6,23cc 2 stroke engine

[xpylonracer]

Hi Kevork
There doesn't seem to be much volume in the cylinder head, or does your design give a lot of squish band to piston clearance ?
Best performance will come from close piston to squish clearance (.012") and more volume in the cylinder bowl.
I would aim for 0.50cc to 0.55cc total combustion chamber volume, measure with a syringe with piston at TDC.

xpylonracer

 

[Kevork BOYACIYAN]

Hi xpylonracer,
Unfortunately there isn't any squish band. My design have a flat crown piston, combustion chamber have combustion dome shape but squish band absent. The clearance at TDC between piston and head is 1,6mm. your advice is aproximiately 0,3mm. I will consider your advice and try to made some modifications on it as soon as possible after my first try to run.

 

[Mechanicboy]

See at the drawing,.. the glow plug is hitting on the edge in the cylinder head.. bad idea. If the glow plug is screwed into the cylinder head then the filament will be scraped off from the glow plug. Oil hole in the wrist pin is not good idea due pressure zone in bearing must be without oil hole where the oil is in high pressure --> Hydrodynamic bearings rely on the high speed of the journal (the part of the shaft resting on the fluid) to pressurize the fluid in a wedge between the faces. The connecting rod is too tight in the piston due heat of expansion and no access to get oil into the wrist pin.

 

[xpylonracer]

Hi
Only reduce the clearance if the combustion chamber volume with piston at TDC is greater than my suggested o.55cc, otherwise you will be over compressed and get pre-ignition.

xpylonracer

 

[Kevork BOYACIYAN]

Hi Jens,
I'm sending my cylinder heads photo with glow plug assembled on it. There is no screw drawn on the 3D model, so it looks like glow plug hit the edge. Actually it looks like in the photo. Do you still think there is a problem or not.

For lub hole on wrist pin and connecting rod wideness those are easy to correct. I will double check before run it.

By the way I saw your signature at the bottom of your message. I've been in Norway Bergen last year at May.

 

[Mechanicboy]

Hi Jens,
I'm sending my cylinder heads photo with glow plug assembled on it. There is no screw drawn on the 3D model, so it looks like glow plug hit the edge.

For lub hole on wrist pin and connecting rod wideness those are easy to correct. I will double check before run it.

By the way I saw your signature at the bottom of your message. I've been in Norway Bergen last year at May.

There is a rule to draw the bolt/treaded hole to understand and produce the product from the drawings or it can come a lot of comments of the drawings who are not correct drawned. (I am educated as metal worker + car mechanic and works as car mechanic)

How do know more rule about drawings (not you only, other newbeginners too ), here is the link: https://blogpuneet.files.wordpress.com/2013/09/machinedrawing.pdf

As rule there is not oil hole in the wrist pin. There is drilled oil hole in top of small end and the other oil hole in bottom of big end. Never in the pressure zone there is largest pressure. If the oil hole was drilled in the pressure zone then hydrodynamic oil pressure will disappear and then the connecting rod/crankpin and wrist pin will be worned out. Except if the engine has oilpump to circulate the oil in the 4 stroke engine, there is the oil hole in near of top of big end, this is to lubricate the cylinder wall in the pressure side where the piston has big pressure on cylinder wall under combustion when the oil hole in crankshaft is aligned with the oil hole in the connecting rod then the jet of oil is directed against the cylinder wall when the piston is on way to TDC.


Yes, i live in Bergen, in fact i'm from Kirkenes (kven people).

 

[petertha]

I'm intrigued by your head design - but I don't quite understand it. What is the rationale for the tiny lip on top of the cylinder liner (red arrow)? This is the hottest part of the engine & where you want minimized distortion. Most engines I have dealt with (RC methanol glow race stuff) the liner has a meaty enlarged diameter flange. The more extreme the running conditions, generally the thicker & wider the flange to maintain circularity. The combustion bowl profile is formed by the head which has an integral male boss that sits snugly within the liner to help minimize distortion & conduct heat away. Alternately a button insert is used which accomplishes the same thing. It then becomes sandwiched between a (mostly heat sink) head top.

This assembly gives a few more tuning options. You can also easily shim up and down to tune compression with no effect to the parts, they fit the same way. On yours it looks like if you increase the shim, blow-over can occur across the little lip & you simultaneously introduce a very irregular combustion chamber shape from with resulting gap. If the shim is decreased or not perfect thickness, then only the lip would contact the head.

 

[Mechanicboy]

As we read here, there is talk of some lack of knowledge to construct such a motor, so I'll post some pictures on how the engine is designed with regard to heat dissipation, adjustment of compression ratios with thickness of cylinder head gasked. Design of transfer canal and ports in the cylinder based on flush effect and timing.
It can be studied from hand books about model engines or by dismantled model engine how they are built together instead of inventing new construction that does not work or is inconvenient to adjust when needed. A little stupid to make parts in advance before you find out that it should not be that way and so it costs to consume materials for no use.

 

[Kevork BOYACIYAN]

Dear Jens, I agree with you completely. In my city there is lots of hobby shops, they all sells brand new engines, spare parts, fuel or some brochures about engines, planes, cars ext. But unfortunately there isn't any handbook to teach building an engine yourself. Altought I do not have any depleted engine to disassemble for educate myself. 15 years ago one of my friends give me his rc car in very bad condition. I didn't hesitate to disassmble it and after cleaning run the engine again with all necessery adjustments. It stayed with me just a few weeks. This was the only occasion for me. I will be very happy if you have any book advice for me, I want to buy it from internet or from you. I find this one, at the attached file, on amazon. Which is explain an engine, but not for build it from the begining as far as I understand.

I want to continue to sharing photos and notes about my engine, while I've already finished it. It makes me happy to hear your suggestions. I will try to correct as much as it allows, at the end, I will made it with new design from the begining.

 

[lohring]

Once again, don't give up on your engine. The fits and finishes are more important than many design details in a small engine. Learning these while building your engine is the most important thing. Most model engines with the exception of the one I posted are far from the current two stroke state of the art. There's plenty of time to study this later.

Lohring Miller

 

[petertha]

Yes keep plugging away, its all part of the journey.

You can get used, reasonably priced, functioning RC engines of almost any comparative size & flavor you are interested to take apart & evaluate. Many are at giveaway prices you wouldn't even feel bad about cutting apart to really understand the certain internal aspects if necessary. Or take it apart, measure everything & sell it for 80% of what you paid. Just look online in an RC forum or Ebay or whatever. Yesterdays treasures are today's refuse unfortunately. I just sold some race engines for 200$ each that are no longer legal as a racing class. Nobody wants these other that they make great sport engines for those not constrained by class rules. The point is you could barely buy the materials for what the engines go for.
https://www.rcgroups.com/aircraft-fuel-engines-and-accessories-fs-w-362/page2.html

 

[Kevork BOYACIYAN]

Hi petertha, initially I've never thought that I needed head height adjustment for change the combustion chamber volume, so cylinder head is consist one pieces and fix its own place with six screw only in one position. My liner has a lip it provide to centering the gasket (I do not need by the way). After your warning, if I need to change the volume or shape of combusting chamber I thought I can reprocess it even a little. Thanks for your remind.

 

[Kevork BOYACIYAN]

Hi petertha again.
Buy an used engine for learning something is acceptable. I may be affected from any engine design, it is normal. But especially I don't want to copy of. I want to make my engine work efficiently by making improvements. I can do it well with your suggestions by the way.

 

[Mechanicboy]

But unfortunately there isn't any handbook to teach building an engine yourself. Altought I do not have any depleted engine to disassemble for educate myself.

There is the book to download where you can read and learn how the engine works and much more. The book is old, but the principle is same as today.

https://rclibrary.co.uk/download_title.asp?ID=1996

And the other book to start to build the engine from drawnings.

https://rclibrary.co.uk/title_details.asp?ID=2139

https://rclibrary.co.uk/title_details.asp?ID=2122

We all here are old fox who are discovering some was wrong in the drawings or constructions hence we are telling to you then do not be afraid to listen from us. You will learn of mistake and improve better by our attentions

 

[Kevork BOYACIYAN]

I am grateful for the information and comments you shared. I wish I had shared my drawings first. In the end, I have a tangible thing in my hand that I can practice and learn from it.
Tomorrow I will share my carburetor.

 

[petertha]

Hi petertha, initially I've never thought that I needed head height adjustment for change the combustion chamber volume.

Well if you get the CR 'mostly right' & you aren't too concerned about performance, then I suppose swapping shims is less a concern. Personally I would want that control. On that note, I can tell by your CAD program that you have the ability to accurately determine squish volume at TDC, therefore you probably already know your CR. The attached table is something I threw together specifically for 4-stroke methanol/glow engines just to give me a feel for CR range, just based on information I had handy. I don't seem to have my 2-stroke list handy, but I seem to recall similar values +/- vintage and application.

Anyway, just eyeballing your combustion dome relative to bore, I would bet if you move the head up or down just a bit in the range of typical shim thicknesses, you might see significant CR changes. 2-strokes are simpler mechanically but more complex from induction/exhaust flow mechanics as others have indicated. Glow is further challenged by choice of plug, nitro content in methanol etc. Not wanting to scare you but I have spend a fair number of years flipping props to see how subtle differences can vary performance. I'm not talking attaining peak power, just fundamentals of running & starting & transitioning. They can be fussy buggers if you are too far outside the range.

 

[Kevork BOYACIYAN]

Hi petertha talking about CR is good point, that I was always in curious. You right, thanks my CAD, I can calculate cylinder volume at TDC or BDC very sensitively, also I can calculate after exhaust port closed. It could be a good advantage that change the cylinder volume at TDC for change the CR, but I can't do with this head design.
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
Also it have 5,38cm3 volume at when exhaust port completely closed so CR is 8,3:1

I guess CR is 10,6:1 but effective CR is 8,3:1 so pumping efficiency is %78, am I right?

Note: It is really hard to turn crankshaft towards the compression by hand with a few drops oil applied to piston.

 

[Kevork BOYACIYAN]

It's time for one of the most sensitive part of the engine, carburetor, when designing years ago I benefited from this forum. It haven't got a simple design as usual ones, it consists twenty different parts, also it have four screw for montage (six screw exist in the picture but I used just four, two for left, two for right and one long pin placed overhead instead of opposed screws, it can seen detailed in photos) and there is five o-rings to prevent unwanted air flow.
Venturi begin from Ø16mm narrows to Ø6mm in 12mm height, it have 42° taper in diameter. It goes 14mm straight alongs the throttle valve and then expands to Ø8mm in 7mm height it have 16,5° taper in diameter. Needle height is 6mm and it have 3° taper in diameter, bottom is Ø0,37mm top is Ø0,94mm. counterhole is Ø1mm, which feed the fuel. It have two brass decagon screws, left is needle adjust at low rpm, right is for fuel quantity adjustment at high rpm, when needle is completely out from the hole. Throttle valve is a brass hexagon ended part. It can turn 45° close to full open. Needle is placed in a cylinder, which have a helical canal and while throttle valve turns it retracts the needle. By the way that cylinder, which have a helical canal can controlled with brass decagon screw, so needle position can adjust back and forth in any time in any position indipendatly from throttle. At the opposite side other decagon screw controlled a disk valve to open or close the fuel line. There is a screw for adjust the idle on the taper side of body (the pink one). It use cage style open air filter.

It has two half circle shapes, can seen on side views, also blok has two holes. It can connected via an o-ring and two pins will lock them together.

At least I'm not sure how many drops fuel I needed at the each rpm.

Briefly like this.

 

[Kevork BOYACIYAN]

This gif will shows how throttle valve pull back the needle, and decagon screw adjust the needle position back and forth independently.

 

[Kevork BOYACIYAN]

Product stage. Unfortunately it lasted couple of years to finish carburetor completely for me, because I was busy and I didn't have enough time for it. I start to build from body, but I broke it when processing the venturi hole, then made a new one. decagon screws have worm gears to control the disks. I used M6 tap to open the threads on the disks. Unfortunatelly there is some absent photos to explain proccessing.