Long lap (and long travel) valves

Question from an "armchair locomotive engineer".
The main advantages quoted for long lap valves are increased steam and exhaust port openings, and this is achieved at most running cut offs.
I have steadily built up a library of technical steam literature with all manner of formulae , valve diagrams etc, which mostly explain valve events and the interactions between, lap, lead, cut off, travel etc (including a fascinating recent publication by Adrian Tester). But they all start off with the blanket statement that long lap gives greater port openings (provided the valve travel is lengthened to accomadate it ). I have never seen a diagram or a simple layman's explanation as to why this would be the case.
e.g If you increased steam lap by, say 1/4", and increased the total valve travel by 1/2" to accomadate that, then at full travel, with full steam port opening, line for line, the exhaust edge of the exhaust valve would have "overtravelled" 1/4" compared to the original valve. So I can imagine how that would give greater exhaust freedom. But I struggle to see how the steam port opening at, say 45% cut off ,would be any greater than the valve without the additional 1/4" lap.
I fear I am missing something simple in trying to visualise this. I'll probably end up doing the sums and just accepting the results, but a visual or practical answer is always nice.
Does anyone have a straightforward answer or know of any literature that would provide the answers.

 

I can't answer your question directly, but you might want to have as look at this site, (or other similar ones) http://www.nmia.com/~vrbass/steam/valvegear.htm. Somewhere in there you should be able to simulate different configurations and see the results.

 

Difficult to put into words, but I'll try! Let's take two examples which are totally theoretical and don't represent any particular loco. A 'short' travel valve with a total movement of 3" and a 'long' travel valve with a total movement of 6" but with otherwise identical valve events. At a particlular cut off, say, the short travel valve will be open 1/8" at maximum opening. At this same cut off, a long travel valve will be open 1/4" simply because it is travelling twice the distance for full movement. This is independent of the full port opening. Even though steam moves very rapidly to fill the available space, it still takes a finite time to do so. If you notch back a short valve travel engine towards mid gear, you end up throttling the supply of steam, a condition known as 'wiredrawing'. If a long travel valve opening is twice that of the short travel valve it can admit significantly more steam in a given time because it is not throttled nearly as much. If you 'notch' the reverser back even more, so that a short travel valve is only fully open (say) 1/128" is will not let in much steam at all. The long travel valve will let in more steam as the valve opening is twice that of the short travel valve at 1/64" Simplistically, the cut off percentage of piston stroke of a long travel valve can be much shorter than a short travel valve for admission of the same volume of steam.
That may, or may not, be an easily understandable explanantion. It's too late at night!!

 

Certain Stephenson gear engines run quite happily in mid gear, - lead steam only.

 

Thanks to all for those responses. If the weather remains inclement I shall while away some time pondering them over the weekend (and a pint of London Pride ).

 

Yeah, we have two GWR 57xx tanks that are well known for that, have seen it done on 7714 with an 8 coach train behind her, never seen it on 5764 yet.

 

Some feedback for the responders and hopefully of interest (and in the spirit of mutual improvement)
I took figures from a book for the valve dimensions of a black 5:- 1.5" lap, lead 0.25", port width 1.75" (apparently later increased to 2" but, not important for this exercise). I knocked up a small spreadsheet and increased the lap by 1/4" to 1.75". I didn't alter the lead, (altering lap alone would alter valve events though). Using formuale from above mentioned books I calc'd the valve gear travels and port openings to steam at different cut offs down to 20%. I assumed that full gear travel would give full port opening (1.75") to steam, and no more. So the valve travel increased 2* 0.25" = 0.5" at full gear.

In all cut offs the valve travel increased about 15.4%.That resulted in valve openings to steam increasing 13.6% at 60% cut off down to 9% at 20% cut off. Still tricky to visualise but the sums do hold true to the blanket statement. Increasing Lap does increase PO to steam, provided that the valve travel is increased to accomadate it.
PO to steam was calc'd from (1/2 valve travel - lap). So it does look like the increase in valve travel is greater than the increase in lap, for a particular cut off, and that gives the greater opening to steam.
The cut off in full forward gear reduces from 75% to 71%.
Anyhow, I shall refrain form further alterations to Mr Staniers masterpiece before I break something.

As for panniers and running in mid gear. If the eccentric rods are "open" then the lead should increase toward mid gear, so mid gear running would be entirely feasible.
Something that hadn't occured to me until recently is that as an engine is linked up , the lead assumes a greater proportion of the valve opening to steam i.e at very short cut offs, most of the port opening is lead.

 

I've not heard the term 'open' eccentirc rods before - I'd interested to know what that means. As I understand it, lead steam increases with Stephenson's gear as you notch up, due to the increased interaction of the opposite eccentric to the direction of travel, this reaching its maximum in mid gear. I've got a feeling that we are probably talking about the same thing in different terms.
As for driving panniers in mid gear, due to their very generous amounts of lead steam, I have seen it done, but usually in error when aiming the reverser for one notch off mid gear! But they do go well enough in that position....

 

Don't forget that there are two sides to a port. My understanding is that with 'modern' cylinders and valve gear (say by 1925 and later), the ports are not open fully to admit steam, even in full gear, but they do open fully to exhaust.

As G. J. Churchward is reputed to have stated - "Any fool can get steam into a cylinder, but it takes a clever man to get it out again."

 

With Stephenson (or any link) valve gear you can have two arrangements of eccentric rods; open or crossed. To establish whether you have open or crossed rods, put the loco with an engine crank on back dead centre. With open eccentric rods, the rods do not cross each other. With crossed rods, they do. That's the only crank position for which this holds true. With outside admission and open eccentric rods, you get increased lead as you notch up; with crossed rods you get decreased lead as you notch up. That is, unless you introduce a rocking shaft when the reverse applies! Then, when you've got your head around that, its all the opposite for inside admission! At least, that's what I think it is. [-o< It's too late at night and I'm quite prepared for someone to tell me that I've got it all the wrong way round! :-k
In theory, any engine will continue to run in mid gear as long as it has lead steam although the power output and smoothness will be quite poor.

 

I fear Mr Chapelon and Mr Porta may not see eye to eye with Churchward on that one. And even they didn't agree on everything. Chapelon experimented with trapezoidal ports that throttled the steam at the point of release, to ease peak flow and lifting of the fire. Porta saw any throttling anywhere in the steam circuit as anathema to the principles of locomotive thermodynamics.

My understanding is that Churchwards valve gear's were optimised for exhaust openings , which might in part account for that wonderful bark of a Western engine (together with the faster travel of his long lap valves)