Time,timing,time area ?

robbie, the pictures are of the old iron barrel i tried doing first , looking forward to getting out on it too, finished the new subframe and seat position now, got given a new front disc yesterday so i'll be making that fit soon , then i think its ready , maybe a test this year if im lucky

track day 2nd of November at lydden which I am thinking of doing keep that in mind Dan

will do, want to give it another crack to find all the problems fixed!

cheers dan,maybe ill get my last barrel re -sleeved after ive butcherd it a bit more.

Courtesy & Property Of Trevor Amos.

Crikey Ed, even by your normal super efficient standards that was rapid indeed , thankyou !

This sketch of trans. ports is pretty much self explanatory and is merely a bit of thinking out loud from way back in the late 70s, it has just had the 175- 58mm stroke added on together with the bore size. The angled top edge to the port worked very well in my first 54 engine and was only superseded by the multi port lay out and staggered timings . Initial flow is fundamentally in the right direction and is established at the furthest point from the ex. port to offer the least possibility of short circuiting of precious, fresh charge , and is wide enough to give cooling to the piston crown. It might be worth considering for you 175/190 runners!

Trevor.

Evening Trevor, just looked at your drawings and wondered about short circuiting of the transfer ports.Can you say which part of the flow from the transfer ports into  the cylinder is more likely to  short circuit.(im assuming caused by the gases being taken out of  the exhaust port)is it the initial flow as the transfers start to open? or later?

Nigel ,
As the ex. port opens, and depending on whether your pipe is working as it should, there will be an appreciable amount of super position of the gas exiting the port and into the header pipe. This wave has a large amplitude , and creates a large depression at the port, at and around BDC. Gas flowing from the trans. ports, at a high pressure, always wants to flow to the low pressure area, the ex. port . This phenomenon is short circuiting. By allowing trans. gas to exit, first, far from the ex. port, and by having the m5 rad adjacent the ex. port gas flow is discouraged from going in the wrong direction . Very large/ wide ex. ports, without a power valve, will always suck large amounts of trans. gas straight out of the ex. port that will play no part in power production, and will only function well at higher rpm, not good for 3 gears !
By having the 1mm inclined roof to the port, in the bore, and with the rear most corner flowing first, the exiting gas core is going in the desired direction and when a mass is in flow it tends to maintain that direction, at least for the few degrees of piston travel before the port is fully open. It never quite manages this ideal, but as they say, every little helps !
If you are doing a re-sleeve in your 175 then this port , at 30-32mm wide, could work well, providing you can match it reasonably well to the transfer duct profile/volume . Hope this helps a bit ?

Cheers , Trevor

Thanks Trevor, when you consider these exhaust gas flow facts, its a wonder the 2 stroke engine works!!


just one more question... would the verticle entry angle of the transfer port edge nearest the exhaust port require it to be angled more so as to kick the flow of gases heading toward the rear cylinder wall allowing for this problem of being drawn towards the exhaust port? or would both verticle entry angles be the same. thanks ..nigel

Yes Nigel , two stroke engines are very far from being simple to understand, they only appear to be so ! Even now, the 4t boys can only dream of the power per litre that the simple 2t achieves .

If I read you correctly then the answer to your question is that the trans. port in the bore should be the smallest area of the whole duct and that the change in area should be constant and proportional , that way flow will be smooth and undisrupted. If the port edge and duct wall adjacent to the ex.port are directed at least to the piston centre and the trailing edge to just in front of the rear cylinder wall then that will do a pretty good job of guiding the incoming charge to form the upward rear gas column and clear away old burnt smoke/gas . The trouble is there is no perfect solution or data, every engine is different and there are always compromises to be made in any trans. flow system . And, it must always be born in mind that the trans. phase is the shortest of any of the events and is consequently the most critical, and the easiest to get wrong ! What happens after that is a whole different scenario !

Trevor

ok, cheers Trevor, thats great thanks.

Thinking a bit more about complex 2t tech matters and slotting into the trans. topic is the follow observation . The rear cylinder scavenging gas column moves at a velocity determined by the engine internal spec. If the column moves too rapidly in relation to mean piston speed, then there will be scavenging losses out the ex. port . If too slowly, then the cylinder will not be adequately scavenged and losses affect power production. This could have implications for the 175 and 125 engines, with the latter revving much higher.
Dead simple these Bantams , don`t you think ?

Trevor

Jawohl Trevor dead simple:   roughly speaking a cup-full of fuel-air-mix is squashed to egg-cup size which is then shoved -- as it tries to expand -- through small holes to travel across the top of a lump of aluminium as fuel-air tries  escape thro´ another big hole but cannot escape entirely because the big hole gets smaller as the lump of aluminium moves ... That is,   as the lump of aluminium attempts to squash it again before it all escapes. Then it does squash it again with the help of a "water hammer effect" which bungs some of the already escaped fuel-air-mix back in on top of that lump of aluminium ....

....which all happens in roughly half-to a-quareter of a two-hundreth of a second .... (12000 rpm...)

...Nah! I don´t believe it -- I think 2-stroke engines only work `cos of magic!

Ah yes! water hammer effect:   it happens when you shut a water valve rapidly. The fast shutting-off  causes a rapid rise of water pressure in the column of water such that a shock wave is developed  which rushes from the valve back along the pipe at the speed of sound. It is the speed of sound being broken that gives the sound the name "Water Hammer"....

Perhaps you could explain what happens better with some more diagrams...?

CheerS!

Courtesy & Property Of Trevor Amos.

There you go John , a nice simulation of the exhaust pulse, pressure energy event in a high speed, high power 2t race engine , My Bantam engine shows much the same , but the values are lower , and the Honda is a "neutral " assessment of happenings !
Also included are other waves which are relevant to the earlier postings referencing trans. ports and reed valve function . I will do a pipe event description later , bit pushed for time at present. Once again our grateful thanks must go to Ed for his continuing patience in posting our offerings !

Trevor

Once the ex. pulse is underway, and it takes a while to be fully formed from the initial port opening, the hot pressure wave propagates down the header pipe, initially at sonic speed , with heat loss and expansion the pressure in the cylinder soon deteriorates to almost atmospheric as the wave reaches the first part of the diffuser. The wave continues its journey, reducing pressure in the header as the trans. ports open. This negative wave needs continuing gradient increases to maintain energy but duration diminishes as energy is used up. Hopefully this negative wave can extend all the way back through the cylinder ports and to the carburettor to maintain useful air flow. The wave then reaches the end of the diffuser and propagates towards the rear cone without further wave reflections. All gas contained within the header now remains after the trans. ports have closed, the rear cone now sends a large reflection back toward the cylinder. This reflection is of reduced duration after having dissipated so much in getting this far, the wave passes the parallel section of the pipe and hits the diffuser in the reverse direction. The diff. then acts as a baffle and reflects a small secondary wave, but the main wave makes its way onward to the ex. port and just before the port slams shut, the waiting header gas is forced back into the cylinder to augment combustion . The remaining wave energy continues to reflect back and forth and eventually disappears out of the tail pipe and the whole sequence is repeated for each rev of the crank, so don`t shut the throttle or this will all go to hell in a hand cart !
The pressure wave picture clearly shows the trans. port reverse flow with a spike at the A port, the first to open, and the higher the port goes the more the cylinder pressure impacts upon the flow. If the port is open when residual cylinder pressure exceeds scavenging pressure, flow cannot take place and will not do so until the ratio of trans. to cylinder pressure reduces to at least ONE. The real downside to this is that the trans ports offer another potential escape route, in addition to the ex. port, and spent gas enters the transfer ducts, this will reduce transfer time/area before fresh, meaningful, gas flow can take place . Subsequent combustion will have to commence with a contaminated charge and power will be compromised, and will do so at higher initial temperature, extreme cases could promote deto which will then be combatted with a rich mixture and/or lower comp. ratio, in both cases reducing power.
As far as gas flow is concerned, the number of degrees the port is open is secondary to the actual, real time, the port is effectively open, reverse flow from excessive trans. timing reduces this time. As engine revs reach a peak above the blowdown available, the transfer time/area suffers further and power drops steeply . This may seems counter intuitive but it happens in reality and the sim image reveals the phenomenon quite clearly .
There are a lot of other additional events going on whilst all of this is happening, some of which have been touched on in earlier posts, but it all goes to illustrate that 2t gas dynamics are not simple, it only seems that way, and as Jan Thiel once remarked, get the Aprilia outside the rev band, both below and above, and the engine just stops working , not a lot of good for a Bantam engine !

Trevor

Thanks Trevor!
I regret having never got into 2 stroke tuning and as I have said many times I left it to Derek Neil who was provided with info from his brother Colin (who raced a Bantam)  who worked for Dr Joe Erhlic on the famous EMC ...

Like I said...  I could only afford time to race and sometimes not then by reason of Simms wanting me over the weekend.

I often wondered about extra cooling of the transfers ... With some of our turbocharged  diesel engines we had an  intercooler cooling the extra-pressured air charge and wondered if something similar could be done on a Bantam. I guess the W/C Bantams have this cooling  anyway -- adequate for requirements like?...  

Yes, a thank-you to Edward for providing a good service. I hope that one of these days I´ll get the hang of sending pics but at the moment I am in trouble with `lodgers´ on my e-Mail software and my PC has to go the dealer to get sorted. It was my own fault, of course, but I had what appeared to be genuine e-mails from known friends which when opened turned out to be cyber-business offers. Of course, I should have deleted but like a dumb-dumb I pressed the wrong key.

If you don´t know this already there´s `Fanbox´who does this -- apparently they are not breaking the law but it has given me a lot of aggro and already cost me in having it partially sorted so I could actually send an e-mail. ***Fanbox keep sending me advice messages saying I am in credit by about $650 and want me to co-operate but my computer dealer/expert warns against it....

Until next time, Cheers!

PS -- Who did all the winning at cadwell?
***I wonder if any others posting on here have been hit with this problem.

Life continually presents us with tricky concepts, those niggley things we feel we should know but can`t quite get a handle on. Angle/area , specific time/area are just such cases in point , a bit of geometry a bit of maths and at the end what does it really mean ?
Time, astronomers use a base value of light years, with Bantams, milliseconds for internal events, it`s all a question of scale.

When assessing barrel porting and their suitable dimensions , the first requirement is to calculate angle/area, and this has no time element, it is simply the integral of the area of the port, or ports, multiplied by the number of degrees of crank rotation from port opening to closing.

Time/area involves rpm, the time element and the real reason you have a rev counter fitted, this is calculated by taking your angle/area and dividing it by rpm. Next, by using the time/area number and dividing that by the cubic capacity of the cylinder in question ( don`t forget to include the combustion chamber volume ! ) we arrive at STA, or specific time area ! Because of the relationship between port area and time it`s easy to see why gas flow is not critically dependant upon how many degrees that the port is open for , but on the real time that it has been open for, double the rpm and you halve the time the port is open, another reason to avoid meaningless revs !

Exhaust blowdown measurement is vitally important and is the portion of the port that is open at the instant that the first, or all , of the transfers open, and is arrived at in a similar way, and it is worth repeating that too much or too little blowdown seriously alters power production and delivery.

The synergistic balancing of all of these calculations is at the heart of producing a powerful and well mannered Bantam engine, the quest for a big dyno number is not where its at, power range is far more important for quick lap times , and notice I said range and not band, a different concept ?

More suitable power can always be found with a calculator, when all else fails !

Catch you all later , Trevor

Time was ...

... oh heck! Soppy nostalgia again...

I used to go to Brands and watch the revs climb to 8000, change up and repeat the procedure -- or not -- depending on what the `new´pipe was doing. I remember one pipe being hopelessly wrong with me making several changes of sprockets until realisation hit that Icarus-1 was always going to `burble-burble-brrp! -- burble-. burble´ with the wrong pipe....

If things looked better than last time I´d be slipping the clutch like mad and getting it to max revs, another 400 of them, and mentally noting which part of the circuit it was that max revs came with the clutch fully engaged. Then keep on doing it lap after lap whilst memorising where it was happening and how long it was taking to happen for my notes later....

All very subjective -- no maths,no dyno testing -- just the straight gut feel each time with me making notes of gear changes and max revs (fully clutched) at which part of the circuit. This gave a good comparison of whatever was new with what had gone before.

Then by the time Andy lent me his 250 ABS I was quite good at this test-recording and we managed in a relatively short time to get the ABS as quick as a Greeves Silverstone without any dyno testing.

Really crude but we did´t have the facilities and I did know I was racing when --for instance -- I was alongside John Senior on his Greeves Silverstone ....

I should delete this -- sickly nostalgia again,

Cheers!

Message for Mike Powell only.

2 issues
1) reference our discussion on your current bikes and reed induction you never responded to.
ln the very well presented Video you have posted on tube and face book, I have to say is excellent, If I were to criticise all I would say is it was a shame, it did not show more or other Bantams racing that day, in the paddock, it seems to show just two. ! it was also good to see both bikes back in their original fairings. I have to say they look so much better than those beautifully made abortions you had changed to, my compliments to you for junking them. Anyway to my second point, both bikes In the video I assume are your current ones, but both are piston ported, your sketches and comments are about reed induction barrels !.

2) The deformation post "alter ego", remove it please.

mjpowell wrote:

Derek things I definitely know about my 'current' championship winning bike
Is it uses the barrel I took to the first midland meeting and you had a good
Look at it and posted some pic's on line and on facebook under your female
alter-ego Bsa Bantam Racer. This bike 001 is a little faster than my reed valve
barrel 0.3 sec at Cadwell and my spare bike 003 is a second slower round Mallory.
Hope that sorts your memory out.
Mike

Derek

Trevor, im sure ive read somewhere ( probably jennings 2 stroke book )that the profile/shape of the transfer port tunnel is only important for the  low to mid range power characteristics of an engine and at higher revs it becomes less important, is this still the  current thinking? if you could redesign  the shape/ profile of a cast barrels transfer tunnels what kind of shape would you consider to be a better?( only allowed two transfers) You have already mentioned that a good bantam engine would be one that has a broader power spread rather than high hp so this transformation of the fuel from the crankcase/ up through the transfer tunnels to the port window, to effect this, must be vitally important.Also would the volume of the transfer tunnel have any influence in trying to achieve a better spread of power. thanks, nigel

Hi all
I too would really like to see what Trevor advises on this, I spent hours on the Dyno doing experiments with the inner profile of the transfers, with advice from Trevor, and certainly got some big jumps in lower end power -increasing the spread of power but more importantly the torque curve.

reducing the volume of the duct always seemed to be at the expense of max power, !! not sure this is an accurate real world stance and it may be to do with the confines of my D1 cylinder, that is a lot smaller than the capabilities using a D10/D14 cylinder and liner it, or even the alloy BTW barrels (if you can afford them!) that have huge possibilities, I think I achieved a compromise with my new Pipe, that was so much better overall the last meeting I raced it, we eventually removed all bar tiny pieces of Plastic padding over painted with araldite, from the ducts inner faces ending up with an almost straight wall, after a largish radii at the entry to the duct inner side just like a dolphin nose ! putting it back in all the way up will give a huge jump in torque, but kill top end (this is only on a cast iron cylinder.

looking at the cast iron barrels by far the best transfers I'm sure! are the D14 in std trim, but Im sure a lot of the barrel vary considerably, getting hold of a with careful work they can indeed be improved. finishing of what I found with more of a results orientated stance was that transfer volume affected power more than the time they opened, but with the biggest volume it was difficult with the constraints of the cast cylinder to get any king of reasonable profile.

regards Derek

Nigel, Derek,
Over the past 30yrs or so 2t power from race engines has increased by around 50% but rpm levels by only 15%, I have an original Yam TD2B barrel and the inlet and exhaust port would not look too out of place in a modern engine . However the transfers are radically inferior compared to 2013`s best offerings . Jennings analysis is as outdated as the Yam. barrel, to suggest that a sweeping turbulent free duct can be equalled by a parallel, flat sided alternative is not realistic , and represents outmoded thinking. After saying that we have the advantage of knowing what we do now, he did not, so the best I can say , and this applies to a lot of what Bell has written as well, is that these books are ok for back ground reading, but go else where for the real deal ! I recon there is more forward thinking stuff here on the Bantam forum than in almost all " tuning books" on 2t engines from the recent past !
The outer duct wall radius needs to sweep in a continuing curve, but the inner wall in very important, and the radius should be as large as the physical restraints of the original design will allow. A large rad will discourage flow detachment thus greatly reducing pressure absorbing turbulence and the Coanda effect encourages flow to cling to the duct inner wall , thereby meeting less resistance and being guided to where it is required to go, and transport a greater mass of mixture into the bargain , a real win win. It is the inner wall that also determines the outer curve as there should be a constant reduction in the duct cross section from the crankcase right the way to the port opening. This is all well and good but we have to deal with a Bantam barrel , and Derek is correct to say that the D14 is the best cast iron option, with the BTW, top of the heap, and you pays your money and takes your pick here ! The Aprilia , as a 125, produces more power than the TD2B did as a 250 twin, and the 50cc Kreidler, produced more power than almost all race Bantams , so we still have a long way to go yet and it is the transfer/ scavenging phase that should receive the most development. I would suggest finding out just how far it is possible to re-profile the outer wall in a cast iron barrel leaving a minimum wall thickness and then , as Derek has successfully done, build up the inner wall to promote a more flowing form . Prior to attempting this, determine the area of the port you intend to use and try to ensure that both are the same and that the duct entries have the same areas. Always leave a radius at the four corners of the duct, to aid flow, tight corners create turbulence and kill flow at this local level, and never leave sharp inner edges to trip up flow, eliminating them always pays off .
Duct volumes have been reducing as a result of recent test results, and it stands to reason that if a large volume of fresh charge is sitting in a duct it will take a lot of energy and time to get it moving into the cylinder. Bantams with their modest power and rpm levels are going to struggle to achieve a meaningful flow rate if the duct volume is excessive , optimising the mass to be moved with the pressure difference available, at any particular rev point is tricky to arrive at when having only 3 gears to play with . And it is very well worth remembering that whilst the port is fully open for a few degrees of crank rotation, the duct is open all of the time. The Honda RS producing 40hp has quite modest duct volumes , so a Bantam at just a half of the figure , won`t have need of excessive ones to be able to perform well within it`s own class .

Although a different topic, it`s also of benefit to point out the woolly thinking on exhaust systems from that era. Whilst the tuned length is based on temperature and port timing, and arrived at within a mathematical formula with a one size fits all constant of 1700ft/sec , the header length is an arbitrary function of ex.port diameter , with no insight as to what that diameter should be ? Examining bell we find almost identical theory with his solution for ex. pipe dimensioning ! Their stuff, by and large, makes for interesting reading but care must be exercised to avoid costly dead ends .

I`ll take some more pics of the Honda barrel to show the trans. ducts and ask Ed if he wouldn`t mind posting them on here for us .

Trevor

Thanks Trevor, good reading.I have been trying to find my silicon castings i made of both my bantam barrel and rotax barrel transfer ports, but they have disappeared. i also deleted my aprilia files which showed the various cylinder head measurements, although i thought maybe those sort of radi ect would not be relevent in terms of the cast iron barrel anyway. looking forward to seeing the honda barrel pics.
I would like to take this opportunity in thanking you greatly for all the time and effort in these posting as i know we all have other important things that need servicing first. The thing i find strange again is the amount of viewings of these posts with only a few people contibuting there ideas or experiances. Still, thanks again.

Nigel,
Back in the 60s George Todd was a one of a number of personalities answering questions on a forum in the " Motor Cycle " weekly paper.
Lots of us showered questions on poor George and he was by far the most popular forum member , enthusiasm for all matters Bantam was enormous and we all gained top class information not available anywhere else . The contrast between then and now couldn`t be more starkly reflected than in the absence of contributions, questions and experiences here on this forum, it certainly puzzles me ?
Shame about the mouldings you made of those trans. ducts, they were quite revealing and put the poor Bantam firmly in the category of commuter bike, have another search I`d love to see them again .
Thanks for the nice words, I have to say that it is something I enjoy doing and if just one reader benefits it becomes worthwhile

Cheers, Trevor