2 stroke thread (with occasional F1 relevance!)

[manolis]

Hello Pinger.

I wrote:
“After the closing of the ports a 2-stroke gives a quite similar plot.”


You write:
“After closing - when compression can commence. 4T inlet valve closing very soon after BDC, your final port closure much, much later. The comparison to 4T is dubious at best.”



This is from Taylor’s book “The Internal Combustion Engine in Theory and Practice”:



Taylor writes:
“after all port are closed, the cycle proceeds through compression, combustion, and expansion, as in the four-stroke engine.”


The argument of Taylor is so simple, straight and obvious that nobody can argue with it.

So,
keeping the exhaust port closed for another 10 crankshaft degrees (from 80 degrees after the TDC in the Yamaha, at 90 degrees after the TDC in the PatATE), you have the same gain in power, in torque etc in the four-stroke and in the two-stroke engines.

The analysis for the gains in the 4-stroke is simple (it is based mainly on the torque plot of Taylor).

By the way, by keeping the exhaust port closed for another 10 degrees at the middle-stroke, there is one more gain in the two-stroke: the pressure and the temperature of the gas at the beginning of the exhaust is lower (because of the extended expansion in the cylinder).

So, the 8% gain on power and on torque is conservative. A 10% seems more close to reality.




You also write:
“So port opening is piston governed and no faster than in any other engine?”


The port opening is not just faster,
It is way faster in the PatATE,
even when the PatATE uses less aggressive timing than the conventional 2-stroke engines.


How?


Just look the port-map of the Yamaha RD350LC:



The exhaust port extends for about 80 degrees along the periphery of the cylinder (this is so because the exhaust port has to leave the rest periphery of the cylinder for the (more than two times shorter) transfer ports).

In comparison, the exhaust port (actually the hybrid port during the beginning of the exhaust) of the PatATE extends along the periphery of the cylinder for more than160 degrees.


This explains why the rate of exhaust port opening is about double in the PatATE than in a racing 2-stroke like the Yamaha RD350LC of ‘86.


This is what “says” the slope of the exhaust curves (red for the PatATE and green for the Yamaha RD350LC):



Thanks

[Pinger]

Taylor writes:
“after all port are closed, the cycle proceeds through compression, combustion, and expansion, as in the four-stroke engine.”


The argument of Taylor is so simple, straight and obvious that nobody can argue with it.

What he says is correct but omits the very obvious fact that the 2T will employ a much lower percentage of its stroke to accomplish this than will the 4T

Comparisons of compression/expansion ratios between 2T and 4T are meaningless until you factor in that one calculates its CR from port closing, the other from the bottom of its stroke, And therein lies the fundamental difference. The 2T has by exhaust port opening expanded its charge by the same amount as the CR. The 4T midstroke is only half way through its expansion so will obviously have more energy still to be tapped.
If the point of how much of stroke each uses for compression is still not accepted, try exchanging combustion chamber volumes between the two and observe what occurs.

So,
keeping the exhaust port closed for another 10 crankshaft degrees (from 80 degrees after the TDC in the Yamaha, at 90 degrees after the TDC in the PatATE), you have the same gain in power, in torque etc in the four-stroke and in the two-stroke engines.

OK, go that route and alter the clearance volume in the combustion chamber as you must, Now delay transfer port opening as must also be done to preserve blowdown period and you are now in the territory many tried before that the limited transfer period occurring later causes transfer gas streams to assume such velocities that at anything other than very high rpm they were squirted straight out the exhaust port.You can of course consider delaying transfer even further but beyond BDC crankcase pressure is diminishing and a blower would be required.

By the way, by keeping the exhaust port closed for another 10 degrees at the middle-stroke, there is one more gain in the two-stroke: the pressure and the temperature of the gas at the beginning of the exhaust is lower (because of the extended expansion in the cylinder).

Basic thermodynamics - no more than that and certainly not applicable to 2T only.

So, the 8% gain on power and on torque is conservative. A 10% seems more close to reality.

Where's the additional 2% suddenly appeared from when the reality is gains in the 2T are more likely to be little more than half of the 8% gain in the 4T?

You also write:
“So port opening is piston governed and no faster than in any other engine?”


The port opening is not just faster,
It is way faster in the PatATE,
even when the PatATE uses less aggressive timing than the conventional 2-stroke engines.

I get that but when this superfast opening port starts opening, is there direct communication between the cylinder and where the gasses are to ultimately arrive? Or is the piston still in the way and said communication does not occur until such time as it too has uncovered the port?

[manolis]

Hello Pinger

You write:
“I get that but when this superfast opening port starts opening, is there direct communication between the cylinder and where the gasses are to ultimately arrive? Or is the piston still in the way and said communication does not occur until such time as it too has uncovered the port?”


Here it is shown, from the exhaust port side (consider the brown “rectangular part” as the exhaust gasket), the hybrid port of the PatATE:

at 90 crankshaft degrees before the BDC (the piston starts opening the hybrid port, the red rotary valve is at the back side of the cylinder, and there are no restrictions / obstacles, at all, between the hybrid port and the exhaust) :



at 81 crankshaft degrees before the BDC:



at 72 crankshaft degrees before the BDC:



at 63 crankshaft degrees before the BDC:



at 54 crankshaft degrees before the BDC:



and at 45 crankshaft degrees before the BDC:




In comparison to the Yamaha RD350LC wherein the same piston, that performs the same piston motion, uncovers an 80 degrees wide (measured along the periphery of the cylinder) exhaust port, in the PatATE the piston uncovers a more than 160 degrees wide (measured along the periphery of the cylinder) “exhaust” port.

With the same timing the PatATE gives a more than double rate of exhaust port area increase, which is crucial for faster blowdown.

Even with a substantially less-aggressive timing (say 150 degrees duration of the hybrid port instead of the 200 degrees duration of the exhaust port of the Yamaha), the PatATE still provides faster increase of the exhaust area at the beginning of the exhaust.

The rotary valve uses the time during the compression and the expansion in order to come in the right angle / position, so that during the opening of the hybrid port by the piston the “working” port are to increase substantially faster than in the conventional 2-stroke engines.

So, besides the early closing of the exhaust, which can reduce substantially the emissions and turn the 2-stroke into a serious competitor of the 4-stroke, the PatATE also provides a substantially more aggressive opening of the exhaust (and closing of the transfer), which can improve the high revving capacity of the engine, as well as the power output.

Thanks
Manolis Pattakos

[Pinger]

Glad we've got that cleared up then!
So, yes, piston controlled but faster opening due to wider port. Not a first though as cross-flows have employed something close to 120 degrees of cylinder circumference for each belt of ports (Blair cites fuller use of the cylinder's circumference as a key cross-flow advantage) and have enjoyed lower port heights as a consequence.

[J.A.W.]

...Just look the port-map of the Yamaha RD350LC...

...This explains why the rate of exhaust port opening is about double in the PatATE than in a racing 2-stroke like the Yamaha RD350LC of ‘86...

Thanks

Hi Manolis..

Just an FYI..
..the Yamaha RD/RZ series machines were very much practical road-legal bikes, & were built down to a price..
& in reality, were fairly far removed from contemporary 'pukka' 2T race bikes, even if.. of nominal 'race-style'..

[manolis]

Hello J.A.W.

Here is the port map of the Yamaha RD500 Racing

Unless I am wrong, it is an official modification of the standard RD500 in order the peak power to increase by 30% and the revs of the peak power to rise from 9,500rpm to 10,500rpm.



Based on the data provided by Yamaha in the port map, the duration of the exhaust is 190 degrees (the exhaust opens 95 degrees before the BDC) and the duration of the transfer is 125 degrees.

The timing appears less aggressive than in the RD350LC.

The exhaust and transfer port areas of the Yamaha RD500 Racing versus the crankshaft angle are quite close to those shown in the PatATE vs the Yamaha RD350LC graph (as in the RD350LC, similarly in the RD500 Racing the maximum transfer port area (piston at the BDC) is some 15% bigger than the maximum exhaust port area).


According the previous, with less aggressive timing, the PatATE achieves a more than double rate of exhaust opening than the Racing RD500.

This is important for the peak power and torque of a 2-stroke engine.
According the previous, the PatATE appears more racing than the racing 2-strokes!


Way more important is that while the RD500 closes its exhaust 32.5 crank degrees after the transfer (providing plenty of time to the fresh mixture to escape from the exhaust unburned), the PatATE closes the exhaust several (like 20) degrees before the transfer.

The early closing of the exhaust is a requirement (it is a must) for “4-stroke-like emissions” and for better fuel efficiency.

For instance, the KTM300EXC of 2018 is OK with the current Euro4 emission regulations. However the same KTM300EXC is gonna have a big problem in 2020 when the Euro5 emission regulations are to be in force.

Thanks
Manolis Pattakos

[Pinger]

The early closing of the exhaust is a requirement (it is a must) for “4-stroke-like emissions” and for better fuel efficiency.

Yes, but to look for it and expect to find it on a race RD500 is nonsensical.

Way more important is that while the RD500 closes its exhaust 32.5 crank degrees after the transfer (providing plenty of time to the fresh mixture to escape from the exhaust unburned), the PatATE closes the exhaust several (like 20) degrees before the transfer.

But the mixture doesn't escape at its operating rpm, The converse occurs when the considerable overspill into the exhaust is returned and a 2bar pressure maintained at the exhaust port as it is closed. Thus, its power potential far exceeds what is possible with a mechanically closing valve.

As I said previously, context is everything.

[manolis]

Hello Pinger.

You write:
“But the mixture doesn't escape at its operating rpm, The converse occurs when the considerable overspill into the exhaust is returned and a 2bar pressure maintained at the exhaust port as it is closed. Thus, its power potential far exceeds what is possible with a mechanically closing valve.
As I said previously, context is everything.”


In all engines there is involved “a mechanically closing valve” that actually traps the fresh charge into the cylinder before the compression.

In the conventional 2-strokes this valve is the piston closing the exhaust port.

In the 4-stroke engines this valve is the intake poppet valve.

In the PatATE 2-stroke this valve is the piston closing the hybrid/transfer port.



The specific torque provided by the 4-stroke Ducati Panigale Desmo (which is based on a pure mechanical control over its poppet valves, without VVT / phaser) exceeds by far the half of the specific torque of any 2-stroke.

In the following plot it is shown the torque of the 1299 and 1199 Ducati Panigale:



The specific torque is 110mN/lt.

The flat torque curve (from 8,000 to 10,500 rpm) is one more characteristic not met in the 2-strokes.

The exhaust helps the induction of the fresh charge into the cylinder of the Panigale only during the overlap of the intake and exhaust valves.

After the overlap, everything is rely exclusively on the intake system.

And judging from the torque provided, the intake system makes an excellent “job”.

As the tuned exhaust of the conventional 2-stroke over-boosts the cylinder (provided the engine is running “at its operating rpm” (tuned?), as you write), similarly the intake system over-boosts the cylinder of the 4-stroke.

And despite the increased mechanical friction of the 4-stroke (one power stroke per two crankshaft rotations), it provides more energy per power stroke than a similar 2-stroke.



Now take the case of the PatATE.

The compression starts when the piston closes the hybrid / transfer port. A properly designed intake system can over-boost the cylinder in a way similar to that of the Panigale.

In such a case the exhaust plays a secondary role: it helps (as the exhaust of the Panigale 4-stroke); however the dominant role belongs to the induction system (as in the Panigale 4-stroke).

Contrary to the conventional 2-strokes, the PatATE needs not to be based on a tuned exhaust (Kaaden).
However its characteristically faster exhaust opening (faster in comparison to the conventional 2-stroke engines) causes a faster blow-down which is crucial at higher revs.


The PatATE seems to combine the characteristics of the 2-strokes (like: one power stroke per crankshaft rotation, simplicity, lightweight, high specific power, low cost, etc) with the characteristics of the 4-strokes (like: better lubrication and lower lube specific consumption, uniform temperature of the cylinder liner and of the piston skirt, scuffing reliability, earlier closing of the exhaust, compliance to emission regulations, better fuel efficiency etc) and with some special characteristic of its own, like the absence of red hot temperatures on the exhaust port or exhaust valve, etc.

Thanks
Manolis Pattakos

[henry]

Manolis,

Why are the PatATE lubrication characteristics any different from a conventional 2 stroke?

Surely a conventional 2 stroke with a tuned exhaust could also have a tuned inlet?

[Pinger]

In all engines there is involved “a mechanically closing valve” that actually traps the fresh charge into the cylinder before the compression.

In the conventional 2-strokes this valve is the piston closing the exhaust port.

A valve that before closing conspires to push half the cylinder's contents into the exhaust unburned.

The specific torque provided by the 4-stroke Ducati Panigale Desmo (which is based on a pure mechanical control over its poppet valves, without VVT / phaser) exceeds by far the half of the specific torque of any 2-stroke.



The flat torque curve (from 8,000 to 10,500 rpm) is one more characteristic not met in the 2-strokes.

Ducati torque is 106lb.ft, 81.5lb.ft/litre. Apply the same specific figure to the E-Tec V6's 200lb.ft and the capacity comes out at 2.45litres - about right. The V6 will have cost buttons to build compared to the Ducati - but yet matches it on specific torque.
It doesn't aim or have to be filling its cylinder to full capacity because it has the luxury of twice as many firing strokes so comparisons with what a 4T must do have no real meaning. This in itself will be the virtue that will permit it to be revisited as 4T cannot now cope with NOx legislation.
However, comparing a lowly tuned outboard with the Ducati engine isn't the comparison that the Ducati should face. Instead, pit it against a race tuned 2T with a BMEP of 12bar (or higher).

And despite the increased mechanical friction of the 4-stroke (one power stroke per two crankshaft rotations), it provides more energy per power stroke than a similar 2-stroke.

Ducati 157.7hp/litre. Hardly the 440hp/litre 2T were delivering a decade ago.
As before the Ducati has to do it all on one stroke in four. Does it provide more per power stroke than a truly similar 2T?

Now take the case of the PatATE.

The compression starts when the piston closes the hybrid / transfer port. A properly designed intake system can over-boost the cylinder in a way similar to that of the Panigale.

In such a case the exhaust plays a secondary role: it helps (as the exhaust of the Panigale 4-stroke); however the dominant role belongs to the induction system (as in the Panigale 4-stroke).

Tall order do do it all with inlet tuning.

The PatATE seems to combine the characteristics of the 2-strokes (like: one power stroke per crankshaft rotation, simplicity, lightweight, high specific power, low cost, etc) with the characteristics of the 4-strokes (like: better lubrication and lower lube specific consumption, uniform temperature of the cylinder liner and of the piston skirt, scuffing reliability, earlier closing of the exhaust, compliance to emission regulations, better fuel efficiency etc) and with some special characteristic of its own, like the absence of red hot temperatures on the exhaust port or exhaust valve, etc.

To be proven....

[Tommy Cookers]

[quote=Pinger]
..........But the mixture doesn't escape at its operating rpm, The converse occurs when the considerable overspill into the exhaust is returned and a 2bar pressure maintained at the exhaust port as it is closed. Thus, its power potential far exceeds what is possible with a mechanically closing valve........[/quote]

is the mean gas pressure in the expansion chamber at high power about 1 bar abs (as in our NA cars etc) ?
or (due to the tiny tailpipe csa) higher than this, eg about 2 bar abs ?

yes, in reposting the link I should read this BLAIR BOOK http://dragonfly75.com/motorbike/2StrokeDesign.pdf

[J.A.W.]

...The specific torque provided by the 4-stroke Ducati Panigale Desmo (which is based on a pure mechanical control over its poppet valves, without VVT / phaser) exceeds by far the half of the specific torque of any 2-stroke.

The flat torque curve (from 8,000 to 10,500 rpm) is one more characteristic not met in the 2-strokes...

Thanks
Manolis Pattakos

Hi Manolis..
As a bit of a Ducati Panagale fan, you might be interested to know that the hi-po ' superbike' variant
is due to be replaced by a new V4 unit, based on the Moto GP design..
& since you've put up a set of 30 year old Yamaha 350 2T road-bike port maps - here is a dyno chart to match..



As for specific power & yet producing a ' tame' but still ' fun' torque curve.. see for yourself..
& an hp/per unit/piston area comparison - likely wont shame the old darlin' 2T either..

[Pinger]

is the mean gas pressure in the expansion chamber at high power about 1 bar abs (as in our NA cars etc) ?
or (due to the tiny tailpipe csa) higher than this, eg about 2 bar abs ?

Due to the returned positive pulse from the convergent cone - or in the case of compact manifolds on 3 and 4 cylinder groupings, from the next in sequence's exhaust pulse entering the manifold - but of lesser magnitude than with an expansion chamber.

yes, in reposting the link I should read this BLAIR BOOK http://dragonfly75.com/motorbike/2StrokeDesign.pdf

It's all in there!

edit PS
The tiny tailpipe (stinger) does maintain backpressure but has to be used sparingly to avoid burning pistons. More there I think for maintaining gas temperature and density both of which are beneficial for the transmission of the sound waves which are the important element.

[manolis]

Hello Henry.

You write:
“Why are the PatATE lubrication characteristics any different from a conventional 2 stroke?”

Because it avoids the high temperatures on the exhaust port and on the side of the piston skirt that slides over the exhaust port.

Think of increasing and increasing the fuel – oil ratio (say from 50:1 to 75:1, then to 100:1, then to 150:1and so on). Then comes a time the scuffing begins. Most likely the scuffing begins wherein the worst conditions are met.


As Pinger wrote, the piston of the conventional 2-stroke is like:
“A valve that before closing conspires to push half the cylinder's contents into the exhaust unburned.”

This is good for the volumetric efficiency at the revs where the engine runs tuned.
But as regards the specific lube consumption, the “reciprocation” of the fresh charge between the cylinder and the exhaust is bad because the oil droplets that pass to the exhaust is difficult to find their way back to the cylinder (they are heavier relative to the surrounding air / gas; the air /gas accelerates the oil droplets towards the exhaust; and as heavy, they cannot return).

In the PatATE the fresh charge passes, right after the red hot burnt gas, thought the same port. Red hot gas, then fresh cool gas with fresh cool oil droplets falling onto the piston ring, onto the piston ring lands, onto the port bridges and the port to cool down and lubricate the most suffering, and prone to scuffing, surfaces. And then red hot gas again, cool fresh gas, and so on.

Does it make any difference to pass both gasses through the same port?

Thanks
Manolis Pattakos

[manolis]

Hello Pinger.

According your plot, the 2-stroke Evinrude E-TEC 150 has less than 20% higher torque at 4,000rpm than the same capacity 4-stroke F150 of Yamaha, and the same torque with the 4-stroke F150 above 5,000rpm.

Not good values for a two-stroke.


You write:
“Ducati torque is 106lb.ft, 81.5lb.ft/litre. Apply the same specific figure to the E-Tec V6's 200lb.ft and the capacity comes out at 2.45litres - about right.”


The Evinrude is a 2-stroke engine: one combustion per crankshaft rotation per cylinder. The Ducati Panigale is a 4-stroke engine with one combustion per cylinder per two crankshaft rotations.

If Evinrude E-TEC was filling its cylinders as Ducati does, Evinrude would make the double torque and power than what it makes now.

Thanks
Manolis Pattakos