Author Topic: Two cycle Engine from Bar stock  (Read 11203 times)

Offline Brian Rupnow

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Two cycle Engine from Bar stock
« on: February 06, 2016, 08:45:02 PM »
I am going to attempt to design and build a 2 cycle engine from bar stock, which uses no castings. I have spent a couple of days searching the internet for a 2 cycle engine which has a relatively low compression ratio, spark ignition, and can be machined using a conventional lathe and mill. I have found what I have been looking for, thanks to a member in the U.K., and although it is made from castings, it is a proven design so I know that the port layout positions are correct. The engine which I will be using as my reference is the "Wombat", originally designed  by Edgar T. Westbury in the 1930's or 40's. My design is going to be a radical departure from the original designers, so I am not concerned about copyright infringement, however, I want to give credit where credit is due. This engine will have a 1" bore, X 3/4" stroke single cylinder configuration with ball bearings on the crankshaft. It will be machined from aluminum with a cast iron liner, and have (gasp) cast iron rings.  It will have rotary valve induction. I am in no rush at all to get back into my machine shop, as I need some recovery time from 4 months of building my twin opposed i.c. engine. Stay tuned--This could be fun!!!---Brian

Offline NickG

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Re: Two cycle Engine from Bar stock
« Reply #1 on: February 06, 2016, 09:38:49 PM »
I'll be watching as usual.

Offline Brian Rupnow

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Re: Two cycle Engine from Bar stock
« Reply #2 on: February 06, 2016, 09:59:14 PM »
Yes, I know--looks pretty strange doesn't it!! This is "more or less" what the original casting would have looked like without the fins on it. It is correct in terms of bore, crankcase i.d. and size and positioning of ports. Now, I wouldn't try to machine this shape from solid in a million years.  However, I have to model this main body, and then all the parts that fit into it as accurately as I can, just to determine to my own satisfaction how everything fits together and how all the parts interact with each other. After doing that, I will be able to determine how I can modify the body shown here to make it more "machineable friendly".


Offline Brian Rupnow

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Re: Two cycle Engine from Bar stock
« Reply #3 on: February 07, 2016, 01:56:34 AM »
There!!! That'll do me for today. All of the other parts which I am modelling, although they too have the same critical dimensions as the cast and machined parts, are being greatly simplified as I model them. This is to make them easy to machine from solid. I have to enter all the math data as I model the parts, so I will pull my detail drawings directly off the parts when I get ready to detail them. Some of the parts may get a bit of cosmetic  work if I think they look too "blocky" but I won't really have a good overview of that until everything is modeled.  I have found one error on the old original drawings. The dimension from the top of the cylinder down to the center of the exhaust port on the cylinder is 21/32:. The dimension on the liner is 11/16" but it is taken from the top of the liner, not the underside of the 1/16" thick liner "lip". This puts the exhaust ports in the liner offset from the cylinder port by 1/32". I have changed the hole position in the liner to match the port in the cylinder.

Offline Jasonb

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Re: Two cycle Engine from Bar stock
« Reply #4 on: February 07, 2016, 07:39:34 AM »
Be very careful about moving ports, it will affect the timing of the engine so may be better to adjust the crancase once you have done a few calculations to see where and when the piston reaches the exhaust port.

Why not have a go at making the crankcase as it is you may enjoy the challenge, I'll let you leave off the cooling fins ;)

Offline Roger B

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Re: Two cycle Engine from Bar stock
« Reply #5 on: February 07, 2016, 09:54:26 AM »
Another interesting build to follow along with  :ThumbsUp:  :ThumbsUp:
Best regards

Roger

Offline NickG

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Re: Two cycle Engine from Bar stock
« Reply #6 on: February 07, 2016, 03:05:57 PM »
It's a complex shape but looks like it could be fabricated in a few steps?

Offline Brian Rupnow

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Re: Two cycle Engine from Bar stock
« Reply #7 on: February 07, 2016, 06:03:36 PM »
At this point I am beginning to get a much better understanding of how this two cycle engine works. The rotary valve is a new concept (for me). I still haven't redesigned the main body to make it more "machining friendly", but I have redesigned all of the ancillary parts to something that can be carved from bar stock on my lathe and mill. I haven't looked too closely at the ignition cam or points yet. I have an idea of how I am going to modify the cylinder head to match the upswept contour of the piston top--I just haven't done it yet.

Offline Brian Rupnow

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Re: Two cycle Engine from Bar stock
« Reply #8 on: February 07, 2016, 06:07:30 PM »
Jason--Leaving off the cooling fins is very tempting. Also the possibility of making it water cooled. I am just about at the point where I can go back and revisit the shape of the main body. I couldn't really do that until I figured out where everything else went.

Offline Brian Rupnow

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Re: Two cycle Engine from Bar stock
« Reply #9 on: February 07, 2016, 07:36:40 PM »
So--My questions are answered. I can get my favorite Chrysler ignition points and adjusting block on there, and one of my store bought Traxxas Pro-15 carburetors. Now that I know that, I can start chopping and hacking at the main body.

Offline NickG

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Re: Two cycle Engine from Bar stock
« Reply #10 on: February 07, 2016, 09:42:03 PM »
Looks pretty neat to me that. Did you decide on air or water cooled yet?

Offline Brian Rupnow

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Re: Two cycle Engine from Bar stock
« Reply #11 on: February 07, 2016, 10:01:06 PM »
This is what it looks like with the cylinder in "super simplified" form. All of the ports, bores, and bolt holes and mating surfaces are the same as the original model, and I have managed to squeeze in a few cooling fins. I can't make it any simpler than this and still make it from one piece and still have a chance to cool it somewhat. I did take the mounts away from the sides of the cylinder and add a separate baseplate to the bottom.




Online crueby

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Re: Two cycle Engine from Bar stock
« Reply #12 on: February 07, 2016, 10:39:01 PM »
Fascinating to watch the design develop like this - thanks for sharing!

Offline Brian Rupnow

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Re: Two cycle Engine from Bar stock
« Reply #13 on: February 07, 2016, 10:57:58 PM »
Fascinating to watch the design develop like this - thanks for sharing!

Crueby--You've been watching it develop in real time today. On a cold winter Sunday in February it's not like I had a whole lot else to do. I love doing this stuff!!! ---Brian

Offline Jasonb

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Re: Two cycle Engine from Bar stock
« Reply #14 on: February 08, 2016, 07:43:39 AM »
Brian, I mentioned it earlier but strongly advise you to look at the port positions, it looks as though you now have the exhaust and transfer ports at the same height. Normal practice is to have the exhaust above the transfer. This can easily be seen on teh cross section at the start of teh articles. I've commented on the inlet elsewhere.

Offline Brian Rupnow

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Re: Two cycle Engine from Bar stock
« Reply #15 on: February 08, 2016, 03:02:21 PM »
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

Offline Niels Abildgaard

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Re: Two cycle Engine from Bar stock
« Reply #16 on: February 08, 2016, 04:03:13 PM »
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.

Offline Jasonb

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Re: Two cycle Engine from Bar stock
« Reply #17 on: February 08, 2016, 04:07:17 PM »
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.

Offline Allen Smithee

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Re: Two cycle Engine from Bar stock
« Reply #18 on: February 08, 2016, 06:21:33 PM »
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
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Offline NickG

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Re: Two cycle Engine from Bar stock
« Reply #19 on: February 08, 2016, 06:33:24 PM »
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.

Offline Jasonb

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Re: Two cycle Engine from Bar stock
« Reply #20 on: February 08, 2016, 06:37:13 PM »
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.
« Last Edit: February 08, 2016, 06:45:31 PM by Jasonb »

Offline Brian Rupnow

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Re: Two cycle Engine from Bar stock
« Reply #21 on: February 08, 2016, 07:00:40 PM »
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

Offline NickG

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Re: Two cycle Engine from Bar stock
« Reply #22 on: February 08, 2016, 07:23:55 PM »
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.

Offline Roger B

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Re: Two cycle Engine from Bar stock
« Reply #23 on: February 08, 2016, 07:46:12 PM »
I believe that the slightly larger 50cc Busy Bee or Bumble Bee was built. Eric is certainly the man with the information.
Best regards

Roger

Offline Brian Rupnow

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Re: Two cycle Engine from Bar stock
« Reply #24 on: February 08, 2016, 08:04:19 PM »
I believe that the slightly larger 50cc Busy Bee or Bumble Bee was built. Eric is certainly the man with the information.

Eric who??

Offline Jo

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Re: Two cycle Engine from Bar stock
« Reply #25 on: February 08, 2016, 08:12:21 PM »
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
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Offline Jasonb

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Re: Two cycle Engine from Bar stock
« Reply #26 on: February 08, 2016, 08:22:46 PM »
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.

Offline Brian Rupnow

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Re: Two cycle Engine from Bar stock
« Reply #27 on: February 09, 2016, 12:00:25 AM »
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.

Offline Pete49

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Re: Two cycle Engine from Bar stock
« Reply #28 on: February 09, 2016, 02:40:38 AM »
Another Brian project to follow. You're getting very constructive in the last couple years  :ThumbsUp:
Hope you can solve the issues as I would love to try making it.
Shrove Tuesday......mmmmmmmm pancakes!!!!!!!!
Pete
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Offline Jasonb

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Re: Two cycle Engine from Bar stock
« Reply #29 on: February 09, 2016, 08:02:06 AM »
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?

Offline Jo

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Re: Two cycle Engine from Bar stock
« Reply #30 on: February 09, 2016, 08:10:43 AM »
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
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Offline Jo

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Re: Two cycle Engine from Bar stock
« Reply #31 on: February 09, 2016, 09:37:15 AM »
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  :headscratch:

Jo

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


« Last Edit: February 09, 2016, 09:48:35 AM by Jo »
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Offline Brian Rupnow

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Re: Two cycle Engine from Bar stock
« Reply #32 on: February 09, 2016, 01:14:28 PM »
Thank you for the information, Jo.---Brian

Offline DTR

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Re: Two cycle Engine from Bar stock
« Reply #33 on: February 09, 2016, 03:51:00 PM »
Watching this with interest :)
Dave

Offline Allen Smithee

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Re: Two cycle Engine from Bar stock
« Reply #34 on: February 09, 2016, 06:45:25 PM »
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
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Offline Jo

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Re: Two cycle Engine from Bar stock
« Reply #35 on: February 09, 2016, 07:01:23 PM »
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  :facepalm: 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  :ShakeHead: 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  :noidea:

Jo


« Last Edit: February 09, 2016, 07:36:59 PM by Jo »
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Offline Jasonb

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Re: Two cycle Engine from Bar stock
« Reply #36 on: February 09, 2016, 07:15:04 PM »
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.

Offline Jasonb

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Re: Two cycle Engine from Bar stock
« Reply #37 on: February 09, 2016, 08:05:45 PM »
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"

Offline NickG

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Re: Two cycle Engine from Bar stock
« Reply #38 on: February 09, 2016, 09:31:25 PM »
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!

Offline Allen Smithee

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Re: Two cycle Engine from Bar stock
« Reply #39 on: February 09, 2016, 11:43:12 PM »
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
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Offline Allen Smithee

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Re: Two cycle Engine from Bar stock
« Reply #40 on: February 10, 2016, 12:01:13 AM »
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!  :Lol:


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Offline NickG

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Re: Two cycle Engine from Bar stock
« Reply #41 on: February 10, 2016, 12:42:01 PM »
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?

Offline Brian Rupnow

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Re: Two cycle Engine from Bar stock
« Reply #42 on: February 10, 2016, 02:05:48 PM »
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.

Offline Brian Rupnow

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Re: Two cycle Engine from Bar stock
« Reply #43 on: February 14, 2016, 09:10:22 PM »
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

Offline zeeprogrammer

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Re: Two cycle Engine from Bar stock
« Reply #44 on: February 14, 2016, 10:18:32 PM »
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.
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Offline Stillyriver

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Re: Two cycle Engine from Bar stock
« Reply #45 on: February 15, 2016, 07:06:16 AM »
 Understand the frustration with Westbury's dimensioning but sorry you're abandoning this. From my own experiences with drafting & drafting checking in the paper & pencil days & Westbury drawings I think there is just a simple drafting error on the liner. That the 5/8 & 11/16 dim's are reversed since we know the exhaust port opens first. So lets assume the exhaust is 5/8 below top of liner for a moment. On the block you are given a 21/32 to middle of port - 3/32 to top of port + 1/16 to top of liner = 5/8. So the exhaust ports will line up that way.
 Now the transfer ports are really dimensioned poorly on the block. It would have been simpler to locate  the ports with one dimension from top of block to top of port but instead you have a daisy chain of extra math relating to the boss and indirectly to the transfer ports: 1/2 + 1/2 - 1/4 - 3/16 + 1/16 liner lip = 5/8. Tilt! So at this point I'd lean towards using the 11/16 dimension & tweaking the less important boss to match the port location.
 On the other hand the 9/16 & 5/8 dimensions Jason provided from Westbury's book might be another choice. Either way it seems clear to me he intended the transfer port to be 1/16 below exhaust. It occurs to me that the lower port position could be raised with a file if the higher position turns out to be the right one.
 Anyway, thanks guys for providing me with enough drawings to spend an afternoon solid modeling another Westbury engine.
  Jeff
 

 

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