Sea Lion engine

[fredrosse]

As to forces on the valve gear, my uniflow engine ran with a single poppet automobile valve with no problems for years. I think, since compression brings the cylinder pressure close to the main steam pressure just before opening the admission valve, and differential pressure on the valve at cutoff is small, the valve gear has light loads while running. On startup with no initial compression the loads on the valve gear have to be considered for strength design, but as far as wear of parts consideration, the single unbalanced poppet has much less differential pressure to deal with.

Automotive poppet valves can easily withstand differential pressures many times more than can occur with our typical steam pressures. A uniflow admission valve with sharp cutoff before seating the poppet upon its seat is available, but with the relatively short cutoff planned on your engine, just a plain automotive type valve would be fine. Sharp cutoff is much more important for engines with the end of admission closer to 50% of stroke.

If you are running with a good vacuum, you may want to consider steam seals on the valve stems and piston rods. That would absolutely eliminate any air inleakage from those points.

[barts]

I had considered the use of balanced or double beat poppet valves but simulation indicated that cylinder compression was likely to be about 150 psi, so the differential pressure is not as big of a factor as on engines with enough clearance to run w/o a condenser. Much more important is the mass of the valve and rocker arms and valve rods because the valve needs to open and close in about 30 degrees of crankshaft rotation. Since the engine runs at say 450 rpm, that means the valve undergoes a complete open/close cycle in 11 milliseconds... like an auto engine running at 4000 rpm. Now, the acceleration of the valve gear depends on the cam profile chosen, but if we chose a harmonic motion then the peak acceleration for the valve is 260 G. A constant acceleration profile yields 110 G - still a big number. This means the valve gear will have loads of 100 to 200 lbs; and need valve springs to match. Keeping the weight of all the components to a minimum is important.
A prototype of the cam and roller follower seems like a good idea; I can use my lathe to drive it and verify proper operation.

With a fixed valve duration of about 30 degrees, short cut-off is achieved by advancing the valve timing to 11 degrees before TDC; this is about 2% of the stroke. The remaining 19 degrees is all admission time, yielding 5.5% cut-off. If we shift the valve timing to opening at TDC, the result is cut-off at 14.5%, using 3 x the steam. In addition, since we're driving a prop, the speed of the engine will go up, increasing power output still further.

I anticipate using o-ring seals on both valve stems and piston rods to forestall vacuum and steam leaks.

Since vacuum is critical for exhaust scavenging and keeping engine compression pressures reasonable, a steam-powered eductor will be used to purge the condenser and generate vacuum for starting. In addition, the cylinder drains will each have two valves in series - one close to the engine for running normally, and one further away to allow use of the extra volume in the pipe as emergency clearance space.

- Bart

[Maltelec]

Since vacuum is critical for exhaust scavenging and keeping engine compression pressures reasonable, a steam-powered eductor will be used to purge the condenser and generate vacuum for starting.

I had thought about this for my engine design. The boat Eplenor has an engine similar to your design, and although it doesn't appear to have any problems starting without a vacuum it doesn't run nice until the vacuum has developed.

[Lopez Mike]

Simple harmonic motion starting from stopped when the valve is stationary produces a spike in the third derivative that goes essentially to infinity. All modern I.C. cams use a polynomial curve for that reason. There is an approach ramp that takes up the clearance and then a transition to the acceleration part of the cam.

When the first 'modern' cams were designed for Jacquard looms in the ninetieth century, the calculations were stunningly involved but with modern computers it is trivial. Ed Iskendarian was the first hot rodder cam designer to get access to a computer in the fifties and trademarked the word Polydyne but the name actually dates back to loom design. The real reason modern cams are designed this way is to reduce vibrations due to infinities in the third derivative which are not likely to be an issue at the speeds and valve train velocities you are intending. You could probably slap it open with a claw hammer at those speeds!