I assumed -- wrongly, of course -- that these two pressure rises were two shock waves and as such would be out of sync with the engine revs. That is, when of course they are just the one, the same one ... The shock wave split in two as it were spread over the crank period in which the transfer port is open ....

This was, as shown in the two traces recently by Trevor & Mick. A shock wave spread over 135° crank angle period is 7/24ths of a revolution... ennítt?? and if at 12,000 rpm means, 7/24ths of 1/200th of a second per rev which is 7/4800 = 0.00146second. A lot happening very quickly!! Touch of magic, if you ask me.

Makes sense now.

Thank-you for helping to keep my brain alert -- its been crying out for sleep lately!!

I´d still like to know the answer to my query of why the modern Bantam exhaust pipe -- as shown in several pics on here -- has its diffuser start at the port or immediately after a short section of parallel pipe....? when my Dr Joe pipe had a positive diffuser -- a positive parellel section -- followed by the all important nozzle (followed naturally by the tail pipe...) ?????

Cheers!

ref Dr Joe'S pipe can you send a sketch of what your refering to - returning waves and cones from exhaust portS or A piston sketch TO END OF PIPE, as a picture is so much simpler to understand.

ed could help load it, so we can all see what you are trying to discribe.

and if "YOU THINK YOU KNOW IT " tell us so we can understand what you refering to.?

you better be quick otherwise THIS WILL SELF DISTRUCT IN 0.00127 SECONDS" (delete FOR EVER).

REGARDS

Derek

Yeah -- I´ll go down to the cellar and see if I can find the original drawing of the pipe. There´s still a load of papers never unpacked from Canada. There ought to be some old Bantam stuff among it....

I thought I had gone thro´ all the description adequately -- as far as the reflecting shock-wave is concerned -- that stuffs the scavenged charge back into the combustion chamber (which would have otherwise been wasted) -- and I´ve long forgetten the diameters and lengths of each section ....

In any case Derek, wouldn´t it be better if you asked a more modern and successful Bantam racer the sizes & data of their exhaust pipes because both my Bantams were Also-Rans.... And maybe Trevor or Mick can describe the events better than I.

I think there have already been several pipe dimensions shown of successful Bantams, more successful than both mine were.

However I´ll see what I can find....

Cheers!

I put on my winter gear and boss-woman warned that it was minus 12 degrees celcius outside and would be the same in our Kellar ...so I came back on here and went to Google and wrote,

"2-Stroke Resonant Exhaust Systems" and the first topic I tried called "2-Stroke Engine" came up with exactly what I wanted to show. No dimensions but a Dr Joe pipe exactly.

-- PLUS -- an animation which I think will serve much better than my rambling on -- on here.

I hope this clears everything up.

Cheers!

Hi All

Prof Blair has an interesting diagram in his books

The first book is available as a pdf download from google here -

http://www.prme.nl/download/engine-0.pdf

The diagram I have in mind is this one from page 273 of that pdf



it's also in the 2nd book slightly modified but essentially the same

Mick

Thanks Mick -- the double diffuser clears up a lot of doubt. If you all look at that Google representation & animation -- I mentioned -- you´ll see how crude my early exhaust pipes were (in comparison with the later pipes and the George Todd sketch & description).

That is with the diffuser and nozzle cone angles very much sharper and a much longer parallel (major diameter) portion. This however, allowed me to make the pipe in two pieces and have the nozzle with its 2" parallel bit capable of being moved about 4" inside the longer portion which was attached to the diffuser. Three slots in the latter, several big hose clips outside and a thick sleeve inside the overhang (of teh parallel portion) plus `Gun-Gum´ (by Holts) goo and tape to seal the slots and Icarus-1 and I were relatively happy at Brands -- except for the cumbersomeness of it making cornering a bit awkward I did get a good idea of what the length of the nozzle-to-the-piston face (LT in the Blair diagram) should be.

Like I said, I just cannot remember the dimensions but I do know that by the method above I got a power band from 5,000 plus something to 8,400rpm after which it misfired which I thought was attributable to CB points making that the best I could do....

I then made the proper pipe with Dr Joe´s recommendation for diffuser and nozzle but with my own established parallel portion length. So the critical bits are obviously the nozzle to piston face length along with the angles of the nozzle and diffuser cones -- both of which I probably got wrong relative the good, high-speed pipes.

Ah Derek! As I am scribbling this it occurs to me what you are really asking ME for.

I think it must be the detail of that `telescoping parallel section´. If so, I shall dutifuly make a sketch and send it to Ed to display.

Another afterthought: "telescoping" is probably the wrong term -- perhaps "Variable Test Length Expansion Chamber" would be better since it was tested in FIXED lengths per test session.

Cheers!

This one is giving me the creeps...

Phenomenon indeed!!

Trevor gives me numbers which are OK but then I look up some more info on Google and see that with the engine running on petrol the exhaust is hotter than when the engine is methanol powered -- which says to me that with the latter, the velocity of sound in the latter exhaust gas is down by about 40% (lower) than with the petrol.

i.e. Petrol engine exhaust, velocity of sound would be at 550 --600metres per second and with the engine on methanol at 330 to 400m/sec (so says the author of this Googled article).

So what is happening to the speed of sound when the engine is running on aviation type fuel ?? Instead of 550 -- 600m/sec it will be PERHAPS? some other number in between the two above?

Then I see the animation of the exhaust proceedings and this shows ALL of the exhaust gas rushing from the exhaust port to the nozzle back thro´the diffuser to gather at the exhaust port. ALL ...? According to what I learned in Hydraulics that is not so -- but then the author of this animation will say Yeah! you blithering OAP! this is gas flow not water... And I shall retaliate with this:- I made a Standing Wave test rig at Walthamstow Tech whereby with suitable baffling-weirs (at places along the sloping channel...) a Standing wave could be made to go with the flow -- or back against the flow. Depending on how quick the weir was lifted and retracted so was the height of the standing wave regulated.

As I understood it the speed at which things happen in the cycle of events the exhaust gas can be considered as a continuous flow with the pressure-head -- described as a `wave´ -- moving `backwards´ thro´this flow -- á la the description above.

So --- correct, or confirm please, and what would be the temperature of exhaust gas when using aviation petrol which would then give the speed of sound of the exhaust gas?

Cheers!

Courtesy & Property Of Trevor Amos.

So , which pipe would you choose ?

One is from Mark`s Cadwell bike !

Trevor

Pipe 1

give me pipe 1 any day!

Pipe 2 looks as if it might have possibilities to rev on -- that´s providing the other engine bits have reliability.

The torque curve looks really good and flat -- still not falling off as it reaches the end of the graph so I reckon it´d be really good for acceleration with slipping clutch at max revs ...

Besides, the mystery of how far it does rev on appeals to me and I´d want to see how far on the revs would go -- out on the track.

Yes, I´d choose Pipe2 and hope the plot would hang together....

pipe 1...

Hi Trevor before I choose, I would really like to see what the BHP/KW graph is graded in on the left 2bhp steps perhaps?.

and besides if your asking which one I would chose, I would say "I chose the bike" any day, a pipe on its own is useless, are you saying that pipe on another engine will be the same ! "no it will not".

so Trevor do you have your own dyno? and if so what are you measuring - air fuel ratio, torque and power / rpm,
or are these graphs that appear to be computer simulated items, but I will point out i'm not challenging the correlation between simulation and results - "this speaks for its self" - just like to know if this is an actuall measured reality or fantasy, if you can forgive my phrasing, I could not think or the correct word to use.

kind regards RH. D.Betts

It would be interesting to see a little past 12000 rpm as it looks like the power from pipe 2 is still increasing but even without that, peak power with pipe 2 is about 1000 rpm higher than that with pipe 1 and at lower rpm both pipes produce much the same power but only about 500 rpm apart.

If you gear pipe 2 about 2 or 3 teeth lower you would have the same power at the same road speed at low to medium rpm but have (at least) an additional 500 rpm at the top end and have more power at those rpm so if engine parts can handle the rpm its pipe 2 for me.

Mick

You are right Derek -- it would be nice to know the difference in horespower or torque between the two graphs. Not necessary to have the actual HP or Torque values -- just the max differences.

This question Trevor poses is a reminder of the answer Maurice Quincey gave Bill Lomas when Bill asked, "There seems to be no rev-counter -- what do you reckon are the max revs of the Walsh Bantam?" to which Maurice replied that no one really knew, that whilst racing it, the Walsh Bantam engine just revved on and on.

I rather like the mystery Trevor has incorporated in the two graphs -- and although I´d rather have a higher torque, at slightly lower speed, than having higher horsepower at very high speed -- I like the idea that if the clutch MUST be slipped with the engine at high revs then have an engine which revs on and on and on...

Keeping in mind, here, of course, that the Slipping-Clutch acts as a Torque Converter giving a higher gear ratio than that available in a 3 speed box which means BETTER ACCELERATION with the chance of being more competitive.

It is acceleration that wins races...

Cheers!

Just been reminded that our drain has blocked again -- so I´ll quickly end by saying, Thanks Mick -- I though I was a lone-rider on Pipe2! Quite a flat torque curve though -- is this for real, Trevor, or simulation??

Cheers!

Glad to see others spotted the torque curve thing I was begining to think "it was me"!!!!!

Mick makes a very interesting point on the parts being capable of taking the extra RPM, I think this one of the original reasons why short stroke bantams were allowed, well this and for the reliability of parts at increased RPM, but another factor was the availability of ? better quality items to do the job. ?

If it were down to me (ever the traditionalist) I think I would vote to have an introduction of rev limiters 10,500 max for Formula Bantams, 8500rpm for 175 to 190cc and max 9,200 for 125c short strokes.
The grids would be fuller, bikes would be much more reliable, the racing would be closer, and more importantly it would stop those with huge resorces and money from disapearing into the distance, also because it would be very easy to police. as you can check what rpm an engine is pulling from the side of the track.


! I will try to do a better job this time to use more appropriate wording for my historic detail, so tring to use the correct words her goes.

"unlike previouse blunderings" on this historic point in the history of our formula Bantam racer. I think its important not to forget, what these genuine people of great intention back in the day, "failed" adismully to take account of and for, was the abilities of the normal bantam racer, at that stage who outweighed the volume going to the island, no account was made towards the resources needed to build properly a formula short stroke motor plant,
There is no doubt in my own mind "this single oversight was the major factor and contributed to our class extintion event,!as in the late 70's Formula Bantams disappeared on mass", to an almost instinct species "as they did" "the records that I have looked at" prove this timing, and not as my previouse mention as a result of the Tom Miller SMS "speeder motors specials incorporation ltd, "still currently winning races". obviously they just did it better, had better resources, and superiour Knowledge. I have been critisised for saing this before, but this is how it should have read, I hope this is not too contravecial but it is factual, We have already seen! what can be achieved and the resuts are only as good as your efforts!!!. I think this last one is a load of "rubbish" effort do not equal results. there is a lot more in bewteen .

just wanted to put that one to bed.

but I still think we should have more limits added, we need to have more of a spectacle in our class, than 2 or 3 one or two laps in front of the rest clearly in my mind "this is not a formula that simply is not working if this is the case.

regards

Derek

some one just PM me" "I agree a return to steel wheels and tanks, too

stricktly on heath and safety grounds !

HR d,betts

Don`t know if anyone`s looked at Mark`s machine , but it still runs steel wheels , and , will do so untill a spare £500 quid or so is available
for an update , anyone like to donate ? Has there ever been a race lost because the wheels were steel and not ally , i think not ! Just a simple matter of perspective , priorities and impecunities !

My pm box is awash with requests for power numbers , thats fine ,i probably would have myself in the past. As i have stated recently
i`m only concearned with differences not absolutes ! The posted graphs illustrate that perfectly , and had the pipe 2 line have continued up
it would have heralded in the first 30bhp Bantam engine , corrected for rear wheel output ! Realistic , of course not , but that is one of the
pitfalls of software , the computer screen is a long way from lydden when the wind is blowing a gale and the rain is thrashing down .

Dynamic and steady state running are worlds apart and a good ,consistent rider on a fast , reliable bike will always be tricky to beat .
Software will never be ameneable to factoring in that dilemma !

Anyone want to speculate on which is Mark`s pipe , or not ?

Catch you all later , Trevor