2-Stroke Design: Port Timing Round 2

[metrogdor22]

Hi all,

I posted here a few weeks back on how the port timing on my (future) 2-stroke SI engine should look. I took some advice from the comments and switched to closer to a loop scavenging approach, and also simplified the machining.

My main question is still how my port timing looks. The exhaust ports open 97.5 degrees before BDC, and the transfer ports open 52.5 degrees before BDC. Both are symmetric about BDC. Exhaust ports are 2x5/16". Transfer ports are 1x7/8" that separates into 3x5/16". I may decide to start with only 2 transfer ports and see how that goes. If I need to add more transfer or more exhaust, I can later. Intake port is 3/4".

Bore is 1.25", stroke is 0.96". 0.08" gap between the head and piston @ TDC. I figure it'll be easier to add spacers and make that gap larger if I need lower compression. 13.02:1 uncorrected compression, 6.89:1 only counting after the exhaust ports close. Crankcase CR is 1.34:1.

Attached are pictures of the piston at TDC, Exhaust Opening, Transfer Opening, BDC, cylinder from the top, and cylinder from the bottom.

Should I move the transfer ports closer to the sides of the cylinder? Do you think this would run as is? Around what size carburetor should I need? I plan to have a reed valve on the intake port, what about the exhaust port?

Thank you for all the help this forum has given me!

 

[jef-lemmens]

I don't think it will work well with the transfer ports and exhaust ports on the same side , the flow will be disturbed I think

Regards Jef

 

[metrogdor22]

I don't think it will work well with the transfer ports and exhaust ports on the same side , the flow will be disturbed I think

Regards Jef

What about something like this? I moved the transfer ports to the sides (90 degrees from the exhaust ports), and added a boost port to 180 degrees from the exhaust port that opens 10 degrees before the transfer ports.

 

[lohring]

Low cost industrial engines use the porting system pictured below. The slugs in the pictures are guides for port modifications. The widths, angles, and timing are important. I can provide details if you are interested. The most powerful small two strokes have exhaust ports that partially overlap the transfers. Engines are being made with exhaust and transfer ports that completely circle the cylinder. See the core patterns for these engines below.

Lohring Miller

 

[jef-lemmens]

What about something like this? I moved the transfer ports to the sides (90 degrees from the exhaust ports), and added a boost port to 180 degrees from the exhaust port that opens 10 degrees before the transfer ports.

This will work better, but the booster port opens at most engines at the same time with the side ports or something later, not earlier .

Regards Jef