of the large ejector when running. The widely used Super-Dreadnought ejector combined a large ejector with two small ejectors in one. If needed, two of the smaller nozzles could produce more vacuum, yet were more efficient in their use of steam then a single larger nozzle. The Great Western Railway was noted for its idiosyncrasies, including the use of a higher brake vacuum than other lines. To maintain this without excessive steam consumption in an ejector, they also favoured the use of a crosshead-driven mechanical pump. Exhausters Diesel locomotives were introduced at a time when vacuum brakes were still widespread. Ejectors are not practical and so mechanical pumps or 'exhausters' are used instead. These are a small rotary vane pump, similar to some forms of vacuum pump. The body is a cylindrical metal casting with a cylindrical rotor within it, but the two axes are offset. The rotor contains a number of sliding vanes, typically six. As the rotor is rotated, the vanes are held against the walls of the cylindrical body. Entry and exit ports at the top and bottom of the cylinder, where the rotor is furthest from and nearest to the wall, provide a vacuum pumping effect. The vanes are held against the cylinder by an internal cam ring or by springs. They are lubricated by an oil feed into the exhauster. As the exhauster is oil-lubricated, the exhaust air is full of oil droplets and so passes through an oil separator before being exhausted to the atmosphere. A simple check valve on the inlet prevents backflow leakage, if the exhauster stops. Compared to the compressor of an air brake, the exhauster is a simpler and more reliable device. It has no valves, so fewer moving parts. There is no pressure control, as vacuum pumping is self-limiting. The exhauster runs cooler as there is little compression of the pumped air. The pump seals are simpler, owing to the lower pressure, and there are no piston rings with a risk of sticking. Exhausters are typically engine-driven, and run continuously. If there are two engines in a locomotive or railcar, two exhausters are usually fitted. They are cheap devices, extra pumping capacity can help to release the brakes more quickly and their redundancy reduces the risk of a failure causing a failed train. On electric locomotives, the exhausters are electrically driven. Some of the first diesel-engined buses and coaches