|Stoker in bottom of C&O 1604 tender
Around 1900 large steam locomotives reached the limit that a fireman could fire by hand. This was somewhere around 5,000 pounds of coal per hour. A mechanical substitute for the fireman and shovel had to be found. The tasks that a stoker had to perform were complicated. It had to bring chunks of coal of various sized from the tender through a flexible joint to the engine and into a very hot firebox. It then had to enable the coal to be spread evenly across the surface of the grates in the firebox. A number of methods were tried. The successful design for a mechanical stoker that emerged was an augur screw that lay in a trough at the bottom of the tender that brought coal through a tube to the firebox and up into a funnel-shaped container where jests of steam were used to disperse the coal throughout the firebox. By the 1920s most medium to large steam locomotives had stokers.
The pieces of coal used in mechanical stokers needed to be smaller than the pieces of coal used for hand firing. As a result, most coal docks had two chutes to each track. Stoker coal was about 1"-2" pieces, while hand fired coal was larger, from about 4" to 6".
Coal stoker augers (feed screws) would often jam on a chunk of coal that would not feed through the tube. The fireman would reverse the stoker engine, which would usually free the jam. Sometimes the chunk that jammed the auger would refuse to break, and the fireman would have to run the stoker engine in reverse until the large chunk was pushed to the rear of the coal bunker. Hopefully it would stay there until the bunker was emptied. Having stoker coal crushed to a smaller size than hand-fire coal was supposed to prevent this, but large chunks slipping through was not uncommon.
The largest coal-burning locomotives before the invention of the power stoker were around the 2-8-2 and 4-8-2 size such as Southern 4501. These would require two firemen when working hard.