The Gas Producer Combustion System (GPCS) is touted on many sites around the Web as “the solution” for burning coal efficiently and cleanly in a steam locomotive.
Apart from Livio Dante Porta’s modified narrowgauge locomotives on the Rio Turbio line, did it ever live up to its promise?
David Wardale’s book “The Red Devil” is replete with charts and tables on the South African Railways 3450 locomotive modified to incorporate a stoker-fed GPCS firebox, but the numbers most often quoted are “28% coal savings, 30%water savings” over the baseline 25NC locomotive.
Let’s look at those numbers. The 30% water saving is impressive, but the “water rate” has little to do with the efficiency of combustion. Rather, it speaks to the efficiency at which steam is used once the boiler raises that steam. One can attribute this efficiency to a somewhat enlarged superheater, increasing the steam temperature and reducing somewhat the pressure drop through the superheater, to the enlarged steam pipes feeding the cylinders, to the enlarged “steam chests” between the pipes and the valves, these storage volumes smoothing out the pressure drops from valve opening, to changes to valve “lap” from new valve geometry and “lead” for modifications to the Waelscharts valve gear, and especially to the low-back pressure exhaust system.
But were the GPCS doing its job, one would expect a proportionately larger coal savings than 30% accounting for the reduction in “carbon carryover” the GPCS is supposed to accomplish. This is explained at great length in Wardale’s book, but a conventional firebox wastes much of the coal as unburnt because the large amount of air flowing upwards through the firebed to support combustion lifts small particles. The GPCS restricts “primary air” through the firebed to prevent this, and introduces “secondary air” above the firebed to finish comb