This project started as an effort to improve my education in an area that I felt the standard WPI curriculum was lacking. I had taken Machine Design, where we were required to design, but not build our designs. I figured that there was a long way between a design on paper and one that makes it all the way to a finished machine. I had taken Material Selection and Processing a.k.a Grunge where we were required to build but not think. I figured there was a big difference between just following instructions to make a pre-planed object and actually doing the planning for yourself. This little project was designed to give me experience building my own design. It isn't as easy as it seems.
I have been fascinated with steam engines for a long while and wanted to build one. As I started to look over some of the existing designs in magazines and on the net I discovered a design that I came to love. The oscillating cylinder design is simplicity itself. In this system, there is no valve box, no eccentric, and no connecting rod. The valve action is provided by the movement of the cylinder itself.
An oscillating cylinder engine has but a few parts to provide all the required functions of a steam engine. There is a cylinder which contains a port at the closed end and a pivot at its midpoint. The piston is a good seal in the bore of the cylinder and connects directly to the crank on the crankshaft. There is a flywheel to store energy for the exhaust stroke of the piston, and a frame to hold it all together. The frame has two ports machined into it. These ports in conjunction with the port in the cylinder form the valve of this engine. When the piston is in its power stroke the cylinder is angled such that the port in the cylinder is aligned with the inlet port on the frame. At the bottom of the power stroke the cylinder pivots and closes the inlet port on the frame. As the flywheel moves the piston through its exhaust stroke the cylinder is angled such that the port in the cylinder is aligned with the exhaust port in the frame. The above animation might help.
From what I read on rec.crafts.metalworking this basic design can be scaled to very small sizes. Someone even mentioned making engines like this small enough to use a thimble as the boiler. I decided to make my engine a little bigger than that. I thought it would be neat to have a small engine chugging away on my desk, so I wanted it to be able to run on the output of an aquarium aerator pump. I finally settled on a design with a 1/4 inch bore and a 1/2 inch stroke. I know the design isn't exactly "square" but I didn't have much choice in the stroke of the engine. The stroke or the engine determines much of the engine geometry. In an engine this small getting sufficient clearance between the inlet and exhaust ports on the frame gets to be a challenge.
The plans were drawn up on paper and checked over for accuracy, then it was off to the shop. A little time with the scrap bin yielded a chunk of brass for the engine, another chunk of brass for the flywheel, and some steel rod for the piston. I wandered over and asked the lab manager for help and the education began.
The piece of stock that I dug up for the majority of the engine had ended up in the scrap bin because it was no longer true. How exactly you warp a piece of 1/2 inch thick brass I am not sure, but someone did. My first operation was to true the stock and reduce it to the proper thickness. Once that was done I marked the frame and cylinder, roughed them out with the bandsaw and milled them to their final dimensions. Of course this was not without it's share of adventure. The frame went well, no fatal errors here, but the cylinder was made three times. The first time went perfectly, then I noticed a flaw in the design. I had specified some decorative relief on the cylinder. Unfortunately, this reduced the area in contact with the frame. As the interface between the frame and cylinder is part of the valve system, I broke the valve system. The relief cuts caused the inlet port to be exposed during the exhaust stroke. Venting your steam supply and getting nothing for it is generally a bad idea. I rebuilt the cylinder without the relief cuts and was almost done when it shifted in the vise and was damaged. The third try was successful. On to the lathe.
The flywheel was rather uneventful, then again, it was simply a disk with a shoulder. The piston was another story.
The piston started as a piece of steel bar stock. I had to turn it to the proper diameter and lap it into the cylinder. Although the plans called for the piston to be 1/4 inch in diameter, you don't just cut it to 1/4 inch and call it good. You custom fit it to the completed cylinder. In my case I turned it to within 0.001 inch of the final diameter and then used sandpaper to slowly reduce the piston diameter until it fit properly. Of course, at this point, I have a precision rod. My plans called for a more complex piston. I narrowed the center part of the rod to produce a piston / "connecting rod" / rod end assembly. The final touch was to drill for the crank pin and mill off the excess stock.
Here is the finished engine. I built a base for it from some oak boards I picked up from the scrap pile at school. The needle valve, also from a scrap pile at school, provides both a convienent way of controlling speed and a place to attach an air supply. Most of the plumbing is hidden in the base, but air makes its way up to the engine through some 1/8 stainless steel tubing. It does actually run on an aquarium aerator pump, but only if the tension on the cylinder retaining spring is backed all the way off. Even then it gets stuck sometimes. The aerator pump claims to be able to produce 4psi and 2000cc per minute. I suspect that is 4psi with no flow and 2000cc per minute with no back pressure and never the two shall meet. The engine runs very well off 5 psi shop air or a can of compressed air for cleaning computers.
On my next design there will be a few changes from this design. First of all the piston will not neck down as deep into the cylinder as it did in this design. I found that as the piston moves the cylinder the head of the piston bites into the walls of the cylinder. A longer head or simply a solid bar for a piston would eliminate that problem.
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