A design of steam locomotive exhaust system designed and patented by the famous French steam engineer André Chapelon, using a second-stage nozzle designed by the Finnish engineer Kylälä and known as the Kylälä spreader; thus the name KylChap for this design.
The Kylchap exhaust consists of four stacked nozzles, the first blowing exhaust steam only and known as the primary nozzle, this being a Chapelon design using four triangular jets. This exhausts into the second stage, the Kylälä spreader, which mixes the exhaust steam with some of the smokebox gases; this then exhausts into a third stage, designed by Chapelon, that mixes the resulting steam/smokebox gases mixture with yet more smokebox gases. The four nozzles of this then exhaust into the fourth stage, the classic chimney bell-mouth.
It was Chapelon's theory that such a multi-stage mixing and suction arrangement would be more efficient than the single stage arrangement hitherto popular in steam locomotive draughting, where an exhaust nozzle simply is fired up the middle of the stack bell-mouth. It would also ensure a more even flow through all the firetubes, rather than concentrating the suction on one area.
The efficiency of the Kylchap system relied on careful proportioning of its components, and perfect alignment and concentricity.
Kylchap exhausts are found on many French locomotives and also on a number of British ones. Sir Nigel Gresley of the LNER was a proponent, and the Kylchap exhaust was fitted to a number of his big Pacifics, including the famous Flying Scotsman and the world record holding Mallard. Later LNER Thompson and Peppercorn designed Pacifics also had them, including preserved Peppercorn A2 Blue Peter, as will the recreated Peppercorn A1 Tornado. The last steam express passenger locomotive built in Britain, Duke of Gloucester, was not fitted with a Kylchap exhaust in service, but one was fitted in preservation when it was realized that poor draughting was one of the biggest reasons behind its poor performance in its service days.
Kylchap exhausts were also fitted to some British-built export locomotives, primarily Garratts for Africa, but the only other nation to take them up in quantity was Czechoslovakia, where they seem to have been quite common.
The Kylchap wasn't the only advanced steam locomotive exhaust: another French design, the Lemaître, had some success in France and England; noted engineer L.D. Porta has designed several, the Kylpor, Lempor and Lemprex designs; and several US railroads including the Norfolk & Western used a concentric nozzle known as the waffle iron exhaust.
Portions of this writeup were assisted by reference to http://www.chapelon.net