Facebook Page
TUSP Exhaust Drawings

TUSP Title Image

Steam Locomotion in the 21st Century

The Recent History of Steam Locomotive Development

Steam Locomotive Exhaust Drawings

Updated 4 February 2022

One of the most important components of virtually all steam locomotives is the exhaust system. Early steam locomotive builders such as George Stephenson discovered the principal upon which virtually all steam locomotives built since have used. By happy accident, they found that directing the "waste" steam exhausted from the cylinders at the end of each stroke up the boiler's chimney greatly increased the air flow through the fire. This caused the fire to burn hotter and faster, allowing a locomotive boiler to generate dramatically more steam than stationary boilers of similar size.

As locomotive design progressed, builders realized that the proportions and configurations of the chimney and the exhaust pipe had a significant effect on how well the exhaust system worked. A major concern was the effect of backpressure on the performance of the locomotive's cylinders. Backpressure was a measure of the restriction of the exhaust steam flow from the cylinders at the end of each stroke. The locomotive exhaust could be built with a small exhaust nozzle, which caused the exhaust steam to jet up the stack at high velocity, which would produce excellent gas flow through the boiler (draft). However, this small nozzle would impede the flow of the exhaust steam from the cylinders, causing excessive backpressure. This backpressure saps power from the locomotive's cylinders, reducing the locomotive's performance. Good draft increased the locomotive's steaming rate making more steam available to generate power, but high backpressure could cancel this out.

This was the chief task of locomotive exhaust designers: how to produce the maximum draft while producing the minimum back-pressure on the cylinders.

Until well into the 20th century, the physics of gas flow were not understood and the theories and laws which could be used to design exhaust systems did not exist. Consequently, for much of their existence, locomotive exhaust systems were developed through a process of trial-and-error. Chapelon, Porta, and others strove to apply principals of engineering and physics to exhaust system design.

early locomotive exhaust This exhaust system, from a locomotive in New Zealand, is similar to that of many 19th Century locomotive exhaust systems. The steam nozzle is at the bottom, and it exhausts through a "petticoat" and finally up through the main chimney (the separate petticoat was a 20th century development). This chimney includes a spark arresting apparatus, which forces the exhaust gases through several turns in order to make sparks and cinders drop out into the bottom of the spark arrestor.

Master Mechanic Exhaust This drawing shows a typical locomotive exhaust system from a U.S. steam locomotive built in the 20th century. The drawing shows a cross-section of the smoke box at the front of a locomotive boiler (the boiler would be to the left of the view). The steam nozzle is at the bottom of the smoke box, exhausting its steam jet up the stack which is at the top. The drawing also illustrates the empirical design formulas which were used to size the components. After building hundreds of exhaust systems, designers decided that the proportions of the components listed above would work best for most locomotives under most conditions.

Kylchap Exhaust This drawing shows an early Kylchap exhaust. Chapelon used the exhaust splitter developed by Finish engineer Kylala, which divides the exhaust stream into four parts. The Kylchap draws in gases from more than one level of the smokebox, which Chapelon believed to be an important feature in providing an even gas flow through the many tubes of the boiler. Later Kylchap exhausts used two levels of entrainment and two or even three stacks.

Kylala exhaust splitter The Kylala exhaust splitter was an important part of the Kylchap exhaust system.

N&W Y-6 exhaust In the U.S., several railways developed improved exhaust systems using annular exhaust nozzles and larger stacks.

N&W waffle iron exhaust nozzle Plan view of the so-called "waffle iron" exhaust nozzle used in the exhaust system shown above.

Lemaitre Exhaust The Lemaitre Exhaust was developed by Lemaitre, a mechanical engineer from the NORD Belge in France. The Lemaitre featured 5 nozzles in a circular pattern exhausting up a large diameter stack, with a variable area nozzle exhausting up the center.

British engineer Bullied used a simplified version of this on the locomotives he built, omitting the variable area center nozzle.

Giesl Ejector

In the late 1940's, Dr. Adolph Giesl-Gieslingen developed a new exhaust design called the Giesl Ejector. He patented this device and it was applied to thousands of steam locomotives all over the world. The Giesl Ejector featured a series of small in-line nozzles exhausting up a thin, oblong chimney.

This system was patented and produced under license, and was applied to many steam locomotives around the world in the 1950's, and 1960's, giving improved performance and fuel economy. Unlicensed copies of these exhausts were also produced and applied to some locomotives in China.

drawing courtesy Stuart Kean

Lempor Ejector This diagram shows the Lempor (Lemaitre-Porta) exhaust developed by Porta and applied to many locomotives. Porta also developed an extensive theory describing the performance and design of these exhaust systems.

Lempor exhaust
                cutaway Here is a 3-D rendering of a Lempor ejector.

The picture shows the full exhaust system with a section taken out of the stack to show the inside. The system shown uses the revised design developed by Porta with a non-tapered mixing chamber (the lower portion of the stack) and a 10 degree angle between the nozzles, as experimentally determined to be optimum by Wardale.

NZR pepperpot exhaust nozzle One of the most recent new exhaust systems was developed for the Garratt locomotives of the Rhodesian Railways. These were known as "pepperpot" exhausts and were later fitted to many Garratts which were overhauled and restored to service in the early 1980's in the new country of Zimbabwe. This nozzle arrangement was used in combination with a larger chimney, and was developed as alternative to Giesl exhausts experimentally fitted to Garratts in the 1960's. The pepperpot exhaust was preferred because (1) it was locally developed (the Giesl was proprietary and royalties had to be paid for its use) and (2) the Pepperpot was less susceptible to unauthorized tampering which tended to cause problems with the Giesls in normal service.

Donna Teresa
                Christina Railway Improved Lempor For the last years of his life, Porta worked on the development of a Lemprex exhaust system, a further advancement on the Lempor. The Lemprex incorporates features to maximize its performance within the limited height available on a steam locomotive. This photo shows one of the last exhaust systems installed under Porta's supervision, on the Donna Teresa Christina Railway in Brazil. This exhaust incorporates at least some features intended for the Lemprex, namely the skewed exhaust and blower nozzles which were to give a swirling motion to the exhaust gases as they passed out of the stack for improved mixing and performance.

                exhaust arrangement Some of the most recent exhaust development work was carried out by Dr. Jos Koopmans of the Netherlands, who wrote his doctoral thesis on exhaust systems some years ago, later published as the book "The Fire Burns Much Better".

In early 2018, Waterloo Central Railway 0-6-0 No. 9, formerly Essex Terminal No. 9, was fitted with a multiple jet exhaust system designed by Dr. Jos Koopmans and fabricated by steam enthusiast Michael Guy.  This system made use of the existing, relatively short chimney, and omitted the two separate petticoats provided in the original exhaust system. A new extension raises the nozzle assembly so that the nozzles are correctly aligned with the existing stack. Performance with this system has been significantly increased over the original exhaust.

In 2016, Dr. Koopmans installed a similar system in the UK on King Edward II 4-6-0 No. 6023. This system successfully restored the locomotive's performance after the stack (chimney) height had to be reduced by 4 inches to meet new clearance requirements.

Photo courtesy of Michael Guy.

Back to Top

Return to the Exhaust Systems Page