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   Common Rail  evaluated  
Whereas in conventional diesel engines injection pressure is generated for each injector individually, a common rail engine stores the fuel under high pressure in a central container ("common rail") and delivers it to the individual injectors on demand. Benefits of common rail injection are reduced noise levels, stronger performance, improved emission control and greater efficiency.
Technology field: Optimisation of traction technologies
close main section General information
  close sub-section Description

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



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


Bosch, Siemens, MTU etc.

close main section General criteria
  close sub-section Status of development: in use
    Diesel engines using common rail injection are available in the power classes needed for railway applications but are still rare in railway fleets. At DB AG, common rail engines are integrated into 400 old shunting locomotives as part of a re-engining programme.
  Time horizon for broad application: 5 - 10 years
    Given the long life-time of rail vehicles, diffusion of common rail engines into rolling stock will be slow. In addition, invitations for tender are usually functional, i.e. they only specify fuel consumption and emission limits but leave it to manufacturers to realise these targets. So in principle there is no bias for common rail technology on the part of railways.
  Expected technological development: dynamic
    cf. Application outside railway sector - Expected technological development outside railway sector
  • High power and torque
  • Emission control
  • Reduced noise emission
  • Low fuel consumption
  Benefits (other than environmental): big

Power and torque

Especially at low speeds common rail systems yield higher engine torques than conventional injection systems.


Reduced noise and vibration.

Reliability and engine life

Constant pressure supply in injection systems puts less stress on material than conventional systems. This leads to longer engine life.

  Barriers: low
    (no details available)
    Success factors:
    (no details available)
  Applicability for railway segments: medium
    Type of traction:  diesel
    Type of transportation:  passenger - main lines, passenger - regional lines, passenger - suburban lines, freight
    In principle, common rail technology can be used on diesel vehicles of all power classes.
    Grade of diffusion into railway markets:
  Diffusion into relevant segment of fleet: < 5%
  Share of newly purchased stock: (no data)
    (no details available)
  Market potential (railways): medium
    Most railways do not plan to pull out of diesel traction in the next decades. So there will be a market for modern diesel technologies in power classes suited for railways even in long-term perspective.

DB re-engining programme

As a part of their re-engining programme German DB AG is currently replacing the engines of 400 old shunting locomotives by common rail engines.

close main section Environmental criteria
  close sub-section Impacts on energy efficiency:
  Energy efficiency potential for single vehicle: > 10%
  Energy efficiency potential throughout fleet: 1 - 2%
    According to manufacturer, the MTU 4000 common rail engine which is fitted for railway applications has a fuel consumption of 195 g/kWh, compared to values between 200 and 250 g/kWh for most diesel engines in service. Improvements in fuel economy to be obtained by common rail technology is highly dependent on point of reference. Automotive manufacturers claim 20% less fuel consumption compared to previous generation of engines (not specified). Compared to other state-of-the-art diesel solutions, improvements will be much smaller. In re-engining programmes for old locomotives, common rail technology may improve fuel economy by up to 30 % compared to 30 year old engines.
  Other environmental impacts: positive
    Reduced emission.
close main section Economic criteria
  close sub-section Vehicle - fix costs: low
    Depends on power class and engine charcteristics. In general common rail is a mature technology which is comparable in price to other engines (at least in the automotive sector).
  Vehicle - running costs: significant reduction

The reduction of running costs depend on point of reference but is generally considerable. If old engines are replaced, both fuel consumption and maintenance efforts are substantially reduced.

The total diesel-engine-related costs are typically composed as follows (Source: Günther 1998):

  • Costs for fuel and lubricants: 65 %
  • Maintanace costs: 15 %
  • Initial investment: 20 %

This gives an idea of the economic relevance of fuel consumption.

  Infrastructure - fix costs: none
    (no details available)
  Infrastructure - running costs: unchanged
    (no details available)
  Scale effects: medium
    Common rail technology in high power classes is still rare. Therefore certain scale effects are to be expected if appreciable numbers are ordered.
  Amortisation: not applicable
    Payback times obviously depend on application context. Re-engining measures typically pay off in about 5 years.
close main section Application outside railway sector
  close sub-section Status of development outside railway sector: in use
    Major automotive manufacturers offer vehicles equipped with common rail technology (e.g. Mercedes Vito CDI).
  Time horizon for broad application outside railway sector: now
    (no details available)
  Expected technological development outside railway sector: dynamic
    There is intensive R&D in industry on the optimisation of the common rail injection system. An example for the development potential of the technology is the development of piezoelectronic actuators at Siemens VDO. As these switching elements operate much faster than conventional solenoid valves, it will be possible in the future to split the fuel volume into several individual injections: Two pre-injections with very small volumes of fuel are followed by the main injection and, if necessary, two smaller post-injections. While the pre-injections serve primarily to build up the pressure in the combustion chamber evenly and thus reduce the noise of combustion, the post-injections are provided for post-treatment of the exhaust gas. Fuel economy will also be increased. Piezoelectronic actuators exploit the behavior of piezoelectric crystals. If an electric charge is connected to such a crystal, the crystal lattice reacts within a few milliseconds by expanding. When discharged the material returns to its original size.
  Market potential outside railway sector: high
    By 2004, 70% of new diesel vehicles will incorporate Common Rail injection systems and 5 million such systems will already be on the road, according to Delphi Diesel Aftermarket.
close main section Overall rating
  close sub-section Overall potential: very promising
  Time horizon: mid-term
    Common rail is state-of-the-art diesel technology. A diffusion into railways is to be expected without external efforts to be made. Common rail engines should also be considered for re-egineering programmes.
References / Links:  Günther 1998;
Related projects:
Contact persons:
 date created: 2002-10-09
© UIC - International Union of Railways 2003
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