effective valve timing for triple expansion

[Peter_A_Lawrence]

The issue I am bringing up is that for a compound steam engine the transfer of steam from one cylinder to the next only happens when both the exhaust valve of the higher pressure cylinder and the intake valve of the lower pressure cylinder are open at the same time (neglecting the volume of the piping and valves themselves).

I am designing/building a triple whose (Joy's Valve Gear) timings are
intake tdc-15...tdc+105, and exhaust bdc-35...bdc+125 <--- specified

HP exhaust 145...305, ie bdc-35...bdc+125, where HP bdc is at 180
IP intake 225...345, ie tdc-15...tdc+105, where IP tdc is at 240

but the HP exhaust and the IP intake only overlap 100-deg,
so the "effective" timings (where both valves are open) are

HP exhaust 225...305 ie bdc+45...bdc+125 HP exhaust opens late
IP intake 225...305 ie tdc-15...tdc+65 IP intake closes early

intake tdc-15...tdc+65, and exhaust bdc+45...bdc+125 <--- effective
are quite a bit different from the original specified timings.

I am most concerned about the late HP exhaust opening,
because being After BDC that implies re-compression,
whereas the early intake closing implies a larger expansion-vs-
intake ratio which is good for efficiency.

but how does one know if the "effective" valve timings are acceptable,
and/or should the timings be tweeked so that the "effective" timings
are closer to a typical non-compound engine, or somewhere in between.

(AFAICT it is impossible for a triple's effective timings to match the non-compound timings, but they could for a double if the cranks were 180-D
phased rather than the typical 90-D.)


I know the initial Joy's Valve Gear timings seem a bit whacky,
but I find the same problem with Stephenson Valve Gear.


Peter A Lawrence

 

[Charles Lamont]

First thing, for a steam engine it is usual to express the valve events (admission, cut-off, exhaust and compression) in terms of percentage piston stroke. Primarily because of the con-rod angularity, they are considerably different from the crank angles, and you may find they are more consistent. With properly designed valve gear the stroke-based events at each end should be closely matched when running ahead.

The valve chest and transfer pipe volumes are supposed to provide enough of a reservoir for the 120 degree phasing of the cranks not to matter.

 

[Steamchick]

Thanks Peter,
Most interesting.
I started to plot this on graph paper, to get some understanding of the timing, overlap between a higher pressure exhaust and a lower pressure inlet... but thinking a bit deeper, realise that gases flow in a NON-linear fashion. Also, valve openings are NON-linear. Everything is sinusoidal for timing, versus volume or port opening area, at a constant crank speed. But gas flows depend on volumes either side of a valve port and pressures either side of a valve port.
SO I must resort to a quite complex iterative model. But this will only be based on a "simple" gas, not steam (which becomes condensing as it expands and pressure drops and adiabatic cooling occurs). Notwithstanding, a good "air model" runs better on steam (usually?).
Considering the valve gear, I am assuming (for simplicity) that port openings will be without overlap/underlap other than as per the timings, so to simplify the model, I shall relate them to the sinusoidal motion derived from the eccentrics. - Even so, the modelling is a bit complex, as the motion of valve opening is a segment of the sinusoidal motion.
I may be some time getting my head around this one...
# But please can you provide the following, so I can build a numerical gas model?
* HP, IP & LP Bores, + stroke (all the same stroke?).
* Inlet port dimensions per HP, IP, LP; steam passage lengths within the cylinder/valve blocks and drilled hole size, inlet and exhaust;
* Interconnecting pipe: passage bore and mean length;
* Dimensions of slide valves (outside) and inside the valve chests - HP, IP, LP.
# Or some picture of drawings if easier?
# Or whatever you can provide and I'll "guess the rest"!
Thanks. If I get a "useful" result, I shall post it for all to share.
If you don't want to be bothered, I don't mind, as we all have things we enjoy and things we don't. And this is a HOBBY, so is ONLY to be enjoyed. (I enjoy studying these things as I once did it professionally, ~40 years ago, when computers did multiple calculations and nothing else!).
Thanks,
K2

 

[Jasonb]

As Charles says the pipework and chest act as a "receiver" so you don't have to have one exhaust open at the exact same time as the next inlet