Reasons why 500's ping, Now 2-stroke porting idea's ;-)

[FourstrokeForever]

Why on earth would anyone consider using a pod uni filter? There is a good reason why manufactures designed and implemented air boxes. First, to allow the filter to be carried by something substantial. Second, to eliminate as much dust as possible. And third, so the bike can run and be ridden in the rain and mud. Solid mounting a carburetor is only going to make fuel frothing worse. All those lumps, jumps, stutters and everything else on the track is going to transfer to the carburetor if it is solid mounted to the frame.

[Daryl Jones]

Hi Lozza,
More to consider:

Intake charge does NOT go from the cases to the cylinder via the transfers in 1 transfer open period. Intake charge only moves to the transfer ducts replacing the charge already sitting  in the transfer ducts. Then goes from the ducts into the cylinder. This was proven by using radioactive traces in fuel with an engine completing 20-25 cycles to move from carb to exhaust not 3-5. Jan Thiel knew that in the 60's but what would he know   

All case volumes fall into a very narrow range 1.25 at the low end and 1.32 at the high end (cyl to case including transfers and under the piston) , its a factor but not over riding.

I do have a problem with this one. 
If I need 250cc of charge and the case holds 250 X 1.32 = 330cc.  I only have 80cc of charge left in the case which must be topped up by fresh intake. Turn over of charge cannot possibly take 20-25 cycles.
Are you perhaps suggesting that 'some' charge is trapped circulating with the crank in the case and fresh charge bypasses this by flowing directly from the intake through the transfers?


That's pretty easy the small hump at TPO is because that is a VERY well developed engine engines that are not so well developed show much higher spikes and more pressure at the exhaust port. As I have said all along that small increase in pressure is from the residual pressure from the cylinder not from the decending piston.

http://www.thumpertalk.com/topic/881202-engmod2t-software-who-else-uses-it/

That is an interesting reference as it supports my understanding of what is happening here:

dmcca:This one is the pressure trace at 8500rpm, just beyond peak power... you can see that as the transfers open (TPO) pressure in the cylinder is greater than in the transfers so they experience a reverse pressure spike, hence the reason power begins to drop at this point.

rayivers: Your results look great! I'm certainly no expert, but it looks to me like the boost ports may be stealing flow from the main transfers (maybe widen the mains?), and also that (a) faster blowdown (raised and/or wider EP, different pipe w/more aggressive initial flare from the manifold, etc.) might reduce that reverse pressure in the transfers and possibly result in more power.

My thoughts too, Over-speed for the Ex dur., Restricted Blow Down and/or Transfer timing a bit long.
I did look at the timing & value of cyl. pressure & reversion pulses in the transfers and at best you get a positive pulse (restricting flow for say, 7deg), a a smaller negative pulse (charging the transfer from the cyl. or case, which ever pressure is greater) finally a very much smaller positive pulse, but the transfer is near fully open.  Key initial transfer time & flow is restricted with the initial positive pressure pulse.

Pre-ignition and detonation are 2 different things, pre-igniton (as it suggests) happens before the spark event, detonation happens after the spark event from end gases self igniting. Pre-ignition is much more destructive and less under stood (it will wreck an engine in seconds) where detonation is measurable and controllable. In some cases a certain amount of detonation is desirable (about 3-5 dets per km) to make the best power

In between, there are plenty of other things to cause 'pinging' noise.

Ignition may be occurring exactly where the designer intended but the engine might be producing too much cylinder pressure, before TDC.
Plenty of different factors (man-made & natural) can speed up combustion to the point where it is having a negative effect.

I once had to diagnose a bike that was 'pinging' at certain revs. Turned out to be a chamber retaining spring that occasionally vibrated enough to touch the front downtube!!

cheers, Daryl

[Daryl Jones]

Here is another opinion on the transfer port pressure bump effect:

It's by Trevor Amos, who is the 2T "subject matter expert" at the UK Bantam Racing Forum, and a friend & colleague of the much quoted Jan Thiel.

"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".


http://bsabantamracing.editboard.com/t1076p90-timetimingtime-area

Well worth a look at the site, there's heaps of interesting tech stuff here, they are still modifying & developing BSA Bantams for racing, including case & side reed engines.

http://bsabantamracing.editboard.com/f1-bantam-racing-forum

Cheers, Daryl

[Lozza]

Looking at 1 thing in isolation is not quite right, because at any 1 time in a 2T engine there are 2 things happening. Flow needs a a pressure differential as soon as that difference is equalised flow stops(yes that flow wil have inertia and continue) but effectively it is zero. From the graphs you can see case/tx pressure is equalised not long after BDC only the pipe is pulling on the cylinder. Soon  after BDC case pressure starts to drop and the disc starts to open we haven't even got the full duration of the tx from a theoretical maximum. Even at a modest 6000 rpm (100Hz) and 120 deg tx duration 0.0033sec is the maximum time the transfers are open - reverse flow, time to start flow + the inertia there isn't much time to jam 250cc from the cases through ducts. Even if 250cc or more goes through the cases while that is happening the cases are being filled. Two strokes don't have clear defined phases unlike a 4T (suck, squeeze, bang ,wheeze) everything is intertwined and connected. A single unit or block of intake charge does not travel through carb,cases, transfers combustion then out the pipe. It takes 3-5 cycles to go from cases to cylinder.

Before and after peak power the pipe influence isn't as strong, look at a 2 stroke road bike black soot in the transfers everywhere, from being ridden at not an optimum rpm.

Yes a spring ringing can even fool a det counter, spent a full day chasing that one , combustion can never happen fast enough, the faster the burn the more power you will make from a) not having to run as much advance b) peak cylinder pressure at or about 15deg ATDC.

Yes deto can occur in the situation Trevor says(I've seen his name on the ESE tuner thread) less likely to happen when the throttle is open. Which bring a neat circle back to Johns origninal problem, which never happened with the throttle open, reeds didn't open cases didn't fill -faster and greater pressure drop was enough to pull on the seal.
Never heard of that one before

[sleepy]

Never heard of that one before

If you have never heard of a 2 srtoke bike ping on closed throttle before because of a leaking seal I think you should stick to your computer sims and stop waisting our time.

[Rossvickicampbell]

lets throw manners out the window and forget what your mum said 

[sleepy]

lets throw manners out the window and forget what your mum said 

Sorry if I seem rude but the last 4 pages of this thread has just been about Lossa disagreeing with everyone else.

[Daryl Jones]

I feel like we are starting to approach "harmonic resonance".
The 'point' gap, at least, seems to be closing.

My own opinion (and the others quoted) is that a transfer pressure bump is detrimental to the 2T process.

A small bump certainly indicates that TPO is at the very maximum position (well, just beyond it) for the exhaust timing (blow-down).

I am prepared to consider, that in some 'very specific' engine configurations, a small burst of hot exhaust gas into the transfer port might compress the colder transfer charge and then cause it to spring back out of the port.
(Still needs the bleed-down/cylinder pressure to have dropped below the transfer port pressure)

This is more a "compressible fluid" effect,  rather than a "sonic pressure wave" effect.

It might allow the power range to extend a little bit beyond the geometric limit.
It will be very engine specific and a delicate balance, getting it wrong or going too far will have all the negatives spelled out by Trevor Amos.

On the other hand:
There is not enough energy in this 'bump' to produce any significant pressure rise in the crank-case.

Cheers, Daryl.

[Daryl Jones]

Another wonderful example of how friendly, honest, open discussion can resolve conflicts & issues.

Look at all the Yes agreement points.

Flow needs a a pressure differential as soon as that difference is equalised flow stops(yes that flow wil have inertia and continue) but effectively it is zero.

Yes

From the graphs you can see case/tx pressure is equalised not long after BDC only the pipe is pulling on the cylinder.

Yes

Soon after BDC case pressure starts to drop and the disc starts to open

Yes

we haven't even got the full duration of the tx from a theoretical maximum.

Yes, We need to modify this pipe to delay the pressure rise and keep it below transfer pressure until TPC.

Then it would be truly "Optimised"

Even at a modest 6000 rpm (100Hz) and 120 deg tx duration 0.0033sec is the maximum time the transfers are open - reverse flow, time to start flow + the inertia there isn't much time to jam 250cc from the cases through ducts.

Yes,  If we remove the reverse flow with sufficient/efficient blow-down we would save a lot of the effective transfer flow time.


Even if 250cc or more goes through the cases while that is happening the cases are being filled.

Yes?  250cc out - replaced by 250cc in.

Two strokes don't have clear defined phases unlike a 4T (suck, squeeze, bang ,wheeze) everything is intertwined and connected.

Well, Four Stroke racing engines are commonly recognised to have 5 cycles, with the processes during overlap behaving much like a 2T.

....and then there is this one:



A single unit or block of intake charge does not travel through carb, cases, transfers, combustion then out the pipe.
It takes 3-5 cycles to go from cases to cylinder.

What? Not 20-25 1960's Radioactive cycles?  And why do modern 2T race engines lower the intake port, cant the cylinders forward, and even relocate gearboxes to provide the most direct flow from intake to transfer ports to the cylinder? Not much point doing all this, if the charge is only sitting/bouncing around for a few cycles, heating up and waiting for it's turn to transfer and burn.

Cheers, Daryl.

[Lozza]

Never heard of that one before

If you have never heard of a 2 srtoke bike ping on closed throttle before because of a leaking seal I think you should stick to your computer sims and stop waisting our time.

It wasn't a 'leaky' seal at all was it. I think if you don't like the thread because you don't want to or can't understand stop reading. Simple.

I feel like we are starting to approach "harmonic resonance".
The 'point' gap, at least, seems to be closing.

My own opinion (and the others quoted) is that a transfer pressure bump is detrimental to the 2T process.

A small bump certainly indicates that TPO is at the very maximum position (well, just beyond it) for the exhaust timing (blow-down).

I am prepared to consider, that in some 'very specific' engine configurations, a small burst of hot exhaust gas into the transfer port might compress the colder transfer charge and then cause it to spring back out of the port.
(Still needs the bleed-down/cylinder pressure to have dropped below the transfer port pressure)

This is more a "compressible fluid" effect,  rather than a "sonic pressure wave" effect.

It might allow the power range to extend a little bit beyond the geometric limit.
It will be very engine specific and a delicate balance, getting it wrong or going too far will have all the negatives spelled out by Trevor Amos.

On the other hand:
There is not enough energy in this 'bump' to produce any significant pressure rise in the crank-case.

Cheers, Daryl.

On the contary every powerful competition two stroke engine uses that to it's advantage.  Four strokes are nothing like a 2 stroke if it was 2T engines would not still be very reluctant to accept EFI . Again you can't modify pipe design  for 1 very tiny part of the phase without throwing all the aspects of a pipe design that work away.  A 2T engine can never have enough blow down angle area, the more your output goes up the more blown down is required (and vise versa). The amount the cases fill is dependent on how much the throttle is open case pressure is lower without the cases filled. The amount the cases fill has a direct bearing on the crankcase pressure. Then the better the pipe works the better the blow down and the better the plugging pulse which means higher cylinder pressures the more critical the blow down angle area.
Single cylinder engines have the cylinder canted over for vibration reasons mainly, there is a flat stone skipping effect off the spinning crank (loose hp if you reverse the direction of the crank against flow) perhaps why cylinder reeds are near universal angle to the crank. 'Stacked' gearboxes are so you can have a longer swingarm ( as was done on the Aprilia RSA 250).
The radioactive test was made in Aprilia Corse racing dept at Noale in Italy in 97 (I think) with a full time staff of 50 with 4 or 5 GP engineers working under the best two stroke tuner ever. Working 8-9 hours a day 5 days a week with near unlimited budgets with only 1 goal......... more hp. Nothing was left to chance nothing was assumed, everything was trialed and tested. That test showed that it took 20 cycles for the radioactive fuel to appear in the exhaust after introduction. Maybe all those blokes got it all wrong but judging by the results they got I very much doubt it   
What their conclusions were was what happened between the intake going out the transfers and  the squish band starting to work was the most important bit, but then again what would  Jan Thiel know

[Daryl Jones]

Again you can't modify pipe design  for 1 very tiny part of the phase without throwing all the aspects of a pipe design that work away. 

Hardly a Tiny part of the 2T process though!

The principle object of an expansion chamber exhaust is to generate a reversion pressure pulse to 'stuff' fresh charge that has been drawn into the exhaust, back into the cylinder. 

Timing of that pulse can be modified by changing the length of the centre section of the pipe, without throwing the rest out of the design away.

Strength & Duration of the pulse is determined by the cone angle. Pressure in the pipe by stinger diam.  Extraction by the header and diffuser will also be unchanged.

If this modification is successful, and we delay our positive pulse until TPC we could increase the quantity charge transferred and captured for combustion. We could make significantly more power.

Other things will undoubtedly change and of course we will need to adjust.
We might decide to reduce the static compression ratio or change the ignition timing. We might need to modify the exhaust port to allow for increased blow down.

But, of course, this is what tuning for improved performance is all about...

Cheers, Daryl.

[Daryl Jones]

Single cylinder engines have the cylinder canted over for vibration reasons mainly, there is a flat stone skipping effect off the spinning crank (loose hp if you reverse the direction of the crank against flow) perhaps why cylinder reeds are near universal angle to the crank. 'Stacked' gearboxes are so you can have a longer swingarm ( as was done on the Aprilia RSA 250).

A 'vibrating' single is still going to vibrate if you cant it forwards, lay it down, or tip it upside down???

HRC was quoted previously as knowing more than a little  bit about this:

Lovely 'balance' job in the layout of the RS cases. Rolling the ass of the gearbox forward didn't change the sprocket position much.





Cheers, Daryl

PS. thanks to Trevor Amos and the Bantam Forum for the pics.    http://bsabantamracing.editboard.com/t925p30-crankcase-compression-debate?highlight=crankcase

[Lozza]

Again you can't modify pipe design  for 1 very tiny part of the phase without throwing all the aspects of a pipe design that work away. 

Hardly a Tiny part of the 2T process though!

The principle object of an expansion chamber exhaust is to generate a reversion pressure pulse to 'stuff' fresh charge that has been drawn into the exhaust, back into the cylinder. 

Timing of that pulse can be modified by changing the length of the centre section of the pipe, without throwing the rest out of the design away.

Strength & Duration of the pulse is determined by the cone angle. Pressure in the pipe by stinger diam.  Extraction by the header and diffuser will also be unchanged.

If this modification is successful, and we delay our positive pulse until TPC we could increase the quantity charge transferred and captured for combustion. We could make significantly more power.

Other things will undoubtedly change and of course we will need to adjust.
We might decide to reduce the static compression ratio or change the ignition timing. We might need to modify the exhaust port to allow for increased blow down.

But, of course, this is what tuning for improved performance is all about...

Cheers, Daryl.

Change the belly length and that throws out the % lengths of the rest of the pipe. Crude and not effective. Change the heat in the pipe and the gas slows down or speeds up. Change the stinger dia you will notice the increase in exhaust gas temp before the pressure, changing the baffle cone angle will change the power characteristics and delivery and more importanly over rev. All well and good hotting up a TS185 but another world on a proper competition engine.

Single cylinder engines have the cylinder canted over for vibration reasons mainly, there is a flat stone skipping effect off the spinning crank (loose hp if you reverse the direction of the crank against flow) perhaps why cylinder reeds are near universal angle to the crank. 'Stacked' gearboxes are so you can have a longer swingarm ( as was done on the Aprilia RSA 250).

A 'vibrating' single is still going to vibrate if you cant it forwards, lay it down, or tip it upside down???

HRC was quoted previously as knowing more than a little  bit about this:

Lovely 'balance' job in the layout of the RS cases. Rolling the ass of the gearbox forward didn't change the sprocket position much.





Cheers, Daryl

PS. thanks to Trevor Amos and the Bantam Forum for the pics.    http://bsabantamracing.editboard.com/t925p30-crankcase-compression-debate?highlight=crankcase

Angle of the cylinder changes you perception of the damaging vertical vibrations, Manx's G50's have near verticle cylinders for their balance factors. ALL single cyl two strokes are inclined 20-25deg(I'd have to check that) from vertical.
HRC did by putting a balance shaft into the NX4(pretty sure they are the pictured cases). HRC did that to make a better handling chassis by changing to a rising rate rear suspension(from cantilever NF4) and a longer swing arm. Engine length was reduced making more space for a better pipe route. More power better handling= Lower lap times

[Daryl Jones]

Well, weren't they Lucky then, all those major engine redesign mods for bike balance & handling.... and the reed port ends up where it is.
(Sounds like PR trying to hide the Engineering)

And there was me.... thinking you would point out the soot in the transfers!!

It's been Fun, but it's 'Back to the Shed' for me, and the TS185's, the Bully's, the Bantams, the mighty Victa, the B31/3 and the 5-cycle SL125.

You can have the World Beater's, I've just got beaters.

Cheers DJ 

[Lozza]

Yes heaps of fun DJ. Stick to the TS185's the fancy ones are 20 times easier to loose HP than make more.

Looking again I think that could be a NF4 engine same as a CR125 basicaly, if there is soot in the transfers there point the finger at the rider. This is the NX cases shorter and taller. The NSR/RS 250's , various kart engines and the ADM engines all had the reed pisitioned verticaly theory is that was much easier for intake charge to negotiate it's ways past the spinning crank and the rod.