alt.energy.renewable

-> I don't know about locomotive engines in particular, but the theory is to
-> a "poppet" valve that fully opens and then closes quickly, perhaps for a
-> variable amount of time depending on conditions. By being either fully open
-> fully closed rather than somewhere in between, you reduce throttling
-> losses.

Reduce, yes, but there are still losses associated with the process.
If the valve is open only very briefly, when the piston is at the end
of its stroke, the effective expansion ratio of the engine is greater
than it is if the valve stays open until the piston has travelled some
distance along the cylinder. So the ideal expansion ratio, matching the
pressure in the boiler, can't be achieved in all cases.

It's better to open the valve for the same amount of time each time it
is opened, but to do it only on *some* of the strokes of the piston. It
makes the engine run "rough", but the efficiency is higher.

dow