In conventional diesel engines injection pressure is generated for each injector individually. A direct injection engine based on the common rail principle separates the two functions pressure generation and injection by first storing the fuel under high pressure in a central container ("common rail") and delivering it to the individual injection valves (injectors) only on demand. This way an injection pressure of up to 1,500 bar (in the future up to 1,600 bar) is available at all times, even at low engine speeds. The high pressure produces a very fine atomisation of the fuel leading to better and cleaner combustion. Moreover, the fuel supply is not dependent on the engine revolutions but can be optimised independently. The time and duration of injection is not fixed (as in older conventional engines) but can be chosen independently for every operation point in order to optimise combustion and emissions. In modern common rail systems injection is split into several individual injections: pre-injection, main injection and post-injection.

Benefits of the common rail principle compared to conventional engines are lower engine noise levels, stronger performance and greater combustion efficiency leading to lower emissions and enhanced fuel economy.

Figure 1: The common rail principle

Source: www.kfztech.de

Four basic components of a common rail system are:

• A high pressure pump with pressure regulator and inlet metering valve.
• A rail which contains a pressurised reserve of fuel.
• Injectors which inject precise amounts of fuel into the combustion chamber as required.
• A diesel control unit – the ‘brain’ of the system, which precisely controls injector flow and timing as well as rail pressure while continuously monitoring the operating conditions of the engine.

As an example for a common rail engine for railway applications the following table gives the technical data of the MTU 4000 engine.

Technical data of the MTU 4000 common rail engine
Power range	760-2720 kW
Power weight	2,7 – 3,5 kg/kW
Power per volume	250 – 310 kW/m3
Maximum revs per min	2100/min
Specific fuel consumption	195 g / kWh

Source: Günther 1998

Manufacturers

Bosch, Siemens, MTU etc.