Compound steam engine.

[Steamchick]

Went shopping with the misses... that cleared my brain.
I have now been re=-reading the following explanation - which uses indicator diagrams and shows the difference between "theoretical" (Missing the real world features, like cooling steam with the cylinder walls, etc.) and REAL.
https://www.engineeringenotes.com/t...struction of theoretical indicator diagram: 1.

Hope this helps?
K2

 

[Steamrguy]

Hi Dean, Jason.
I like the simple approach you are taking. However, "Life isn't like that" is what I heard for decades every time I tried to "calculate" what should happen.
As a lover of numbers - but not the most competent - I enjoy all these number crunching exercises. And learn a lot in the process. Simply:
The pressure explanation is a bit too simple. While it expresses what the simplest theory should predict in "work preformed" by each cylinder - and Jason, I agree with your calcs. - this isn't really true of a more complex (more real?) model. You need to consider real cut-off, mean cylinder pressure, expansion into intermediate cavities... etc. to get a better numerical model of what the steam can do. In fact a lot of energy is lost to cylinder walls, and passage walls, while transiting the engine, so this "loss of efficiency" should be estimated - even if you use a fudge factor of 50% or something at each stage. As the steam expands into the HP cylinder, then from that to an intermediate chamber volume when the exhaust valve opens, followed by the LP cylinder, these expansions must be considered to guesstimate the mean pressures in both cylinders for the period when they can "accept the work from the steam" and translate this into thrust on the piston to produce motion power. I tried to estimate this in a "thought model" in post #19... In fact it isn't so clear that I find it confusing to read now... (Sorry about that!).
But the bottom line is that the 100psi supply steam - if exhausting at 33psi - would only be applying 66psi AVERAGE pressure for the length of the stroke. At exhaust, the steam is expanded into the transfer passage volume - reducing pressure further - and therefore the inlet steam to the LP cylinder is a much lower pressure than you have modelled.
So maybe a real set of sizes needs to be computed to better understand this? (I'm not clever enough to work it all out).
Conclusion: I think the simple "volume" based calculations say that the 2 confirgurations that Richard 1 postulates are effectively the same. But when further losses of transfer passages, and heat loss are considered, the larger piston LP is the way to go, because there is less cylinder wall to loose the heat in this configuration. (Maybe that is why it has always been done that way?).
K2 - Now I need to lie-down and rest the grey cells!

I agree with what you have said. I was speaking "conceptually" and staying out of all the fiddly bit of a design. I think, since we are talking about a model (?) the fiddly bits are mostly academic. The model suggested would run I think and make an interesting display.

 

[Steamrguy]

Dean as torque is lbs.ft the crank throw possibly comes into what torque is being applied to the crankshaft

HP 100lbs force x 0.5" crank throw 4.16 lbs.ft
LP 33.3lbs force x 1.5" crank throw = 4.16 lbs.ft

Yes! your right! I did not see that, I didn't think that through quit far enough.

 

[Steamchick]

I'm sure a well made engine will run. From my experience of the compound twin, the small bore, long stroke, LP cylinder may not do a lot of work. It is a huge drag when cold and condensing the steam rather than allowing the pressure of the steam to push a piston. But the HP cylinder will always do the work with enough steam at high enough pressure.
The club had a display in the park today. I tried to run the compound from a different boiler. However, it needed 30 psi to run (as a single HP cylinder) and I could only maintain 20-25 psi with that boiler at the steam demand. Previously I have run successfully at 20 psi (different boiler with a lot of superheat) while the LP cylinder warms, when I can reduce to 15 psi when the compound (LP cylinder) starts to work. I think the problem with today's boiler is a minimal superheater. It may only be acting as a steam drier....
But I do find the drag of the compound when the LP is too cold to work a challenge for boilers.... Hence the small bore long stroke design may be problematic without a lot of superheat!
K2