Wobbler Steam or Air Engine

[mklotz]

Most steam locomotives had D or piston valves.

If you have a piston valve, the cylinder must be outfitted with blowdown valves. Condensation after engine shutdown can lead to water in the cylinder. If you attempt to start the engine this can lead to a hydraulic load on the piston and the valve. In the worst case something bends or breaks. Opening the blowdown valves at startup provides an escape route for water. You can see the blowdown valves in this three poster...

On the other hand, in the D valve the 'D' piece is free-floating on the rod that pushes it back and forth. Condensed water forced backward at startup will simply lift the 'D' and the water will have a clear path to the exhaust stream. You'll notice that the marine engine has no blowdowns which indicates that it has D valves, as indeed it does.

Another tidbit that making the marine engine taught me...

If a single cylinder steam engine stops at exactly TDC or BDC it has no lever arm to turn itself over on startup. This means it would have to be hand cranked into a starting position. When the engine is down in the bilges somewhere, that's neither an easy or appealing thing to do.

The marine engine has its two cranks out of phase by 90 degrees. No matter where it stops at least one of the cylinders will have leverage to restart the engine; all that's needed is to apply steam.

The rope drive engine has offset cranks for a similar reason. Hand cranking a big engine isn't an easy job. In fact, some of the early single cylinder engines had separate small "barring" engines that were used to turn over the big engine. A gear on the little engine engaged, via a clutch, gear teeth on the periphery of the big engine's flywheel. Once the big engine got going the small engine was disengaged.