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 Thought for the day!

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Trevor Amos



Number of posts : 870
Registration date : 2010-08-13

PostSubject: Thought for the day!   Tue Oct 17, 2017 4:08 am

Time/Area is fundamentally a mathematical concept.
In the cylinder blowdown T/A, the element of time is the crank rotational period (and rpm) between the opening of the exhaust port and the opening of the transfer ports. The area is the exhaust port open area at transfer port first opening. Blowdown gas flow therefore is directed into a deep, full port duct volume which then creates swirls, eddies and general turbulence, the cumulative effect of which is detrimental to the rate of out flow. Raising the floor of the exhaust port and duct in a blending area change to the header, will measurably reduce these unwanted effects and provides a far more efficient, smoother blowdown outflow coefficient. Such a change will materially affect the arithmetic of the T/A value, but the important result is that the whole blowdown cycle is enhanced. As none of the gas flow events in an engine function entirely independently we can offset one negative effect and override it with a positive, so that the consequences are ultimately beneficial to power production.

Trevor
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john bass

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PostSubject: Thought for tomorrow   Wed Oct 18, 2017 5:39 am

Hi Trevor -- I was wondering where you'd got to-

Thought for tomorrow:-
   
    "d(T/A)  over dt" would surely mean the acceleration of port opening which would be nearly equal to the rate of pressure change  before and after the port wouldn't it ....?

Perhaps a graph ....?

Perhaps I'll go back to sleep....        

Glad to see you are back...

Cheers!

JayBee.
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Trevor Amos



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PostSubject: Re: Thought for the day!   Thu Oct 19, 2017 1:29 am

Oh crikey John, I was hoping that differential calculus had been left behind way in my past! The times I cursed Newton for being so damn clever in devising such mental torture on poor inadequate brains like mine.
I`m not certain if the piston is actually accelerating, it`s just about at maximum velocity at exhaust port opening and can only slowdown from there on. The exhaust valve in a four stroke certainly will and it, and the exhaust gas, are at zero velocity just prior to valve opening, with the piston yet to achieve max velocity.
The two stroke application is just so multi-faceted with the gas forcing pressure varying, the port opening varying, gas temperature varying to say nothing of energy pulse coming back to the exhaust port later in the cycle, and of course the time element of varying rpm. Throw in as well the complication of the geometry of stroke and rod lengths adjusting where maximum piston velocity actually occurs, I could get a migraine just thinking about it all.
Trying to find a derivative for incremental steps to solve the problem is just so complicated, at least for me it would be, but then, is it even necessary? The requirements of achieving a 9bar Bantam engine are so straight forward today that plain, simple arithmetic can solve all of the problems.

Trevor,
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nigel breeze

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PostSubject: Re: Thought for the day!   Thu Oct 19, 2017 5:01 am

Evening Trevor, welcome back to "bantam world", cheers missed your posts mate. so blow down T/A is still important although the total exhaust opening time is not? which makes you wonder how high the exhaust port floor can be raised ,along with an enhanced duct exit profile with the negative creating a positive.
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john bass

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PostSubject: Sorry Trevor    Thu Oct 19, 2017 5:39 am

Sorry Trevor...and Jimmie & Nigel,

It was the computer modelling which Jimmie -- Nigel contributed and I was suddenly taken with a fit of thinking in terms of what happens with 4-Stoke valves & cams when they suffere an acceleration of an acceletaion which has the plot differentiated twice and becomes a PULSE and my brains***went into hysterical, violent turbulence.
That's a good point you make about the piston being somewhere at constant velocity where piston acceleration is zero -- or near zero. I was quite hopeless at tuning -- my excuse being that I had to argue the toss with Leyland's engineers about Simms pump cams having Constant-Velocity, hybrid or constant accelertion shape and 4 days a week of such hi-tech had me gölad to get on Icarus-! or the ABS and get rid of all dull care.

Cheers,

John-Boy.
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Trevor Amos



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PostSubject: Re: Thought for the day!   Thu Oct 19, 2017 9:55 pm

Thank you all for the compliments, both here on the forum and privately.
The following may go someway towards a suitable answer Nigel but the next "Thought for the day!" will be more expansive, this is a reply I did a short while ago by email to a fellow enthusiast.

 Question: I`ve read that cylinder blowdown should be as big as possible, but doesn`t this narrow the engine`s power band too much? Should it really be as big as possible in racing engines and is it the same with the exhaust port timing?

Blowdown depends upon four principle and significant factors and they are, exhaust port timing, transfer port timing, total exhaust port width and the coefficient of exhaust flow, surprisingly the latter is not much referenced  in currently available two-stroke literature, but is so fundamental in understanding efficient gas flow! Blowdown time/area is the product of port timing and port area divided by rpm.
For optimum pipe resonance to occur, the exhaust port timing should be 180*effective, where the port is open for the same timing duration as it is closed, but that would have to extend to 190* geometrical for a more realistic operational timing. This is the compromise between true resonance and blowdown angle/area with emphasis on blow down. So if you have a good pipe and a gearbox with enough ratios to keep the engine in its power band then 190* is good for most Bantam competition applications!
Transfer timing should be such that cylinder pressure has dropped below available scavenging pressure by the time the transfer ports first crack open. No scavenge mixture can begin to flow until this pressure differential has been achieved, irrespective of the actual timing.
With exhaust port height determined by its timing the only way to achieve more blowdown angle/area is to utilise more of the bore`s circumference. However the very real danger in doing this, in the absence of the mitigating influence of a power valve, is of losing more of the fresh mixture charge straight out of the big port through short circuiting during mid rpm running. It may not be immediately obvious but that short circuited and unburned mixture serves to cool the hot exhaust gas consequently the engine loses rpm, the pipe then begins to cool and all of those careful, mathematical pipe calculations, based on the local gas velocity, become momentarily redundant! That is until a period of high speed engine running restores the thermal equilibrium along the length of the pipe. Overriding these events is the presumption that carburation and ignition timings are somewhere near being optimised; sadly, the mandated straight line ignition for the big Bantam engines is not one of those cases!
The following analysis may serve to explain some of the short comings of just such a restrictive, non-corrective ignition set up. The most positive response from the engine at low rpm and wide open throttle is achieved with a lot of ignition advance, 25/30*is not excessive, but it certainly would be at high rpm! The downside here is, when running a lot of advance at low rpm and at partial throttle opening, detonation can very quickly take hold.
If then the ignition is retarded to keep the high end safe, throttle response becomes sluggish at low rpm because of slow combustion, caused by partial cylinder filling and a modest dynamic compression ratio.
Bantam engines are very prone to this because numerically they  are gear ratio restricted therefore are unable to adequately maintain engine rpm in an optimum range commensurate with required track speed.
The pressure fluctuations in the typical exhaust pipe to a large extent determine the nature of an engine`s power band.
With the engine rpm at its optimum of peak torque, the exhaust pipe`s wave action will create a low pressure in the cylinder at around BDC that assists scavenging, then a high pressure between transfer closure and exhaust port closure to minimise over-scavenged mixture loss and reintroduce that washed through mixture from the duct/header back to the cylinder before exhaust closure. This movement of mixture back into the cylinder also has the added beneficial effect of creating extra turbulence within the cylinder which is so beneficial to the effectiveness of the ensuing combustion process.
At somewhat lower engine rpm and up to about 2/3 of maximum torque rpm the high pressure pulse reflections from the pipe`s rear cone arrives back at the cylinder much too early, and crucially, when the transfer ports are still open! This pressure rise will at best disrupt scavenge flow, perhaps delay it a bit or much worse, push mixture back into the crankcase together with hot, spent exhaust gas, the resulting adverse effects again disrupt the torque curve. The higher the transfer ports are, the worse the adverse effects this scenario then become.

So it behoves the designer/tuner to think carefully about what the real needs are for good performance from a severely handicapped racing Bantam engine!

Trevor
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nigel breeze

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PostSubject: Re: Thought for the day!   Sat Oct 21, 2017 5:34 am

Having recently tried to ride my bantam, with the cagiva mito  barrel on, with no power valve, i found that while i had to slip the clutch to get it on the pipe  to get away( and it fly's)  it was a nightmare to keep it the power band through the gears (4 speed standard gear box). so ive bought another barrel with the power valve set up intact to try on the fitted barrel.( why buy a barrel? it was the power valve i was interested in and  it turned out considerably cheaper to buy both,than the power valve mechanism on its own???) im going to set it up to manually, via a small choke type lever , to  close the power valve when required at low rpm. i could use the the whole electrically operated set up to do this but at this point but i don’t want to incur the cost!! im only testing.... looking at the barrel i purchased with power valve, i noticed that the auxiliary exhaust ports are not controlled by the power valve only the main exhaust port. The power valve itself closes off a good 1/3 of the main port area when closed giving very little blow-down , if fact if the auxiliary ports were not there it would almost be zero! This cagiva barrels power valve doesn’t normally open until around 8000 rpm and i cant wait to see how the engine pulls up to this point with the power valve fully closed and ithen let the power valve open fully.Sadly i don’t have a torque curve for the cagiva mito  engine to  examine but with the very little blown down im surprised how high it will rev(8000 rpm) with such little blow-down to get on the pipe and power valve to open to take the engine to the next level.
 ill be back:study:
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Trevor Amos



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PostSubject: Re: Thought for the day!   Sat Oct 21, 2017 9:18 am

Nigel, do you still have the hot mail account , if so I can send you by attachment some Aprilia pictures illustrating exactly what you are experimenting with, it might just help.
The two stage power valve starts to lift at 10,000rpm and is fully open at 12,000rpm, depressingly, at least for us Bantam strugglers, the engine pokes out near enough 30hp at 10,000!

I would normally just upload them to the forum but Photobucket are now demanding $400 for the privilege, which I refuse to pay, so they have taken down everything I have previously posted! Any of you clever guys out there can help out here with advice as to an alternative to get around my problem I would be most grateful?
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nigel breeze

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PostSubject: Re: Thought for the day!   Sat Oct 21, 2017 10:34 am

Trevor. still nigel.breeze@hotmail.co.uk. thanks i think i did post you a power valve opening /bhp graph with my prediction of bhp @ 9/10,000 rpm 193* ep open timing giving 35 bhp with the aprilia rsw engine set up.without the power valve even coming into play... wow that will do bounce
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Alan Bateman



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PostSubject: Re: Thought for the day!   Wed Nov 15, 2017 3:12 am

Hi Nigel.
I have been watching this, don't wish to ask silly question's but this post on "power valves"!- really has me.

One question,? did you manage to install a power valve on your cast Barrel yet ?, if so how about some photo's?.

I have an iron cylinder with a reed valve, I'm sure it's seen a better life. Sadly my Bantam falls a little flat after about 10,800 slowly running on to 11,000. Although this is less of an issue in 1st/ 2nd gears.

Top gear seems to be the problem, revs drop to low, and it takes time to pull back past 8,000 rpm where it takes off again.!

I believe a power valve would help it. ! This could also lift the revs ?- "My fast "road tuned" Aprilia125 would pull to 13,500 all day long!. Until it went bang, "mind it had done 28,000 miles". If I were to pick a barrel to use I would prefer the Derbi 125 barrel. ? "what problems did you encounter when you fitted the RS Barrel to your Bantam.?

Regards Alan
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Trevor Amos



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PostSubject: Re: Thought for the day!   Sat Nov 18, 2017 1:27 am

Hello Alan,
Welcome to the Bantam Forum, it`s always good to have a new contributor posting, be assured there are no silly questions on here, someone will always be grateful that you`ve brought up a particular issue, so you are in good company.
Although you have specifically sought the advice of Nigel, the acknowledged power valve man, feeling that just such a device will help with your engines power characteristics, but the answer may be more fundamental and lie elsewhere.
A drop in rpm from 11,000 down to below 8,000, let`s say 7500, is quite significant, has the engine always behaved like that? There are plenty of racing 125s that don`t have a power valve, the Honda RS 125 for instance, and it is very marginal as to the benefits one would bring, and those engines rev out cleanly to 13,500!
You may have a pipe dimension problem, or ignition, carburation or a combination of two or more of these variables. My suggestion would be to solve that problem first to get the engine revving out cleanly and then think about the complication of a power valve installation.
Many years back when Steve Hall and I were developing engines together we had a similar occurrence to yours. Steve had built an up rated, water cooled version of his good air cooled motor but it just would not rev any sense. It should have been a step change up from the previous one. We talked it over and decided it was a pipe problem, a new pipe was designed, constructed and fitted and that engine went on to win three championships with that same unchanged pipe, it pulled like a train and revved out almost 1,000rpm higher, a quite simple solution to an initially worrying dilemma!

Let us know what you think and how you get on,

Regards, Trevor

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john bass

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PostSubject: What about the gears?   Yesterday at 5:33 am

Alan! I humbly forward a SILLY suggestion when you fellows are speaking sensible engine stuff...

   First: Are you speaking of dyno readings or are these figures from the road...?

   If so,  perhaps the 2nd to top is a big ratio jump meaning that changing into top against a slight headwind or shallow uphill slope might need some clutch slip... Er...Er! . This difficultzy shows when comparing downhill (say Cadwell when changing up after Park) the revs should go up rapidly compared with how they are on the flat.

  Maybe the the total engine friction losses have increased. I say that  by your saying "...slowly running on from 10,800 to 11,000....
  Piston and rings are about 40% of total engine losses. There's always an optimum time of piston, rings and bore working together in perfect harmony to give the best-sealing in conjunction with optimum power output at max during the working speed range. I mention this because the Walsh Bantam with megaphone exhaust had one off, 1.5 mm piston ring which was changed beteween practice and race and between races where it beat normally race winning 250s.

OK -- OK! So the Walsh was on methanol fuel and had a special crank but according to Eric Walsh, Bill Lomas and the Aussy riders it "revved on and on..."

Hmmm!

I'm going back to sleep.

Cheers!
Jaybee.
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