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

[Lozza]

Not even worth the time to argue, your so far from understanding what is going on inside a 2 stroke. You can't even see what is happening on the graph

Well that's your opinion backed by....................old wives tales? Even Bells book talks about this.

As explained before the highest pressure in the crankcase is lower than residual cylinder pressure. 

Stated repeatedly, but neither 'explained' or 'proven'.

Please explain how the pressure in the cylinder (after blow-down) is higher than the pressure at the exhaust port.

Cheers, Daryl.

They are the same never said that port pressure was higher. I said higher than crankcase pressure

A trip down to Spec Savers must be on the cards   The blue trace clearly shows a spike after transfer port open,

Let's try to keep our discussions to Real Facts,
Colour Blindness cannot be corrected with glasses from Spec Savers.


As we have been given no details on whether these are Real measurements, what the sample rate is, or if this is just a simulation, a small bump at this point is hardly proof of the proposition that: "Residual cylinder pressure ALWAYS exceeds crankcase pressure".
l
The graph suggests that the 'effective' TPO point is actually several degrees after the point indicated on the X axis.

Cheers, Daryl.

The blue and green overlay each other just shows the green  . The traces come from EngMod2T, that is taken from Blairs calculations validated by a South African Professor (who worked in Yamaha's GP program from the mid 80's to mid 90's)  at the uni with about 30 Kistler pressure transducers connected to the YZ250 engine. Same 4T software is used by EVERY V8 Supercar team. The results are very much validated by simulated outputs perfectly matching "at the wheel" figures. Feel free to contact http://www.vannik.co.za/index.htm

I see so trying to convince me that with the transfers OPEN no matter how small that opening the decending  piston will cause case pressure rise and "force" charge through the transfers?

Fuel is roughly 80deg C in the cases, measured pressure has never increased by 5 times regardless of expansion and evaporation.

Look at the black trace just as the transfer opens the resonance in the inlet tract moves into the crankcase and again just as the case pressure drops from the ascending piston. red trace shows a corresponding rise in pressure.

I see pressure in the inlet tract rising as soon as IPC . (Piston ported engine, port closed)
Pressure rise from stopping the flow.
Pressure wave bouncing back and forward along the inlet.
No effect on case pressure at second pressure peak. (Piston ported engine, port closed)
Pressure rise in crank-case at 3rd peak corresponding to IPO. (Piston ported engine, port open)

Perhaps some-one can explain what is driving the intake pressure so Negative from 60degrees ATDC.??
The gradual pressure rise to TDC and beyond doesn't look like it's got the energy to do it.

Cheers, Daryl

Assumptions are mistakes awaiting discovery  Would be the first piston port engine ever  with asymetric  inlet timing  The black trace is the pressure trace at the end of the inlet tract. The inlet resonance peaks and troughs shift with rpm that's is just 1 rpm point 12,000 in a Rotax 128 engine. The resonance is not as important on the disc valve as on a reed valve, which (with a reed engine)you can with varying degrees of success shift the peaks to where you want them.

Yes effective opening of the transfers is a tiny bit later than when the window opens, as the port that opens first flows last.

Lozza, you wrote the "transfers are purple, green & blue", would the green & blue be each side transfer and the purple a rear boost/C port?

Excellent question, John.     

The purple (pink) one is responding directly to the pressure drop at the exhaust port.

The green & blue appear to be responding to the pressure in the crank-case.

Cheers, Daryl.

Rotax 128 John @12,000 rpm.
.........and that pressure comes from the cylinder not the decending piston.  Here's an old MOTA shot showing velocity in a transfer port just after the transfer is open. Yes getting that mass in the transfers moving is the secret to a twostroke........... well one of them anyway.

[Tim754]

Lozza's technical's make sense , perhaps reminding me of  my wasted life opportunities.

[sleepy]

Hi Sleepy,

I love this stuff.

Do you have any ideas on what's driving the dramatic negative pressure in the intake AFTER TDC?
It'd suck the air cleaner through the carby.

Perhaps the graph is just a simulation, with some strange parameters.

Cheers DJ

PS. Get some Sleep, I only have to do work around home tomorrow.

I did get some sleep thanks.

The inlet port trace I think is showing the effect of harmonics. I haven't seen much on 2 stroke inlet harmonics but on a 4 stroke the lenght of the intake track can be designed to make use of these pressure pulses. getting the positive pulse to arrive just at the correct time during the inlet phase can push the volumetric efficiency well over 130%. I've got a book here somewhere call "Scientific design of intake and exhaust systems" you can borrow it if you like reading that sort of thing.
A lot more info on the motor in question would be needed before to much more could be thought about, things like intake type, size and RPM etc.

John. You asked about air flow causing pressure drop. Absolutely correct. The more you try and stuff through a hole or transfer port the greater the pressure difference will be at either end.  The differing flow in the tranfers has a lot to do with the different shape coloured traces on the graph.

Got to get back to work now the cylinder rebore count is back up to 15 not to mention all the other work.

[Daryl Jones]

Hi Lozza,

The quote from Bell proves the point that crankcase primary compression is a factor.
The factories moved away from it because it required very short transfer duration periods and very high reving engines with narrow powerbands.

You asked the question as to how the case pressure can increase with the transfers open, it can, the graph shows it can.

I think what you have been trying to suggest is that residual cylinder pressure can also pressurise the crankcase through the transfers.
I have no doubt that in some situations it probably can (especially off the pipe).
In this case, opening exhaust ports later and boost ports earlier (how about a tiny one before EPO) and significantly bigger case volumes should dramatically increase charge transfer, though I imagine the balance point is pretty delicate.

My problem with what you have presented so far is that you have still not explained how the residual pressure in the cylinder can be larger than the falling pressure recorded at the exhaust port which, even on your supplied chart, shows to be pretty well equivalent to crankcase pressure at effective TPO. Also the tiny pressure bump shown in the transfers is not even as high as the case pressure rise, the volume of the crankcase is far greater than the transfers, this is not going to produce the indicated case pressure rise. If it spiked significantly higher than max case pressure I would be more convinced.

I'll give you 1 point for the rotary valve Rotax. It was late & I didn't check the timing symmetry but the lack of response to the second harmonic peak eliminated a reed engine.

Cheers, Daryl.

[sleepy]

You asked the question as to how the case pressure can increase with the transfers open, it can, the graph shows it can.

Can I answer this for you.
That little thing called the piston is still on it's way down the bore and even though the transfers open what is in the crankcase can't get up into the cylinder fast enough to stop the pressure increase. I think a school kid doing science could even work that out.

Keep up the good work Daryl.

[Daryl Jones]

Assumptions are mistakes awaiting discovery  Would be the first piston port engine ever  with asymetric  inlet timing 

They certainly are aren't they:   

Probably not the 'first ever'.

A piston port engine fitted with a reed valve and NO compensating holes in the skirt can have asymmetric intake timing.
(Reed can close on reversion well before bottom of piston skirt closes the port).
Perhaps the worst of both worlds, but it would stop old Bully's loading up.

Cheers, Daryl.

[Daryl Jones]

I think what you have been trying to suggest is that residual cylinder pressure can also pressurise the crankcase through the transfers.
I have no doubt that in some situations it probably can (especially off the pipe).
In this case, opening exhaust ports later and boost ports earlier (how about a tiny one before EPO) and significantly bigger case volumes should dramatically increase charge transfer, though I imagine the balance point is pretty delicate.

Now this has got me thinking, I might have to try this on the Victa.   

A tiny hole (say 3mm) drilled above the exhaust port has been used to reduce compression for easier kick-starting of big bore, high compression 2-strokes.  No 'seat of the pants' detection of any power loss when running.

Lets drill a hole on the inlet side, after the point of maximum con-rod/crank mechanical leverage and before EPO and pipe it into a transfer duct or the crank-case, a small volume of very hot, high pressure exhaust gas to directly pressurise the crankcase.   A side benefit should be a reduction of NOx (from exhaust gas recirculation). That applies to the exhaust stroke, on compression it's just recycling a portion of the charge).

We should have plenty of crankcase pressure available at TPO, (Way in excess of any residual cylinder pressure), we can even delay the TPO to reduce charge losses, (though the engine might get a bit pipey).

We'll work on the chamber design, to get as much charge back into the cylinder as possible and get the 'effective' compression ratio as high as possible.

Now all we need.... is a solution to stop the 'Pinging'. 

[mick25]

I throught pinging was from to much compression I have found this on four strokes I was told to use higher octane fuels Av gas etc , but not always the case .

A few years back I was talking to a race engine engineer he said you can buy a 11 to one ratio piston off the shelf , but it won't be the case when fitted because all barrels are all different from the factory he has to machine some off the bottom of some barrels and some times put two base gaskets on or make a 2 / 4 mm alloy plate to get the right compression rate .

And then there is the head how many times has been skimed / decked over the last 40 years to these old engines.
Anyone can slap some new rings and piston in and thing it's 11 to 0ne or what ever it is but all this has to be measured my a pro .
This may be a bit of topic sorry .

[mick25]

Is it pinging only under load like labouring it at times john .
I found this happening to me on one of my bikes years back .

[John Orchard]

Is it pinging only under load like labouring it at times john .
I found this happening to me on one of my bikes years back .

It turned out to be sucking air through the cranks seals, at high rpm/small throttle openings the crank vacuum was drawing the main seals, all fixed now  :-)  Thanks.

[Daryl Jones]

This may be a bit of topic sorry .

No...actually is the topic.

Pinging sound is due to cylinder pressure that is Too High Before TDC.

Many causes, including compression too high, ignition too advanced, lean mixture, low octane fuel.......Worse still,  hot spots causing pre-ignition and/or detonation.

Experts can elaborate.

Cheers, Daryl

[Lozza]

Hi Lozza,

The quote from Bell proves the point that crankcase primary compression is a factor.
The factories moved away from it because it required very short transfer duration periods and very high reving engines with narrow powerbands.

You asked the question as to how the case pressure can increase with the transfers open, it can, the graph shows it can.

I think what you have been trying to suggest is that residual cylinder pressure can also pressurise the crankcase through the transfers.
I have no doubt that in some situations it probably can (especially off the pipe).
In this case, opening exhaust ports later and boost ports earlier (how about a tiny one before EPO) and significantly bigger case volumes should dramatically increase charge transfer, though I imagine the balance point is pretty delicate.

My problem with what you have presented so far is that you have still not explained how the residual pressure in the cylinder can be larger than the falling pressure recorded at the exhaust port which, even on your supplied chart, shows to be pretty well equivalent to crankcase pressure at effective TPO. Also the tiny pressure bump shown in the transfers is not even as high as the case pressure rise, the volume of the crankcase is far greater than the transfers, this is not going to produce the indicated case pressure rise. If it spiked significantly higher than max case pressure I would be more convinced.

I'll give you 1 point for the rotary valve Rotax. It was late & I didn't check the timing symmetry but the lack of response to the second harmonic peak eliminated a reed engine.

Cheers, Daryl.

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. 
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.
The graph shows the cases getting pressure from the inlet AND from the cylinder, even the clunky old MOTA program shows reverse flow in the transfers at the point of opening.

Now this has got me thinking, I might have to try this on the Victa.   

A tiny hole (say 3mm) drilled above the exhaust port has been used to reduce compression for easier kick-starting of big bore, high compression 2-strokes.  No 'seat of the pants' detection of any power loss when running.

Lets drill a hole on the inlet side, after the point of maximum con-rod/crank mechanical leverage and before EPO and pipe it into a transfer duct or the crank-case, a small volume of very hot, high pressure exhaust gas to directly pressurise the crankcase.   A side benefit should be a reduction of NOx (from exhaust gas recirculation). That applies to the exhaust stroke, on compression it's just recycling a portion of the charge).

We should have plenty of crankcase pressure available at TPO, (Way in excess of any residual cylinder pressure), we can even delay the TPO to reduce charge losses, (though the engine might get a bit pipey).

We'll work on the chamber design, to get as much charge back into the cylinder as possible and get the 'effective' compression ratio as high as possible.

Now all we need.... is a solution to stop the 'Pinging'. 

I suspect what would happen would be the same as the starting hole and perforated rear cone pipes, that those holes would effectively disappear with rpm.

Here are traces from a optimised engine within the confines of the cylinder.
off pipe


on pipe


Pressure in the cases gets higher later when on the pipe.

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

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

You asked the question as to how the case pressure can increase with the transfers open, it can, the graph shows it can.

Can I answer this for you.
That little thing called the piston is still on it's way down the bore and even though the transfers open what is in the crankcase can't get up into the cylinder fast enough to stop the pressure increase. I think a school kid doing science could even work that out.

Keep up the good work Daryl.

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 

[Lozza]

A piston port engine fitted with a reed valve and NO compensating holes in the skirt can have asymmetric intake timing.
(Reed can close on reversion well before bottom of piston skirt closes the port).
Perhaps the worst of both worlds, but it would stop old Bully's loading up.

Cheers, Daryl.

Then it's a reed valve no longer piston port , pistons don't need ports on reed engines case reed engines seem to run just fine.  Yamaha in 1980..something the last year of their piston port OW-31?? YZR500 stopped the fueling up with what must have been a LOT of time spent on the size, shape and length of the inlet tract, that and King Kenny won a title on a 'old tech' bike.

[chrisdespo]

Been thinking about this thread for a few weeks before weighing in my three cents worth, have been a bike mechanic since i started my trade way back in the seventies so have a small idea of things and i always tend to look for the easy things first. big bore twostrokes had few tuning problems a few used to ping a bit but mostly when something was wrong like a bad jetting job or various other problems generaly man made. i like the idea of the fuel foaming or airateing in the fulel bowl that john O came up with and think that he is right on the money someone else may have said this earlier but today i dont have time to read all the posts in this thread but keeping with the KISS principal i came up with this the other day just adjusting an air mixture screw, with my trusty 42 year old screw driver that i do the air screws and pilot jets i put it on the head of the air screw and couldnt keep it in the slot it kept vibrating out of the slot, not something that would normally happen but i put it down to me getting old and blind. a thought crossed my mind that i cant remember carbies haveing that much vibration and with a bit more thought i think that the fuel is foamed by vibration due to the carby and airbox rubbers being as hard as resin i feel that if the rubbers were soft and supple like on a new bike that they would isolate a fair bit of vibration from the carb. may be naybe not but i think this would be a good starting point i always tell my apprentices look for the simple things first as that is generally the cause of the problem.

Just a thought

[mick25]

Been thinking about this thread for a few weeks before weighing in my three cents worth, have been a bike mechanic since i started my trade way back in the seventies so have a small idea of things and i always tend to look for the easy things first. big bore twostrokes had few tuning problems a few used to ping a bit but mostly when something was wrong like a bad jetting job or various other problems generaly man made. i like the idea of the fuel foaming or airateing in the fulel bowl that john O came up with and think that he is right on the money someone else may have said this earlier but today i dont have time to read all the posts in this thread but keeping with the KISS principal i came up with this the other day just adjusting an air mixture screw, with my trusty 42 year old screw driver that i do the air screws and pilot jets i put it on the head of the air screw and couldnt keep it in the slot it kept vibrating out of the slot, not something that would normally happen but i put it down to me getting old and blind. a thought crossed my mind that i cant remember carbies haveing that much vibration and with a bit more thought i think that the fuel is foamed by vibration due to the carby and airbox rubbers being as hard as resin i feel that if the rubbers were soft and supple like on a new bike that they would isolate a fair bit of vibration from the carb. may be naybe not but i think this would be a good starting point i always tell my apprentices look for the simple things first as that is generally the cause of the problem

could the carby be braced off the frame to stop vibration ?
 if you don't use the org air filter box and use a uni filter this will make more carb vibration and movement ,stress on the rubber manifold over bumps and jumps etc , and could cause the manifold to crack and suck  air .