Diesel Multiple Unit

Overview

A diesel multiple unit is a passenger train that carries its own power and needs no separate locomotive. Each motored car has an underfloor diesel Underfloor Diesel Power Pack driving a Hydrodynamic Transmission that turns the wheels of the Powered Bogie through a cardan shaft and axle gearbox. Because the heavy equipment hangs beneath the floor, the whole length of the car stays available for passengers, and trains can be made up of as many or as few cars as the timetable needs. This makes the type the standard choice for non-electrified regional and suburban lines where stringing overhead wire would not be justified.

The unit is built around the Carbody Shell, carried on a Powered Bogie at one end and a Trailer Bogie at the other. Fuel comes from the Fuel System, the engine is kept in range by the Engine Cooling System, and exhaust passes through the Exhaust & Aftertreatment line to meet emissions limits. The Brake System, Saloon HVAC Unit packs and Sliding Door Pair units serve the passengers, while the Train Control System supervises the train and the End Coupler & Gangway joins cars together. Compared with an electric multiple unit, the difference is the whole traction chain: there is no pantograph and no main transformer, only a diesel engine, a gearbox, fuel and a cooling pack.

Diesel power pack

The Underfloor Diesel Power Pack is a horizontal turbocharged six-cylinder diesel slung under the floor, typically rated between 290 and 390 kW. The Crankshaft inside the Engine Block (inline-6 crankcase) is driven by six Piston Assembly units, each breathing through a Cylinder Head. A single Turbocharger recovers exhaust energy to boost intake air, and the Fuel Injection System system feeds it through a high-pressure Common Rail. The Lube Oil System keeps bearings supplied, and the whole engine sits in a Engine Cradle / AV Mount on anti-vibration mounts so that combustion noise and shake do not reach the saloon floor directly. A Starter / Charging Alternator cranks the engine and charges the battery.

Transmission and drive

Power from the engine flows into the Hydrodynamic Transmission, a hydrodynamic gearbox. Its Hydrodynamic Torque Converter uses an oil-filled Converter Impeller and Converter Turbine Wheel to multiply torque smoothly without a mechanical clutch, which suits the frequent starts of stopping service. The Reversing / Output Gear Stage selects ratio and direction through hydraulic Hydraulic Shift Clutch elements, and the output turns the Cardan Drive Shaft running to the Final Drive Axle Gearbox axle gearbox on the powered bogie. Transmission oil runs hot and is held in range by the Transmission Oil Cooler, with the Transmission Control Unit unit sequencing the shifts. Some fleets use electric transmission instead, where the engine drives an alternator and traction motors, but the hydrodynamic layout is the most common on regional stock. The torque converter is forgiving of the constant stop-start duty of suburban service because it has no friction clutch to wear, and its Converter Reactor (stator) Wheel redirects oil flow to multiply torque most strongly at low output speed, which is exactly when a train pulling away from a platform needs it. As speed rises and the converter becomes less efficient, the Reversing / Output Gear Stage locks up or shifts to a higher ratio so that the engine is not left spinning fast against a stalled load.

Fuel, cooling and emissions

The Fuel System stores diesel in an underslung Underslung Fuel Tank with internal Tank Baffle plates, and a Fuel Lift Pump feeds the engine through a Fuel Water Separator. The Engine Cooling System carries engine and charge-air heat to a Radiator pack, moved by Radiator Cooling Fan units that a Coolant Thermostat switches on as temperature rises. Exhaust gas leaves through the Exhaust & Aftertreatment line, where a Diesel Particulate Filter traps soot and a SCR Catalyst fed by the DEF (urea) Dosing Unit urea dosing unit cuts nitrogen oxides to meet current limits.

Running gear and braking

The Powered Bogie takes drive through the Final Drive Axle Gearbox onto its Wheelset assemblies, which run in Axlebox bearings sprung by Primary Suspension Coil coils. The Air Spring (secondary) secondary suspension supports the carbody and holds floor height steady through the Height Leveling Valve. The Trailer Bogie is the same without the gearbox. Braking is pneumatic: the Electronic Brake Control Unit commands Disc Brake Caliper units fed by the Air Compressor and Air Reservoir tanks, and a Hydrodynamic Retarder in the transmission absorbs energy on long descents without wearing the friction pads.

Interior, climate and control

The Passenger Saloon is fitted with Seat Assembly units, Grab Pole supports and Passenger Information Display screens. The Saloon HVAC Unit packs on the roof heat and cool the cabin through a HVAC Compressor and HVAC Blower units. Passengers board through the Sliding Door Pair, whose Door Drive Mechanism runs on a Servo Motor and Drive Belt. The Train Control System ties it together: a Central Control Computer talks to Remote I/O Module units over Train Ethernet Switch hardware, managing engine, transmission, braking and doors and presenting status to the driver. Across the trainset the End Coupler & Gangway passes control signals so that several units can be driven together from one cab. This ability to couple and split units quickly is one of the main operational advantages of the type: a long train can run in the peak and divide into shorter portions for branches or off-peak running without any shunting locomotive. Diesel multiple units do carry penalties that electric stock avoids, including the weight and noise of the Underfloor Diesel Power Pack, the fuel and maintenance cost of a diesel engine, and the emissions handled by the Exhaust & Aftertreatment line, which is why operators convert busy routes to overhead electrification once traffic justifies it. On lines that stay diesel, newer variants add battery or hydrogen hybrid power packs that let the engine shut down in stations and tunnels, but the running gear, Saloon HVAC Unit and Sliding Door Pair patterns stay the same as the conventional design.