DMU is an abbreviation for Diesel Multiple Unit. It commonly covers diesel powered units with two types of transmission, mechanical (DMMUs) and hydraulic (DHMUs). Normally differentiated, vehicles with electric transmission are referred to as DEMUs.
Diesel Multiple Units, as the name implies, are designed to run in units (sometimes called sets) normally comprising powered vehicles and trailer cars. There are many different formations possible, although the common arrangement being two powered cars with a trailer car in the middle or one power car and one trailer, although many other formations have been used. These units can be worked in multiple with other units with the same coupling code, up to a maximum of six power cars, depending on the control system design.
The method that is used to take the output from the diesel engine and make the wheels turn.
Mechanical |
uses a fluid flywheel, freewheel, gearbox and final drive coupled by cardan shafts |
Hydraulic |
uses a torque converter and final drive coupled by cardan shafts |
Electric |
uses a generator and traction motors coupled by cables |
There were various types of engines used on mechanical and hydraulic vehicles. All engines were underfloor mounted, and were of the horizontal type. All power cars except Classes 112/113 had two engines.
| AEC | 150hp | 6-cylinder |
| Leyland | 125hp | 6-cylinder |
| " | 150hp | 6-cylinder |
| Leyland Albion | 230hp | 6-cylinder |
| Rolls Royce | 180hp | 6-cylinder |
| " | 238hp | 8-cylinder |
The general layout of the power unit, transmission, etc. for a two engine-gearbox car is shown below. No. 1 engine and transmission is always on the driver's left-hand side when sitting in the driver's seat, and No. 2 engine and transmission on the right-hand side.
Fixed to the output of the engine is a fluid coupling. This flywheel comprised of two halves and was filled with oil. The two halves contain membranes, and the oil is forced outward between these so the input half turns the output.
The output is connected by a cardan shaft to with a freewheel. The freewheel is a protection device should the road wheels rotate the drive train faster than the engine allows, for example when going downhill.
The freewheel / shaft connects to the 4-speed gearbox. The epicyclic gear trains allows ratios from 4.28/1 in 1st gear up to 1/1 (direct drive) in 4th.
The gearbox is connected by another shaft to the final drive, mounted on the inner axle of the bogie. To allow for independent movement of the units, and for the pivoting of the bogies when rounding curves, Hardy Spicer universal couplings are fitted to the shafts. On the final drive the drive is transmitted through one of two bevel pinions, selected according to the desired direction of travel.
The vehicles are controlled by an electro-pnuematic (EP) system. Most of the cab controls work by electricity, which send signals to EP valves throughout the train which control the distribution of compressed air. The air operates the engine throttles, gearboxes and final drives, and auxiliaries such as the horns and wipers.
As DMUs evolved so did the control systems, meaning some of the early types were incompatible with others, so only vehicles with the same coupling code could be used in multiple.
Coupling Codes |
|
Red Triangle |
The first system used for the West Riding hydraulic Derby Lightweights. These were withdrawn in 1964 and the code was reused in the '70s for Class 127s, although they were still wired as blue square. |
Yellow Diamond |
The rest of the Derby Lightweights, the Met-Cam Lightweights and the Class 129s. These had the 'A' type control system. All the EP valves operated down the train without relays, powered by the battery in the car being driven from, which proved troublesome due to voltage drop. |
White Circle |
Used on the Class 126 Swindons only. In truth there were two batches of these cars which were incompatible. The first had 'A' type controls and was based on Yellow Diamond but with relays. The second had 'L' type controls and was based on Blue Square. Modifications were done two allow the two to run together. |
Blue Square |
The most common code. It introduced relays, so that the driving car only sent a signal to the other cars which then used their own battery power to operate the EP valves. |
Orange Star |
Used only on Class 125 hydraulic sets. |
There also some 'sub' variations on these codes over the years. Click here for some more information on coupling codes.
Electric connections between the cars were by jumpers, normally 4 between each car. Other connections between cars were two brake pipes, on most cars an air pipe, and on some a water pipe. Along with the coupling, this meant that between 6 and 8 connections between each car!
All diesel multiple units used the Gresham & Craven twin pipe system. As on coaching stock the brakes operate on 0 - 21" of vacuum, using standard cylinders. But on DMUs the exhauster is belt driven from the engine, meaning that at the time the brakes require to be released, when the vehicles are stopped, the engines are at idling and the exhausters are running slow. The two pipe system incorporates large vacuum reservoirs which store vacuum at up to 29" ready for when required. The second pipe is the 'release pipe' to distribute this. Also associated with the brakes is the AWS.
As well as the exhauster other components are required.
Compressor - these are normally engine mounted and so run constantly. They supply air to operate the throttle motors, gears, final drives, windscreen wipers/washers and horn. Air is stored in reservoirs.
Dynamo/Alternator - belt driven from the cardan shafts, above a certain speed they generate electricity to charge the batteries and run the controls, lights and heaters.
Heaters - these diesel burning heaters each run on a single electric motor and distribute warmed air through ducts to outlets in the vehicles floor. They can be controlled individually, or all at once, and can also feed through unheated air.
Batteries - used to start the engines and operate the controls / heating / lighting when the vehicle is stationary or at slow speed. They can be formed of either lead acid or alkaline cells.
Training Documents
There's some useful training sheets here
which help understand the basic systems.