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| Conversion of 4 Cycle Utility Engine to Steam |
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| vtsteam:
I've begun work on a project to convert a Tecumseh H50 5 horsepower horizontal shaft motor to steam. It's been a great learning process for me. The goal of the project was to take one of 3 defunct engines I received in a trade and do the conversion. I didn't want to do just a simple "see it rotate" conversion like many of those shown on Youtube, but wanted to have proper valve events for both inlet and exhaust. To explain that, many of the conversions I've seen are inlet open for 360 degrees, and exhaust closure is the only thing that provides drive during part of the cycle. The usual procedure for the simpler conversions is to add weld metal to the back side of the exhaust cam lobe and file it to a profile to match the existing lobe. The camshaft on utility 4 cycle engines rotates at half the rate that their crankshaft turns, and this is accomplished (on my Tecumseh engine) by a set of gears of 30 and 60 teeth with a pitch of 5 TPI. Because single acting steam engines are essentially 2 cycle engines, at least by analogy, the existing cams on a 4 cycle IC engine are rotating at half the rate needed, and in addition, the valve events themselves are quite a bit too long for an efficient running steam engine. In other words the cam lobe lift(s) are too wide. With suggestions by an experienced steam engineer, my intended valve timing for the steam engine eventually resolved to an inlet opening at 10 deg BTDC and closing at 90 ATDC (100 deg dwell) and exhaust opening at 15 deg BBDC and closing at 15 deg BTDC (180 deg dwell). Before moving forward with my present plan, I'm going to first retrace here all of the steps (mostly missteps) I've taken so far, including the many mistakes and changes of direction as I gradually discovered problems with my various ideas. I'm sure this is all old hat to those more experienced in steam and IC engine design, but it may be entertaining for that reason! I'm definitely beyond Plan B -- maybe I'm now at Plan F. But it has been wonderfully educational, particularly with regard to cam design, for me at least, and I enjoy learning this way. It's the old take apart the alarm clock, and try to put it back together before the folks get home kind of approach to education! Below: 1.) Candidate engines -- all Tecumseh 1 seized, 2 with thrown rods. 2.) The star pupil -- a seized 50H type 3.) Crankshaft and camshaft gears 4.) Camshaft and lifters 5.) The existing IC timing marked off |
| vtsteam:
My first plan was to cast a new cylinder head, with a boss for a piston intake valve that would be activated by a bash type pin welded to the exhaust valve. This would still require adding a second lobe to the exhaust valve cam location -- opposite the present one. At the time I didn't realize that the exhaust valve cam would be far too wide to work well for this. The old inlet valve was to remain closed, achieved by removing the inlet valve tappet. I drew out a wooden pattern for the head to be cast in aluminum, tracing around the old head gasket with a magic marker -- the thick marker outline was a simple method for adding a shrink allowance for casting the actual head. Approximate head bolt locations were marked from the gasket, although these would change due to shrinkage. But they were close enough to give me locations for adding bosses to the casting at the bolt locations. I also drew out the cylinder and valve spaces. |
| vtsteam:
I carved out a passage in the pattern for the new single inlet/exhaust valve, and added bosses to the pattern for head bolts and a base for the new piston valve and its cylinder -- which would be added as a separate assembly. |
| vtsteam:
The valve boss was a little tricky to locate and align because the exhaust valve is inclined 5 degrees toward the engine cylinder. I also added a boss for a relief valve over the cylinder center. Finished pattern below, contrasted with the old IC finned head. |
| vtsteam:
At about this time I began to realize that a combination exhaust and inlet valve actuation had some limitations. It seemed that the inlet event would have to be symmetrical with the center of the exhaust event. They didn't have to overlap, but a single actuator simply means that those events must be tied in regard to their center points in a cycle. Or at least that's what I thought. This stuff can get tricky to imagine. I also started to think that a 3/4" thick aluminum casting would absorb a lot of steam heat, at least initially, even if lagged. So I began to think about adopting a suggestion I'd had to fabricate a steel plate head with a bash type ball valve for inlet, actuated by a pin added to the top of the valve, and retaining the old poppet exhaust valve. This would require adding two new lobes to the cam -- one each for inlet and exhaust. It would also require filling in the carburetor manifold with something (lead was suggested) to reduce the head space left by actuating the inlet valve, but using it only as a pin to actuate then newer ball valve. Because my sadly rusted used milling machine purchase was in a somewhat exposed storage shed at the time -- surrounded by layers of odds and ends and unwired, and because temps have been in the sub zero (F) here in Vermont lately, I wasn't able to use it at the time. (Happily, I've since got it wired, cleaned, and operational, as well as closing in the shed it is in) Instead I decided to make the plate head as two laminated pieces, brazed together. This would allow me to make a pocket for the exhaust and inlet valves by drilling the bottom piece -- the other would act as a cover plate. |
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