Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Here it is! I wanted a 2 cylinder Märklin and luck would have it, a compound as well, so I made one with 2 orphans.



The compound twin engine took a bit to accelerate and warm up. It runs the table saw very well. Surprised how well it works. 😊


Remember cut once measure twice? When I first tested it running in simple mode and not chained, one went a lot faster than the other as I thought they were identical displacement.

(The bigger displacement engine runs well: )

I needed to measure at least once!

The flywheels are the same, but the displacement definitely isn't!

Measurements:

Stroke 20mm vs 12mm and I measured approximately bore guesses of 17.5mm vs 16.5mm so:
4.811 cm3 idealized
2.566 cm3 idealized

 That is close to proper compounding. =)

Wikipedia article states: In non-condensing engines, the HP:LP volume ratio is usually 1:2¼.  https://en.m.wikipedia.org/wiki/Compound_locomotive

No wonder  the cylinders I have work for compounding.

When the cylinders are 180 degrees out of phase (Woolf compounding), then the plumbing is simple. Hence my current arrangement.

Even though the engine has a lot of power and a lot of displacement (biggest engine I have), it isn't steam thirsty in my opinion.  Admittedly, the Wilesco hedgehog boiler helps counter steam thirst. The Wilesco hedgehogging is so simple and elegant, could easily be added to all pot boilers. Runs better than my flued boilers whose heat tends to flow thru too fast imho.

For self starting, it will be complicated... one needs 90 degrees phasing with buffers called receivers as well as proper plumbing.

http://ecoursesonline.iasri.res.in/mod/page/view.php?id=2450

In order to reduce this pressure variation, the receiver is made at least 1.5 times the H.P. cylinder volume. The connecting steam pipe may suffice though.

I did a test of pumping water  (hand turned) to see if it would hydro lock. I did at 180 deg and 90. The 90 was a bit stiffer than the 180 but I didn't notice hydro lock. (I've had Wilesco engines hydro lock and they lock up!)

Then on air, I tested 90 degree phasing  and 180. I think it runs better as 180 than 90. As for self starting... It doesn't work to self start in compound mode at 90. Why? This actually reminded me of the discussion I had with a British steam roller owner. I asked about the simple/compound switch. I asked "does it give more power in simple mode?" His replied was "Perhaps, but I mostly use it for starting." At the time, I thought it was to get over inertia. Actually no: in compound mode, the first cylinder acts as a valve for the second. Simple mode is needed for improved self starting when bdc, tdc HP position.

For me to do the plumbing, I do have extra brass fish tank valves. They are all binary valves. Ideally I'd have an all in 1 complex valve but in lieu of that, it'll be an extra 3 valves. I wonder if I can get a 3 in 1 air valve?

Another question, anyone know the but size for Märklin crankshaft ends?

Nice to see someone else having fun with compounding!

That is an interesting valve setup, i know the pain of warming everything through which takes awhile.

You need to make a separate condenser for it now!

I will be attempting a compound soon from castings
I am starting with a Stuart D10 .
Will start with a new crank shaft ( 180 * ) and may if need sleeve one cylinder and bore second as far as I can go,

Plenty of room to stuff it up .

Cheers
Dennis

Nice work and you sure are having fun!


Gil