The earliest steam locomotives were fitted with tall chimneys, intended to provide sufficient draught to burn the fuel needed to raise steam through the natural effect of rising hot air. It was discovered by the end of the 1820s that directing exhaust steam from the cylinders up the chimney to provide a steam blast increased the draught and consequently provided significantly better performance. However, the invention of the steam blast is a matter of controversy. Although it is often credited to George Stephenson (largely due to the writings of Samuel Smiles) and was used on his Rocket (1829), it was by that time already in use by Timothy Hackworth, who had incorporated a steam blast into the Royal George (1827). Even Richard Trevithick was aware of the usefulness of the steam blast; his Pen-y-Daren locomotive of 1804 exhausted steam into the chimney. Sir Goldsworthy Gurney also laid claim to the invention.
Soon after the power of the steam blast was discovered, it became apparent that a smokebox was needed beneath the chimney, to provide a space in which the exhaust gases emerging from the boiler tubes can mix with the steam. This had the added advantage of allowing access to collect the ash drawn through the fire tubes by the draught. The blast pipe, from which steam is admitted, was mounted directly beneath the chimney at the bottom of the smokebox.
The steam blast is largely self-regulating: an increase in the rate of steam consumption by the cylinders increases the steam blast, which increases the draught and thence the temperature of the fire. Modern locomotives are fitted with a blower to release steam directly into the smokebox, for use when a greater draught is needed without a greater volume of steam passing through the cylinders. (An example such situation is when the regulator is closed suddenly, or the train passes through a tunnel.)
Little development of the basic principles of smokebox design until 1908, when the first comprehensive examination of steam-raising performance was carried out by W.F.M. Goss of Purdue University. These principles were adopted on the Great Western Railway by Churchward. A later development was the so-called jumper-top blastpipe which controlled the area of the blastpipe at different steaming rates to maximise efficiency.
The aim of blastpipe modification is to obtain maximum smokebox vacuum with minimum back pressure on the pistons. The simplest modification is a double chimney with twin blastpipes but many other arrangements have been tried. Towards the end of the steam era, the Kylchap exhaust was popular and used on the Nigel Gresley's Mallard. Other designs include Giesl, Lemaître and Lempor blastpipes.
- P.W.B. Semmens and A.J. Goldfinch (2003). How Steam Locomotives Really Work. OUP. ISBN 0-19-860782-2.
- L.T.C. Rolt (1978). George and Robert Stephenson: The Railway Revolution. Pelican. ISBN 0-14-022063-1.