how equal is steam chest pressure for each cylinder

[gregc]

wondering if steam chest pressure for each cylinder needs to be considered independent of each other or if the connection between them is large enough that they are essentially at equal pressure

[Overmod]

The problem with what I think you're asking is that the two cylinders of a typical 2-cylinder DA are in 90-degree phase, so any loss and recovery of nominal steam-chest pressure as a 'balance' between mass flow into and out of the steam chest will be at a peak in one cylinder while relatively low in the other.  You can see one of the issues here: the two chests are fed from the same header and throttle.  The supply of steam can be easily traced from the dome, through the elements and throttle on a modern engine with a multiple poppet throttle, and then down through the pipes in the smokebox -- the only balancing difference down to the steam chest being the flow characteristics of the pipes from the branch near the throttle 'on down' to the chest.  The flow will determine the available pressure and hence mass flow vs. pressure drop through admission to admission cutoff (after which of course the steam pressure can equalize).

An assumption, which Sinclair noted was not always borne out in practice, was that the steam-chest pressure side-to-side would very quickly balance at short cutoff or at mid.  There is a certain amount of 'flowing' that ensures there is little 'flow stall' at or near the branch from one side to the other, e.g. at slow speed and gear 'down in the corner' where cyclic is low and mass flow very high.  That implies pulsations in the flow to the chests, which is one stated reason why relatively large steam-chest volume (very well insulated) is desirable for general working.

[gregc]

what is the typical size of a steam chest relative to cylinder size?

guess the question is how much variation is there due to the 2 cylinders drawing steam singularly or overlapping?

[Overmod]

There has always been something of a tradeoff between steam-chest size and shape, valve diameter and port opening, and port and passage shape.  The steam chest when properly arranged has had time to fill with steam mass coming up to pressure that can then act as a reservoir for admission reducing pressure drop in the supply pipe (which is of smaller diameter than apparent and usually not well flow-streamlined) which would otherwise start to starve mass flow by the time the valve closes to steam and expansive working begins.  You can perhaps recognize the effects when you consider an engine working at its highest speed (say 540rpm at about 41% cutoff).

One place you see steam chest volume relied upon (perhaps a bit unwittingly) is in the early Lenz/Lentz-derived Franklin System of Steam Distribution.  The actual path from the throttle to the valve transit is almost fiendishly convoluted, and the valves themselves are designed to mimic the very rapid port opening of long-lap long-travel valves very quickly after opening, so without reasonable 'reservoir' space there just won't be enough moving mass to go through the more freely-breathing valves...

Meanwhile we get to another timeless topic: how large should the valves be and how long should their travel be?  For a while the 'design idea' was to reduce inertia and throw as much as possible, and you see this in early slide-valve practice where the valve moves centrally with only a couple of inches between admission and exhaust events.  Then you see various Trick ports leading up to a waffle-grate sort of valve that is essentially a multiple grouping of small and short-throw valves in parallel for large mass flow -- the limit being what could be arranged over a reasonably-flowing passage at the cylinder head ends.  (I have seen versions of this resembling the Franklin chest approach, with two rectangular chests centered over the cylinder ends with a simple glanded valve rod between them, probably with a riding cutoff arranged above the actual slides for some probably necessary fine control...)

Then radial valve gear makes piston valves desirable, and within only a few years Churchward and disciples... reportedly looking at American practice although whoe practice I don't know... tumble to the idea that accelerating a valve gently and smoothly in an extended blind bore so that it's moving quickly when it actually crosses the admission edge is a Great Secret for high-speed operation (it would take Gresley two decades and some horrible humiliation along the way to accept this, but he made up for it when he did).  Now you can have port openings an appreciable percentage of  circle around the valve face as opposed to what a slide valve provides, and in fact Bulleid increases this to full circle using sleeve valves with essentially no passage longer than about 2" through a Meehanite sleeve, so essentially the best possible case for determining how much of high-speed flow restriction is attributable to valves.  (It turned out not to be the 'killer app' expected...)

Near the very end of steam, the opposite end of a stroke was addressed, by providing enormous fabricated exhaust valves in parallel -- Willoteaux valves.  When you combine these with reversible compression control you can optimize some of the issues of 'starved admission' by having cylinder pressure at the moment of valve admission equal to or only slightly below chest pressure.

Wardale re-introduced the idea of separate valves at either end, joined by an articulated structure, and then the comparatively obvious step of using a pair of separately-timed piston valve assemblies side-by-side to separate admission flow from exhaust flow.  Even here you'll see 'larger than traditional' valve diameter for better port streamlining with restricted dead space issues...

[gregc]

so how big is the steam chest  ??

[Overmod]

Meanwhile, to answer the question that was asked, there is little concern with any average resultant between steam to the left and right chests.  The left 'side' takes steam from the flow down its branch pipe -- the concern being that you have to carefully avoid any induction of reverse mass flow from one side even at the peak of flow drawn by the other.  (I have read some accounts of early practice that seemed to think this was a good thing, not a bad!)

One thing this does imply is very careful design of the zone where the exit from the multiple-throttle header actually starts to branch and then divides the flow.  Here a certain amount of 'tuning' can assist with the left-right 'steering' of instantaneous high mass flow without reflected shock, choking, reverse flow, etc.