2 Cycle Single with a disk rotary valve.

[nx06563]

Just got my own design for a 2 cycle with a rotary disk intake running fairly well. It took a long time to get running since I honed the piston in without a ring and it took a while for it to loosen up enough to run. I was afraid that a ring would get caught on the exhaust port. Had quite a time figuring out the port timing and machining dimensions to get it right. Used Fusion 360 and indexed the model to the correct angle for the port timing and calculated the dimensions from that.
Here are a couple pictures. I will try to get the prints in a presentable form and make them available. Not too tough of a build once I got port timing figured out.

 

[Brian Rupnow]

nx06563--that is a wonderful accomplishment. Congratulations. Right now I'm having a terrible time with a conventional overhead valve engine, so when I see someone else having a success it gives me hope.---Brian

 

[nx06563]

Ok, I need an expert in two cycle compression ratio calculating.
My two cycle in this post runs great, starts easy, and has great throttle response. I am busy designing and building a two cylinder version of this engine.
Here is the question. The compression ratio on my completed engine calculates out to 4.4 using the total swept volume and only 3.6 using the swept volume above the top of the exhaust port.

I am not one to quibble with success but this seems exceeding low for an alcohol burning engine (I am using model airplane fuel). Is the functional compression ratio actually higher because the inlet charge is forced into the combustion chamber by the crankcase pressure? Would the engine actually run better with higher compression.

I have to say the original engine was designed using a math error I had committed and I thought the actual compression ratio was 6.5. Part of model engine building has to be DAL. ( Dumb Ass Luck )

 

[petertha]

You might want check out this link for analogous engines & their respective CRs. I think it safe to say most methanol engines are higher CR than what you are running.
http://sceptreflight.com/
But I've also seen reference to theoretical & effective CR on 2-strokes which I presume the latter factor considers the port timing? I'm actually not sure. And makes me wonder which is being reference when only one number is stated. Random example but but of the pattern engines I used to run back in the day (YS-61) references 13.2:1 and 9.2:1 respectively.

 

[Chris Murphy]

Your stated compression ratio is about half what a typical run of the mill two-stroke model engine would use-one running on methanol based fuels anyway-the model aero and marine model diesel run about 16-18:1 CRs....even the higher performing glowplug engines are running in the 11:1-12:1 sort of range-depending on nitromethane content...........on the early traditional petroil mix and spark ignition type model engines the typical CR would be around the 7:1-8:1 mark....a 4.4:1 nominal and 3.6:1 actual CR is extremely low

ChrisM

 

[Steamchick]

Just "because it was logical" I understood the compression ratio to be the compression from closing the last port opening to TDC. However, I do remember a discussion in the early 1970s, about the time that Yamaha engaged Queen's University Belfast to dynamically re-engineer the porting of all the suck and blow phases, using the gas dynamic technology available back then. They considered that the intake charge from atmosphere was defined by the stroke below the closure of the intake port, when an engine was tuned so there were effectively no losses down the exhaust port during transfer. So using exhaust back pressure and exhaust reflected shock waves to stuff exhaust back into the combustion chamber they almost eliminated losses of intake charge down the exhaust pipe. But also they contrived bigger and more numerous transfer ports, etc, reed valves, and exhaust valves so that the performace increased dramatically over the next decade. Disc valves really developed during 1950s 2 stroke racing did a similar thing on the intake port to maximise the initial charge intake, timed differently to a simple piston port crankcase intake.
Therefore the initial charge volume - theoretically - is the voume of the stroke of the piston from disc port closure to BDC, plus combustion chamber volume, divided by the combustion chamber volume. But this is only effective if the exhaust is suitably tuned, otherwise the volume of charge lost down the exhaust port before that closes, means you could be running at your 3.6 or lower... I think it is covered in the book "Tuning for speed" - by Phil Irving - and used by many motorcyclists in the 1960s. Also Austin Rodgers book -chapter 3 - search the web for "disc valve tuning 2-stroke engines".
I had a race tuned Yamaha LC350 that pushed 17:1 compression on the gauge at kick-over. Needed Avgas to keep the pre-ignition under control. After burning away 4 pistons I used a cylinder base spacer to drop compression to below 15:1 when it was good with 98 octane and octane booster.... I saw 11500rpm and still pulling in top gear one day, as the highway developed bends, and it seemed that the traffic had nearly stopped. Silly me!
K2

 

[nx06563]

What got me started on a two stroke was the fact that I had raced outboards in the 70's and ran a Konig with a disk rotary valve. At that time I could mechanic but not engineer so I did what I was told as far as tuning goes. Guess that is still true to some extent.
This engine uses a shaped piston, hemi head, and a open exhaust port so I got something right.
I think the next one I will reduce the piston to cylinder top dimension and reduce the combustion chamber size and up the compression a bit. Check on me in a year or so before it get it done and I will report my results.
Here is a short video of how this one runs and a couple of the prints.
Thanks for the input.

 

[Steamchick]

Easy to up the compression by skimming the head - until the piston crown gets too close. I suspect the top of the piston does not encroach into the head at all, so maybe skim the head 0.05" ~0.1" ~0.15" and watch the compression rise - and see how that improves the running?
What is the head gasket? Thick Copper or Aluminium? If so you could try come very thin aluminium foil (Baking foil from the Kitchen) and see what difference that makes to the actual compression? - Then the compression can be estimated per axial length of cylinder combustion chamber... if you can handle the numbers?
If you make another head, I suggest the radius for the dome should be half the distance between the centre of the head and the lowest point of the piston top face at BDC. Imagine a sphere from BDC to head, but with the piston centre ridge protruding up into that sphere. The imaginary sphere is a crude interpretation of the flame front as it expands to BDC. The intent is that it reaches all corners of the cylinder simultaneously. But not being a 2-stroke expert, maybe it is better to use the point at which the transfer port opens? - or even higher where the exhaust port opens? Can you model the whole combustion chamber at those 3 stages so we can see?
Regards,
K2

 

[Steamchick]

A friend had some papers on 2-stroke design. Maybe of interest?
There are more on related subjects...
K2

 

[Steamchick]

What is used as a starter?
K2

 

[nx06563]

Haven't looked at the thread for a while but thanks for the replies.

I will print and read the info from steamchick since there is so little information for people starting from scatch.
Starter is an electric drill with a starter hub attached to the flywheel and and a little driver chucked into the drill.

 

[Steamchick]

Better than a pull chord I guess!?
K2