The little bit of reading I've done on the Rio Turbio engines, one huge benefit was the abiltiy to burn inferior quality coal with practically no clinker formation, due to the cooler firebed temperature. Also, the great reduction of abrassive material drawn through the tubes and blasted out the stack, greatly extended the life of boiler componants. Since boiler maintenance is a huge part of steam locomotive cost, I'm thinking this may be enough to promote using a GPCS.
As you've stated, so much of the improvement in efficiency could be attributed to better steam flow, improved valve events and reduced back pressure.
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 combustion of the hydrogen and carbon monoxide gases generated by the thick fire bed and the oxygen-starved fire.
OK, The Red Devil added a feedwater heater, which is good for what, a 7 percent saving on the fuel needed to raise a certain amount of steam? But the 30 percent water savings, by whatever means, results in at that much reduction in the combustion needs, which would result in reduced carbon carryover with a conventional firebox?
Perhaps what the GPCS contributed was reduced coal particle loss at the upper end of combustion and power output? Reduced smoke?
But much of Wardale's book expresses his frustration that the GPCS, for a multitude of reasons, didn't quite do what it was supposed to do. Wardale also explains that getting good GPCS fires took considerable care on the part of the crews -- when "it worked", it was fantastic but getting it "to work" required many conditions to break the right way. Wardale concludes the book with the notion that pulverized coal combustion (there have been recent discussions around here on the drawbacks of that) was a better solution.
Apart from the many Web sites touting GPCS, what work has been done since Red Devil?
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
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