2nd order wave phenomena , and Dr Joe !

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well i recon one of these for the cylinder pressures.... what would you use in the pipe.?????

if i read this correctly are you all saying these graphs are simulated ?? and not actually measured.
can they be measured.

Mick I see Ian says on face book - you have "still got a pipe to design", an engine to rebuild, a frame to weld up, and fit body work, blimey when you doing all this and how about some pictures/ a sketch of the pipe would be good and is it a 125 or 175cc?.


John "no not arrested" - I have done the rolling test though - with different rear wheels repeated, 3 times - stopping each time within a few feet, but boy the differences between different rear wheel setups was unberleavable, and could certainly detect slight dragging of rear brake, also massive difference- especially if you remove the drive chain to the back wheel,

john "ive gone gone quiet again /where have I been?
all will be revealed soon, not sure when just yet, I will let you know so you can see for yourself, what I have been doing, although it has been six years since done this and boy am I stiff and very tired. "o" I litrally nearly broke the camera with the broom handle.


John "no" actually I think this is a fantastic subject, I'm really excited about the potential of learning how all Bantams will be going very quickly to form an equal "Pro Am" Bantam series - all bantams fighting for the lead would be a proper spectical.

but will Trevor share all that is needed for this but Ican hope, and the otherb guy seems cock sure of himself. ! I wish to have fun and compete or get closer than I'am now.

regards Derek

Regards Derek

Yes, like I said, the diagrams are from a simulation. The "sensor" is just software capturing values at the appropriate point and in the case of the exhaust pipe pressure trace it shows values at 1mm from the cylinder.

In this next one Black is the cylinder pressure and red is the exhaust port pressure. Same engine near max power rpm. Does that help John?

Mick

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The instrumented spark plug Nobby showed does give in-cylinder, above piston pressures but have a limited life. I got them from Vibrometer GB Ltd and also from Prof List in Switzerland -- both addresses may have changed or been taken over -- its a long, dark time ago. We also used a Piston-Way tranducer from Vibrometer which with the spark plug signal printed out a beautiful PV (Pressure-Volume)diagram....

Glad you have tried the these Rolling Resistance Tests Derek -- I really could imagine you on a hill in a Public park preparing to do a dead-engine coast-down using a megaphone to warn the parents to keep their kids and dogs under control -- in THE INTERESTS OF SCIENCE!

I did similar (Coast Down) tests on a proper test track with Leyland Tankers -- used by Silver Roadways for sugar beet transport -- where the tyres were pumped up with nitrogen. The idea being that the inert nitrogen would prevent the tyres getting hot and varying tyre temperature... Their test showed a lot of advantages but it was said the cost of using nitrogen instead of air was prohibitive of adoptinmg the idea overall yet I knoiw Silver Roadways did use nitrogen in their tanker tyres for some time and stated there was a fuel-economy gain which meant the Rolling Resistance was reduced.

Of course, I´d not recommend use of nitrogen for racing tyres because you always wantr them hot -- or do you??

Cheers!

Thanks Mick -- as I see it simulation is OK but as Trevor said (or I think he said) the formulae is only a guide to get down to practical exoperimentation.

I have only one regret about the time spent with BRC -- I did not get into this Bantam development I see on here -- I was too busy with big Auto companies problems to even consider more than the experimentation which was mostly rushing around Brands-- which was like taking an exciting holiday....



CheerS!

It suddenly occurred to me -- as second thoughts -- on the explanation for the phenomonen of resonant exhaust pipes -- that I had not been clear enough and should have left it to Trevor ...

...however since I´d already committed myself, I´ll add this which I hope helps to give a clearer pic than I already have --WITHOUT any fancy formulae or calcs....

We assume the exhaust gas is a continuous flow which when measured by an anemometer would appear to be so. In actual fact the flow stops & starts continuously.

Referring to Mick´s diagrams shown, when the exhaust port closes there is a stop of flow for a crank angle rotation period of 165°*** to when it opens again*** and the flow of fresh exhaust gas goes thro´a short piece of parallel to then diffuse and expand where the pipe widens, thus reducing its pressure to then come to the nozzle where its volume is suddenly contracted(squashed) and pressure thus rapidly increased. This change of pressure produces a shock effect which rushes as a pressure-wave back through the moving gas -- at the speed of sound (which is 343meters/sec in air at 20°c & 1014hPa -- about 770mph...) to block the exhaust port before it closes -- thus doing what I had said earlier of shoving back into the cylinder the part of the Fuel-Air charge that would have gone out with the exhaust gas flow... Since there is a change of pressure (certainly not 1014hPa -- 14.7lbs/²) of the gas in the exhaust pipe as this happens it is exhaust gas at a fairly high temperature (certainkly not 20°c), the exact speed of this pressure wave is not so simple to calculate as when dealing with pure air.

Of course, it can be done using the PV = RmT formula that Trevor quoted sometime earlier where the PV (Pressure X Volume) = R (Gas constant) x mass x absolute temperature and using formulae derived for divergent and convergent nozzles....

No one answered my query as to whether bumps of weld and the inner surface roughness of the pipe -- something to do with gas flow drag relative pressure over smooth or rough surfaces -- had ever been subected to experiment...


Didn´t have a dyno avaialable so that would have been more laps around Brands -- so who cares??

CheerS!


*** in equal amounts either side of TDC...

so, if you only had the facility to use a spark plug type of pressure tester and sync it with timing duration and rpm would you be able statically test wether your exhaust was actually retuning a strong pressure wave and at the most effective time? even if the pressure wave wasnt too clever getting what youve got to arrive at the appropriate time would be of benefit, wouldnt it....

Hi Nobby -- Yes!

If you had the instrumented spark plug installed (in the head, of course) the pressure trace would surely show the effect of the pipe because your X-axis on the monitor would show the crank angle & Y-axis the combustion chamber pressure. You then have a Combustion Pressure-Crankangle diagram: all that´s needed to do is run with the resonant pipe then run with an open pipe and compare the two traces ... The instrumented spark plug in the exhaust would surely show a trace but far better would be the diaphragm or flat disc (Washer) type of pressure transducers -- sunk into the surface so as to not interfere with the waves....

...I fink!!

There are in the Auto industry a fascinating range of transducers and sensors available. A quick glance at Formula One cars shows they go from measuring tyre-pressures, thro´ every known engine & transmission parameter to measuring the driver´s pulse, blood-pressure and urine-level.***

The dyno curves I have been allowed to see so far have shown NOTHING below 7,000 rpm and I wonder why? Is the power so small? It would be interesting -- as suggested above -- to see how much power a Bantam burbles out during the whole open-pipe speed range...?

I´d also be really interested if someone put a couple of diaphram type of pressure sensors in the exhaust pipe because my original question was whether the other lesser waves could be of influence -- or are the tiertary(Trevor!?) waves too small to be mistaken for the main wave?

Perhaps I´ll be jumped on for that -- "We are practical racers," comes the cry, "not Research & Development engineers."

No! but while you have your Bantam on the dyno can´t you just include a little more ...????

Cheers!

**haven´t you heard the Pits calling a driver on their radio? Hey Lewis! take it easy -- time for a pee...!"

Hi John ,
Just a few numbers for you to play around with ! A good ball park acoustic velocity number is 550mt/sec , a figure suggested
by a twostroke designer/tuner of world renown as being a good average number for less than GP engines . If you rearange the
equation for velocity you come up with an average pipe temp of 500*c .

thus : sq rt (s x r x t) : sq rt (1.35 x 290 x 773 ) = 550 mt/sec

temp is in Kelvin = t + 273

s= specific ratio of heats

r= universal gas constant

Keep rockin pops ! Trevor

Hi Trevor!

Thanks for the info. I think the 1.35 index for specfic heats a bit suspect with the pressures and temperatures involved but bow to superior knowledge.

Pops indeed!!

Cheers!

bettsd wrote:

if i read this correctly are you all saying these graphs are simulated ?? and not actually measured.
can they be measured.?

but will Trevor share all that is needed for this but I can hope, and the other guy seems cock sure of himself. ! I wish to have fun and compete or get closer than I'am now.

Regards Derek

all looks great plenty being shared so far but I'm having difficulty following some of this techi stuff, so are you saying this wave can actually be measured and therfore these simulations could be re-produced with real data.

! I would be very interested to volunteer or get involved to make this happen or even do a vidio to show on here of the results with any other volunteers, sure it may cost a few quid but it must shureley be worth it, making 200 pipes seems a waste ! of money and Idont have 8 to 10 years left, so to check this properly if some one would be good enough to point out the correct equipment or help detail a proper way of checking/doing this.

the offer is there ?
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Hi John you old pro you !
I see you make a very interesting and Ifeel very valid point, your comment on whats it doing below 7000 rpm, is a smoke screen, I was thinking (this is always dangerous) that it does not really matter what the engine/pipe simulation of the waves are doing at 10,000 or 10500 I think its more important what its doing well below this upto this point, from say about 7,000 to 10,800 "up to" - but please correct me if you feel i'm missing the obviouse here.

regards Derek

Quite correct John , did a bit of digging around and the s/r figure for 500*c is 1.357, not 1.35 as i suggested , and is 1.4 at 20*c
Good to get these thing established with some kind certainty , it`s already figured into the memory on my calculator .

Thanks for that ! Trevor

Right Trevor!

I looked it up in a table by Rogers & Mayhew (were lecturers at Bristol Uni...) that I found in the Kellar. It gives for AIR at 850°K (577°c) the index of 1.349 which means your number must be about right but I still wonder about the composition of exhaust gas having similar index as air -- and still (maybe) be a bit different...??

I made Icarus-1´s pipes to info coming originally from Dr Joe and had to spend a lot of time at Brands -- as I said -- getting it right...! With Icarus-2, I got the motor to rev to 11,000rpm (only in short periods -- its reliability was crud) and all I did there was to copy Icarus-1`s pipe dimensions except to shorten the distance to the nozzle -- plus more time at Brands and it was OK. Like a dumb-dumb I used Standard BSA pistons which used to grow longer in a race because of the third (pulse) differential -- the acceleration of acceleration like of the piston at TDC & BDC....

At least Icarus-2 succeeded in scaring-the-pooh out of my old buddy*** The Wobbly Man who did actually race it at Llandow -- for about 3 laps -- only discovering he was on Icarus-2 when the race had started!! He happened to be suffering concusion at teh time ....

I wonder now if the info I got in those oldhat, dark days was really right and that I might have been experimenting to some 2nd order pressure wave -- which would obviously be weaker...That´s why I asked about your tierartary(??) waves --- and now I am a bit leery about doing the calcs to see if I were really wrong in those OldHat days.

It would account for ALL the cutchslipping I did -- which the modern Bantams seem to do far less of....

CheerS!

***I should have used "friend" there -- because he was!