Stephenson's Link Hookup Question

[farmerden]

I'm reading all this and realizing how little I know[but I knew that ] If one shortens cutoff ,doesn't the LP in a compound get less steam ,as the steam in a shortened cutoff would have expanded more? Den

[fredrosse]

Pat, your reply :

“You can't dismiss any restriction in a steam engine, all will limit the power produced.
You can have too small a steam supply line, too small of ports and passages, too small an exhaust pipe, all can greatly limit the engine power.”

That statement is entirely true, and I did not intend to imply anything contrary to that.

I was having difficulty with the statement “He (the Stanley owner) says the shorter cutoff at higher speeds is not for efficiency, but due to a minimal sized exhaust port and pipes, and the shorter cutoff releases the backpressure from the engine after release, and allows the engine to make more power."

What I was trying to say is that, for any given engine (in this case a Stanley Engine with restricted exhaust), longer cutoff will always give more power, assuming all other relevant parameters are being held constant. The relevant parameters in this case being inlet steam pressure, engine RPM, and exhaust passage size . If steam inlet pressure, RPM and exhaust passage size are held constant, then longer cutoff will always give more output power, and this is true if the engine has excessive exhaust restriction or not. This is also true at any steam inlet pressure, and at any given RPM.

The attached P-V diagram illustrates the point, two PV diagrams, the Blue curve ( Short Cutoff and lower exhaust pressure) and the Red curve ( Long Cutoff with extra exhaust pressure buildup due to restricted exhaust passage) The red curve shows extra work of long cutoff, giving more diagram area “A”. The Red curve also suffers a loss of work due to exhaust pressure buildup, area “C”. However, it can be shown that the area “A” (gain) is larger than area “C” (loss), thus there is a net increase of power output by increasing cutoff. The Blue curve has higher efficiency, but lower gross output power.

There can be cases where the application of longer cutoff would lead to lower power output, for example if the boiler was undersized and not capable of keeping up steam pressure. All parameters being equal, longer cutoff does indeed consume more steam, so the boiler pressure would drop off as longer cutoff was applied with an undersized boiler. In this condition, shorter cutoff could be applied, and the engine’s efficiency will improve, and output power would increase. But here the reason for the increased power is the better efficiency generally associated with higher steam pressure and shorter cutoff, contrary to the Stanley owner’s statement.

[SteamGuy]

Fred-

I guess I don't really follow your arguement very well (my lack of knowledge).

I will have to ponder those ideas for a while.

Den-

I am also confused about changing the cutoff using the links in a marine compound, since it was said that the lp cylinder really did not need a shorter cutoff, but then someone else said they operate their marine engine with the throttle full open, and regulate the engine speed using the links to shorten the hp and lp cutoff?

A bit confusing.

So does that mean you remove the lap from the lp valve? or do you just shorten the cutoff on the lp cylinder as the link is adjusted?

[steamboatjack]

Folks,
I am afraid I have to take issue with one or too points above. I don't like to argue with such learned people as J. Griffiths but “crossed rods” are not usually used with inside admission (either can be used) more likely they are the result of the link suspension point as I mentioned earlier, of course with inside admission “open” rods appear crossed at BDC (all very confusing).
Regarding Don Ashton's work, I have agreed to differ on a couple of points, Basically on a locomotive, the valve gear is designed to use the same lap at both ends of the cylinder and his work achieves this. However with marine engines usually employing much shorter connecting rods relative to the stroke, this is not practical. Usual marine practice has been to have different laps for each end to compensate for the errors of angularity which are greater due to the short rods. I also believe that his theoretical system does achieve improved valve events but at the expense of more rather than less “link slip”.
The principle reason for “Lead” on any steam engine is to cushion the moving mass of the piston as it stops at top & bottom dead centre, In a small launch engine running at modest speeds this force is not great.
As for the effect of adjusting the cut off of individual cylinders on the performance of the other cylinders (compounds) This is a complex subject which requires much study before getting the tools out.
I have some books showing details of open/crossed rods and of the method of adjusting the valve gear on large engines, I can post these however I am waiting for #1 son to show me how the new scanner works!!!
A steam loco needs the cut off to be at minimum say 70% to ensure starting (the figure is always expressed as a percentage from piston start point to cut off point) This can be reduced to 30 or even 20% when moving with favourable conditions. This gives a high rate of expansion and thus saves steam, on a compound this expansion is achieved by using progressively larger cylinders so there is no need to have an early cut off function. The advantage of compounding is that the cylinder is not subject to a large temperature change and therefore the condensation is minimised, that's it. Any more is just waffle.

Regards
Jack


Regards
Jack

[SteamGuy]

Jack-

That is some good information.

I have heard a similar comment about Don Ashton's work applying more to locomotives than to marine engines.

I continue to study the subject of valve gear, and I know I have a long way to go before I get a good feel for Stephenson's gear and rod arrangement.

It seems like Don mentions shifting the suspension point of the link to try and equalize both admission and cutoff for the entire range of the link, while at the same time minimizing link slip. At the same time, I think he uses a change in the suspension point to correct for connecting rod angularity to correct for asymmetrical events about the forward and reverse stroke.

Some have mentioned that such a thing is not necessary for a marine engine, but from a knowledge standpoint I would say that if you are designing engines with Stephenson's link, it would be a good thing to at least understand how to design the arrangement for perhaps a maximum of 3% inequality of event across the link range Don claims to achieve 1% if I recall correctly.

And if you can achieve 1% equality across the range, then why not design for that?
Perhaps for a large marine engine that generally runs forward, you would want to optimize for the forward run, but from what I am hearing others say, they run at an increased cutoff, and so the potential for significant unequal events it present at an increased cutoff.

I hope to get a simulation of a Stephenson's link going so that I can play around with rod lengths, suspension points, link slip, crossed rods, rod angularity effects on events, etc. and get a better feel for it.

At this point there does not seem to be a lot of agreement about valve gear, and as I understand it, there never has been a consensus about valve gear in the design community.

I would think most could agree on basic facts about Stephenson's valve gear, if I can figure out what those basic facts would be.
Trying to understand Stephenson's as applied to a compound marine certainly is an increased challenge.

Much studying to do on my part.

[farmerden]

As usual I spoke without thinking! If i shortened cutoff on the HP ,I would also shorten it on the LP because my Stephenson's links are connected .In the pic the engine is presently in reverse and the three forward notches are shown at the top of curved bar that the reversing handle glides on. So based on Fred's notes I would assume that shortening the cutoff would allow the boiler to increase in pressure and we might have the same or greater power than running the link flat out! This is only in my application as my boiler can only maintain 100 to 120 psi under perfect conditions with a wide open throttle. I will try this out this summer and see if theory holds up to reality! Den[/img]

[DetroiTug]

Den,

That is a beautiful engine.

On the mention of holding steam; did you ever try the economizer? It made a world of difference in my set up.

-Ron

[Lopez Mike]

Gee, my original post about the valve events getting 'screwy' at short cutoffs sounds sooo simplistic now!

I stand by my assertion that all of this is of lower importance compared to better insulation of all hot stuff and, as has been suggested, adding things like economizers and feed water heaters.

I love this discussion though and am connecting all sorts of bits of information I have heard and read over the years. From railroad practice, I can tell you that an engineer on a hand fired coal burning locomotive who forgets to hook up as the engine speed increases will surely hear about it from the fireman. The reduction of fuel consumption from shorter cutoff settings is startling!

[Mike Rometer]

I stand by my assertion that all of this is of lower importance compared to better insulation of all hot stuff and, as has been suggested, adding things like economizers and feed water heaters.

Mike, I've long held the belief that all those old time guys such as Churchward, Stanier etc whilst arguably briliant engineers in the own right would have had better results, efficiency-wise, had they concentrated on improving the insulation on their locos, rather than trying a load of other stuff, but that's a purely personnal view.

[Lopez Mike]

What is more surprising is that insulation is far more important in small power plants than in large ones.

The heat generated goes up as the cube of the linear dimensions while the heat lost goes up as only the square of those dimensions. That's why mice need fur coats and eat a ot for their size while elephants spend a lot of time flapping their ears and splashing water on themselves to keep cool.

Also the main reason why small turbines are seldom worth the trouble for efficiency reasons. About the only successful turbines in small sizes are in helicopters where power to weight ratios dominate everything.

One of the telling modern anecdotes about steam heat generation came out of the recent wars in the Balkans. One of our F16 fighter planes was patrolling at night and picked up a huge infrared signal. The only time the pilot had seen such a large signature was when there was a structure fire but his wonderful data system said that it was moving at 40 kph! Upon closer examination, he found it to be a steam locomotive.