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A steam locomotive's exhaust system is perhaps the most innovative of all the ideas that underpin the "Stephensonian" concept. It's cleverness derives from its automation whereby the draught that provides the oxygen to generate heat from the fire is automatically governed by the work that the engine is doing. The harder it steams, the greater the exhaust and thus the greater the draught and greater the heat produced.
In his introduction to FDC 12, Wardale describes the exhaust system (as it relates to the 5AT) as follows:
“The exhaust system, dynamically connecting the boiler and cylinders, is thermodynamically the heart of the locomotive and must therefore be as good as possible within practical limitations. That the exhaust entrains sufficient combustion air to sustain the combustion rate necessary to match the steam demand throughout the boiler’s evaporative range is a cardinal point for good performance from any steam locomotive, and that it does this with the minimum of exhaust steam energy is the key to optimum performance. This point is especially important on the 5AT as the locomotive is to operate mostly at high speed with full throttle and low cut-off, giving high heat conversion to mechanical work in the cylinders and therefore limiting the amount of energy available for draughting work in the exhaust steam (it is common for locomotives to steam adequately at long to medium cut-offs but not at short, for this reason), and this is compounded on the 5AT by the use of piston valves with exhaust lap, delaying release.”
In most First Generation Steam (FGS) locomotives, the exhaust system was developed largely through rule of thumb with very little application of scientific or engineer theory. Exhaust steam from the cylinders passes through passages (usually cast into the smokebox saddle) which meet below a blast nozzle through which the steam is ejected upwards through a "petticoat" mounted under the chimney, the petticoat acting as a venturi that creates a partial vacuum inside the smokebox that draws combustion gases from the firebox and mixes it with the exhausting steam. The size of blast nozzle and the geometric arrangement of the nozzle and petticoat were critical in obtaining a good draught and therefore good steaming - a smaller nozzle causing greater exhaust pressure which creates a greater blast and thus a greater vacuum and draught.
A typical FGS arrangement is shown in the cut-away drawing below copied from a 1930s children's book. Click on image to see full-size enlargement.
Relationship between Exhaust Pressure and Power Output
Improving a locomotive's steaming by reducing its blast pipe diameter and thereby increasing its exhaust pressure, has a very serious and measurable effect on a locomotive's power output. Conversely, fitting an improved exhaust system that maintains smokebox vacuum when using of a larger blastpipe orifice, can significantly boost a locomotive's power output or reduce its fuel and water consumption at a given power output.
The benefit derived from an improved exhaust with enlarged blastpipe can readily be seen by inspecting a typical indicator diagram and bearing in mind that the area contained within its curve represents the power delivered by the cylinder. The hypothical (simplified) diagram below, plotted using an Excel spreadsheet, illustrates a 14% increase in power output resulting from a reduction in exhaust pressure from 75 kPa (10.9 psi) down to 25 kPa (3.6 psi).
The "Double Chimney"
During the 1950s, the double chimney was seen in Britain to be a major technological advance bringing significant performance improvements to older designs such as the Kings and Castles from the GWR. The basic aim was to increase the blast pipe nozzle area through the use of two nozzles each exhausting through its own petticoat venturi, thereby reducing cylinder back pressure and increasing power output without loss of draught (or alternatively, increasing draught without loss of power).Unfortunately, it seems that when exhaust system improvements were being planned for post-war locomotives in the UK, too little attention was paid to earlier developments that had taken place in France where in 1926 André Chapelon had developed a much more sophisticated and scientifically-based exhaust system design that he called the "Kylchap" in recognition of a Finnish engineer by the name of Kyösti Kylälä on whose ideas Chapelon had based his design. In fact, in 1938 Gresley fitted Kylchap exhausts to four of his then new A4 locomotives, one of which (Mallard) demonstrated its superiority by achieving the world speed record for steam. Presumably it was the financial constraints of the notoriously cash-starved LNER that deterred the widespread fitting of Kychap exhausts to other members of the class until the 1950s, by when they had become part of BR's fleet. It is to be regretted that Kylchap exhausts were not more widely used since the performance benefits and fuel cost savings that they offered must have outweighed their capital cost many times over.
Modern Developments - The Kylchap, Kylpor and Lempor Exhausts