Timing for SI 2-stroke?

[metrogdor22]

I've decided that I want my first project to be a basic 2-stroke. Nothing fancy, just a single cylinder spark-ignition 2-stroke. I'd be elated just to get it to run.

So I'm currently deciding on placement of the transfer and exhaust ports. I know there's a lot left (head, entire carburetor, rings, clearances, etc.) but I want to make sure I have the size and placement of the ports correct, at least enough for it to run. Attached are pictures of the piston in the cylinder at different positions.

The bore is 1", the exhaust port is 0.175"x0.75". The transfer port in the crankcase is the same size but splits into two 0.15"x0.4" slots that angle upwards. Stroke is 0.4" The lines radiating out from the crankshaft are at 30 degree intervals. There's also a simple flow simulation with the piston at just below the transfer ports opening. It looks to me like the gas would hang around in the cylinder enough for most of it to get compressed and ignited. I'm thinking I'll make the exhaust port smaller to help keep the rings from bulging into it, maybe 0.60".

What do you think?

 

[jef-lemmens]

This is the timing that I use on my motor, you can make the exhaust port with a bar in the middle if your worried about the width .

Regards Jef

 

[metrogdor22]

Thanks Jef! I definitely appreciate the input from an actual running engine.

 

[lohring]

This may be a little late, but a better design for a cross flow scavenged engine is the QUB crossflow design. It has a better combustion chamber shape and should be easier to build.

Lohring Miller

 

[metrogdor22]

This may be a little late, but a better design for a cross flow scavenged engine is the QUB crossflow design. It has a better combustion chamber shape and should be easier to build.

Lohring Miller

Thanks for the advice. I remember seeing the acronym QUB somewhere but I can't find anything that really says what it is. Is it a port arrangement, or a combustion chamber shape?

 

[jef-lemmens]

Diffirent flow's

 

[metrogdor22]

Diffirent flow's

Which one of those is QUB?

 

[lohring]

QUB is Queens University of Belfast. The late Gordon Blair did a lot of the basic research there that lead to the modern two stroke design. Unlike most two stroke developers, he published his work and developed ways to simulate two stroke designs on computers. An early version of his book "The Basic Design of Two Stroke Engines", is available on line. The modern version of his two stroke simulation is EngineMod 2T. I've used this program extensively in developing tuned pipes and matching port timing for model racing engines. It has a significant learning curve and would be expensive unless you do a lot of high performance two stroke development. If you are interested in some history and the future of high performance two strokes, I wrote a series of articles for the North American Model Boat Association. The links are below: Download the .pdf file for easier reading.

High Power Two Stroke Design
High Power Two Stroke Design - Part 2
High Power Two Stroke Design - Part 3

Lohring Miller