Model Engine Maker

Help! => Specific Engine Help => Topic started by: Roger B on December 15, 2015, 11:54:20 AM

Title: Two stroke engine ports
Post by: Roger B on December 15, 2015, 11:54:20 AM
I am currently designing what will be a fairly low speed two stoke and would like to see what other members think about port designs. This will be a loop scavenged engine with the transfer port on one side of the cylinder and the exhaust port on the other side with a coned piston.

1) My initial thoughts put the ports across the crankshaft axis. This means that they will be on the thrust faces of the piston. Is this likely to be a problem? If they are aligned along the crankshaft I would have to have a solid gudgeon (wrist) pin so it does not act as a passage from the transfer port to the exhaust port.

2) Do the piston rings always need to be pinned to keep the gap away from the ports or is this only necessary if the ports are wider than a certain amount? I am initially thinking of each port covering 90 of the cylinder wall and separated into four slots so that each opening will be less than 20. (I will have to work out if I can actually make this  ::) )

3) If the rings have to be pinned what is the best way to do this? The piston will be 25mm diameter and the rings ~1mm square.
Title: Re: Two stroke engine ports
Post by: Jo on December 15, 2015, 12:49:00 PM
I was told not to waste my time worrying about where the gaps were  on the piston ring as they will go round and round when the engine runs  :insane:

Your piston is about the same size as the Feilgiebel which is not pinned. But it does have the ports in two halves with a centre bit remaining to prevent the rings falling into the ports  ;).

Title: Re: Two stroke engine ports
Post by: Jasonb on December 15, 2015, 01:16:57 PM
Roger you could always drill the pin in from either end and just leave say 1mm in the middle if you are worried about gasses passing through.

I have not pinned any of my rings, you need a larger gap for a pin so more chance of gasses passing. On larger rings you can shape the ends to fit around a pin but on a small ring its not easy
Title: Re: Two stroke engine ports
Post by: Alyn Foundry on December 15, 2015, 01:21:58 PM

I spent a few years working at a local motorcycle breakers who also did repairs. I got heavily involved in the reconditioning side of the business. Back then the Japanese two stroke was in vogue but due to operator error ( mainly) they were always needing a rebore.

Most were reed valve induction via the crankcase transferring to ports. All pistons were just domed, non damming type. But the piston rings were always pinned, usually at the port divider in the cylinder.

It might be that over a certain cylinder diameter pinning becomes a necessity as the ring ends can catch in the port, but from my own experience all two strokes I have worked on have had piston ring pins.

My favourite two strokes are the Petter range ( M ) types. Are you going to use premix or try out the Petter approach with a sprayer in the transfer port?

Kind regards, Graham.
Title: Re: Two stroke engine ports
Post by: gbritnell on December 15, 2015, 01:58:07 PM
As the performance of 2 cycle engines increased the width of the ports got enlarged to allow more gasses to flow, both intake (transfer) and exhaust. In doing this small bridges were cast across these wide ports to prevent the rings from catching the edges and breaking the rings. This was just the side of the ring opposite the split. If the split side was to come around into the wide port there surely would be disaster so the pistons have small pins to prevent the rings from rotating.
As far as the wrist pin (gudgeon pin) construction goes the top of the piston controls the timing events. 1. The piston/rings seal the cylinder. 2. As the piston moves down in the bore it starts to pressurize the crankcase. 3. The top of the piston starts to uncover the exhaust ports and the exhaust gasses start to flow out. 4. Depending on the timing events of the engine with the piston's downward travel it will then uncover the transfer ports, all the while continuing to add more pressure to the crankcase. 5. Once the transfer ports are uncovered the pressurized mixture in the crankcase will be forced into the cylinder and also be drawn by the scavenging affect of the exhaust flow as the exhaust ports are still exposed. It's at this point that 2 cycle engines become polluters. 6. As the top of the piston covers the exhaust ports on it's way back up the cylinder the charge then starts to compress. Upon reaching the ignition timing point the compressed mixture is ignited and the cycle starts over.
Port timing/location along the cylinder controls the performance of a 2 cycle engine. By moving the ports up and down the engine can be made to produce it's power at different rpm's. The exhaust configuration also adds to the performance due to the scavenging affect it can add.
Actually 2 cycle engines to the uninitiated can seem quite simple but engineering wise are quite complex.
Title: Re: Two stroke engine ports
Post by: Allen Smithee on December 15, 2015, 02:56:53 PM
Well your alternative is to bridge the ports. According to my copy of Blair if the ring(s) are pinned you can have port widths up to 60 degrees of cylinder circumference before they catch, but for unpinned rings the recommendation is individual ports should be no wider than 25 degrees.


Title: Re: Two stroke engine ports
Post by: CHP on December 15, 2015, 03:39:57 PM
These books is free and does answer all your question.  :cartwheel:
something to not forget is the primary compression ratio :o   :ThumbsUp:   :ThumbsUp:
Title: Re: Two stroke engine ports
Post by: Roger B on December 15, 2015, 07:56:29 PM
Thank you all for the information and the links. There is plenty to think about  :headscratch:  ::)

This is intended to be a full diesel, similar in layout to the Field Marshall tractor but using the combustion chamber from the Petter Atomic diesel. The original has bridged ports which I am intending to copy. A quick sketch suggests that the angle will be less than 25 so pinned rings may not be essential, especially if the top of the ports are curved.

The crankcase will be quite compact and as I am planning on a CI piston the balance weights should take up sufficient space to maintain reasonable crankcase compression.
Title: Re: Two stroke engine ports
Post by: Admiral_dk on December 15, 2015, 09:23:51 PM
To me it will not make any sense if you choose a exhaust duration less than 90 degree - though it might still like to run faster than planned. The transfer port needs to have shorter duration than the exhaust, so perhaps 65-70 degree.

It's at this point that 2 cycle engines become polluters.

Hmm - yes and no .... Any high performance two stroke engine only has a high performance combined with a tuned expansion chamber exhaust system. Such a system draws (sucks) the gas from the crankcase out the exhaust during the time both the transfers and exhaust port is open. When the transfer ports close, you should have a returning pressure pulse from the resonance chamber pressing all the fresh gas back into the cylinder and holding it there until the exhaust port closes => no pollution from fresh gas.

Honda did even one better and has the patent to prove it (and not producing anything with it) + they raced a bike with such an engine and won. It's a two stroke with an exhaust valve (no not like on a four stroke), that changes the exhaust timing - seen before, but in such a fashion that it becomes a diesel though running on gasoline from 1/3 to 2/3 throttle - it even shut down the ignition in this mode. So except in idle, it runs just as clean as a four stroke.