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AC Cars Railbus

Power Train

Extensive use was made of BUT power transmission equipment and controls, many components of which were exchangeable with those on standard DMUs.

The railbus was powered by a B.U.T. AEC 220AJ 150 h.p. six-cylinder horizontal diesel engine. It drove, through a 20" fluid flywheel and freewheel unit, a four-speed epicyclic Self-Changing Gears R14 gearbox. A universal joint shaft then took the drive to the reversing final drive unit mounted centrally on one of the axles. The torque reaction was taken by an arm attached to the underframe which incorporated rubber buffers to absorb shock loads.

Combustion air for the engine was taken through a large air filter and then to the manifold by flexible pipe. The engine cooling water was fed through a large radiator mounted at the side of the underframe. The fan was driven by a universal joint shaft from the engine and enclosed in a cowl attached to the radiator body. A header tank incorporating a filler was mounted above the engine to ensure that no steam locks formed in the cooling system.

A 50-gallon fuel tank was suspended on one end of the underframe and the filler was easily accessible from rail level. The fuel was passed through a filter when leaving the tank.


Engine throttle, gearbox and forward-reverse gear control were all operated through electro-pneumatic valves, the air supply being drawn from the system after the brake reservoirs had reached a pre-determined pressure. The compressor, of 15 cu. ft./ min. capacity was not engine mounted as on most DMUs, but was a larger type driven through vee-belts from the input end of the gearbox. Air was drawn through a filter and anti-freezer, and connections to and from the compressor were flexible to allow for belt adjustment and vibration.

The generator was also driven from the input end of the gearbox, in which position it was unaffected when the car was freewheeling. J Stone (Deptford) Ltd supplied the generator and control panel. The NiFe Batteries Ltd 200 amp/hour alkaline battery formed of 19 cells were suspended at the side of the underframe in a wooden battery box, with a hinged side for servicing.


The railbus was heated by one Smith's combustion unit mounted on the underframe and operated from the same fuel as the power unit. Fresh air was drawn through a filter before heating and then ducted to be omitted at a number of points beneath the seats.

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