2-stroke piston window, rear boost port.

[John Orchard]

The piston window is continually open to the intake/rear transfer.

John, that's my point, or I've misunderstood your first post.
You've put the window in the piston to allow hot gasses to escape OUT through it.
So if it is open to the intake port aren't the escaping hot gasses from under the piston crown traveling in the opposite direction to the fresh charge coming in through the intake port?
If you want hot gases to escape from under the crown wouldn't you put it on the exhaust port side?

Soon after the pistons starts its downward stroke, the reed petals close, stopping mixture coming in (or going back out) the inlet port, the piston hasn't uncovered the transfer/boost ports yet so it starts to compress the mixture in the crankcase, as the transfer/boost ports are uncovered in the combustion chamber the crankcase compression, combined with the suck from a properly tuned expansion chamber expel much of the mixture from the crankcase & under the piston up the transfer/boost ports into the combustion chamber.

In a modern reed valve engine with a rear transfer (or boost) port, the intake port becomes part of the transfers while the piston is basically on its downward stroke.

[Daryl Jones]

The piston window is continually open to the intake/rear transfer.

John, that's my point, or I've misunderstood your first post.
You've put the window in the piston to allow hot gasses to escape OUT through it.
So if it is open to the intake port aren't the escaping hot gasses from under the piston crown traveling in the opposite direction to the fresh charge coming in through the intake port?
If you want hot gases to escape from under the crown wouldn't you put it on the exhaust port side?

As the piston descends and pressurises the crank-case, the reed valve closes, the intake flow stops and the port becomes a transfer only.
Pressurised charge flows out the new hole in the back of the piston and up into the chamber. Movement of this charge past the underside of the piston crown ( inc. vapourisation) should remove some heat from this area.
Increases of transfer port area/flow can be beneficial to broadening mid range power.

None of the process is as simple and straight forward as above, but that's the basic idea.

I type too slowly... just read Johns reply above.

Cheers DJ

[Lozza]

At no stage ever does the decending piston 'pressurise' the crank case to any great degree or force the intake charge through the transfers.
All the work is done by the pipe at BDC,  it pulls intake from the TRANSFER ports not the crankcase, it can open reeds but that only replaces the intake charge in the crankcase before it moves to the transfer ducts.
What the piston port does is what does it line up with  at BDC, intake charge to travel from under the piston to the C port duct is the answer.
Its a very 80's idea that I suppose superceded now by case reed engines and a proper C port duct. Never found any real measurable gain on cylinder reeds with combined C port and inlet port. It can't hurt though.

[Daryl Jones]

OOPS!  I better explain to my Victa that it can't possibly start or run without an expansion chamber doing 'all the work' at BDC. :'(

[Momus]

I did a bit of homework & thought regarding piston window's, thought I'd give it a try.  Reports state an increase in 1 to 2 horsepower on a 500, plus a cooler running piston crown from escaping hot gases from under the crown.

Not worried about any more power but thought anything that may give cooler running is worth a try plus even a slight reduction in piston weight wouldn't be a bad thing?

To tell you the truth, I wouldn't notice any increase in power, I'm not getting what I've got to the ground now!  It does seem a touch cooler in the combustion chamber; plug is a little darker.  And my imagination is feeling there is less vibration (I'm a wanker lol).

Any thoughts?

I see the piston exhaust port helping refresh the under crown and improving the lube and cooling of  the small end bearing.

I wonder about the effect of the under piston mixture exhausting from the new outlet into the scavenge stream from the standard boost ports.
Does the piston slot stream affect the aim and velocity of the standard boost ports mixture flow?

Back when I did the same mod to a YZ80C and lo and behold the following year the YZ80D incorporated the horizontal piston slot. It was very difficult to tell if there was any change in the relatively lack lustre performance these single transfer per side engines gave. When the design changed to the 4 transfer (then six like the CR) arrangement with the E? with the big side cavities the engines really came alive

At no stage ever does the decending piston 'pressurise' the crank case to any great degree or force the intake charge through the transfers.
All the work is done by the pipe at BDC,  it pulls intake from the TRANSFER ports not the crankcase, it can open reeds but that only replaces the intake charge in the crankcase before it moves to the transfer ducts.
What the piston port does is what does it line up with  at BDC, intake charge to travel from under the piston to the C port duct is the answer.
Its a very 80's idea that I suppose superceded now by case reed engines and a proper C port duct. Never found any real measurable gain on cylinder reeds with combined C port and inlet port. It can't hurt though.

Aussie engineer Gordon Burford made thousands of these 2 strokes. They never had an exhaust pipe, much less a chamber. The inlet porting is rotary valve and there is no connection between the carburettor and crankcase at BDC, as with a piston port engine.

[djr]

I thought the fresh fuel charge in the crank was moved up through the transfer ports to the combustion chamber by the descending piston increasing (compressing) the air pressure in the crank, not being sucked up by escaping exhaust gasses..
8 pages here we come....


  You are right , but the best answer to that is Yes & No,
 while it is true that a good exhaust on a 2 stroke will pull mixture up from the crankcase through the transfer ports , an expansion chamber only works when the engine is firing and burning air/fuel mixture.
To get the engine to  Start Running SOME crankcase compression is required to get some air/fuel mixture into the cylinder, after that
the exhaust can take over I suppose.
Also 8 pages is not enough , maybe 80 ......

[Lozza]

OOPS!  I better explain to my Victa that it can't possibly start or run without an expansion chamber doing 'all the work' at BDC. :'(

Box mufflers still provide (albeit very limited) a suction and plugging pulses. I order for the piston to push anything through the transfers, pressure in the crankcase would have to be higher than what is above the piston. That will only happen long after the transfers are open

  You are right , but the best answer to that is Yes & No,
 while it is true that a good exhaust on a 2 stroke will pull mixture up from the crankcase through the transfer ports , an expansion chamber only works when the engine is firing and burning air/fuel mixture.
To get the engine to  Start Running SOME crankcase compression is required to get some air/fuel mixture into the cylinder, after that
the exhaust can take over I suppose.

The exhaust only pulls mixture from the transfers, mixture in the transfers is replaced by what is in the crankcase. A air/fuel molecule will take 15-20 cycles to go through the engine not 1 or 2. No case compression is need to start an engine, fuel evaporisation is whats needed.

[Daryl Jones]

I(n) order for the piston to push anything through the transfers, pressure in the crankcase would have to be higher than what is above the piston. That will only happen long after the transfers are open

Pressure above the piston is able to fall dramatically when the exhaust port opens.
If the engine is cold and not yet running it will return to atmospheric.
(With a running engine and a suitable pipe it can go negative).

The volume in the crankcase reduces due to piston displacement down from TDC.
Pressure WILL increase until the transfer ports open. (P1xV1xT1 = P2xV2xT2)
(no matter how negligible this may appear to be (to some), including losses due to back flow through the inlet port).

Once fully operating on a tuned pipe, a 2 stroke is more like a pulse jet engine, until it is, it's just a basic air pump.

I merely wished to provide a simple explanation to evo550 why the window was in the back of the piston not on the exhaust side.
I think we have managed that.

I am happy to agree to disagree on the rest.

Cheers, DJ

[sleepy]

Got to love some of the 2 stroke theory here.

John I believe you have it correct in the cooling benifit for the piston crown. Quite a few good 2 stroke have been done that way in the past. 250 Maico's are a good example although they used the piston hole to feed the rear transfer port without an inlet port involvment. Not sure about the HP gain but the crown temp will help with detination control not to mention more consistent combustion.

Lozza has a unique theory, and if I get the gist of it we seem to not need a crankcase but should just plumb the carb straight to the tranfer port through a reed. Can't wait to see this new technology in action.

[GMC]

My understanding is that it increases the inlet port duration and will only work with a reed valve motor.
It would bring the motor closer to case reed specs (inlet port nearly always open)

A piston port motor would suffer blowback into the carby which is why the Maico motors that Sleepy talks of would have the port holes lining up with the transfers.

[djr]

OOPS!  I better explain to my Victa that it can't possibly start or run without an expansion chamber doing 'all the work' at BDC. :'(

Box mufflers still provide (albeit very limited) a suction and plugging pulses. I order for the piston to push anything through the transfers, pressure in the crankcase would have to be higher than what is above the piston. That will only happen long after the transfers are open

  You are right , but the best answer to that is Yes & No,
 while it is true that a good exhaust on a 2 stroke will pull mixture up from the crankcase through the transfer ports , an expansion chamber only works when the engine is firing and burning air/fuel mixture.
To get the engine to  Start Running SOME crankcase compression is required to get some air/fuel mixture into the cylinder, after that
the exhaust can take over I suppose.

The exhaust only pulls mixture from the transfers, mixture in the transfers is replaced by what is in the crankcase. A air/fuel molecule will take 15-20 cycles to go through the engine not 1 or 2. No case compression is need to start an engine, fuel evaporisation is whats needed.

Okay then so what you are saying is - crankcase compression is not required & has no effect {even for starting} ,
and the exhaust system is the only thing pulling air/fuel mixture up from the crankcase.    Is that right ?
so a 2 stroke engine with the exhaust removed will never start & run ?
 
that's an interesting theory that I have never heard before, you learn something new every day , thanks 

[Lozza]

Pressure above the piston is able to fall dramatically when the exhaust port opens.
If the engine is cold and not yet running it will return to atmospheric.
(With a running engine and a suitable pipe it can go negative).

The volume in the crankcase reduces due to piston displacement down from TDC.
Pressure WILL increase until the transfer ports open. (P1xV1xT1 = P2xV2xT2)
(no matter how negligible this may appear to be (to some), including losses due to back flow through the inlet port).

Once fully operating on a tuned pipe, a 2 stroke is more like a pulse jet engine, until it is, it's just a basic air pump.

I merely wished to provide a simple explanation to evo550 why the window was in the back of the piston not on the exhaust side.
I think we have managed that.

I am happy to agree to disagree on the rest.

Cheers, DJ

The pipe still 'works' without the engine running not to the degree when it is
The pressure gauges I connect to superkart engines show a massive +/- 1-1.5psi on the pulse pump connection. More than 10 psi in the cases you will blow the crank seals, compare that with maybe 1500psi at max compression and maybe 500psi at exhaust port open, then a mere 30deg of crank rotation later pressure above the piston is going to be less than 1.5psi?

Lozza has a unique theory, and if I get the gist of it we seem to not need a crankcase but should just plumb the carb straight to the tranfer port through a reed. Can't wait to see this new technology in action.

Just have a look at a 2015 Modena ICC kart engine that has a 24/7 inlet valve that does basicaly that. Unique in the sense that HRC, Jan Thiel, Jorge Muller and a host of kart engine manufacturers know what I'm talking about 

Okay then so what you are saying is - crankcase compression is not required & has no effect {even for starting} ,
and the exhaust system is the only thing pulling air/fuel mixture up from the crankcase.    Is that right ?
so a 2 stroke engine with the exhaust removed will never start & run ?
 
that's an interesting theory that I have never heard before, you learn something new every day , thanks 

atmospheric pressure is still the lowest regardless, even if the engine doesn't fire higher pressure is still in the combustion chamber until the exhaust port opens. Each time the piston ascends you fill the crankcase including the transfer ducts.

There is never going to be high enough pressure in the crankcase to 'push" intake out the transfers especialy on a running engine.

[80-85 husky]

I didn't get past "maybe 1 or 2 hp" with a 500 I thought why lessen the piston integrity for f*kall gain ? but im a lazy basket....

[bigk]

I put fuel in the tank & it somehow magically gets to where it's supposed to go in the engine & my bike moves forward. That's all I need to know.
K

[sleepy]

There is never going to be high enough pressure in the crankcase to 'push" intake out the transfers especialy on a running engine.

If the pressure in the crankcase isn't greater than the pressure in the cylinder it is a pysical impossibility for the motor to ever run, simple physics. I didn't need to spend 4 years at UNI to work that one out.