Definition of a Modern Steam Engine

The definition of a modern steam engine is outlined in the latter part of this video:

http://www.youtube.com/watch?v=mhE_3d3WSvs&feature=BFp&list=PLD4A0A9AFD30E1190

Key characteristics of a modern steam engine

 * Poppet valves
 * Different passages. One path for the steam to enter the expansion chamber and a separate path for the steam to exit. This ensures the cold exhaust steam doesn't cool down the metal the new steam is about to pass through, cooling down the steam that is supposed to be hot before it gets into the expansion chamber.
 * Low clearance volume. I believe this means that the piston-swept volume is pretty much the entire cylinder volume, so the ports and cylinder head do not add significantly to the volume of the cylinder. This minimizes the amount of space the hot steam has to fill up before it can start to push on the piston.

Controls for a modern steam engine
Although there might me some dissenters, we expect that a modern steam engine will involve digital electronic feedback control for functions such as speed governing, fuel flow regulation, and valve cutoff timing. This is especially valuable for an engine that works under dynamically varying load conditions, as it relieves the human operator from attention to maintaining safe and efficient boiler and engine operating conditions.