2 stroke thread (with occasional F1 relevance!)

[manolis]

Hello Gruntguru

Here is the Evinrude E-TEC direct injection 2-stroke marine engine:



which seems to be the most advance 2-stroke today (and one having quite big cylinder capacity, too).

From the photo and some youtube videos it seems that the exhaust opens more than 80 crankshaft degrees before the BDC (Bottom Dead Center), which appears as an aggressive timing (if anybody knows the exact port timing, please let me know it).

It seems they sacrify some expansion for the sake of freer breathing.


In the case of the PatATE with the rotary valve, an obvious modification is a VVT (phaser) between the bevel gear on the crankshaft and the crankshaft.

A few degrees (of rotation of the bevel gear on the crankshaft relative to the crankshaft) at one direction, causes the earlier opening of the transfer port, the increase of the transfer port valve-time area and the earlier closing of the intake port,

while a few degrees at the opposite direction causes the late opening of the transfer, the decrease of the transfer port valve-time area and the later closing of the intake port.

Thanks
Manolis Pattakos

[manolis]

Hello Pinger.

The early opening of the exhaust port is a requirement in all high revving 2-stroke engines.


For instance, in the RD350LC the exhaust port opens before the middle stroke:



It is strange, however according the port-map at bottom right, the exhaust port of the Yamaha RD350LC opens 100 degrees before the BDC (only 47% of the piston stroke is given to expansion). This is 200 degrees exhaust port duration. The transfer duration, according the same port-map, is only 125 crankshaft degrees.

Note: the height of the exhaust port is 29mm, while the height of the transfer port(s) is only 12mm (42% of the exhaust port height).


For higher revving the PatATE needs a long hybrid port duration, too.

However there is a significant difference between the PatATE and a conventional 2-stroke (like the RD350LC): the conventional needs the long duration for the exhaust not for the exhaust itself, but in order to enable a significant duration (and height of the ports) for the transfer, while in the PatATE the height of the transfer is the same as the height of the exhaust (it is the height of the hybrid port).

On this reasoning, the PatATE can extend the expansion for several crankshaft degrees (say, 15 or 20) to milk more energy from the fuel without loosing breathing efficiency or revving capacity.




In another forum they blame the added complexity of the PatATE.

Let's suppose that the:

"The whole point of a 2 stroke is to remove mechanical complication, not to add to it"

is correct.



Then this:



appears the ideal 2-stroke, because it is the simplest one.



For as long as the added complication is justified by the improved operational characteristics it brings, it is worthy / welcome.

An example: the reed valve.
A reed valve adds mechanical complication, however the engine becomes substantially better than with the intake controlled by the piston (as in the figure above). This justifies the added mechanical complication.



And what about the mechanical complication added by the PatATE?

A PatATE can be regarded as the simplest 2-stroke (above image) with the addition of a rotating part (the rotary valve).

In the question:

“is the added complication (due to the rotary valve) justified by the improved characteristics of the engine?”

the answer is:

“much more than in the case a reed valve is added”.


Because the PatATE architecture besides improving the intake, it also improves the exhaust and the transfer.

And because (and this may prove the most important) the PatATE architecture "frees" the 2-stroke engine from extreme local temperatures (those around the exhaust port and on the piston side sliding over the exhaust port)

For instance, imagine the piston rings of the conventional 2-stroke sliding over the red-hot exhaust port (like being in the hell), and the same piston rings sliding over the hybrid port of the PatATE.

The substantial drop of the peak temperatures on the cylinder liner and on the piston enables a substantially lower lube specific consumption, the engine becomes less prone to seizure, the engine can have higher compression ratio (no hot spots in the combustion chamber), the engine needs less cooling, the fuel efficiency increases, etc, etc.

Thanks
Manolis Pattakos

[Pinger]

the exhaust opens more than 80 crankshaft degrees before the BDC (Bottom Dead Center), which appears as an aggressive timing (if anybody knows the exact port timing, please let me know it).

It seems they sacrify some expansion for the sake of freer breathing.

More likely, given the exhaust will be arranged to serve banks of three cylinders, the exhaust duration is more aggressive to enable a longer time for the positive pulse from an opening port to traverse the (compact) manifold to the closing port. The large bore of 98mm will have lengthened each exhaust branch making the manifold less 'compact'.

[Pinger]

Hello Pinger.

The early opening of the exhaust port is a requirement in all high revving 2-stroke engines.

A chainsaw type engine has conservative timing despite circa 9500 rpm being its operating rpm.

However there is a significant difference between the PatATE and a conventional 2-stroke (like the RD350LC): the conventional needs the long duration for the exhaust not for the exhaust itself, but in order to enable a significant duration (and height of the ports) for the transfer, while in the PatATE the height of the transfer is the same as the height of the exhaust (it is the height of the hybrid port).

Yes, to enable sufficient blow down period before opening transfers designed with maximum breathing and power in mind. But if lower cylinder filling is acceptable (as it is with the chainsaw) then the transfer ports can be lower and thus the exhaust port also.

On this reasoning, the PatATE can extend the expansion for several crankshaft degrees (say, 15 or 20) to milk more energy from the fuel without loosing breathing efficiency or revving capacity.

Laudable aim if achievable. Definitely separating more effectively the exhaust and transfer events (more like a 4T does) is desirable, but for as long as crankcase pumping is the source of pressure differential then that objective is seriously hampered much after BDC due to the rising piston and consequent falling crankcase pressure.
Exhaust tuning will be limited by the inability to return a sufficiently late (in the cycle) -ve pulse as the exhaust port will already be closed.

In another forum they blame the added complexity of the PatATE.

Let's suppose that the:

"The whole point of a 2 stroke is to remove mechanical complication, not to add to it"

is correct.



Then this:

http://www.pattakon.com/tempman/2_cycle_simple.jpg

appears the ideal 2-stroke, because it is the simplest one.



For as long as the added complication is justified by the improved operational characteristics it brings, it is worthy / welcome.

An example: the reed valve.
A reed valve adds mechanical complication, however the engine becomes substantially better than with the intake controlled by the piston (as in the figure above). This justifies the added mechanical complication.



And what about the mechanical complication added by the PatATE?

A PatATE can be regarded as the simplest 2-stroke (above image) with the addition of a rotating part (the rotary valve).

In the question:

“is the added complication (due to the rotary valve) justified by the improved characteristics of the engine?”

the answer is:

“much more than in the case a reed valve is added”.


Because the PatATE architecture besides improving the intake, it also improves the exhaust and the transfer.

And because (and this may prove the most important) the PatATE architecture "frees" the 2-stroke engine from extreme local temperatures (those around the exhaust port and on the piston side sliding over the exhaust port)

For instance, imagine the piston rings of the conventional 2-stroke sliding over the red-hot exhaust port (like being in the hell), and the same piston rings sliding over the hybrid port of the PatATE.

The substantial drop of the peak temperatures on the cylinder liner and on the piston enables a substantially lower lube specific consumption, the engine becomes less prone to seizure, the engine can have higher compression ratio (no hot spots in the combustion chamber), the engine needs less cooling, the fuel efficiency increases, etc, etc.

Thanks
Manolis Pattakos

Complexity is in context.
GG suggested that the cylinder you are showing is small and suited to high rpm - hi-po application. In such an application are you really offering anything valued when the pursuit of 440hp/litre is the objective and can be achieved with a mere three moving parts all of which are proven to survive in the prevailing environment?

For other applications with less emphasis on specific output and where the gains in fuel efficiency and emissions reduction are sought then more complexity will get a fairer hearing. But much of what is being shown is expected to survive in a lubrication environment of scanty oil mist and proving that it will (including variable timing devices) to someone who's life may depend on the engine's reliability will be needed. If a manufacturer is compelled to employ complexity to achieve its goals the 4T is already proven technology.

For each of your designs Manolis, they would be more easily understood if you stated where they were pitched (eg cylinder size, rpm, specific output (or BMEP) exhaust tuning options, etc). You are displaying complexity not suited or required for a race engine, ruled out on cost grounds for a chainsaw type, but possibly ideal for an outboard or automotive engine. But even those last applications wont readily welcome bulky expansion chambers (for example) if required.
Final words to better explain my point. The most efficient 2T (indeed, reciprocating) engine is the 'cathedral diesels' in ocean going ship with an operating speed of 80rpm. No way does its technology scale to a 125cc cylinder.

[Tommy Cookers]

regarding the exhaust port timing .....

Rajdoot in India assembled/made a detuned so-called RD 350 in 1983-1990 for police etc use
the earlier 30.5 bhp version with exhaust ports at 32 mm
the later 27 bhp version with an exhaust ports at 36 mm
http://www.aircooledrdclub.com/smf/inde ... ic=38560.0
http://motorbreathing.tripod.com/rd350.html

now I find that the YR5 had 32 mm exhaust port height
(the RD350B and afaik the identical RD350 had 29.5 mm and interestingly, the RD350LC had 30 mm)
unlike the earlier YR1 - 3 350s the YR5 had a stroke common to the 250 - all these made 36 bhp

the above is meat and drink to me ......
being convinced since 1975 that the YR5 was a nicer and better 'real-world' machine than the/my 1975 fuel-guzzling RD350B
nicer through having in some way a smoother power curve and so better 'speed stability'
also discovering the 750 Kawasaki's huge power spread (riding the 1972 UK road-test machine before journalists eg Dave Minton did)

btw the R5 & early RD350 frame design was famously the same as their race brothers but of course made of 'heavy' mild steel
mild steel and fancy steels having equal elastic modulii, the road frames (heavier ie thicker wall) would have been stiffer (as with 'featherbeds')
having not then discovered 'clever' steering geometry/tyre geometry those Yamahas steered better than anything before or since

[manolis]

Hello Pinger.

The 9,500rpm of a chainsaw are not high revs if the piston stroke is short.

Typically the piston speed is what defines the high and low revving range of an engine.

For instance, with 30mm stroke, at 9,000rpm the mean piston speed of the chainsaw is only 9m/sec, i.e. about as high as the mean piston speed of a giant Wartsila X92 marine 2-stroke (piston stroke 3,468mm) revving at only 75rpm.



Based on the port-plan of the RD350LC of ’86, in the plot at the bottom of the http://www.pattakon.com/pattakonPatATE.htm web page they were added the curves of the RD350LC:



The green curve is the exhaust port area versus the crankshaft angle, while the yellow curve is the transfer port area versus the crankshaft angle.

The duration of the hybrid port of the PatATE is only 180 degrees, while the duration of the exhaust port of the Yamaha RD350LC is 200 degrees (which means the PatATE has an about 18% longer (measured as piston travel) expansion, and additional mechanical energy on the crankshaft).

Despite the shorter duration of the hybrid port of the PatATE, the rhythm (or rate) of the exhaust opening in the PatATE is almost double as compared to rate of the exhaust opening of the RD350LC.

While the exhaust port of the RD350LC opens 10 degrees earlier (at 80 degrees after the TDC),
40 degrees later (i.e. at 120 degrees after the TDC or 60 degrees before the BDC) the exhaust port area of the PatATE (which started opening 10 degrees later) is more than 50% larger.

The differences are even larger as regards the transfer (see the plot).

The previous “say” that the PatATE can use way more conservative timing than a conventional 2-stroke without loosing anything in breathing and high revving capacity.

It is strange, however the PatATE idea started as a way to clean the exhaust of the 2-stroke engine.

Thanks
Manolis Pattakos

[denktank]

Ryger engine internals:

http://www.motor-forum.nl/forum/list-me ... akt-1.html

From the forum , it already set some records

[J.A.W.]

Ryger engine internals:

http://www.motor-forum.nl/forum/list-me ... akt-1.html

From the forum , it already set some records

Yeah D-T, we have had LucF as a member here,& he'd been hyping up the 'Ryger' with teases of reveals..
..but his last post was ' bout a year & a 1/2 ago, he'd yet again stated "soon" - when asked for actual data..

Feel free to cite confirmation of new 'lap records' - & a link to proof - as shown by official competition telemetry..

[J.A.W.]

...For instance, in the RD350LC the exhaust port opens before the middle stroke:

http://droxine.net/RZ/RZ-RD-Ports.jpg

It is strange, however according the port-map at bottom right, the exhaust port of the Yamaha RD350LC opens 100 degrees before the BDC (only 47% of the piston stroke is given to expansion). This is 200 degrees exhaust port duration. The transfer duration, according the same port-map, is only 125 crankshaft degrees.

Let's suppose that the:

"The whole point of a 2 stroke is to remove mechanical complication, not to add to it" is correct.

Then this:

http://www.pattakon.com/tempman/2_cycle_simple.jpg

appears the ideal 2-stroke, because it is the simplest one.



For as long as the added complication is justified by the improved operational characteristics it brings, it is worthy / welcome.

An example: the reed valve.
A reed valve adds mechanical complication, however the engine becomes substantially better than with the intake controlled by the piston (as in the figure above). This justifies the added mechanical complication.



And what about the mechanical complication added by the PatATE?

A PatATE can be regarded as the simplest 2-stroke (above image) with the addition of a rotating part (the rotary valve).

In the question:

“is the added complication (due to the rotary valve) justified by the improved characteristics of the engine?”

the answer is:

“much more than in the case a reed valve is added”.


Because the PatATE architecture besides improving the intake, it also improves the exhaust and the transfer.

And because (and this may prove the most important) the PatATE architecture "frees" the 2-stroke engine from extreme local temperatures (those around the exhaust port and on the piston side sliding over the exhaust port)

Thanks
Manolis Pattakos

Hi Manolis,

A couple of things..

Where the Yamaha RD 350 diagrams are captioned with the suffix 'YPVS (RZ)' - this indicates variable exhaust port
timing is present - & is operating via a electronic servo-controlled rotary-drum valve..

BRP/Evinrude have announced that they will be introducing their own version of such a set-up ( presently in
use on BRP/Ski-Doo E-TEC snowmobiles) to the 'High Output' variants of their outboard engines, too..

Precise EGT control via exhaust pulse reversion helps current 2Ts with the "hell" heat factor of firing @
TDC on every cycle.. as Pinger has hinted at..

When you do build & test an 'in metal' engine to your rotary sleeve design, how it copes with heat-flows will
be a point of interest to many readers here, I'd reckon..

[manolis]

Hello J.A.W.

Here is an Opposed Piston PatATE:





In case of divided load (Portable Flyers with two intermeshed counter-rotating propellers, Electric Power sets with two counter-rotating generators, Marine Outboard engines driving two counter-rotating screws etc) the synchronization gearing (not shown) runs unloaded and the basis of the engine is perfectly rid of vibrations.

The compact combustion chamber is shared between the two opposed pistons (the instant pressure on the two piston crowns is the same).

The scavenging is not of the through or uniflow type.

Thanks
Manolis Pattakos

[Pinger]

Hello Pinger.

The 9,500rpm of a chainsaw are not high revs if the piston stroke is short.

Typically the piston speed is what defines the high and low revving range of an engine.

For instance, with 30mm stroke, at 9,000rpm the mean piston speed of the chainsaw is only 9m/sec, i.e. about as high as the mean piston speed of a giant Wartsila X92 marine 2-stroke (piston stroke 3,468mm) revving at only 75rpm.

Well, seeing as you have chosen to depart from your earlier statement that:''The early opening of the exhaust port is a requirement in all high revving 2-stroke engines.'' we'll discuss it in terms of stroke instead. The short stroke chainsaw gives up less of its stroke to exhaust than does an engine designed for maximum power - all proportionately.

The duration of the hybrid port of the PatATE is only 180 degrees, while the duration of the exhaust port of the Yamaha RD350LC is 200 degrees (which means the PatATE has an about 18% longer (measured as piston travel) expansion, and additional mechanical energy on the crankshaft).

18% more piston travel propelled by an average of say 2bar pressure. The torque gain will be slight.

Despite the shorter duration of the hybrid port of the PatATE, the rhythm (or rate) of the exhaust opening in the PatATE is almost double as compared to rate of the exhaust opening of the RD350LC.

While the exhaust port of the RD350LC opens 10 degrees earlier (at 80 degrees after the TDC),
40 degrees later (i.e. at 120 degrees after the TDC or 60 degrees before the BDC) the exhaust port area of the PatATE (which started opening 10 degrees later) is more than 50% larger.

Despite the valve opening the port very fast - is the port not still obscured by the piston?

[manolis]

Hello Pinger

You write:

“18% more piston travel propelled by an average of say 2bar pressure. The torque gain will be slight.”



Here is the pressure (and the torque) versus the crankshaft angle of a typical 4-stroke engine:



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

After the closing of the ports a 2-stroke gives a quite similar plot.


Spot on the pressure at 90 crank degrees. It is about 1/6 of the peak pressure. Nothing to do with 2 bars.

Spot on the torque at 90 degrees.

While at 90 degrees after the TDC the pressure is several times lower than the peak pressure, at the same 90 degrees the eccentricity of the connecting rod from the crankshaft axis maximizes, explaining how, with six times lower pressure, the torque at 90 degrees is only two times lower than the peak torque.

The mechanical energy provided by the engine during the rotation of the crankshaft from an angle f to an angle f+df equals to the torque (at the angle f) times the angle differential df.

The total mechanical energy provided to the crankshaft from 0 degrees to 180 degrees equals to the area underside the torque curve, which is about equal to the torque at 90 degrees times 180 degrees (according the plot of Taylor, the torque at 90 degrees is about the mean torque during the expansion stroke).
About one quarter of this mechanical energy is consumed during the compression stroke (torque curve before the TDC).

According the previous, extending the expansion from 80 degrees after the TDC (RD350LC) to 90 degrees after the TDC (PatATE), another 6% of mechanical energy arrives to the crankshaft during the expansion stroke.
And because the compression is already "paid", this 6% is "clean", which means the mechanical energy to the crankshaft (as well as the power of the engine) increases by some 8%.

An 8% increase of the power and of the torque (and of the mileage etc) cannot be considered as insignificant.

As for the above rough calculations, they favor the conventional, not the PatATE.




You also write:
“Despite the valve opening the port very fast - is the port not still obscured by the piston?”

No.

It is not the valve that opens the hybrid port.

Several degrees before the piston starts opening the hybrid port, the rotary valve has uncovered the outer side of the hybrid port.

See the following animation.
Spot on the hybrid port and on the rotary valve when the piston is around the TDC: the piston keeps the hybrid port closed, however the rotary valve keeps rotating to get at the right angle / position when the piston will start opening the hybrid port.


Quote from http://www.pattakon.com/pattakonPatATE.htm (near the end of the web page):


Rate of Exhaust Opening and of Transfer Closing

In the following graph it is shown the Exhaust Port area and the Transfer Port area versus the crank angle:



In the following animation it can be seen the position of the rotary valve of the PatATE just before the opening of the hybrid port by the piston.

During the compression and the expansion the rotary valve keeps rotating.

At the beginning of the exhaust the outer side of the hybrid port is fully "uncovered" by the rotary valve, so that the complete hybrid port is dedicated to the exhaust.

The rate of the exhaust opening is about double as compared to the rate of the exhaust opening in a similar conventional 2-stroke.

According the Yamaha RD350LC port-map (click http://www.pattakon.com/tempman/Yamaha_ ... rt_map.jpg to download), the exhaust port extends on the periphery of the cylinder liner for some 80 degrees while the hybrid port of a similar PatATE extends along the periphery of the cylinder liner for nearly 180 degrees.

The graph at http://www.pattakon.com/tempman/PatATE_ ... orting.png shows the difference.

With similar timing with a conventional, the blow down of the PatATE is substantially faster.

For similar blow down, the PatATE needs substantially more conservative timing (which also means longer expansion, more power and torque, better fuel efficiency etc)..



End of Quote



If something is confusing, please let me know to further explain.

Thanks
Manolis Pattakos

[Pinger]

After the closing of the ports a 2-stroke gives a quite similar plot.

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.

Spot on the pressure at 90 crank degrees. It is about 1/6 of the peak pressure. Nothing to do with 2 bars.

1/6 of peak pressure when nearly the entire stroke is utilised for compression. It will be much lower at the same CAD when only half the stroke is used for compression and the compression ratios are the same.
Ballpark 2T numbers: Compression commences from 1bar. Compressed to say, 10bar. Fired, combustion raises the pressure fourfold to 40bar. Expand back to point of exhaust port opening and 4bar. Pressure never falls below 1 bar so 3bar differenc the average of which is 1.5. 1.5+1 = 2.5 bar average over the extended stroke. Less actually as there is 1bar of pressure on the piston's underside. So 2 bar isn't far from the reality of what you are chasing.

While at 90 degrees after the TDC the pressure is several times lower than the peak pressure, at the same 90 degrees the eccentricity of the connecting rod from the crankshaft axis maximizes, explaining how, with six times lower pressure, the torque at 90 degrees is only two times lower than the peak torque. .

One tenth, not one sixth. The gain is about halved already.

According the previous, extending the expansion from 80 degrees after the TDC (RD350LC) to 90 degrees after the TDC (PatATE), another 6% of mechanical energy arrives to the crankshaft during the expansion stroke.
And because the compression is already "paid", this 6% is "clean", which means the mechanical energy to the crankshaft (as well as the power of the engine) increases by some 8%.

An 8% increase of the power and of the torque (and of the mileage etc) cannot be considered as insignificant.

So your design is pitched at the same operating speed as the RD350LC namely 8500rpm? (For the comparison to be valid).

You also write:
“Despite the valve opening the port very fast - is the port not still obscured by the piston?”

No.

It is not the valve that opens the hybrid port.

Several degrees before the piston starts opening the hybrid port, the rotary valve has uncovered the outer side of the hybrid port.

So port opening is piston governed and no faster than in any other engine?

[henry]

So port opening is piston governed and no faster than in any other engine?

I believe faster refers to the rate of port area growth and not the rate of port exposure.

[manolis]

Hello Henry.

According the plot:



the RD350LC needs 20 degrees to go from 0 to 7.5 opening, while into 20 degrees the PatATE goes from 0 to 16 (more than double).

Something similar to the 4-stroke Ducati Desmo engines wherein the valves open faster than in the rest 4-stroke engines.
But in the PatATE the rate of exhaust opening not only increases but doubles.

Thanks
Manolis Pattakos