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Asymmetric Timing in the Two-Stroke engines


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#1 manolis

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Posted 07 June 2014 - 03:17

Hello.

The plot:

PatAT_Timing.gif

shows, at left, a typical symmetric timing of a two-stroke.

With symmetric timing, the transfer port opens after the exhaust port and closes before the exhaust port.

Time is required to pass from the opening of the exhaust port till the opening of the transfer port (translated into crankshaft degrees) in order the pressure inside the combustion chamber to fall substantially.

The same time passes from the closing of the transfer port till the closing of the exhaust port (case of zero offset), giving the chance to a good part of the fresh air or mixture to escape from the open exhaust port under the action of the upwardly moving piston, reducing the torque and increasing the emissions.


At http://www.pattakon....ttakonPatAT.htm it is presented the PatAT:

PatAT1.gif

In the PatAT the source of pressurized air of mixture (the crankcase in most cases) communicates with the combustion chamber through transfer ports disposed in series with respective piston ports.
The transfer ports are controlled by the piston.
The piston ports are controlled by the connecting rod.

The source of pressurized air or mixture can, additionally, communicate with the combustion chamber through conventional transfer ports (that open by the piston after the exhaust port).

PatAT1A.gif

As the piston moves "downwards", it initially opens the transfer ports; but with the respective piston ports closed by the connecting rod, the combustion chamber cannot communicate with the crankcase (or, in general, with the scavenge pump).

The piston continues its downwards motion and opens the exhaust port; the pressure inside the combustion chamber drops quickly; the crankcase continues to remain sealed from the combustion chamber.

Later the connecting rod opens the piston ports (and the piston opens the conventional transfer ports, if any). The transfer takes place.

As the piston moves upwards, it initially closes the conventional transfer ports (if any).
Later the piston closes the exhaust port.

The crankcase is still communicating with the combustion chamber: after the closing of the exhaust port, air or mixture continues to enter (through the "connecting rod controllable" piston ports and their respective transfer ports) into the combustion chamber untill the transfer ports to close by the piston.


A high revving model / RC engine with ringless piston and the PatAT would be interesting.

Also the comparison of the emissions and of the torque of a conventional two stroke and of its modified to PatAT version.


With external scavenging pump (say a turbocharger), the PatAT can combine the four-stroke lubrication with the asymmetric timing (say in Diesels, in Direct Injection spark ignition etc), like:

PatAT2.gif

or like:

PatAT3.gif

The separating plate at the middle of the piston seals the space underneath the piston crown from the crankcase.

Through openings in the cylinder and in the piston, the pressurized air or mixture enters into the combustion chamber either directly or indirectly (it initially goes into the space underneath the piston crown and then, through a piston port controlled by the connecting rod and then through a transfer port controlled by the piston, it gets into the combustion chamber).

If the "stroke" of the oil-scraper-ring and of the lowest compression spring overlap (as in the animations above) and if the area of the cylinder liner wherein the oil-scraper-ring slides is rid-of-ports, the lubracation and the scuffing resistance get quite similar to those of the four-strokes.


Thoughts?

Objections?

Thanks
Manolis Pattakos

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#2 Speedman

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Posted 07 June 2014 - 07:57

Hello Manolis

 

A other special solution for asymmetric port timing  by two stroke engine.

 

I think the upper example is the better construction. This example is simpler and has less less seal areas.

The problem is, the transfer ports are not open in the crankcase. This is not optimal for a good performance. All good race two stroke engine has  transfer ports open in the crankcase without exception. And for several transfer ports is this system not whole simple. A good two stroke engine has 3 till 8 transfer ports.

 

 

The lower example is not for high RPM. Has to much seal areas and zu much oszillierende masse.

In practice, this engine will not run as good as in the theory

The service life will not be very high.

Little 2-stroke engines and turbos are not good solutions. Turbo is to expensiv, to larg, need a Oil-pump and turbos need a regular service. Maybe for big ship diesel two stroke engine.

 

 

All in all are this examples a good approach for a asymmetric two stroke engine, but not yet the final solution, but I'll think about it.

 

Maybe axial piston or rotary piston engine are better?

 

 

best regards

 

Speedman


Edited by Speedman, 07 June 2014 - 09:24.


#3 manolis

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Posted 07 June 2014 - 11:46

Hello Speedman.


The conventional transfer port of the upper solution (the port at the right side of the cylinder) can be open to the crankcase, as in the conventional two-stroke.

The other transfer ports (let’s call them "PatAT transfer ports") communicate with the crankcase through the space inside the piston.

Think the “time-port-area”, not only the “port area”.
See the timing plot, at the top of the http://www.pattakon....ttakonPatAT.htm web page.
With the "PatAT transfer ports" being open for several degrees after the closing of the exhaust port(s), there is much more time for the inroduction of the air / mixture into the cylinder, without the risk of escaping though the open exhaust.



The other version (that with the scraper ring at the middle of the piston) is a different approach: it can use clean air (rid of oil) for the scavenging, with a controllable (as in the four strokes) lubrication of the compression rings.
A Diesel engine (or a direct injection spark ignition engine of normal size for cars / motorcycles) is not for high revving: the power of the Diesels drops steeply after 4,000 – 4,500 rpm. The inertia loads at these revs (even at 7,000 rpm wherein most spark ignition car engines are deep in their red) are way weaker than the combustion pressure loads:
With 80mm bore and 120 bar peak pressure, the peak pressure-force on the piston is 6 tons.
With 80mm stroke, in order to have 6 tons peak inertia force at 8,000 rpm (21.3 m/sec mean piston speed), you need a "piston/wrist pin/upper con-rod" mass of nearly 2Kg. Even a solid aluminum cylinder (80 diameter, 120 height) is lighter.


Regarding the piston mass: the piston is a little longer, but it is not proportionally heavier. Do I miss something?


So, please take another look at this arrangement not as an RC engine, but as a normal size engine.


As for the “too much seal area” you see, please explain. What do you mean by "too much seal area"?


With forced lubrication of the bearings of the crankshaft and of the connecting rod,
with good splash lubrication of the piston skirt (wherein the thrust loads are taken),
i.e. with four-stroke lubrication,
the service life is expected to be long.
Please explain where you see the reliability problem.


With asymmetric timing the turbocharging becomes an attractive solution.
The OPOC of EcoMotors (Bill Gates with 23.5 millions is one of their investors) is a two-stroke opposed piston that uses an electrically-assisted turbocharger (unlike the Detroit Diesels turbo-charged engines, there is no volumetric -roots- pump in the OPOC).
During the cranking, also during operation at low revs and loads, the battery rotates the turbocharger with an electric motor. At higher revs and loads, the electric motor turns to electric generator (compound).
The OPOC has a substantially asymmetric timing and fits with turbocharging.
The PatAT has a substantially asymmetric timing, too. It just needs not a second piston to achieve the asymmetric timing.


The asymmetric timing of Hossack square piston engine is interesting, but the rest structure of his engine introduces more problems (side effects) than it solves.


The Primavis engine uses a pair of cylinders in Vee - 90 degrees, with the one being the compressor cylinder and the other being the combustion cylinder.
It also uses a rotary valve (rotating at the crankshaft speed) to open and close the “top transfer port” (located higher than the exhaust port) in order to give an asymmetric timing to the engine.
Their slogan / motto comes from Leonardo Da Vinci: "Simplicity is the ultimate sophistication".


Does anybody knows an other “conventional” two-stroke achieving, with a single piston, true asymmetric timing?

Thanks
Manolis Pattakos

#4 Speedman

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Posted 07 June 2014 - 18:28

Hello Manolis

 

Sorry i have too less time. I have to install a new computer.

 

 

<As for the “too much seal area” you see, please explain. What do you mean by "too much seal area"?>

 

Sorry my english is too bad.

 

I wanted to say: The under example has a lof of seal areas. This ares are very difficult to seal. This areas become quickly leaky.

In theory simple to seal, in practice not simple to seal.

 

 

<The asymmetric timing of Hossack square piston engine is interesting, but the rest structure of his engine introduces more problems (side effects) than it solves.>

 

Yes the Hoosak engine has a very strong surface pressure

 

 

<Does anybody knows an other “conventional” two-stroke achieving, with a single piston, true asymmetric timing?>

 

No conventional but unconventional the rocking piston engine from swiss engineer Mr. W. Salzmann. With square piston and with round piston. The patent is with square piston. I have with Mr. salzmann earlier phoned times. He is a very good engineer. He lived about 20 km away from me.

 

He has a construction a little concept car (1956) for automobile salon geneva (Switzerland). This had to cheap, small (500kg) car with much interior and a very little engine. The engine is under the rear seat (see the picture)

 

http://www.zwischeng...ittelmotor.html

 

http://www.tagesanze.../story/16463951

 

http://de.wikipedia....iki/Soletta_750

 

 

< Leonardo Da Vinci: "Simplicity is the ultimate sophistication".>

 

Thanks I did not know, thats said Leonardo.

I said a long time. Simple solution are most (not every)  the better solution.

Paul Roche BMW Turbo F1 engineer said something similar.

 

 

When you want a two stroke engine with oil (oil sump lubrication) in the crankcase the is this example interessting.

 

Stepped piston engine.

 

http://users.breathe...ooper/opads.htm

 

https://data.epo.org...1/document.html

 

 

< forced lubrication>

 

Is a good idea. but the problem is: A small amount of oil remains stuck at the ports. Four stroke engine haven't ports in the cylinder.

So to much bad exhaust gases through Oil. This problem is well-known.

A two stroke engine with EURO 6 emission standards is very very difficult and expensiv.

 

 

I have found a very simple und very interessting engine. I am very impressive. The engine is very simple, very sma, very cheap and very silence.

Good for a hyrid range extender.

 

Norbert Mueller describes his wave disk generator

Michigan State University 2011 President's Report: Revving an automotive revolution

 

Why simple,  when it can be so complicated?  Ähhhh......  vice versa.  Why difficult just when it can be so simple?

 

 

 

 

Sorry I can only report back when I have again  time.

 

 

best regards

 

Speedman


Edited by Speedman, 07 June 2014 - 19:48.


#5 Powersteer

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Posted 08 June 2014 - 07:34

The shock wave part is very interesting, velocity ramming against the wall to create high pressure igniting combustion.



#6 manolis

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Posted 08 June 2014 - 07:48

Hello Speedman.

 

Edit: The part about the rocking piston engine of Mr. Salzmann was removed for a better look.

 

Regarding the step-piston engine, it has not asymmetric timing; it also does not have four-stroke lubrication: it needs a lot of lubricant in the space wherein the air / mixture for the next cycle is compressed, because the thrust loads – see wherein the wrist pin is - are taken in this space.


The four-stroke lubrication has to do especially with the way the piston skirt (wherein the thrust loads are taken and the torque is “generated”) is lubricated.
The compression rings require a thin oil film (they say that 0.01mm is adequate), the piston skirt needs to “float” (to ski) on a thick oil film (more than 0.05mm) otherwise the friction and the scuffing increase.

Taking the thrust loads bellow the oil scraper ring by a well lubricated piston skirt (plenty of oil is splashed to the lower side of the cylinder liner, which is rid of ports) things change a lot.
As for the upper side of the cylinder liner (wherein a thin oil film is necessary to prevent the compression rings from getting in contact with the cylinder liner), only a tiny quantity of oil can go to the exhaust (just like a tiny quantity of lubricant that passes above the oil scraper ring of the 4-stroke is burned / consumed: read the technical article of Wartsila at http://www.pattakon....takonPatMar.htm ) .


Regarding the extensive sealing surfaces and the expected degrading of the sealing, I can’t see why the PatAT has a substantially bigger sealing surface than the conventional with the same number and size of ports.


PS.
Speedman thank you for responding: you give me the opportunity to explain how things work (according my way of thinking, of course).

The question remains: does anybody know a functional reciprocating piston two-stroke with asymmetric timing?

Thanks
Manolis Pattakos


Edited by manolis, 08 June 2014 - 09:06.


#7 Speedman

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Posted 08 June 2014 - 11:07

Hello Manoli

 

Too many text for me. I don't understand not all text, but i try.

 

<When you meet or call your fellow citizen / compatriot, tell him that a guy from Greece says the above reasoning in the patent is wrong:>

 

I don't know, but i don't found his adress. I think he is died. He is not a young man. He has construction his concept car 1956.

But I spoke many years ago with  two men, who has seen motors run. Both men said: the engine ran very quiet and almost no viberationen without external balancing shafts and only 1 and 2 cylinders. Mr. Salzman said me, he had made two-stroke and four-stroke engine with rocking piston.

No sealing problem by engine with round piston.

Mr. Salzmann write: The seal is simpler than in the rotary engine. With short pistons is the mass forces 1. order and 2. order maximum 50% of a conventional engine either 40 %.

 

The men saw not this engines.

 

http://img5.fotos-ho...trdmi8tzpnv.jpg

 

http://img5.fotos-ho...th431a02krp.jpg

 

 

<The question remains: does anybody know a functional reciprocating piston two-stroke with asymmetric timing?>

 

I am not sure, but i belive i had see patent reciprocating piston two-stroke with asymmetric timing, but it's been too long.

 

 

<read the technical article of Wartsila at http://www.pattakon....takonPatMar.htm ) .>

 

I know Wartsila. Wartsila  has bought the Swiss companySulzer Ship Diesel .

 

 

 

 

best regards.


Edited by Speedman, 08 June 2014 - 11:17.


#8 manolis

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Posted 08 June 2014 - 13:17

Hello Manoli

 

Too many text for me. I don't understand not all text, but i try.

 

<When you meet or call your fellow citizen / compatriot, tell him that a guy from Greece says the above reasoning in the patent is wrong:>

 

I don't know, but i don't found his adress. I think he is died. He is not a young man. He has construction his concept car 1956.

But I spoke many years ago with  two men, who has seen motors run. Both men said: the engine ran very quiet and almost no viberationen without external balancing shafts and only 1 and 2 cylinders. Mr. Salzman said me, he had made two-stroke and four-stroke engine with rocking piston.

No sealing problem by engine with round piston.

Mr. Salzmann write: The seal is simpler than in the rotary engine. With short pistons is the mass forces 1. order and 2. order maximum 50% of a conventional engine either 40 %.

 

The men saw not this engines.

 

http://img5.fotos-ho...trdmi8tzpnv.jpg

 

http://img5.fotos-ho...th431a02krp.jpg

 

 

<The question remains: does anybody know a functional reciprocating piston two-stroke with asymmetric timing?>

 

I am not sure, but i belive i had see patent reciprocating piston two-stroke with asymmetric timing, but it's been too long.

 

 

<read the technical article of Wartsila at http://www.pattakon....takonPatMar.htm ) .>

 

I know Wartsila. Wartsila  has bought the Swiss companySulzer Ship Diesel .

 

 

 

 

best regards.

 

Hello Speedman.

 
I was reading the US 5,769,048 patent granted to
“Salzmann; Willy Ernst (Zug, CH)”, June 23, 1998.


Quote from the US 5,769,048 :


Rocking_Piston.gif

“The present rocking-piston engine is the result of many decades of theoretical and practical research and development, partially together with the Federal Institute of Technology, Zurich (Switzerland). The aim has been to achieve decisive benefits with respect to simplicity, compactness, weight, manufacturing costs, smooth running, response, consumption, emissions, servicing and recycling. Applications involving engines of any size and configuration seem to be universally sensible, indeed essential for land and water vehicles (and airplanes), if their needed reduction in size and simplification are to be made at all possible .”
. . .
“The semicylindrical connecting rod cover 55 is designed as counterweight to the piston and upper part of the connecting rod and, with regard to its moment of inertia, designed such that the centre of percussion of the oscillating parts 33 to 55 (possibly without slide valves 48) lies at least approximately at the centre 2 of the connecting rod bearing. Thus, the centre 46 of a hypothetically unguided piston would of its own accord trace out an elongated figure eight. The fine transverse oscillations which would thus occur are taken up by the piston guide springs 25. Since hardly any transverse forces caused by gas forces occur between hovering piston and cylinder front walls 26 because of the arced shape of the piston crown 1 whose centre 2 coincides with that of the connecting rod bearing 3, except for its frictional moment, the frictional losses and oil consumption are many times smaller than for conventional plunger pistons. This is of very great significance, especially for two-stroke engines.--For the relatively small external diameter of the connecting rod cover 55 (the degree of charging of the "connecting rod charger" up to inlet closure is in this case only approx. 1.5) a dense material such as steel or brass is necessary, in order to achieve the required rotative moment. Fine adjustment can be achieved using the void 56 in the connecting rod cover 55 or the piston plate 33, as well as by using steel screws 35 of various lengths; this can be checked on a horizontal vibrator. The connecting rod screws 56 are inserted from above; for the engine casing as in FIG. 7 a screwing from underneath is necessary for certain numbers of cylinders, in order that the crankshaft can be fitted and removed. In order to seal the connecting rod charger, e.g., injected plastic plugs 60 fixable by a pin 51 which is slightly kinked in the middle are necessary. The external surfaces 17 and 55 of the connecting rod charger are finely machined, and, e.g., galvanized or coated with PTFE, and make a seal as a result of minimal clearance.”


I thought there was a mistake regarding the inertia thrust loads on the “cylinder wall” but I was wrong.

 

Willy Salzmann is right: he solved a crucial problem of the rocking piston engine.

By increasing the mass of the semicylindrical cover of the piston, he moves the “percussion point” to the crankpin center. This makes the difference: as the crankpin pushes the rocking piston to the left and to the right, the center of the “crown” of the rocking piston has no tendency to leave away from the “cylinder axis”.

Simple and brilliant.

The cylinder wall does not apply forces to the rocking piston.

 

Imagine you keep a heavy stick and you hit a rock. If the stick hits the rock with its percussion point, your hands take no shock. If the stick hits the rock at a point away from its percussion point, your hands will take a strong shock.

 

There are other issues in the rocking piston engine (like the sealing, the manufacturing, the vibrations), but as regards the thrust forces it is perfect.

 

Thanks
Manolis Pattakos



#9 Speedman

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Posted 08 June 2014 - 15:15

You have explain this engine very good.



#10 Kelpiecross

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Posted 09 June 2014 - 04:45


I notice in both timing diagrams that the transfer ports are open at the same time as the exhaust for 90 degrees. The only real difference seems to be having the intake stay open for 10 degrees after the exhaust has closed. Would this make a big difference to the amount of intake air (or air/fuel) lost out the exhaust?

Otherwise a good simple and probably practical idea - especially if it does something useful. Maybe you should build one and test it?

Edited by Kelpiecross, 09 June 2014 - 04:46.


#11 Speedman

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Posted 09 June 2014 - 07:29

Hello Kelpiercross

 

This is only a patent-paper not a construction draw. 90 degrees is too little, either 180 degrees for exhaust.

Each shorter the piston/rod, then bigger the difference.

And each wider the piston then more difference (more tilt).

 

 

 

 

best regards


Edited by Speedman, 09 June 2014 - 11:53.


#12 Kelpiecross

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Posted 09 June 2014 - 12:03


I meant both the transfer ports and exhaust port were open at the same time for 90 degrees. Manny's 130 degrees for the exhaust opening seems about average for a two-stroke - 180 degrees for a very high RPM race engine maybe?

#13 Speedman

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Posted 09 June 2014 - 13:55

<80 degrees for a very high RPM race engine maybe?>

 

180 degrees is quite sporty.



#14 manolis

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Posted 09 June 2014 - 16:31

Hello Kelpiecross.

 

The symmetric timing is a necessity for the conventional two-stroke.

 

The transfer needs duration (and port area). If it is too short, the engine cannot operate at high revs and the peak power is low.

 

Then the exhaust port must open substantially earlier than the transfer port. Due to the architecture of the conventional two-stroke, the exhaust extends too much (equally at the two ends of the transfer).

 

With the PatAT asymmetric timing, the exhaust can be substantially shorter. In the plot, the exhaust duration drops 20 degrees (from 130 to 110 degrees). The transfer duration increases substantially (from 90 to 100 degrees) and ends – if desirable – substantially after the exhaust port.

These differences are anything but small as regards the breathing of the engine.

 

The plot is only indicative. It is too sporty for big Diesels, it is too mild/soft for sport high revving engines.

 

The opposed piston engines (Junkers Jumo, Junkers-Doxford, TS3, EcoMotor’s OPOC, Achates power etc) use asymmetric timing because it is better and because they can.

 

Now with a single piston, the two-stroke has the option to operate with substantially asymmetric timing.

 

 

PS.

Did you see the stereoscopic plot in the PatEff CVT?

Or the stereoscopic photos of the PatRoVa rotary valve prototype engine?

 

Thanks

Manolis Pattakos



#15 Kelpiecross

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Posted 10 June 2014 - 03:50

Yes, I did see the stereoscopic pictures - very nice as always.

Your asymmetric two stroke engine does look promising - you should build one. It should cost little more than a normal two stroke to build - which would be a huge plus for the idea.

#16 gruntguru

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Posted 10 June 2014 - 08:12

At first glance asymmetric timing looks ideal for forced induction and low-medium specific output 2 strokes. Your typical race 2 stroke uses an expansion chamber to "supercharge" via the exhaust port - after the transfer port has closed. Clearly this won't work if the exhaust closes before the transfer port.



#17 Speedman

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Posted 10 June 2014 - 16:34

Hello

 

Seen today, it needs to read but still

http://www.borderlan...T0712S24-27.pdf

http://www.pelz-motorenentwicklung.de/

 

Best regards


Edited by Speedman, 10 June 2014 - 16:36.


#18 manolis

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Posted 10 June 2014 - 17:11

Hello

Seen today, it needs to read but still
http://www.borderlan...T0712S24-27.pdf
http://www.pelz-motorenentwicklung.de/

Best regards

Hello Speedman.

I think the arrangement of the crankshafts / connecting rods / piston used in the opposed piston of Achates Power is similar:

Achates_Power1.gif

Thanks
Manolis Pattakos

Edited by manolis, 10 June 2014 - 17:14.


#19 manolis

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Posted 10 June 2014 - 17:42

At first glance asymmetric timing looks ideal for forced induction and low-medium specific output 2 strokes. Your typical race 2 stroke uses an expansion chamber to "supercharge" via the exhaust port - after the transfer port has closed. Clearly this won't work if the exhaust closes before the transfer port.


Hello gruntguru.

I think the Kaaden effect can be used even when the exhaust closes before the transfer.
The cylinder is "supercharged" by a quantity of air / mixture that returns to the cylinder from the expansion chamber, then - with the exhaust closed - the transfer continues to bring compressed air / mixture to the cylinder.

On the other hand, asymmetric timing does not necessarily mean that the transfer ends after the exhaust port closing. Suppose the exhaust opens 65 degrees BBDC, the transfer opens 40 degrees BBDC, the transfer closes 55 degrees ABDC and the exhaust closes 65 degrees ABDC. The timing is substantially asymmetric, however the exhaust closes after the end of the transfer.

The interesting with the PatAT is that with its built-in low-cost control over the transfer process, several options (not existing before) get into play: turbocharging, supercharging, Kaaden, clean exhaust, over-expansion etc.

Combined with forced / splashed four-stroke lubrication in the crankcase and on the piston skirt /cylinder liner, things get even more interesting.

Thanks
Manolis Pattakos

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#20 gruntguru

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Posted 10 June 2014 - 23:16

Thanks Manolis. As usual you have showed me some things I hadn't thought of.



#21 manolis

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Posted 11 June 2014 - 03:36

Thank you gruntguru.
I would like having more objections from you.
The PatEFF CVT, for instance, is still waiting and is an interesting (and fuel saving) project.



The following animation has been added at http://www.pattakon....ttakonPatAT.htm :

PatAT4.gif

It is Cross-Radial (Radial-4) with forked connecting rods.

It is as vibration free as the best Vee-8,
it has firing intervals equal to those of a Vee-8 four-stroke,
it has four-stroke lubrication (plain bearings, forced / splashed lubrication in the crankcase, oil scraper rings),
it can utilize a central scavenging pump (a turbocharger, for instance), etc.

Depending on the use, a second compression ring may be necessary.

As a turbocharged Diesel it has the qualifications for extreme power to weight ratio and, at the same time, for top fuel efficiency (airplanes, helicopters, paragliding etc). The crankshaft is as lightweight and reliable as in the Bristol Radials (Hercules, Centaurus etc).



Hello Kelpiecross.

Building a prototype takes money and time; sometimes lots of them.
This is why taking a second, a third etc opinions / objections before staring, is so important / precious.

Thanks
Manolis Pattakos

#22 MatsNorway

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Posted 11 June 2014 - 20:57

Dont bother with two strokes manolis. And this is too much ducting. I much prefer the popped valve in the piston then. At least in teory. Too much fiddling during service ++



#23 gruntguru

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Posted 11 June 2014 - 23:25

Mats - there are still a lot of 2 strokes in the world. At my home there is a weed wacker and a blower-vac. The big marine diesels are mostly 2 stroke.



#24 manolis

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Posted 12 June 2014 - 04:01

Hello MatsNorway.

In a week the USPTO (US patent and trademark office) is expected to give the number of the patent for the HyDesmo at http://www.pattakon....akonHyDesmo.htm (the invention is already approved; the granting of a patent number is typical).

"Hy" from Hydraulic and "Desmo" because it needs not restoring valve springs.

The HyDesmo is the evolution of the PatAir, which is the evolution of the MultiAir / TwinAir (evolution in the meaning that it does more than its predecessor).

It revs reliably at higher revs than the MultiAir of FIAT / Alfa Romeo / Chrysler.
It also provides additional fuel-efficient modes: all the modes of the MultiAir / TwinAir plus unlimited Atkinson/Miller modes (see the thread about Toyota’s non-hybrid Atkinson engines).

The electronic control offers such flexibility and accuracy the conventional mechanical VVAs (like the valvetronic of BMW, the valvematic of Toyota, the VVEL of Nissan) cannot even imagine.

The HyDesmo is about poppet valves and their control.


On the other hand, the PatRoVa rotary valve at http://www.pattakon....akonPatRoVa.htm seems more than promising: I would not bet on the poppet valves, especially for sport engines.
Imagine replacing the two cylinder heads (they are more than half of the complete engine) of the Ducati Panigale by others PatRoVa ones, then imagine increasing the revs as much as the underneath mechanism of Ducati Panigale (pistons, con-rods, crankshaft, casing) can stand (say 30%? Say 50%?).


On the other hand, when it comes into "power to weight ratio" (the last two days I was talking with a few paragliders), the two-stroke is at its own class.

The OPRE tilting at http://www.pattakon....akonTilting.htm combines top power density with true vibration free operation (important for such applications).


The EcoMotors (Bill Gates etc) and the Achates Power (Walmart etc) are both working on the evolution of the two-stroke (of the opposed piston Diesel) because “they see light at the end of the tunnel”.

The OPRE and the PatOP projects of pattakon are also for opposed piston Diesels.

The two-stroke fits with the Diesel cycle more than the four-stroke: besides the fact that during the scavenging only “clean” air can escape from the exhaust, it is also the internal friction of the engine that drops to almost half improving substantially the already high fuel efficiency of the Diesel (especially at light loads).


The weight of the Cross Radial PatAT of the animation in my last post is way less than half of a similar power conventional four-stroke. This means that in a small airplane or helicopter you can carry a spare engine without increasing the weight, if you like so, or better you can carry more fuel or passengers.


On the other hand, imagine the emission reduction and the fuel saving if in the several millions of auxiliary two strokes used everyday in motorbikes, chainsaws, lawnmowers, tools etc a low-cost control over the transfer can reduce substantially the mixture that escapes unburned through the exhaust port.

Thanks
Manolis Pattakos

#25 gruntguru

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Posted 12 June 2014 - 05:50

No doubt asymmetric timing will increase the part load stability - reducing skipped cycles etc?



#26 manolis

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Posted 12 June 2014 - 13:18

No doubt asymmetric timing will increase the part load stability - reducing skipped cycles etc?

Hello gruntguru and thanks for the interesting question.

The following animation has been added at http://www.pattakon....ttakonPatAT.htm

PatAT5.gif

as well as the following timing plot:

PatAT5_Timing.gif

(there is also a windows exe controllable animation at the bottom of the above web page).

When the asymmetry of the PatAT timing gets extreme, as above, the exhaust port can be closed before the opening of the transfer port. The fresh mixture has no chance to escape from the exhaust because the exhaust is closed when the transfer opens:

PatAT5.jpg

The engine seems closer to four-stroke than to two-stroke (stability of combustion, emissions etc).

With a part of the burned gas remaining in the cylinder, the radical ignition at partial loads gets into play (HCCI).

In applications wherein the clean exhaust is more important than the peak power, the above low cost and simple two-stroke seems interesting.

Thanks
Manolis Pattakos

#27 Speedman

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Posted 12 June 2014 - 15:35

Hallo Manoli

 

The engine is good, but the airflow trough the transfer port is not optimal. The airflow from crankhaft-case  through  overflow-channel in the port is many better.

By y our system, flow the air/fulel mixture from underneath  through  the piston, this does not allow for good gas flow.

Your system hasn't a good airflow for powerful two stroke engine. And a powerful two stroke engine required 5 or more transfer ports.

5 ports with your system is possible but not opimal. The airflow in two stroke engine is much more complicated and susceptible than four stroke engine.

 

Perhaps sufficient for a normal motor, but not for a sport engine.. The hydrocarbon-jetting-will certainly be much lower.

 

best regards


Edited by Speedman, 12 June 2014 - 16:07.


#28 manolis

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Posted 12 June 2014 - 16:23

Hallo Manoli
The engine is good, but the airflow trough the transfer port is not optimal. The airflow from crankhaft-case through overflow-channel in the port is many better.
By y our system, flow the air/fulel mixture from underneath through the piston, this does not allow for good gas flow.
Your system hasn't a good airflow for powerful two stroke engine. And a powerful two stroke engine required 5 or more transfer ports.
5 ports with your system is possible but not opimal. The airflow in two stroke engine is much more complicated and susceptible than four stroke engine.
best regards


Hello Speedman.

I know that the airflow of the last arrangement (the one with the zero overlap) is not optimal for a powerful two-stroke.

On the other hand, take a look at his arrangement from a different viewpoint:

Not looking for a powerful two-stroke having nearly double power than a good four-stroke of similar capacity, but looking for a cheap and simple and efficient and green alternative of the four-strokes.

Take, for instance, the case of a two-cylinder four-stroke 500cc, 40bhp engine of a scooter. The cylinder head is half of the rest engine (sometimes more); it is also the most expensive part and needs frequent service.

Replace the above four-stroke by a PatAT zero-overlap two-stroke:

PatAT5.jpg

of the same capacity.

The weight and the size and the cost of the two-stroke is substantially lower.

Suppose the two-stroke (because its airflow and its transfer ports and its exhaust are not optimized) makes the same power as the four-stroke.

Isn’t it interesting?

Suppose the two-stroke makes even less power than the four-stroke. Even in such a case you have the option to increase the capacity of the two-stroke to, say, 750cc without exceeding the weight or the size or the cost of the initial four-stroke.

Isn’t it more interesting?

So, please take a different look and let me know what you see.

Thanks
Manolis Pattakos

#29 Speedman

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Posted 12 June 2014 - 20:35

Hi

 

You are right.

I also belive the 4 strokes engines for scooter are too expensive and too heavy. 

 

 

best regards


Edited by Speedman, 12 June 2014 - 21:28.


#30 gruntguru

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Posted 12 June 2014 - 23:21

I doubt that zero overlap would be optimal for even the mildest engine. All four strokes have some "mechanical" overlap even when zero "effective" overlap is desired in gas exchange terms.



#31 manolis

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Posted 13 June 2014 - 03:58

Hello Speedman

The following drawing has been added to the PatAT web site:

PatAT_Overlap.gif

The combination of the zero-overlap PatAT with additional controllable transfer ports provides different modes of operation: green/economy mode at left, power mode at right.
With the yellow throttle blocking the "conventional" transfer port, the engine runs without overlap between the transfer and the exhaust.
With the yellow throttle down, the engine runs with all transfer ports active.


Hello gruntguru.

I agree.
The zero overlap is not the optimum.
But when you can have from zero to 100% overlap (the typical two-strokes run at 100% overlap between the transfer and the exhaust), the choice is yours.

The conventional four-strokes utilize some overlap because with their unique valve lift profile they have to idle and to operate from partial to full load and from low revs to red-line revs.

The MultiAir of FIAT operates – at some conditions - with substantially negative overlap. It is the “late opening” mode. The intake valve opens substantially after the TDC.
The valvetronic of BMW can also operate at substantially negative overlap (with their vanos VVT).
And I think that the new Toyota engines that use the Atkinson / Miller cycle cannot help running without negative overlap at light loads (the opening and closing of the intake valves are shifted substantially later).


Thanks
Manolis Pattakos

#32 Speedman

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Posted 13 June 2014 - 07:11

Hallo Manoli

 

I also thik the transfer-timing is too litte.  A little bit overlapping of exhaust-timing and transfertiming has almost no negative impact.

The time would too short that fresh gas is lost.

 

 

best regards.


Edited by Speedman, 13 June 2014 - 08:48.


#33 manolis

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Posted 14 June 2014 - 11:01

Hallo Manoli
I also thik the transfer-timing is too litte. A little bit overlapping of exhaust-timing and transfertiming has almost no negative impact.
The time would too short that fresh gas is lost.
best regards.


Hello Speedman.

I agree.

But think the following case:

You have on a scooter a carburetted two-stroke PatAT with some transfer ports having zero overlap with the exhaust, and other conventional transfer ports that can be blocked completely by a diaphragm:

PatAT_Overlap.gif

At idling and low revs / light loads, the diaphragm keeps closed the conventional transfer port. The mixture cannot escape to the exhaust.

The engine operates this way until a load (an angle of the throttle valve of the carburettor).

Then, together with the throttle valve, the diaphragm is displaced and the conventional transfer port gets into play progressively.

It is like the way the modern car engines are tuned: green and fuel efficient at the official urban and extra-urban cycles, and powerful (and gas guzzlers) when lots of power are required.


The interesting thing is that with the PatAT various options, not existing before, are to be investigated.

Thanks
Manolis Pattakos

#34 Speedman

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Posted 24 June 2014 - 08:43

Hello Manolis

 

 

I remember nowthere is two stroke engine with asymetric port-timing. A conventional two stroke motor with a rotary valve in the exhaust. This rotary valve makes asymetric port-timing.

 

 

best regards



#35 manolis

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Posted 24 June 2014 - 14:52

Hello Manolis
I remember now, there is two stroke engine with asymetric port-timing. A conventional two stroke motor with a rotary valve in the exhaust. This rotary valve makes asymetric port-timing.
best regards

Hello Speedman.

A rotary valve (the CCD at left of the upper cylinder) is used in the primavis engine:

primavis.jpg

to control the top transfer ports (and not the exhaust, i.e. it runs cold) providing asymmetric timing:
When the exhaust port opens, the top transfer ports are still closed by the rotary valve.
The lower transfer ports close before the exhaust port.
The top transfer ports (those controlled by the rotary valve) remain open a little after the closing of the exhaust port.

But a rotary valve (either on the exhaust or in the transfer ports) needs to rotate in synchronization to the crankshaft (i.e. it needs a gearing), it also needs bearings, it adds friction, cost etc.

The idea in the PatAT is to have asymmetric timing with the least possible complication and cost.

The following animation has been added to the http://www.pattakon.com web site:

PatAT6.gif

and shows how the connecting rods and the pistons of a Radial-4 PatAT are machined to provide wide bearing surface wherein the heavy loads are taken, keeping at the same time the crankpin short.
A single “plane bearing” (the yellow part around the crankpin) serves all the connecting rods in an "unconventional" way (it rotates inside the big ends of the connecting rods, being secured to the crankpin).

A Cross-Radial having articulated connecting rods cannot be well balanced, and the piston motion (or piston displacement) vs. the crankshaft angle is slightly different for different cylinders.
In comparison the arrangement of a Cross-Radial shown at http://www.pattakon....atAT/PatAT4.gif is as balanced as the best V-8 four-stroke engines.

Thanks
Manolis Pattakos

#36 Speedman

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Posted 24 June 2014 - 19:20

Hello

 

 

Is a famous opposite piston two stroke engine (principle Junkers). A old system, not very simple, but a very nice studenproject.

 

The counter-piston engine „4cl-alpha“ is a single- cylinder two-stroke engine based on the Junker-priciple.

http://cad.burg-halle.de/5912.html

 

motor manufacturing
 
Der Motor „2cl alpha

 

 

 

best regards


Edited by Speedman, 24 June 2014 - 19:26.


#37 gruntguru

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Posted 25 June 2014 - 00:14

Hi Manolis. Is there a reason you avoided the traditional master/slave connecting rod solution for your radial engine? The only disadvantages I see are the small variation in piston motion and the difficulty of lubricating articulated joints in a 2 stroke engine with it's reduced reciprocating forces. OTOH the master/slave would have to be simpler, cheaper and lighter than your solution in addition to being more easily expanded to radial layouts with higher cylinder count.



#38 manolis

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Posted 25 June 2014 - 04:07

Hello
Is a famous opposite piston two stroke engine (principle Junkers). A old system, not very simple, but a very nice studenproject.
The counter-piston engine „4cl-alpha“ is a single- cylinder two-stroke engine based on the Junker-priciple.
http://cad.burg-halle.de/5912.html
motor manufacturing
http://cad.burg-halle.de/5913.html
Der Motor „2cl alpha
http://web.hs-merseb.../Der-Motor.html
best regards

Hello Speedman.

Quote from http://www.pattakon....takonPatPOC.htm (a patent for the Junkers-Doxford opposed piston engine):

US1679976.gif

And here is the EcoMotors OPOC engine (two basic modules connected by a "clutch" for "variable capacity"):

OPOC_variable_capacity.jpg

(the OPOC is actually a pair of Junkers-Doxford sharing the same crankshaft for the sake of a better balancing).

In the same web page it is presented the PatPOC engine:

PatPOC_crosshead.gif

The PatPOC also uses two opposed pistons per cylinder. But it needs not two opposed cylinders to get well or full balanced (1st and 2nd order inertia forces).
Imagine the near to the crankshaft piston of the PatPOC made of steel (it controls the exhaust ports and is hot, so it is better if made of steel), and the distant piston made of light alloy (intake). Alternatively you can have different strokes for the two pistons (long for the lightweight one, short for the heavier) for a full (case of crankpins at 180 degrees) or good balancing.

Even when the crankpins are not exactly at 180 degrees (for the sake of an asymmetric timing) the vibrations of the single cylinder / two pistons PatPOC are comparable to the vibrations of the two-opposed cylinders / four pistons of the basic module of EcoMotors.

Compare the length of the PatPOC crankshaft with that of the other single crankshaft opposed pistons.
In the PatPOC the connecting rods are disposed substantially inside the cylinder footprint.
The typical failure of the Junkers-Doxford multicylinder engines of the old British ships was the necessarily long crankshaft because two of the three connecting rods had to be arranged at the sides of their cylinder, outside the cylinder footprint. With the PatPOC architecture, these old engine would have half-length for the same number (and bore) of cylinders.

As for the long “rods” (green) connecting the upper and lower sides of the distant piston, they are loaded in tension only (and they perform a simple reciprocation “along their long axis”).

If you know the guys making the "cl-alpha" engines, tell them for the PatPOC.


If you have to use two opposed pistons in order to have asymmetric timing of the ports, the PatAT that achieves the same with a single piston is preferable in most cases.

Thanks
Manolis Pattakos

#39 manolis

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Posted 25 June 2014 - 05:33

Hi Manolis. Is there a reason you avoided the traditional master/slave connecting rod solution for your radial engine? The only disadvantages I see are the small variation in piston motion and the difficulty of lubricating articulated joints in a 2 stroke engine with it's reduced reciprocating forces. OTOH the master/slave would have to be simpler, cheaper and lighter than your solution in addition to being more easily expanded to radial layouts with higher cylinder count.


Hello Gruntguru.

When the number of cylinders of a Radial is big enough (seven, nine etc) the “small” variation of the piston motion caused by the articulated rods is OK.
But with only four cylinders, things are different.

Think the case of a “Radial”-2 with master/slave connecting rod and compare it with a flat two-cylinder with forked connecting rods and common crankpin. In the second case the two pistons (m is the mass of piston, of the piston pin and of the upper part of the connecting rod) are equivalent with a 2*m mass reciprocating harmonically (take the center of gravity of the two pistons: the triangle from the crankpin to the one wrist pin and to the other wrist pin is isosceles, i.e. the center of gravity of the two pistons is at the projection of the center of the crankpin on the cylinder axis). Only a 1st order inertia force remains unbalanced. All the rest inertia forces are balanced.
In the case with the articulated rods (master / slave) the motion of the second piston is substantially different from the motion of the first piston. The different motion of the two pistons leaves unbalanced inertia forces of various orders.

In the case of the Radial-4, if you try to keep the even firing, you have to put the pins in the master rod at specific angles, which is not the best for the balancing. If you optimize the balancing, you lose the even firing. The pair of the two side (relative to the master rod) pistons is a more difficult to balance (as compared to the other pair).


And why four cylinders and not fewer?

Because with only two cylinders and even firing (i.e. common crankpin) the engine is unbalanced (it needs a pair of counter-rotating external balancing shafts) to get rid of the 1st order unbalanced inertia forces, while it suffers from a heavy inertia torque).

And because with only three cylinders and even firing (common crankpin) there is a strong unbalanced 2nd order rotating inertia force and a significant 3rd order inertia torque.

The Cross Radial (Radial-4) with forked connecting rods has a perfect balancing of the inertia forces, and a very good (as the best V-8 four strokes) balancing of the inertia torques.

For the manufacturing, I think that making the forked connecting rods is not difficult: you start from the same basic “full” connecting rod and remove material depending on which cylinder this connecting rod is to serve. The unconventional unique “plane bearing” seems a simple / functional solution.


And why four cylinders (per crankpin) and not more?

Having a fully balanced even firing Cross-Radial two-stroke, you don’t need more cylinders per crankpin.
If you need more power, a second Corss-Radial (rotated by 45 degrees for cooling) is disposed around a second crankpin of the same crankshaft/

Thanks
Manolis Pattakos

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#40 manolis

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Posted 25 June 2014 - 10:09

Hello Gruntguru.

I think the following GIF animation helps:

Radial_4_Articulated_Rods.gif

The master rod (red) has a center to center length 1.6 times the stroke (i.e. it is not short).

Each slave connecting rod has center to center length equal to the stroke.

The engine is even firing.

The two side cylinders (cyan and green connecting rods) run necessarily at longer strokes.

Look at the angle between the cylinder axis and the respective slave connecting rod (thrust loads), and compare it to the angle of the master rod with the axis of its cylinder (-18.2 to 18.2 degrees). The one side cylinder compresses quickly, the other expands quickly.

It seems the Radial-4 (and the R-5 and the R-6) with master / slave connecting rods has serious unevenness of various kinds.

In comparison the Cross-Radial PatAT with the forked connecting rods seems way superior;
imagine it as Direct Injection Diesel with a turbocharger as scavenging pump (for an airplane or helicopter or paraglider it is not necessary a volumetric scavenge pump) and four-stroke-like lubrication.

Thanks
Manolis Pattakos

#41 gruntguru

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Posted 25 June 2014 - 23:05

Thanks Manolis. Clearly the typical aircraft radial engines must use a much higher l/r ratio. That would also make sense when you are trying to fit 9 cylinders into 360 degrees.



#42 manolis

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Posted 26 June 2014 - 06:58

Thanks Manolis. Clearly the typical aircraft radial engines must use a much higher l/r ratio. That would also make sense when you are trying to fit 9 cylinders into 360 degrees.

Hello Gruntguru.

Typical connecting rod to stroke ratios in the airplane Radial engines is 2 to 2.2. Smaller values would be preferable (despite the resulting asymmetry) because the diameter and the front area of the engine would reduce, however the l/r is big to avoid neighbor piston collision at the BDC.

Using the crank-arm, the master connecting rod length and the slave connecting rod length of the Bristol Centaurus (nine cylinders per crankpin; the connecting rods were longer than the Bristol Hercules with the seven cylinders per crankpin to avoid neighbor pistons collision at the BDC) the following drawing was made:

Radial_4_Long_Rod.gif

The drawing (made for a Cross-Radial) reveals several interesting things about the Radial engines used in airplanes.

While the stroke of the piston driven by the master rod (and of the “opposite” piston) is 140mm (two times the crankshaft arm), the other two pistons (the side pistons) have a stroke of 150mm (7% more)!

While the angle between the master rod and its cylinder axis varies from –13 to +13 degrees, the angle between the slave rod of the “opposite” piston and its cylinder axis varies from –22 to +22 degrees (more than 50% heavier thrust loads on the cylinder liner of the “opposite” piston).

At 90 degrees the “master rod” piston is 78mm away from the TDC, while the “opposite” piston is 89mm away from its TDC (14% more piston travel in the same time)!

The previous asymmetries exist in all Radial engines with articulated rods.

Despite all these, the Radial engines were achieving extreme power to weight ratios.

By the way, in the poppet valve Radial engines they were using a rotating (concentric with the crankshaft) second order balancing shaft. In the Bristol Sleeve valve Radial engines no such balancing was used (maybe because there was not space with all the gearing for the motion of the sleeve valves).

In comparison, the symmetry of the structure of the two-stroke Cross-Radial PatAT seems as a great advantage (think, for instance, to try to optimize the combustion or the breathing of cylinders having substantially different capacity and substantially different piston-velocity-profiles).

Thanks
Manolis Pattakos

#43 manolis

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Posted 28 June 2014 - 12:08

Hello.

This:

Radial.gif

is a slide taken from this http://www.pattakon....pman/Radial.exe windows exe program.

It shows a conventional Radial engine (master / slave rods architecture) wherein the number of cylinders varies from 1 to above 30 (keep pressed the up-arrow on the keyboard), wherein the L/S ratio (connecting rod to stroke ratio) varies from below 1.5 to about 3, wherein the eccentricity of the wrist pins on the master rod vary, etc.

It also calculates (press the c key) and presents grafically the motion of the center of gravity of the set of all pistons.

To stop the motion press the Space Bar key. To accelerate the motion keep pressed the . key.

Thanks
Manolis Pattakos

#44 manolis

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Posted 06 July 2014 - 06:05

Hello.

A new "chapter" has been added to the PatAT at http://www.pattakon....ttakonPatAT.htm

Now the PatAT Asymmetric Timing is applied to the Intake, too:

PatATi_Timing.gif

The piston, in cooperation with the connecting rod and the cylinder, controls the intake timing.

PatATi.gif

The intake port communicates with the crankcase through a piston port controlled by the connecting rod:

PatATi_2.gif

At http://www.pattakon....atAT.htm#PatATi you can also find controllable windows exe animations.


PS.
Strange that nobody comment on the http://www.pattakon....atAT/Radial.exe windows exe program for the Radial engines.
Did you know about the various asymmetries in the Radials?
Did you see, for instance, the significant (and substantially variable) offset of the center of gravity of the reciprocating parts from the "crankpin-center to crankshaft center" line?
Did you see the substantially different strokes in the various cylinders?

Thanks
Manolis Pattakos

#45 manolis

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Posted 20 July 2014 - 09:15

Hello.

In the following drawings it is shown a PatATi model engine (or RC engine) wherein the crankshaft is made of two pieces to enable assembly, with the casing (comprising the cylinder head, the cylinder, the cooling fins, the crankcase and the complete main crankshaft bearing) being one piece (only a simple cover - the green sliced part at left - needs to be added to seal the crankcase) :

PatATi_model1_cover.gif

PatATi_model1.gif

There are five transfer ports (the two asymmetrical) and two inlet ports:

PatATi_model1_ports.jpg

Thanks
Manolis Pattakos


Edited by manolis, 20 July 2014 - 09:41.


#46 gruntguru

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Posted 20 July 2014 - 23:18

Hi Manolis. Could you post a timing diagram showing all 4 port types?



#47 manolis

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Posted 21 July 2014 - 04:36

Hi Manolis. Could you post a timing diagram showing all 4 port types?


Hello Gruntguru.

The following timing plot is added at the bottom of http://www.pattakon....ttakonPatAT.htm

PatATi_model1_timing.gif

It is not shown in the plot, but just like the transfer, the inlet is also comprising two "parts":

initially (from 75 deg after the BDC to 40 deg before the TDC) the air / mixture is suctioned into the crankcase through the lower piston ports that are controlled by the connecting rod,

then (from 40 deg before the TDC to 40 deg after the TDC, i.e. till the end of the admission) the air / mixture is suctioned into the crankcase through two paths: through the lower piston ports mentioned above and directly through the inlet ports (as in the simple two-stroke).

Thanks
Manolis Pattakos

#48 manolis

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Posted 24 July 2014 - 05:41

Hello Gruntguru.

If it helps,
here is the timing of the OS .18 TZ (3cc / 0.18ci, 2.28 bhp/30,500rpm):

PatATi_OS18TZ_timing.gif

for comparison.

From 35 deg after the BDC to 57 deg after the BDC (i.e. for only 22 crankshaft degrees, and with the inlet and transfer ports only partly open) the engine is like an "open pipe".

At http://www.osengines....st-rcnitro.pdf you can see the flat torque curve (from 19 to 31 Krpm) of this extreme power density ( 750 bhp per liter ) engine.

By extending properly the lower end of the inlet ports of the PatATi model engine, the desirable overlap (the part of the cycle wherein the engine behaves as an "open pipe) is taken, for instance like the one of the OS.18TZ.

Thanks
Manolis Pattakos

#49 Speedman

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Posted 24 July 2014 - 09:13

Hello peoples

 

A very nice sound. Is it ether a theoretical test.

 

Murnan Modified RBR3 on the Nitro Engine Dyno

 

I thik the engine is after the test  heavily worn.

 

best regards


Edited by Speedman, 24 July 2014 - 11:09.


#50 manolis

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Posted 31 July 2014 - 13:17

Hello.

Achates Power (Wal-Mart, nearly 100 millions US dollars invested so far, their Opposed Piston prototype with the two synchronized side crankshafts is claimed achieving in their lab 45% BTE (180gr/kWh) and 0.1gr/kWh specific lube consumption) explains in their article:

"Non All Two-Stroke Engines Are Created Equal"

several things about the timing in the two-stroke engines:


Does a conventional two-stroke have the same efficiency advantages as the opposed-piston, two-stroke (OP2S) engine? If you answered “no”, you’re right. But, do you know why?

One reason is due to heat transfer. As we highlighted in an earlier post and technical paper, the favorable surface area-to-volume ratio of the OP2S contributes to the engine’s inherent thermal efficiency benefits. So too does the architecture itself, which uses a different scavenging method than other two strokes.

For example, loop scavenged two-stroke engines use a cylinder head with two intake and exhaust valves. To reduce the short-circuit flow, the intake valves are masked—guiding the flow away from the exhaust valve during scavenging. Standard uniflow-scavenged two-strokes have intake ports that are controlled by the piston and four valves in the cylinder head that are used to manage the exhaust flow. The OP2S, on the other hand, relies on separate intake and exhaust ports that are controlled by the motion of the pistons.

scavenging_methods-300x228.jpg

To determine the gas-exchange performance, we need to evaluate the effective flow areas of the intake and exhaust ports or valves during the scavenging process. The ultimate goal is to deliver the same trapped air mass for all configurations. In a two-stroke engine, the intake and exhaust ports are open at the same time. This differs from a four-stroke engine since the piston motion has little effect on the mass flow through the engine during scavenging.

To compare the three different two-stroke configurations for their reduced effective flow area, you first need to define the effective flow areas for the valves and ports. Using flow bench measurements and computational fluid dynamics, we have confirmed that for the ports, the flow coefficient of constant 0.75 is a good average approximation for different port lifting.

The gas exchange period of a two-stroke engine is significantly shorter than that of a four stroke. Therefore, for two-stroke engines with valves, the cam profile has to be developed to open a valve within the design limits of velocity and acceleration in order to avoid excessive friction and valve tossing. When the opening and closing of intake and exhaust are assumed to have timing similar to that of the opposed-piston engine and velocity and acceleration limits are imposed, it leads to the valve lift profiles illustrated below.

valve_lift_profile_loop_scavenged_2.jpg

Based on the reduced flow capacity shown for the loop-scavenged configuration, you can see how all valve-controlled two-stroke engines are compromised. The same is true for the uniflow-scavenged configuration with exhaust valves on top. The symmetric opening and closing of the intake port around bottom dead center means the compression stroke is always significantly longer than the expansion stroke, which leads to a penalty in gross indicated efficiency.

valve_lift_profile_uniflow.jpg

In an opposed-piston engine, however, the cranks are phased to enable an exhaust blowdown event, meaning the intake opening and closing is not symmetric. As a result, the difference between the compression and expansion stroke will always be lower than for a uniflow-scavenged engine and, therefore, the gross indicated efficiency loss caused by a difference in compression to expansion stroke is lower.

uniflow_scavenged-300x196.jpg

When comparing the effective flow areas of the intake and exhaust valves, the OP2S also has an advantage. Due to the OP2S crank lead, the intake port closing is later than the uniflow poppet valve engine for the same exhaust port or valve closing. Therefore, the scavenging process is extended and more fresh charge can enter the cylinder.

After viewing the effective reduced flow areas for all three configurations, it’s clear that loop-scavenged engines are not as efficient, since they have much lower blowdown and scavenging effective time areas as compared to OP2S engines, which have the highest scavenging time areas. The opposed-piston port engine also starts scavenging earlier, reaches a higher level and ends the period later than the uniflow-poppet configuration. As an integral over time, the advantage in flow area of the OP2S in this example is 22.2%. To get the same mass flow, the intake manifold pressure and/or the pressure difference between intake and exhaust has to be raised for engines with loop-scavenged or uniflow-poppet configurations, leading to higher pumping losses. Based on this comparison, the opposed-piston engine produces the lowest pumping losses of all configurations.

effective_flow_areas.jpg

With reduced heat transfer, a better relation between compression and expansion ratios, a higher effective flow area and higher scavenging efficiency, the OP2S truly is more efficient than other two-stroke powertrains.

Thanks
Manolis Pattakos