Making waves

Reflecting on Exhaust Waves

                         

The following article is the result of a casual conversation between Kevin Cameron and Professor Gordon Blair and was reported a couple of years ago in Cycle World magazine, where Cameron is the technical editor as well as being the author of very many related books………….

The late Professor Blair brought mathematics to the analysis of two-stroke exhaust pipes. One evening in 1976, with a bottle of Bushmills Irish whiskey between us, I asked him how a small change in exhaust-pipe centre section length could make the big difference in horse power that is often seen on the dyno.

“That is because the pipe contains not just a single resonance but two” he said. He went on to note that the major resonance is a wave that is created as the exhaust port begins to open, releasing a pulse of exhaust of perhaps 100 psi. It travels the length of the pipe to the convergent cone at the far end, where it is reflected with no change of sign (it remains a positive wave) back to the exhaust port just in time to halt and reverse the outrush of fresh charge that has meanwhile been looping into the cylinder through the two or more transfer ports.
There is a secondary wave, which bounces back and forth between the divergent cones nearer the front of the pipe and the convergent cone at the rear. It typically makes three oscillations while the primary wave makes one. And this explains why a small change to the centre-section can have a significant on engine power: If the two are nicely in step, their amplitudes add, creating a stronger “stuffing wave” that can stuff escaped fresh charge back into the cylinder.
If the two are out of step, however, the amplitude of the secondary is subtracted from that of the primary, resulting in a much weaker stuffing wave, less escaped charge pushed back into the cylinder, and reduced engine torque. In this way then, a 5mm change to centre-section length can have a big effect.

Astonishingly, that conversation took place 43 years ago, and all things associated with two-stroke exhausts still occupy the minds tuners today with seemingly undiminished interest, at least, judging by the number of hits on the “One size fits all” topic here on the forum, it most certainly does!

Trevor.

Hi Trevor
If a small change in the centre section can have a significant effect on engine power, what about the end can spec? Photos of racing Bantams show a selection of fat/thin/long/short designs - personal choice,depth of pocket, or what?
Jim

Jimmie,
I`m a little unsure as to what you are specifically referring to with `end can spec`? Could this be a miss-spelling of end cone or the silencer, which is sometimes called a can. Could you clear up my confusion please and I can give you an answer.

Regards, Trevor

Apologies, should have used 'silencer' instead.
Jim

Hello Jimmie, happy Easter.
                                                 21st century silencer design is pretty much nailed down now with little restriction to flow with minimum power loss associated with them. However there are one or two caveats to apply, a restrictor needs to be inserted into the extreme end of the rear cone, most 40 + hp 125 engines spec a 23mm insert.  The more modest output of a well cooled Bantam could use a smaller diameter with safety, in large part this diameter is determined by, and so is proportional to the quantity of exhaust gas produced. More power needs more fuel equals more, and hotter exhaust gas!
 What works for today is a tail pipe larger in diameter than the insert with the silencer inner perforated tube that is a couple of mm larger in diameter again, this make the restrictor diameter the dominant feature and not the impedance of the tube, the outlet from the end of the silencer needs to be a couple of mm larger again. This might seem counter intuitive but the these steps up in diameter give better acoustic attenuation than none at all, a step down at the outlet from the perforated core tube may increase wave amplitude but most certainly gives more noise!
When I first built my w/c reed-valve engine the silencing consisted of a perforated tube tail pipe welded to the end-cone with an outer shell packed with absorption material, much later with more noise restriction in place the whole thing was replaced with the tail pipe and silencer from an RS Honda. Not only was the bike quieter but revved a little better as well, which perhaps proves the point?
Brief spec of the silencer/ tailpipe:
Insert 22.5mm, tailpipe pipe 24mm  , perforated pipe 25mm dia of 275mm length, perforations 4mm dia with 4 holes per 25mm of length, can diameter 62mm, length 300mm, outlet 30mm.
These dimensions might be variable for the whole thing was not ever truly round, being second hand when we had it donated in a job exchange.
When correctly proportioned for the individual engine`s needs, this type of set up can render the actual tailpipe length pretty much irrelevant.

Trevor

Hi Trevor
And a Happy Easter to you and yours.
Thanks for the information which I shall try to understand after a couple of reads. I attended a talk  given by Dr Blair some 40 plus years ago and was  fascinated by the high speed photos of the pulses exiting the pipe. He made the complicated seem so simple in principle but it was still way over my head.

Trevor
Is the of wall thickness (within reason) of any component important?

Morning Jimmie,
Apologies for not replying sooner but other matters have taken priority this week. I have always made my pipes myself from 1mm or .8mm mild steel sheet, but as my gas welding skills are pretty poor the larger thickness gives me a better chance of not blowing holes along the seams. Various brackets are made of a heavier gauge, say 16, and the cylinder/ pipe slip joint is again a heavier wall thickness.
Where barrel construction is concerned, for instance, there needs to be sufficient rigidity imparted back into the whole, after chopping of all the various ducts, to avoid heat distortion and the continual vibration that over a period of time can fatigue, then break thinner sheet fabrications. In these areas I always use 3mm steel and affix them with a generous weld prep and fillet weld. Another process I always use is to stress relieve the barrel after each welding session by cranking up the kitchen cooker to its maximum, heat soaking the barrel at least a couple of hours then switching it off and allowing it to cool slowly overnight. The last thing you want is for the barrel to start to de-stress its self, then distort after a few laps of a race as the whole thing gets hotter.
So I guess the short answer to your question is, it depends on the application more than anything else!

Regards, Trevor

Trevor
Many thanks for the tech details. Much appreciated and clearly expressed.
Jim