6,23cc 2 stroke engine

[Mechanicboy]

X

 

[Kevork BOYACIYAN]

continue with some photos more...

 

[Kevork BOYACIYAN]

Photos of air filter and complete carburetor.

 

[petertha]

Lets talk about my engine compression ratio; It have 6,89cm3 volume at BDC and 0,65cm3 volume at TDC so CR is 10,6:1

I think 10.6 is correct if I understand your description, but verify these stated volumes against more typical formula that defines 'displacement volume' & 'clearance volume'. The chart I attached shows 7-9 range but I am aware of engines around 10 assuming typical RC methanol/oil fuel & moderate/low nitro%.
https://en.wikipedia.org/wiki/Compression_ratio

 

[Kevork BOYACIYAN]

Dear petertha, I’ve already known the cylinder volume calculation formulas since childhood, which you shared on the link. I’ve reviewed my engine and I saw that I could convert it sandwich system between cylinder and head with squish band and circular combustion dome added, as you mentioned before, thus I could adjust also the CR.

In your previous mail you shared a picture which showed some kinds of engines CR. I’m curious of that what the CR should approximately be for 21mm bore 18mm stroke engine. If I know this ratio I can machined nearly. Then I can adjust more or less by just changing the shims.
If you have any further idea about ratio I will be pleased to hear it.

 

[petertha]

I recommend you do a search using key words 'compression ratio rc glow engines'. The glow plug + methanol combination has different implications for CR than other fuels & ignition systems like gasoline & spark. Why? because glow engines are more akin to self-dieseling, keeping the glow plug lit, burn rate of that fuel type vs. triggered spark. 2S self induction through cylinder wall ports and muffler vs. reflective wave or 'tuned exhaust' is maybe a bit different that 4S with valves opening relative to piston placement vs. The CR might be similar mathematically, but the breathing of engine behind the scenes is different.

So don't quote me on this but I would guess non-high performance sport engine, some simple noise abatement muffler (ie. not tuned exhaust) maybe 7-10 CR. High performance, tuned exhaust, specific nitro%, very application specific induction porting maybe 10-13? Others here on forum may be able to get you more complete answers or point you to more detailed calculation tools. I know there are simulators out there, some free, some cost. Some of those books look very interesting, I would like to read them myself.

Some discussions/USA vendors of model airplane race engines. Just recognize different competition classes & rules & no modifications. So in the end, all we get to play with is shims & plugs. But even within very limited range of say 0.001-0.010" shims (1) 'under sleeve' shim =raising or lowering the entire cylinder therefore timing since the piston motion is fixed and (2) between head & flange = affecting CR, you can go from a totally messed up engine that detonates & consumes plugs to a sweet running engine to runs but blah power to...
http://www.rcuniverse.com/forum/extreme-speed-prop-planes-104/9102061-nelson-40-info-needed.html
https://www.aeroracingengines.com/
http://www.dubjett.com/engines2015.html

some interesting history to FAI race engines (now very old news)
https://www.mh-aerotools.de/airfoils/pylon_engines.htm

CR in general with more home shop information
http://www.mecoa.com/faq/compression/compression.htm
https://www.model-engineer.co.uk/forums/postings.asp?th=83083
http://www.rcuniverse.com/forum/glow-engines-114/11643344-compression-ratio-fai-fuel.html
http://www.rcuniverse.com/forum/glo...do-compression-ratio-glow-plug-heat-have.html

 

[petertha]

But first, have you done a basic search here on the forum using search tool? There is a ton of information. Just bear in mind the same CR question may vary from a gasoline hit & miss to some very sophisticated engine project.s You should focus on the methanol/glow comments IMO.

 

[Kevork BOYACIYAN]

Hi to all,
After your comments and suggestions I made some researches on internet. I read a couple of threads from HMEM or different forums and some books, which sended from Mechanicboy or some others that I found them already. I'm not read all that books completely, just some articles that I needed. I'm sure that there is still lots of things to learn. At the same time I examined my engine, that how can I modify its shape to achieve acceptable or at least running design with minimum changes.
First I will machine a new cylinder liner and piston set, it have an exhaust port, which opening at 90° and four transfer and one boost ports opening at 120°. Transfer ports have a 25° up angle and boost port have a 45° up angle. New piston will be 1,5gr lighter and 1mm shorter then the old one and it have a dome shape with R40mm. I will be revised the cylinder it will have more bigger exhaust outlet, and more inner space for all of the transfer ports. I will be revized cylinder head, it will have a space for buton head and I will be fixe my glow plug to the buton directly. I could adjust CR by adding shims, now it is 10,6:1. I will revize my cranckshaft inlet port so it will open 140° before TDC and close 60° after TDC.
Here are some 3D captures from the screen.

TDC

TDC+90° EXHAUST OPEN

TDC+120° TRANSFER OPEN

BDC

TDC

TDC+90° EXHAUST OPEN

TDC+120° TRANSFER OPEN

TDC-140° INLET OPEN

TDC+60° INLET CLOSE

 

[Kevork BOYACIYAN]

I want to thank xpylonracer, petertha, Mechanicboy, Lohring.

I will be pleased to hear your comments.

TDC

TDC+120° TRANSFER OPEN

BDC

 

[lohring]

That's a great start. The crankshaft intake port timing should work well. The cylinder head looks good. The large flange on the liner can be even thicker. This helps hold the very important upper cylinder area round at operating temperature. The outer area of the head button (squish area) should have minimum clearance of around 0.2 mm or less. You can adjust the total volume a little with shims, but I would reshape the dome for large changes. It's not uncommon to experiment with several head buttons with different volumes.

However, the 4 side transfer ports should have different angles than those you have drawn. The boost port width should be around 30% of the bore and have around a 55 to 60 degree up angle. For the best performance the inner transfer wall should be a radius with a matching outer wall. See below. This is difficult to machine and I wouldn't worry about it now. If you stick with your current transfer passage shape, I would angle the side transfers flat across the piston. The idea is the flows collide to form a slowly rising column that pushes out the exhaust. The flow from the boost port goes over this to clean out the combustion chamber so it should be aimed there. Because of your transfer passage shape the flow out of the side ports will actually be a wide spray. Our experiments as well as others showed that a 0 degree up angle gives the best power with that transfer passage type.

The horizontal angles of the side transfers are very important. The picture of model engine transfers I posted earlier is a great starting point. The picture below shows the best and worst designs tested by Gordon Blair a very long time ago on Yahama cylinders. Model engine transfer passages are usually the #12 shape. The #14 is good start point, but the rear transfer ports are wider than the front ports on modern designs. See the second picture for a more modern design. Note the hook on the rear transfer. A more gentle radius works better but the liner thickness is a good start point for the hook depth.

Lohring Miller

 

[lohring]

All the above is secondary to really good piston and liner fits. There are several ideas on what this should be. The simplest is to make the liner 0.15 to 0.2 mm larger diameter than the piston at the base of the liner tapering to zero clearance a little (1 mm or less) below the top of the stroke. This gives a squeak as you turn the engine over. You can lap this area in with very fine, non embeding compound or let it break in by running under load. With the right materials there will be a very small clearance at operating temperatures. The piston needs to be a high silicon alloy (US designation 4032) and the liner should be hard chromed brass.

Lohring Miller

 

[Mechanicboy]

See at the image where you need some corrections..

 

[Kevork BOYACIYAN]

Dear Lohring Mechanicboy,
So transfer and boost ports shape will be like turbo snail. Those are wider at the beginning and narrower at the bore side. So gas speed up and cools down before entering.
I changed my drawings as I can. There is no infrastructure in the drawing I just trying to copy the picture #14, which sended by Lohring. Transfer angles are same 25 degree, boost is converted to 55 degre and boost will open a little bit later then transfers.

Boost port

Transfer near boost

Transfer near exhaust

 

[Mechanicboy]

Much better..

 

[xpylonracer]

Nice work Kevork, even looks powerful in the model !!!

 

[lohring]

Since you reshaped the transfers I would reduce the up angle on the side transfer nearest the boost port to 10 degrees. Otherwise the passage shape looks great. Do you have a cross section through the transfers? I would be interested in looking at the horizontal angles.

Lohring Miller

 

[lohring]

The internet archive is a wonderful place. https://web.archive.org/web/20030210205741/http://rcboat.com:80/past.htm is the index web page for a lot of engine information from one of the most successful boat racers. It has great information on fits and timing for the various sizes of nitro race engines as well as more than you wanted to know about other facets of model boat racing.

Lohring Miller

 

[Kevork BOYACIYAN]

Angles that I used.But I found a mistake on my model so try to fix it. I will draw it from the beginning.

 

[lohring]

That should be very workable. You might consider making the walls of the boost port parallel and adding a hook to the rear transfer. You do that by cutting the port like your drawing in the liner and match the transfer passage to it like in my drawing. Once again, the liner to piston fit means more than all these details.

Lohring Miller

 

[Kevork BOYACIYAN]

Hi to all, at the end I finished my drawings, thanks for Lohrings (and others also) guidance. I don't want to make this topic boring so I hope those are the last pictures of cylinder and liner. Finally boost port walls is parallel, transfer ports near the boost have a hook (hook angle needs to be reduced for to coincident with center) and I saw that this drawing is more suitable to produce, there isn't so much reverse angles.
Thanks to all...

Note: Last there pictures name shows the right sides transfer port