Engines > Your Own Design
Four Valve Non-Releasing Steam Engine
Captain Jerry:
Last month I started talking about designing and building a Corlisss type steam engine and started a discussion in the "Twenty Questions" section with the hope of getting some input from forum members. As you might expect, I got lots of good information and suggestions so I am ready to start this new project. Thanks to all of those who contributed to the pool of information.
I started thinking about building a model steam engine based on the Corliss Valve design but as I got a little further into the design, I realized that there are problems that cannot be resolved if I try to build a true, working, Corliss style valve mechanism to be powered by compressed air. S0....I am going to build a model of a later design that is often confused with the Corliss mechanism, (at least by me). The value of the Corliss design is the sharp steam cut-off at variable crank angles, under governor control. By closing the steam valve early in the cycle, the steam expansion is used more efficiently, and by varying the angle of cut-off, the power can be matched to the load to maintain a set speed. The Corliss design changes the steam cut-off angle without affecting the exhaust valve setting. The most recognizable feature of the design is the ?wrist plate? on the side of the cylinder and the four radial rods that operate the four valves. The ?Four Valve, Non-Releasing ? engine is based on a valve design that appears quite similar to the Corliss valve and overcomes some of the shortcomings of the Corliss design. It uses a similar cylinder form and the four valves are operated with rods radiating from a wrist plate. BUT it does not incorporate the ?quick drop cut-off? of the Corliss design. In its simplest form, power and speed are managed by a throttle valve under governor control. In its most complicated form, it uses two eccentrics and two wrist plates. One of the eccentrics and the associated wrist plate controls the exhaust valves and the other eccentric and wrist plate controls the steam valves. The steam eccentric uses a governor to advance the eccentric angle, much like an automotive distributor uses manifold pressure to advance or retard the spark.
The shortcomings of the Corliss design that this engine overcomes are:
*
*
* Initial cost
* Maximum speed
* Later cut-off Initial cost is obviously higher with all of the additional parts required for the Corliss quick cut-off mechanism. The maximum speed of a true Corliss engine is also limited by the reaction time of the dashpots and return springs to between 100 and 150 RPM. Later designs increased the maximum speed to 200 RPM. This is fine for mill operation where belt and shaft transmission control the final shaft speed but it is too slow for some other applications such as electric power generation.
The most important shortcoming, from the viewpoint of this model, which will operate on compressed air is that the Corliss quick cut-off cannot be configured to occur much later than about 50% of piston travel. Compressed air does have a small component of expansion but not that much. Early cut-off just does not work with compressed air.
This is obviously an oversimplified description of these two designs and if you want to get deeper into the subject with a more qualified source, you should read Audels Engineers and Mechanics Guide 2 available here:
http://books.google.com/books?id=5_pYAAAAYAAJ&printsec=frontcover&dq=editions:pi7LBdCfU5wC&hl=en&sa=X&ei=3uERUKXAKajF6wGDn4HoAg&ved=0CDgQ6AEwAjgK#v=onepage&q&f=false
Thanks to Arnold for this link.
This is what my design looks like at this point.
I am using Alibre' 2011 for all 3D modeling and 2D drawings. The details of the design may change as the build progresses but I will try to post updated details as they occur.
I actually got some work done today. I started with the cylinder because it offers the most new challenges. The first thing is to get the stock to size. The finished size if the cylinder block is 2" long by 1.5" high and 1.25 wide. I used my table saw to get the stock to rough oversize:
And then brought it square and to dimension with a flycutter. I then used my edge finder to locate the center of the face to drill for the wrist plate pivot and the four valve bores.
The valve bores are .25" diameter finished. I located them from the center point and and then drilled 3/16", 7/32" and 1/4". I know from previous use that this 1/4" bit produces undersized holes which will be reamed to size in a later step. The last operation in this setup is to return to the center and drill a flatbottomed hole with a 1/4" end mill for use as a locator for the wrist plate pivot.
The next step is to bore the 5/8" cylinder bore, centered in the end face. I do not have a lot of confidence in ability of the X2 mill column and dovetail slide to stay vertical for this operation. The bore is 2" deep and there is enough flex in the column to give problems, even with a boring head so I decided to do this job in the lathe.
That is a piece of 2" square aluminum tube bolted to the lathe cross slide. The cylinder block is bolted to the square tube with shims to bring it up to height. It was set parallel to the lathe ways using a dial test indicator swept against the far edge.
The part was centered horizontally using a center finder to locate frond and back faces and the dimension divided by two, just as you would do on a mill table. Sorry, I forgot to photograph this operation.
The hole was pierced with a 3/16" drill bit and the progressively opened with 1/4", 5/16", 3/8", 1/2" and 5/8" drill bits. For each of these operations, the tailstock feed was used to drive the part into the drill bit to avoid any off center forces that would cause the part to lift or twist.
The final passes of the 5/8" bit were driven by the carriage feed lead screw with the bit rotating at 2000 RPM and well lubricated. The end is as good as or better than I have ever been able to achieve with a boring bar. Honing with abrasive will be done later.
The final job before leaving the shop for the day was to put a tapered top face and spigot on the top plate and to mill a rebate on the edges for hold down bolts.
It is late and I have l small glass of Jack Daniels with an Ice cube trying to get my attention.
Jerry
steamer:
That's a flying Start Capt!
Dave
zeeprogrammer:
Wow, what a start!
That's going to be a good looking engine.
Dean W:
What the other guys said; A great start. This should be an interesting engine to watch running. Lots going on.
swilliams:
I agree; this looks like it will be very interesting
Steve
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