Reply To: Jos Koopman Conundrum
If we were talking about a turbine engine, I would agree entirely with you.
For a simple expansion engine (and probably a compound) the engine is unable to fully expand the steam, and there is considerable residual energy in the steam when the exhaust valve opens. Porta’s argument (as described in “Red Devil”) is that the de Laval nozzle utilises this pressure close the dead centre position of the piston (when back pressure doesn’t matter) and as a trade off, the back pressure can be lower during the piston mid stroke. Obviously with a two cylinder engine, both pistons will be subject to the same back pressure, and Porta introduced the “Kordina” which was a crude ejector arranged so that the draught from one cylinder reduced the back pressure in the other cylinder.
(Strictly speaking, a Kordina is a concentric device. The more common arrangement where the two exhausts merge as two semi circlular pipes is known as a “Goss wall”)
My suggestion is that the de Laval “chokes” should be located in the Kordina, to reduce the back pressure even further, but I don’t think anyone has designed such a system.
Regarding the argument of “off beat” engines, you obviously have to consider the overall efficiency of the locomotive, and I’m sure the engine with correctly set valves would be better in this respect.
It is true that de Laval nozzles were rare. Apart from the Lempor, the only other example I am aware of is the 4-8-4 Niagara on the New York Central.