Over-fire air jets ?

[Anonymous]

My understanding is that they weren't intended to be used when the locomotive was producing power; the normal draft was expected to do the job. They were intended to be used in yards and engine terminals to reduce smoke output when the locomotive was sitting still or operating with minimal throttle. The air introduced over the firebed was intended to mix with the products of combustion to burn the carbon more thoroughly, thus reducing the amount of unburned carbon going through the flues.

They worked pretty well, too.

Old Timer

[Overmod]

If you want to see some up close, come to Collierville, Tennessee. The rebuilt Frisco 2-8-2 on display there has them.

There were versions of these jets that used brake air -- as indicated, these induced problems with cold air. The steam jets mentioned in the C&O article required external air feeds so they could entrain air and preheat it -- it makes some sense to inject steam to improve combustion and heat transfer, but generally the place you want to do it is under the grate (as in GPCS) rather than over the fire (nominally for NOx reduction -- it works better in boiler geometries that are different from 'traditional' locomotive firetube boilers, cf. the beloved TurboFire XL.)

All overfire systems involved high-pressure, with nozzles, and hence amazing amounts of noise. The "hats" or "cans" on the guns didn't do that much to minimize the noise, I suspect. Probably didn't get as much turbulent mixing and complete combustion done in the gas coming from the center of the fire, either -- I've never seen a flow diagram of the likely effects of these overfire jets performed in combustion software, but I'd expect that much of the injected or entrained air would remain fairly close to the sidesheets and wrapper... right where there's already an issue with early combustion quench...

[nanaimo73]

During 1948 ALCo built their last steam engines- 7 Berkshires for the P&LE. On the side of the firebox were seven protrusions which were over-fire air jets. These were supposed to improve fuel combustion. How did they work, and were they controlled by the fireman ? Lima's last steamer was the 779 built for Nickel Plate a year later and I don't see them there. Lima proposed to build some 4-8-6s for C&O during 1949 which would have had them. Did many locomotives have them ?

www.steamlocomotive.com/berkshire/ple.shtml