Torque talk Trevor...

Weird but maybe successful.


I could only get the first page with the action shown.
It could be that reliability suffers with excessive  Hertz Stress, which  was the problem of cam driven pistons in previous arrangements....

Thanks for an interesting looky-looky, Les...

The 2000rpm optimisation might be a clue to how good the reliability is....


Cheers!

so.... the transfer port opening and closing period is short and sweet..... if it could be extended for the whole of the transfer port open duration at the same crankcase initial pressure , how would that effect the torque of an engine? : if it were possible (unlikley ) would the exhaust port area have to change proportionally, due to increased transfer port flow duration?

Now that`s an interesting hypothesis Nigel, I think I see where you`re going with this, or maybe not!
Is that a constant transfer area for the same timing, and what is the piston doing?

Any increase in transfer port area can offer the prospect of greater efficiency, this is why we now use 5 ports instead of the old two; greater bulk flow for the same time frame! Be great not to have the piston edge getting in the way and spoiling the fun, conventionally the only time the ports are fully free flowing and mixture is at maximum velocity is the dwell period around BDC, and con-rod length influences even that brief stay!

Taking the last point first, there will be no greater duration, angular duration that is, assuming the transfer ports remain the same height, but there will be a much greater mass flow for that fixed duration.
With all other aspects remaining the same, the potential is there for retention of a far purer combustion charge, the scavenge streams having greater area bulk flow potential for displacing more of the hot impurities of spent combustion gas out of the exhaust port, loss of some charge fresh might be worthwhile, the piston crown will be cooler for it! The result would be higher pressure and temperature in a shorter and more complete burn time. Shorten the burn time and ignition can be timed later and the exhaust pipe will see higher energy level and higher acoustic gas speed. All of which can be exploited for making more torque! The forgoing assumes, for the moment, that trapping efficiency stays the same.
This is where the exhaust port height, blowdown duration, pipe diffuser and tuned length action combine to retain more of that nice clean, fresh, cool and pure mixture. Even with the same trapping efficiency power rises by virtue of more fuel molecules bonding with available oxygen and as the mix is cooler and therefore, denser, it becomes a win-win all round!
There has to be cylinder blow-down to enable the rubbish to be extracted and to allow cylinder pressure to drop below the crankcase pressure prior to the transfers opening. Failure to do this converts the transfers ducts into extra exhaust ports and when pressures equalize, the emerging scavenge stream is hot and heavily contaminated. Hot always migrates to cold and high pressure to low, no way around it without physical barriers. It make no difference to the cylinder pressure where is escapes to, but can it makes a huge difference to the performance potential and possible thermal loads!
I don`t think the exhaust port would necessarily need to change but if overall efficiency is upped then it might be possible to get the timing to a point where the pipe tuned length is closer to it`s natural acoustic resonance of a  simple fraction of the revolution,- -180*; but the tuned length alters with rpm and temperature?
As for crankcase pressure, when you are running in a high torque band the case is no longer a pump, it becomes a resonator with the exhaust pipe orchestrating mixture movement and the diffuser is the conductor! But, with your scenario torque should improve from a lower rpm up to peak revs, but down there, a bit of assistance from the case always helps. The $64,000 question is, how are you going to arrange 24/7 constant area transfer ports and still have a piston going up and down? If you can do it lot of extra torque is there for the taking.

Quieten down there at the back, the challenge for you today is to………….???  

Cheers, Trevor

Heres a graph taken from another web site.. it shows the rise an fall of pressure in  two sets of transfers. the red curve a standard engine and the yellow curve, of the same engine, with the modified transfers. This graph is taken from an sae paper of work by Manuel Sevilla Sanz.

Wonderful pictures  of engines´ "breathing,"  Nigel...

Thank-you.

 -- and am  I correct saying the area under the curves is the true indication of the efficiency of transfer -- not that the pressure represents the air-charge amount.

The yellow curve  proves the point that not only is it transferring for the whole period of transfer port being  open but also more effectively by the pressure being fairly constant throughout the port being open....
 
... I´m thinking Bounilli comes into it here with high pressure (red curve at start) meaning slower velocity -- which proves the modification (called "Tuning"  outside the hallowed walls of IMechE) is  telling the tuners they are doing the right thing -- nearly?...  

The SAE paper ought to mention the instrumentation (telemetry)  used Nigel -- is that so? I was wondering where the pressure sensor(s) would be located ...

Inside the  transfer port would  suggest there´s a leakage  as shown by pressure rise before the port opens and similarly pressure drop  after transfer port closes which adds another feature for the tuner to think about --which would suggest  the port "works" for quite a bit longer period than the measured time of of open to close ...  


Cheers!

JayBee.

PS -- if to scale then the modifird port is actually open for 1 & 3/4 its `Tmed` period of opening & closing. If positive pressure indicates  flow, then doesn´t this add another tricky bit to the overall  'Tuning'  parameters ? ...

...that then raises the question -- how did the author measure the opening and closing  as compared with how Bantam tuners dmeasure transfer timing....?

Cheers!

It might be useful to establish a little back ground to this engine and the investigation which produced these results. This work dates from 1984 and was published in 85; so it is 30+ years old! The aim was to modify induction flow through the inlet and transfers; asymmetrically relative to BDC, whereas a normal engine has symmetrically timed transfer and exhaust ports!
The actual engine is “blower” equipped through an integrated, auxiliary cylinder which also serves to counteract the out of balance forces of the main, single cylinder, engine. Conceptually, the engine was aimed for the motorcycle Trials fraternity where road and engine speeds are very low so efficiency at elevated performance level was not considered.
The graphs reflect case pressure, the red curve being the standard engine and is fairly typical, and the yellow, the modified engine. Even though less air enters at a lower pressure the timed blower continues to deliver more air well after BDC and on up to port closure. The benefits for low speed operation are plain to see. The best us Bantam tuners could hope for is a well performing exhaust pipe to extract case mixture and have reeds opening for a little extra, input!
Whilst a conventional two stroke engine has a lot of inherent short comings, it is difficult to improve on without adding mechanical complications, at least where utilitarian use of engines is concerned. The yet to be revealed Ryger may change all that?

Trevor

Sorry John i haven’t access to the sae paper only snippets

but i can see that the reduced pressure for the yellow line at initial transfer opening is maybe half the that of the red line, telling me that the flow would indeed be slower and that possibly less of the charge would be lost out the exhaust and maybe upsetting the scavenging as well but the secondary phase (second peak of yellow line ) would extend the flow of transfer for a longer period enhancing the trapping efficiency?

The red line transfers pressure falls as it releases the fuel into the the cylinder and apparently up to 30% out the exhaust port, wouldn’t it be beneficial to open the transfers to the atmosphere for a duration to raise the pressure in the cylinder? Even if pure ish air were to be used then having a richened initial charge would keep the mixture to the correct A/F.?

Thanks Nigel...

Opening the port to atmosphere -- intersting concept alright!!***

With my  RE 125 work Hack in 1951  I drilled One, one-eighth-inch hole, low in each transfor port -- not really knowing what the hell I was doing -- just for the heck of it like. What seemed to happen was that on throttle opening the flow of tansfer charge sucked in pure air to weaken off the mixtire . The RE Flea did buzz a lot better, sounded more punchy  and the plug did not oil up at all in years of alter service. Trouble was oil seepage was all over my feet when shutting the trottle -- it oozed out in frothy heaps -- so I wore plastic bags over my foot wear which were handy when it rained ....
Fact is the engine was running weaker than RE would like for their historical reliability and in its way was bit of "Tuning" -- very chancy of course.

I reckon you are right though, the A/F ratio could be corrected.

How is the tranfer port timing measured?  -- I thought the accepted way  was when the upper edge of the piston crown was level with the  port edge and somehow niggling is the piston ring probably being the true cut-in and cut-off points of beginning and end transfer....

Put me right -- somebody --please!
 
Trevor has introfduced another aspect on that special engine of -- hmmm-- would you call it supercharging???...

*** A follow on from that would be the holes connected by pipe to an oxygen bottle with an oil-sump lower down from the hole so that the heavy oil dropped to the bottom and the engine with a puff-puff of oxygen at every combustion  would craete a phenomnal power increase.

I think I shall have to delete that last bit as being repugnantly cheating....

Cheers!

JayBee.

The concept 30 years old -- so what, Mike Powell just said his current winning engine is over that in age and still winning...

OK, OK!

I´ll go back to sleep....

John, i was thinking a more controlled situation, perhaps a one way valve being opened and closed at the appropriate time .seem to remember someone saying " 2strokes love air"... supercharging is not spoken about in the rules.

What is being seen here is case pressure set against crank rotation, measured using a pressure transducer, interfaced to a computer, and it will only reflect the highest pressure and cannot differentiate between fluctuations within the whole, under piston volume. Only multiple transducers will expand the picture. Neither is the transfer flow efficiency represented, the designed in geometry of the engine ducting does this. The spike in the yellow line indicates a pressure increase and this is most probably the timed blower beginning to doing it`s job.

It is only the ratio of pressure differences created within the design of engine that moves mixture, and the port timings contribute to determine the quantity of mixture moved within their timing constraints. The introduction of an external, pressure producing device, the blower, changes the whole dynamics of gas flow, this is what the image represents!

The piston ascending from bdc increases case volume and pressure falls, the very reason why induction happens; atmospheric pressure set against sub-atmospheric pressure, low against high. The tail end of the yellow graph indicates case pressure is still high after transfer closure, the blower is still operating. The, out of phase, asymmetry of engine operating parameters creates the conditions for greater mixture flow in this low rpm engine. In a conventional engine with the piston rapidly closing the transfer ports and case pressure falling, efficiency falters, the blower chiming in at this point maintains pressure and subsequent cylinder filling.

To a certain extent the absence of a resonant exhaust system, pulling mixture from the crankcase right through the transfer phase, is supplanted by the blower, the use that trials engines are put to is unsuited to the application of tuned pipes. Simple, waste exhaust gas extraction is all that is available to be exploited.

Incidentally, the engine in question is a much modified 250cc Spanish Montesa and as such is a specifically used for low speed trialing over rough terrain with constantly varying throttle use providing control. The `Instituto de Technicus Energeticas` were consultants to Montesa and were responsible for the work involved in this project.

Trevor

From what you say Trevor it sounds like one of those papers similar to the Bill Lomas criticism of Royal Enfield´s Chief Development Engineer running odd experiments so he (CDE) could read a paper at the IMechE Institute -- SAE, of course, copy.. they really do get some extraordinary presentations where sometimes the wheel is reinvented ....


Nigel,  I was not thinking of rules .... but there seems to be no rule against having an air squirter into the combustion chamber but it would have to be capable of squirting air at a pressure greater than  combustion pressure.

... Rules  are only made to be broken I think is an unfair comment because some competitors do break the rules even if only for a tin-pot at season end.

What you said about a valve controlling  extra air was what I had intended and the little holes were just to see  what would happen over a prolomgred period of running and at the transfer pressure a timed solenoid arrangemant would do --I think!?. At the time (early fifties)  electronic control -- timing -- was by large, glass  thermionic valves and I was so busy building grass specials and falling off them I never got round to doing  anything  further than two small holes....

I have no doubt,  if you attached such to the transfers and beat them all up someone would find you guilty of breaking the rules....

... with that in mind -- looking at the 125 formula -- the `barrel has to be "Standard" and can only be mofified internaly'  which means bolted-on tranfer-tunnels, or something bolted onto existing transfer-tunnels  must be illegal.
Reading that bit, in the Formula again where a sentence is in brackets (after standard barrel) it rather looks as if "liquid-cooled" is also not permitted.

Other things about tuning to  Bantam Formula  have me puzzled but I´d better shut up -- for now!! maybe Trevor
would pick up on one or two a bit later -- like port shape, particularly exhaust ....

Cheers!

John-Boy.