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

[sleepy]

I meant the gearbox part of the crank case As explained before the highest pressure in the crankcase is lower than residual cylinder pressure.  Throttle response more carburation than anything else

Pressure in the crank case(red),transfers(purple green blue) inlet (black) and exhaust(brown)

                                                                                                Pressure in cylinder(red) exhaust(black) crankcase(green)

I've got to disagree again. From the graph it is clear that the crankcase pressure stays higher than both the transfer and the exhaust port pressure for the entire tranfer open period which allows the fresh mixture to flow up into the cylinder. It is also nice to see the chamber doing it job to scavange the cylinder/ exhaust port pressure down to almost equal that of the crankcase just before the transfer open piont and then to see the exhaust port reverse pulse peak just before the exhaust port closes. If the cylinder pressure stayed high as you say the moment the transfers opened their pressure would spike high which they don't.
The graph also clearly shows that the peak pressure in the crankcase of around 1.6 bar which is .6 bar over atmospheric is greater than the amount it drops below of about .7 which is only .3 below.

[Lozza]

I see crankcase pressure exceeding exhaust port pressure between TPO & IPO.
Cylinder pressure = exhaust port pressure, after blow-down.

Second graph shows nothing useful, because the scales of the measurements are so different.
Resolution of a combustion pressure transducer is not suitable for very low & negative pressures.
(Explained by the manufacturer, previously).

Cheers, Daryl

No not at all. The cylinder pressures and the other traces can't exist on the same graph as the cylinder pressure dominates that turns the other traces into flat lines at the bottom of the graph

I've got to disagree again. From the graph it is clear that the crankcase pressure stays higher than both the transfer and the exhaust port pressure for the entire tranfer open period which allows the fresh mixture to flow up into the cylinder. It is also nice to see the chamber doing it job to scavange the cylinder/ exhaust port pressure down to almost equal that of the crankcase just before the transfer open piont and then to see the exhaust port reverse pulse peak just before the exhaust port closes. If the cylinder pressure stayed high as you say the moment the transfers opened their pressure would spike high which they don't.
The graph also clearly shows that the peak pressure in the crankcase of around 1.6 bar which is .6 bar over atmospheric is greater than the amount it drops below of about .7 which is only .3 below.

A trip down to Spec Savers must be on the cards   The blue trace clearly shows a spike after transfer port open, crankcase pressure peaks AFTER the transfers open how can the crankcase pressurise with the transfers open?  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.

[sleepy]

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

[John Orchard]

I guess these graphs will give a broad idea on what happens in a two-stroke engine, things would change quite radically from engine to engine.

Lozza do you know any details of the engine (model, mods, cc etc) that the graphs are from?

[John Orchard]

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?

[Daryl Jones]

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.

[Daryl Jones]

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.

[Daryl Jones]

... crankcase pressure peaks AFTER the transfers open how can the crankcase pressurise with the transfers open? 

This is probably not of any value to Lozza, but others might be interested in this stuff.

When the transfer ports open, as the piston descends, the initial opening is Very Small.
The transfers are still a restriction at this point, they are not fully open until BDC.

The fresh charge in the transfer ports is stationary. It has Mass that needs to be accelerated.
It takes time to get it moving across the cylinder toward the exhaust port.

Meanwhile....the piston is still descending, reducing the volume in the crankcase.
Volume decrease = pressure increase.

Other stuff is happening, the Intake port has closed.
Air & fuel previously drawn into the Crankcase will warm up, from say 20C. to maybe 100C or even more.
This is through contact with cases, crank, rod, lower part of cylinder and the piston, especially the underside of the crown.
As the pressure increases the heat transfer increases even further ( like pushing the steak onto the BBQ plate).
Increase of Temperature X5 = effective pressure increase X5.

That would be significant enough if we were inducting just air, but unless we are running on LPG,
we have a significant proportion of raw fuel particles in the charge. The change in volume of petrol,
as it vapourises in the hot case, is X160. More opportunity for pressure increases in the crank-case.

As the transfers open wider (increasing the volumetric flow), temperature increase stabilises and the rate of volume change falls, as the piston approaches BDC, then the pressures in the transfers and the crankcase equalise. (visible in the graph).

There are a lot of thing going on in a 2-Stroke engine Temperatures, Pressures, Mass flows, and we haven't even looked at fixed amplitude sound waves...yet.

Cheers, Daryl.

[Daryl Jones]

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.

[sleepy]

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.

Daryl you have a lot more patience than me.

I would think that the green trace would be the rear boost port. It normaly opens a little earlier than the others which could account for the slight pressure spick. As the piston is going down at that piont and the cylinder pressure is dropping fast and it would be quite conseivable that the first opened port could see a small back flow increasing it's pressure for a split second.

[Daryl Jones]

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

[Daryl Jones]

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.

[djr]

... crankcase pressure peaks AFTER the transfers open how can the crankcase pressurise with the transfers open? 

This is probably not of any value to Lozza, but others might be interested in this stuff.

When the transfer ports open, as the piston descends, the initial opening is Very Small.
The transfers are still a restriction at this point, they are not fully open until BDC.

The fresh charge in the transfer ports is stationary. It has Mass that needs to be accelerated.
It takes time to get it moving across the cylinder toward the exhaust port.

Meanwhile....the piston is still descending, reducing the volume in the crankcase.
Volume decrease = pressure increase.

Other stuff is happening, the Intake port has closed.
Air & fuel previously drawn into the Crankcase will warm up, from say 20C. to maybe 100C or even more.
This is through contact with cases, crank, rod, lower part of cylinder and the piston, especially the underside of the crown.
As the pressure increases the heat transfer increases even further ( like pushing the steak onto the BBQ plate).
Increase of Temperature X5 = effective pressure increase X5.

That would be significant enough if we were inducting just air, but unless we are running on LPG,
we have a significant proportion of raw fuel particles in the charge. The change in volume of petrol,
as it vapourises in the hot case, is X160. More opportunity for pressure increases in the crank-case.

As the transfers open wider (increasing the volumetric flow), temperature increase stabilises and the rate of volume change falls, as the piston approaches BDC, then the pressures in the transfers and the crankcase equalise. (visible in the graph).

There are a lot of thing going on in a 2-Stroke engine Temperatures, Pressures, Mass flows, and we haven't even looked at fixed amplitude sound waves...yet.

Cheers, Daryl.

Thank you    Daryl Jones , for a clear & rational explanation,  interesting reading.

[John Orchard]

Yep very interesting stuff, bizarre stuff happens in a two-stroke  :-)  I am wondering about another point; the faster air moves, the more the drop in pressure, could that be changing pressure readings?

[FourstrokeForever]

... crankcase pressure peaks AFTER the transfers open how can the crankcase pressurise with the transfers open? 

This is probably not of any value to Lozza, but others might be interested in this stuff.

When the transfer ports open, as the piston descends, the initial opening is Very Small.
The transfers are still a restriction at this point, they are not fully open until BDC.

The fresh charge in the transfer ports is stationary. It has Mass that needs to be accelerated.
It takes time to get it moving across the cylinder toward the exhaust port.

Meanwhile....the piston is still descending, reducing the volume in the crankcase.
Volume decrease = pressure increase.

Other stuff is happening, the Intake port has closed.
Air & fuel previously drawn into the Crankcase will warm up, from say 20C. to maybe 100C or even more.
This is through contact with cases, crank, rod, lower part of cylinder and the piston, especially the underside of the crown.
As the pressure increases the heat transfer increases even further ( like pushing the steak onto the BBQ plate).
Increase of Temperature X5 = effective pressure increase X5.

That would be significant enough if we were inducting just air, but unless we are running on LPG,
we have a significant proportion of raw fuel particles in the charge. The change in volume of petrol,
as it vapourises in the hot case, is X160. More opportunity for pressure increases in the crank-case.

As the transfers open wider (increasing the volumetric flow), temperature increase stabilises and the rate of volume change falls, as the piston approaches BDC, then the pressures in the transfers and the crankcase equalise. (visible in the graph).

There are a lot of thing going on in a 2-Stroke engine Temperatures, Pressures, Mass flows, and we haven't even looked at fixed amplitude sound waves...yet.

Cheers, Daryl.

Thank you    Daryl Jones , for a clear & rational explanation,  interesting reading.

And that would pretty much sum it up for me too. Thanks