Hi everyone. I think this has been discussed on RYPN somewhat before, but I think within the context of a broader topic. At any rate, could someone explain to me the advantage of piston valves over slide valves, if there is one? I take it there is, since newer locomotives were built with piston valves and older slide valve locomotives were typically rebuilt with piston valves. I take it piston valves came on the scene around the time superpower was emerging; is this correct? Also, were the Franklin rotary cam poppet valves an improvement over traditional piston valves, or did they never get past experimentation? I suspect they came too late in the steam era to have an impact, right or wrong? Looking forward to your insights. Thanks. Seth

Well you can buy the 611 and get it all set up
and run some tests! ;-)
Right now there are two poppet valve engines
active in England. A fair amount of information
is available.
In the long run poppets may have worked out but
then it may be that standard piston valves
are equally effective and cheaper. There actually
are several kinds of valves extant world wide.
Like valve motion, there is more than one way
to skin a cat.

Hi Seth, The earlier steam locomotives that used saturated steam used mostly slide valves which were adequate at the time. When superheated steam began to be used, it was found that the piston valve was more effective, because the pressure of the steam was being distributed in a cylinder type valve, thus keeping the pressure around the valve equal, rather than all on the top which would tend to wear the face more rapidly. A good example of an upgrade were the Boston and Maine Moguls. They were delivered as saturated steam engines, but were later superheated and used Franklin Economy steam chests, which was a way to convert slide valves to a piston type valve.
Leverett Fernald