Two cycle Engine from Bar stock

[Brian Rupnow]

Okay---I'm hung up!! The original Westbury drawings I am working from seem to have a discrepancy on them related to the port positions. In a cut-away assembly drawing, he shows the exhaust ports quite clearly as being closer to the top of the cylinder than the upper inlet port. In his detail of the cylinder, he dimensions the exhaust port and top inlet port as being directly in line. Something is not right, and I have no other frame of reference to clarify which is correct. Since the port positioning is critical to the function of the engine, this project has come to a halt until I can either get clarification or abandon this project.---Brian

[Niels Abildgaard]

You can adjust the timing with  varying the edge of piston or make something new .

http://www.modelenginemaker.com/index.php?action=dlattach;topic=4547.0;attach=38811;image

Two or three cam experiments and You have a worlds first.

[Jasonb]

Maybe Jo can give Eric a prod as he is our best authority on Westbury's engines. Also have a look at te e-mails I sent you.

[Allen Smithee]

I don't know this specific engine, but I'm fairly familiar with two-stroke engines of this sort of size. For a "mild" low-power engine without a tuned exhaust a general guide on port timings would be:

Exhaust - open for 135-155 degrees (less for low-revving, more for higher revs)
Transfer - open for 120-135 degrees (match it to the above range, so if the exhaust is only 135 use 120deg of transfer, and increase it pro-rata if you use longer exhaust timing)

Induction - opening is not critical; anything from 180dgeBTDC to about 100degBTDC won't make much difference on an engine like this. Closing can go from a but before TDC up to about 30 after, but you only need the extended closing time for power at higher revs. In real performance engines you'd be looking at 65-68ATDC, but that gets rather touchy at low revs and would probably need a tuned pipe to work.

If you want to use a tuned pipe you can go much wilder - the exhaust could go to 195deg with the transfer at 155-160deg, but this may not even run at low speeds and would want to run mostly in the range 18-28,000rpm (which I'm guessing is not what you're looking to do!).

Final thought - the vertical finning on the head could probably be much deeper (more fin area) with more/thinner fins - at least double the depth you have there. In this kind of engine MOST of the heat dissipation that doesn't go into the exhaust gas goes through the cylinder head; comparatively little goes through the fins on the sides of the cylinder. When using two-strokes in aeroplanes I would always carefully duct air through the head fins and the area of the exhaust port, and often block direct airflow to the front of the cylinder as this can make the cylinder distort through differential cooling (causing the piston to pick up on the ports).

€0.03 supplied,

AS

[NickG]

Hi Brian, i'm looking at a drawing of atom minor now and the centre of the exhaust port is 1/32" higher than the transfer but the port is 1/16" larger in diameter so the start of both ports opening are the same dimension up from the bottom.
There is also a diagram earlier on in the book showing the bottom of the exhaust port inline with the bottom of the transfer port. Again, the exhaust ports are larger but the bottoms are inline.

[Jasonb]

This is the liner drawing as you can see the inlet is shown above the transfer as is usual but the dimensions put it below.

Also crankcase drawing has 1/2" between transfers but liner only 7/16, also the dimensions on the crankcase put both ports on the same level.

Finally the section through the GA this has teh exhaust approx 1/16" above top transfer  but notice the two transfers are not symetrical about the cover bolt holes but they are on the crankcase drawing.

[Brian Rupnow]

Okay, that's it!! There are enough anomalies in the original design, combined with the fact that as near as I can tell there are no records of anyone having built and ran this engine, that I am abandoning it. Not really a big deal, as it gave me a Sunday's worth of entertainment and did contribute somewhat to my knowledge of two cycle engines. Now it is time for me to ask for help. I was hoping to get by without laying out any money for plans, but that hasn't worked out. I have searched the internet for plans that I could buy, and that search came up dry as well. I even went to Chapters Bookstore in Barrie and to the Hobby Store in Barrie and they couldn't help me. So--This is what I want. Plans for a single cylinder, spark ignition, two cycle model engine that runs on gasoline or Coleman fuel, and has been actually built and ran by someone. I do not want plans for a glow engine nor a diesel engine. I do not want plans that have been made by someone but never "proven" by being built and ran. If you have a set of plans for what I want, and wish to help, please send them to brupnow@rogers.com . If you know of a book that has these plans, please tell me the name of it, and possibly where I can purchase it. There is a lot of work in building a model engine, and as much as I enjoy it, I do not want to build a prototype of an engine which I don't as yet fully understand.---Thank You---Brian Rupnow

[NickG]

Brian, I would recommend the book Model Petrol Engines by ET Westbury if you want to learn about the design principles. The drawings might not be the best but there is some useful information in there. It mentions the fact that the "transfer port opening should not follow too closely on the heels of the exhaust port, or high-pressure gas may be forced down the passage, retarding the transfer of the charge, impairing the scavenging, and  even tending to cause crankcase explosions." I don't believe the rest is as critical as you might think. OK if you're going for high performance to beat some speed with a hydroplane, but not to get a running engine. Having the bottom of the ports level just helps the fresh charge flush combustion fumes out of the cylinder which while not terribly efficient, should make it run better.

[Roger B]

I believe that the slightly larger 50cc Busy Bee or Bumble Bee was built. Eric is certainly the man with the information.

[Brian Rupnow]

I believe that the slightly larger 50cc Busy Bee or Bumble Bee was built. Eric is certainly the man with the information.

Eric who??

[Jo]

From Eric:

The Wombat was the last of Westbury's engines which he designed in the last few years of his life when he was not well. It is arguably a 1940's design built in the 1960's at a time when commercially built engines were more capable that this engine was ever likely to be, so there was never a lot of interest in it.

We know that there have been a number of attempts to make this engine, we even know of the location of a set of patterns for it  But as you have found there are numerous mistakes in the drawings. It is believed that it could be made to go if you put together a timing diagram for it and redesigned accordingly.

If it was built and made to run it would be a very collectable/valuable engine.

Jo

[Jasonb]

Thanks Jo, I did e-mail Eric but as you have a directy line to your supplier thought you may get a quicker answer

Brian, Eric is one of the Motor Boys

I had suggested Brian make a timing diagram and sent him an example of one.

[Brian Rupnow]

Really, I didn't find "numerous mistakes" when I modeled everything. The only discrepancy I discovered was in the exhaust port position, both relative to the intake ports and relative to the holes in the liner. My solid models showed no other mistakes, or at least none that I found. I wish I knew more about what I was doing in terms of the exhaust port position, because it does make quite a workable engine in every other respect. Luc from Quebec takes great exception to the way the carb inlet comes in at 90 degrees to the crankshaft axis, but although that may keep the engine from being "high performance", I can't see it keeping the engine from running.---And--It does give a lovely spot to mount the ignition points.

[Pete49]

Another Brian project to follow. You're getting very constructive in the last couple years 
Hope you can solve the issues as I would love to try making it.
Shrove Tuesday......mmmmmmmm pancakes!!!!!!!!
Pete

[Jasonb]

Really, I didn't find "numerous mistakes" when I modeled everything. The only discrepancy I discovered was in the exhaust port position, both relative to the intake ports and relative to the holes in the liner..

Brian, did you not notice 7/16 between transfers on the cylinder liner and 1/2" on the crankcase? What about the transfers being symetrical about the cover bolts on one drawing and not on another and I've only had a quick look. I have mentioned the differences about what you have drawn for the intake and what is on Westbury's drawing elsewhere.

Jo does Eric Know if Westbury was able to build one of these or was it a design that he did not get the chance to actually make?