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High-pressure steam locomotive
A high-pressure steam locomotive is a steam locomotive with a boiler that operates at pressures well above what would be considered normal. In the later years of steam, boiler pressures were typically . High-pressure locomotives can be considered to start at , when special construction techniques become necessary, but some had boilers that operated at over .
==The reason for high pressure==
Maximising the efficiency of a heat engine depends fundamentally upon getting the temperature at which heat is accepted (i.e. raising steam in the boiler) as far as possible from the temperature at which it is rejected (i.e. the steam when it leaves the cylinder). This was quantified by Nicolas Léonard Sadi Carnot.
There are two options: raise the acceptance temperature or lower the rejection temperature. For a steam engine, the former means raising steam at higher pressure and temperature, which is in engineering terms fairly straightforward. The latter means bigger cylinders to allow the exhaust steam to expand further - and going this direction is limited by the loading gauge - and possibly condensing the exhaust to further lower the rejection temperature. This tends to be self-defeating because of frictional losses in the greatly increased volumes of exhaust steam to be handled.
Thus it has often been considered that high-pressure is the way to go to improve locomotive fuel efficiency. However, experiments in this direction were always defeated by much increased purchase and maintenance costs. A simpler way to increase the acceptance temperature is to use a modest steam pressure and a superheater.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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