Compressed on L'Ouest- The Ouest Air Locomotives.
A new underground railway station by the side of the Seine, called Invalides had just been built, to handle lines serving Britanny and Normandy and deal with congestion in Montparnasse. These were normally worked by electric locomotives that drew 650V DC from a raised third rail. This voltage was already widely used on French tramways. However as the Company had concluded agreements with the City of Paris that excluded the use of steam engines inside the new station, other methods were needed to rescue electric trains stranded by loss of traction current- hence the compressed-air locomotives.
Above: A L'Ouest air locomotive.
These locomotives had two four-wheel bogies. They could be driven from either end. Air was stored in 33 air tanks (presumably cylinders) in the central part of the locomotive. There was an intermediate reservoir working at 20 kg/cm2. (284 psi) Each bogie had two cylinders in compound, the HP cylinder working at 20 kg/cm2, while the LP cylinder worked at 10 kg/cm2. (142 psi) Air charging attachments were fitted at each end; possibly these are the otherwise unexplained structures on the roof. There appear to have been bouillottes at each end to reheat the stored air before use, increasing power output and preventing freezing as the air expanded. For more on bouillottes and compressed-air vehicles in general see Compressed Air Propulsion. The locos were fitted with both Westinghouse and manual brakes.
Their performance appears to have been inadequate as they were replaced by electric locomotives in 1910. Presumably these were battery-operated, as otherwise they would not have been much use for rescue purposes when the power supply failed.
THE COMPRESSED-AIR SUPPLY
Compressed air was generated in a plant near to the esplanade of Invalides station, with three independant groups of machinery, each group comprising a three-phase electric motor energised at 5000V, supplied from a power-station at Issy by the general supply circuit, and a vertical Mekarski compressor, turning at 100 rpm and absorbing 200 HP. These took compressed air at 6kg/cm2 (85 psi) bought from the Compagnie Parisienne*, and raised it to 100 kg/cm2. (1422 psi) The compressors had four cylinders; and each compressor could supply 2000 kg of air per hour. (Is that right? It seems like an awful lot of air) The air at final pressure was stored in six groups of 20 tanks, storage being essential as compressed air to run the locomotives would be needed when the electricity supply failed. Adjacent were special facilities for the bouillottes; presumably for filling them with hot water.
*The Compagnie Parisienne distributed compressed air for power purposes through a network of pipes under the Paris streets.
From the French journal Nature for 1901, 2nd issue.