Wobbler port specifics questions - size + timing.

[Buffalo_VR]

Hello Everyone,
I have searched and read as many things as I could about this and have not found a specific answer. Perhaps I simply missed the answer.

I'll preface this with, the engine I wish to develop will only be run on air, never steam.

First question; Port size. For air use is bigger better? Or are there other considerations, like sealing problems with large ports? Or is there a mathematical optimum? It seems the ports I see in pictures are relatively small with more than 1 port dia. worth of space between them. There is more to this question but it is dependent on question 2.

Second question: Port timing
I have read of 3 different timings.
(1) crank journal @ 90°, before and after TDC = full open port
(2) crank journal @ 120°(4+8 o'clock) before and after TDC = full open port
(3) crank journal is located so a right angle is formed between crank shaft, crank journal and cylinder pivot = full open port. Is one technically correct? Is one better for air?

Third question,
Slots for ports? Not in the cylinder, just in the body.
I was thinking that having as much duration as possible would be better than a fixed hole. Providing of course that there is no overlap.
I have not seen any examples with slots, so perhaps I'm missing the examples..or..I'm just missing things in general

Great informative forum btw.

thanks in advance
bvr

 

[Tin Falcon]

The best explanation I have seen on wobbler valving in in Building Simple Steam engines book 2 by Tubal Cain.

my suggestion before trying to develop your own engine is build a few of existing designs. There are dozens if not hundreds of variations of the oscillator out there many of the plan sets are free. Why reinvent the wheel when you do not understand how and why it spins.

as far as ports I have only ever seen holes for an oscillator.
the crank is more like 2 o'clock and 10 o'clock when the ports line up.If the ports overlap the air will bypass the cylinder and leak through. if too small they will constrict flow. for a given stroke the distance between the ports will be determined by the pivot location. I am sure there is some golden ratio but do not know what it is off hand.
when I started out I built a lucy engine then two variations of the same engine the mechanical proportions are all the same but I redesigned the frame or standard and changed the orientation of the pivot and crank.

If you realy want to understand the math take a few sets of plans and do the math. that you give some working parameters.
Tin .

 

[Buffalo_VR]

Tin Falcon.
Thanks for the reply. I understand pretty well how motors function. I have built more than a few drag bike engines, both 4 strokes and 2 strokes(port timing 2 strokes is so much foredom fun). I also understand that an air or steam driven motor has different requirements, hence my questions.

As far as reinventing the wheel. Its already being done..so to speak. Additive manufacturing(3d printing) now allows us to make say.... a double acting wobbler engine with a square piston+con rod thats one piece and already captive in the cylinder. While it would be tough, it is possible. With nothing more than CAD. This opens up possibility's for the average hobbyist that weren't available or affordable until a few years ago. Yes there are limitations to the process and that's exactly what I'm doing, finding the limits.

So, how about slots for the ports?

thank you again
bvr

ps. I already scoured a few drawings, that's how I obtained my basic layout. I'm working on animating my model in blender as I type. If it turns out decent perhaps I'll share it.

 

[Ken I]

Timing port sizes pretty much follow from the selection of bore and stroke - which is typically 1.5 / 2.5 :1.

The cylinder port is then located so that it just "peeps" out above the deadlengthed piston - ie the piston should almost close the gap between the head and the piston.

The resultant angle of the wobble and the position of this port then determine its diameter - since its maximum size is prescribed by the fact that the cylinder port must not overlap both inlet and exhaust ports simultaneously (bypass).
In practice you want to keep the ports smaller than this maximum to minimise bypass leakage.

Obviously the ports could be "wedge" shaped for improved performance but in practce the size of the pipes / gallery ports are the greater restriction.

The photo below is the porting of my double acting V4 wobbler - which is available in the downloads section. (10mm bore x 15mm stroke)

Regards,
Ken

 

[lensman57]

Hi,

The advice of TIN and the explanation of KEN I are excellent, you may also wish to have a look at the design of the TiNY no 23 by Elmer, the location of the ports by a simple jig is really easy and fool proof , you might wish to adapt the idea to your design.

Regards,

A.G