Valve Gear Simulation

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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PatJ
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Re: Valve Gear Simulation

Post by PatJ » Sat Mar 12, 2022 11:35 pm

Bill Hall has both Stephenson's and Walschert's valve gear programs.

And Alan Wallace has a Stephenson's program (or maybe just a Walschert's program).

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fredrosse
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Re: Valve Gear Simulation

Post by fredrosse » Sun Mar 13, 2022 3:11 am

The graph presented on my 31 Jan 2022 post within this thread shows the performance of the Marshall/Bremme (or Klug) valve gear on my engine. There are 7 useful link-up notches forward, and 7 useful notches in reverse. Cutoff from 30% to 85% stroke is illustrated throughout this range, in forward and reverse, and in the head end as well as the crank end of the double acting cylinder.

I would like to see the same graph generated for Stephenson valve gear, as well as others. I think that information could illustrate the cutoff performance & variation of the various valve gears at some "linked up" and full gear positions. Computer generated motions do all the time consuming work here, and could serve to actually quantify the differences in performance. I have not seen this analysis done for any valve gears except the one I generated for my engine, does anyone have access to anything similar?
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Last edited by fredrosse on Sun Mar 13, 2022 5:26 am, edited 1 time in total.
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PatJ
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Re: Valve Gear Simulation

Post by PatJ » Sun Mar 13, 2022 3:50 am

I made this graph in Excel for a Stanley 20hp, but unfortunately I am not far enough along in valvegear understanding to program in the Stephenson's link.
I can't verify that the labels are correct, but they are my understanding of the various points.

I am getting there, but am definitely not there yet as far as valvegear analysis.

I wonder if you could attach linear travel sensors on the piston and valve of an existing engine, and pick off the travel data for various link positions?
Sort of like the indicator.

Also attached is a diagram from Verbal Sketches for Marine Engines, which helped me a lot to understand valve movement vs piston movement.

You can also calculate and plot the acceleration of both the piston and valve.

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PatJ
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Re: Valve Gear Simulation

Post by PatJ » Sun Mar 13, 2022 4:59 am

I have followed Don Ashton (and others), but mainly Don because he seems to deep dive into valvegear subjects to a greater extent than many/most.

So read his 8th paragraph here:
http://www.donashton.co.uk/html/other_gears.html

Does this jibe with what you describe as the benefits of the Bremme gear?
Don seems to indicate the Bremme is one of the few valve gear variants that is useful, and perhaps an improvement on the Stephensons?

I found your video on your boat gear, and while I see it operating, I have not yet wrapped my head around it yet.
Perhaps one day I will catch up, but don't hold your breath.
https://www.youtube.com/watch?v=h_wKu8klZDA

As I recall, Don has downloads on the Stephenson, and perhaps the Bremme too, such as spreadsheets.

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Re: Valve Gear Simulation

Post by fredrosse » Sun Mar 13, 2022 7:28 am

Don's writings... "The primary saviour comes in many variations of the Hackworth family of gears and near relatives like the Bremme and Greenly lever systems." This refers to the ability of the Bremme gear to properly to be a "saviour" of Hackworth-like valve gear which has inherent error in timing events (Head end vs. Crank end, Forward vs. Reverse)

What Don apparently does not address is the simplicity of construction and ability to make very strong, and very accurate motions with the Bremme arrangement. This is possible as the Bremme Gear uses only pure rotary pinned joints, which are both easy to make and precise with respect to minimum wear, and very little "free play". Not so with motions involving sliding surfaces, either straight or curved.

One significant issue with study of valve gear performance is to use actual engine stroke displayed on the graph, accounting for both main connecting rod length, as well as eccentric rod length. Indicator diagrams, and valve timing event diagrams should be presented vs stroke rather than crank angles.

19th century valve diagrams often are based on the approximation of pure sine wave motion, but now we can do a days arithmetic in one second with a computer. As we know, with short connecting rods, the instroke vs crank degrees and the outstroke vs. crank degrees is quite different. The sine wave approximation error is much less pronounced with relatively long connecting rods, (long with respect to the motion's stroke) such as typical eccentric to valve stem rods. Almost all valve gear models use a sine wave approximation for the valve gear eccentric, but with a computer, accounting for the connecting rod actual motion distortion, more realistic results are available. Both the power piston as well as the slide valve stroke displacements can be programmed without relying on the approximation of a sine wave.
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