The units are known as 'diesel electric' because a diesel engine provides the power which is transmitted to the driving wheels by electrical means. The diesel engine drives a main and auxiliary generator and a radiator fan. The current produced by the main generator is supplied to two bogie mounted traction motors, which drive the traction wheels through gearing mounted on the axles.
The English Electric 4SRKT type diesel engine is mounted in an engine room behind the DMBS vehicle driving cab and is rated at 500HP. On the Hampshire and Oxted DEMUs it was uprated to 600HP early on by upgrading the turbo charger. The main and an auxiliary electric generators are mounted directly on the end of engine. The auxiliary generator provides 90 volt current for the control, battery charging, fuel pump, air compressor and lighting throughout the train. There are electric heaters throughout the train, the current being taken from the main generator.
The DMBS has a heavy framed variety of bogies (8' 9" wheel base) with two EE507 type electric motors only fitted on the 'inner end' bogie (to counter-balance the engine weight). The 'outer end' bogie has no motors. The trailer coaches have a lighter framed version of the bogies with a wheel base of 8' 6". These are all of BR MK4 type although in the early days these were MK3A type.
Mounted on the underframe are two 350 gallon fuel tanks, two battery cabinets, air resivoirs, air compressor and two control equipment cabinets.
