Two cycle Engine from Bar stock

[Jo]

By the time Westbury was writing up this history of model engines that the Wombat formed part of he was in no fit state to make model engines any longer.

It is notable as being the only engine he did not provide a timing diagram for. I am also aware that the ports are considered to be too small for the engine

Jo

[Jo]

More details:

This is a set of patterns (a bit tight) or castings for the Wombat 

Letter from David: The distance from the top of the liner to the edge of the transfer port and the exhaust port are given as differing by 1/16" but as drawn the 11/16" dimension is shown as smaller that the 5/8" dimension. I would have thought these could be coincidental with the body. Exhaust ports are given an angular position and width of 7/32".. The drawing just does not look right. Enclosed is a 4X layout giving what I think they should be in relation to the exhaust port in the body. The 11.5 degree angle is taken from the layout. The 31.4 from (180- (2* 11.5))/5.

Transfer ports have the radial position given and 1/4" wide ports. there is no way these have any possibility of matching the 11/16" transfer port and at the same time having reasonable metal thickness between its openings. Again the X4 layout shows what is possible with the changed angles and port width reduced to 7/32".

As I have not found a timing diagram the one cannot be check against the other.. Am I missing something?

Attached David's redesign of the liner, not sure if the drawing with the measurements ports has been updated 

Jo

P.S. I am told Westbury didn't have time to make any engines after the war.

[Brian Rupnow]

Thank you for the information, Jo.---Brian

[DTR]

Watching this with interest

[Allen Smithee]

The actual calculation of port positions for a given timing is a bit involved, but if it's of interest I have a simple spreadsheet that I used to use in my racing days that just needs you to put in some easily measured stuff (stroke, con-rod-length, piston deck height) and then it will give you the port timing for any given port position (measured as the distance from the top of the port to the top of the cylinder).

Attached below

AS

[Jo]

Stroke (throw): 19.05
Con-rod- length = 39.7
Piston deck height = 58.74 + 1.6 - 9.53 - 39.7 - 11.1 = 0

Unknown distance from top of the port to the top of the liner    If 15.88, port duration = 75.6, 14.3 = 107, 12.7 = 133

I think we have another error  the centre line of the gudgeon pin for the piston is given the same dimension down from the crown of the piston as the top of the port (in red above), which is not what is shown in the drawing  If it is drawn out I would expect this port to move down the piston which might correspond with the crankcase which is 12.7mm between the edges of the ports 

Jo

[Jasonb]

Jo you have not included the 1/16" or 1.6mm thickness of the lip on the liner so deck height actually equals zero. Oh you edited that fast

I suppose if we took Westbury's gerenral timing diagram from his book for a rotary valve engine we could see what heights are needed to give that timing and then see how they compare with teh various dimensions on the drawings.

[Jasonb]

In his Book Westbury suggests as a general purpose timing for two strokes 130deg exhaust and 110deg transfer. By playing about with teh port heights in Pete's spreadsheet to get these angles gives the following

Top of Exhaust port to top of liner 14.5mm or 0.570" say 9/16"

Top of transfer to top of liner 16mm or 0.630" say 5/8"

[NickG]

And that sounds about right giving enough chance for the exhaust before the fresh charge is squeezed in. I think the window to get a runner will be fairly forgiving as long as the exhaust leads slightly. Must be pretty sensitive on the small CI engines, if you're say 0.25mm out with your port on a 0.8cc diesel it would probably affect the duration quite a bit, not to mention the build up of all the other tolerances, however plenty of people have made good runners. It's not an exact science as E.T.W says in his book, they are trying to extract the best performance they can yet most people are just happy if the thing runs!

[Allen Smithee]

Stroke (throw): 19.05

This may be just a difference in usage, but I've always understood the "crank throw" to be the distance from the centre of the shaft to the centre of the crank pin, which is half the stroke.

AS

[Allen Smithee]

Must be pretty sensitive on the small CI engines, if you're say 0.25mm out with your port on a 0.8cc diesel it would probably affect the duration quite a bit

Well a "square" 10mm bore and stroke gives a 0.78cc engine. Assume a con-rod length of 20mm (because I don't have a DC Merlin to hand to measure it!) and set the timing for 133.7deg with zero deck for simplicity and you get a port height (to the top of the liner) of 7.5mm.

The timings for 7.25 and 7.75mm are 140.5 and 126.6 respectively. so call it +/- 7%.

But I'd rather hope cutting the ports on a liner could be done to better than +/- 10 thou (in old money). When tweaking the ports on race engines (usually with a needle file, followed by a diamond file and then a small circular stone and a lap to remove and burs and the sharp corner) I could easily get the top of the port within a couple of thou of the target, and I have club fingers when it comes to hand-tools!

AS

PS - Aren't spreadsheets are wonderful tools for scratchpad calcs, Marv! 

[NickG]

10 thou would be good for me on a drilled hole! Maybe I shouldn't attempt one! But would it still run if that far out? Probably?

[Brian Rupnow]

When I was a kid, I had an old Evinrude 2 cycle boat engine. It ran slow, idled slow, and started easily. This is far more in line with what I want t build. I can't believe that there are no engine plans for model versions of that type of engine out there somewhere.---Brian.

[Brian Rupnow]

I have abandoned this thread because of inconsistencies in the original drawings I worked from, and the realization that I don't really understand two stroke engines well enough to embark on building this sophisticated a model. I have instead turned my efforts to the design of an engine who's full size counterpart was built over a hundred years ago. If you wish to follow my efforts to make a working two stroke engine, then go to this link.---Brian
http://www.modelenginemaker.com/index.php?topic=5852.0

[zeeprogrammer]

I have abandoned this thread because of inconsistencies in the original drawings I worked from, and the realization that I don't really understand two stroke engines well enough

I can understand the 1st part...not so much the 2nd.
Always a bummer...but looking forward to seeing your thread.