Common Rail Fuel Injection

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  • Common Rail Fuel Injection

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    o Common Rail Fuel Injection System Components

    o Common Rail Injection System Pressure Control

    Abstract: In the common rail system, fuel is distributed to the injectors from a high pressure accumulator,

    called the rail. The rail is fed by a high pressure fuel pump. The pressure in the rail, as well as the start

    and end of the signal that activates the injector for each cylinder are electronically controlled. Advantages

    of the common rail system include flexibility in controlling both the injection timing and injection rate.

    Introduction

    Common Rail Concept

    Common Rail System Dynamics

    Control of Common Rail System

    Common Rail Injection Systems for Large Engines

    Introduction

    The merits of the common rail fuel injection system architecture have been

    recognized since the development of the diesel engine. Early researchers,

    including Rudolf Diesel, worked with fuel systems that contained some of the

    essential features of modern common rail diesel fuel injection systems. For

    example, in 1913, a patent for a common rail fuel injection system with

    mechanically actuated injectors was issued to Vickers Ltd. of Great

    Britain [McKechnie 1913]. Around the same time, another patent was issued in the

    United States to Thomas Gaff for a fuel system for a direct cylinder injection

    spark ignition engine using electrically actuated solenoid valves. The fuel was

  • metered by controlling the length of time the valves were open [Gaff 1913]. The

    idea of using an electrically actuated injection valve on a diesel engine with a

    common rail fuel system was developed by Brooks Walker and Harry Kennedy in

    the late 1920s and applied to a diesel engine by Atlas-Imperial Diesel Engine

    Company of California in the early 1930s [Walker 1933][DeLuca 2010][Knecht

    2004][Aird 2001].

    Work on modern day common rail fuel injection systems was pioneered in the

    1960s by the Societe des Procedes Modernes DInjection (SOPROMI) [Huber

    1969]. However, it would still take 2-3 decades before regulatory pressure would

    spur further development and the technology would mature to be commercially

    viable. The SOPROMI technology was evaluated by CAV Ltd. in the early 1970s

    and was found to provide little benefit over existing P-L-N systems in use at the

    time. Considerable work was still required to improve the precision and

    capability of solenoid actuators.

    Further development of diesel common rail systems began in earnest in the

    1980s. By 1985, Industrieverband Fahrzeugbau (IFA) of the former East

    Germany developed a common rail injection system for their W50 truck, but the

    prototype never entered series production and the project was abandoned a

    couple of years later [Sachsisches Industriemuseum 2010]. Around the same time,

    General Motors was also developing a common rail system for application to

    their light-duty IDI engines [Williams 1982]. However, with the cancellation of

    their light-duty diesel program in the mid-1980s, further development was

    stopped.

    A few years later, in the late 1980s and early 1990s, a number of development

    projects were initiated by engine OEMs and later taken up by fuel injection

    equipment manufacturers:

  • Nippondenso further developed a common rail system for commercial

    vehicles [Miyaki 1988][Miyaki 1991] that they acquired from Renault and that

    was introduced into production in 1995 in Hino Rising Ranger trucks.

    In 1993, Boschperhaps due to some pressure by Daimler-Benzacquired

    the UNIJET technology initially developed by the efforts of Fiat and Elasis

    (a Fiat subsidiary) for further development and production [Stumpp 1996].

    Boschs passenger car common rail system was introduced into production

    in 1997 for the 1998 model year Alfa Romeo 156 [Jost 1998] and C-Class

    Mercedes-Benz.

    Shortly afterward, Lucas announced common rail contracts with Ford,

    Renault and Kia with production starting in 2000.

    In 2003, Fiat introduced a next generation common rail system capable of

    3-5 injections/engine cycle for the Multijet Euro 4 engine.

    Further information on the history of common rail systems can be found in the

    literature [Knecht 2004][Petruzzelli 2013].

    The aim of these development programs started in the late 1980s/early 1990s

    was to develop a fuel system for the future diesel powered passenger car. Early on

    in these efforts, it was apparent that future diesel cars would utilize a direct

    injection combustion system due to the clear advantage in fuel economy and

    power density relative to the then prevalent indirect injection combustion

    system. The objectives of the developments included driving comfort comparable

    to that of gasoline fueled cars, compliance with future emission limits and

    improved fuel economy. Three groups of fuel system architectures were under

    consideration: (1) an electronically controlled distributor pump, (2) an

    electronically controlled unit injector (EUI or pump-nozzle unit) and (3) a

    common rail (CR) injection system. While the efforts around each of these

    approaches lead to commercial fuel systems for production vehicles, the common

    rail system provided a number of advantages and would eventually come to

  • dominate as the primary fuel system used in light-duty vehicles. These

    advantages included:

    Fuel pressure independent of engine speed and load conditions. This

    allows for flexibility in controlling both the fuel injection quantity and

    injection timing and enables better spray penetration and mixing even at

    low engine speeds and loads. This feature differentiates the common rail

    system from other injection systems, where injection pressure increases

    with engine speed, as illustrated in Figure 1 [Hawley 1998]. This

    characteristic also allows engines to produce higher torque at low engine

    speedespecially if a variable geometry turbocharger (VGT) is used. It

    should be noted that while common rail systems could operate with

    maximum rail pressure held constant over a wide range of engine speeds

    and loads, this is rarely done. As is discussed elsewhere, fuel pressure in

    common rail systems can be controlled as a function of engine speed and

    load to optimize emissions and performance while ensuring engine

    durability is not compromised.

    Figure 1. Relationship between Injection Pressure and Engine Speed

    in Different Injection Systems

    Lower fuel pump peak torque requirements. As high speed direct injection

    (HSDI) engines developed, more of the energy to mix the air with fuel

    came from the fuel spray momentum as opposed to the swirl mechanisms

    employed in older, IDI combustion systems. Only high pressure fuel

    injection systems were able to provide the mixing energy and good spray

    preparation needed for low PM and HC emissions. To generate the energy

    required to inject the fuel in approximately 1 millisecond, the conventional

    distributor pump would have to provide nearly 1 kW of hydraulic power in

    four (in a 4-cylinder engine) 1 ms bursts per pump revolution, thus placing

    considerable strain on the drive shaft [Breitbach 2002]. One of the reasons

  • behind the trend toward common rail systems was to minimize the

    maximum pump torque requirement. While the power and average torque

    requirements of the common rail pump were similar, high pressure fuel

    delivery is to an accumulator and thus the peak flow rate (and peak torque

    required to drive the pump) does not have to coincide with the injection

    event as is the case with the distributor pump. Pump discharge flow can be

    spread out over a longer portion of the engine cycle to keep pump torque

    demand more even.

    Improved noise quality. DI engines are characterized by higher peak

    combustion pressures and, thus, by higher noise than IDI engines. It was

    found that improved noise and low NOx emissions were best achieved by

    introducing pilot injection(s). This was most easily realized in the common

    rail system, which was capable of stable deliveries of small pilot fuel

    quantities over the entire load/speed range of the engine.

    References

    Aird, F., 2001. Bosch Fuel Injection Systems, HP Books, ISBN: 1-55788-365-3

    Breitbach, H., 2002. Fuel Injection Systems Overview, Delphi Corporation, March 2002

    DeLuca, F., 2010. History of fuel injection, internet, retrieved May 25,

    2010, http://www.disa.it/pdf/01HystoryOfDieselFuelInj.pdf

    Gaff, T.T., 1913. Explosion-engine, US Patent

    1,059,604, http://www.google.com/patents/US1059604

    Hawley, J.G., C.J. Brace and F.J. Wallace, 1998. Combustion-Related Emissions in CI Engines,

    In: "Handbook of Air Pollution...", Editor: E. Sher, Academic Press, Boston, 1998, 280-357

    Huber, R., 1969. Electromagnetic fuel-injection valve, US Patent

    3,464,627, http://www.google.com/patents/US3464627

    Jost, K., 1998. New common-rail diesels power Alfa's 156, Automotive Engineering, January

    1998, 36-38

    Knecht, W., 2004. Some historical steps in the development of the common rail injection

    system, Trans. Newcomen Soc., 74, 89-107

  • McKechnie, J., 1913. Improvements in and relating to the injection of liquid fuel in internal

    combustion engines, GB Patent 24,153

    Miyaki, M., et al., 1988. Fuel injection system, US Patent

    4,777,921, http://www.google.com/patents/US4777921

    Miyaki, M., et al., 1991. Development of new electronically controlled fuel injection system ECD-

    U2 for diesel engines, SAE Technical Paper 910252, doi:10.4271/910252

    Petruzzelli, A.M., 2013. A STORY OF BREAKTHROUGH. THE CASE OF COMMON RAIL

    DEVELOPMENT, 35th DRUID Celebration Conference 2013, Barcelona, Spain, June 17-

    19, http://druid8.sit.aau.dk/acc_papers/9qly98k6m68rc4r3gog33mbjlpu0.pdf

    Sachsisches Industriemuseum, 2010. Versuchsmotor mit Common Rail-Einspritzsystem,

    Sachsisches Industriemuseum, Chemnitz, Germany; internet, accessed: March 18,

    2010, http://www.saechsisches-

    industriemuseum.de/c1/c1/redaktion?latestVersion=true&workshop=-1&URLID=6213

    Stumpp, G., Ricco, M., 1996. Common rail, an attractive fuel injection system for passenger car

    DI diesel engines, SAE Technical Paper 960870, doi:10.4271/960870

    Walker, B., Kennedy, H.E., 1933. Magnetic valve, US Patent

    1,892,917, http://www.google.com/patents/US1892917

    Williams, D.L., 1982. Electromagnetic diesel fuel injector, US Patent

    4,360,163, http://www.google.com/patents/US4360163

    Abstract: The components of a common rail fuel injection system include the rail, a high pressure pump

    and fuel injectors. Radial, unit and in-line pumps are used in commercial common rail systems. High

    pressure pump designs are evolving to achieve higher efficiency of the fuel injection system and to

    facilitate accurate rail pressure control. Several types of injectors can be used in common rail systems,

    including servo controlled electrohydraulic injectors and direct acting injectors.

    Piping System and Rail

    High Pressure Pump

    Injectors

    Piping System and Rail

    In modern common rail systems, the injector supply pipe dimensions and rail

    volume are critical parameters that can affect injection system

    dynamicperformance. The sizing of these components has a significant impact on

    critical fuel injection variables such as the dwell time between multiple injections

  • and the minimum fuel injection quantity. With increased use of multiple

    injections and the need to accurately control small fuel injection quantities

    starting at about the Euro 4 phase, manufacturers have paid more attention these

    seemingly mundane components.

    The rail is a thick walled tube designed to act as an accumulator to prevent

    significant pressure drop at the full fueling rate by providing hydraulic

    capacitance to the high pressure circuit. The volume of the rail varies from only a

    few cubic centimeters in passenger cars, to as much as 60 cm3 in heavy-duty

    applications. In most cases, a metering valve at the high pressure pump controls

    the high pressure fuel delivery to the rail. The rail pressure can be controlled to a

    value that depends on the needs of any particular engine operating condition. In

    some cases, rail pressures can reach 300 MPa.

    Just as is the case with P-L-N systems, common rail systems are also prone to

    effects related to wave dynamics in the rail and in fuel lines. Waves generated by

    sudden changes in pressure in one part of the system, such as when injection

    needle valve is opened, may become reflected at rigid terminations in the system

    and return to their origins, causing unwelcome consequences such as reduced

    injection pressure and variations in injection quantity.

    In order to better control the pressure at the injector nozzle, some common rail

    injectors include an additional accumulator volume in the injector.

    Injector Inlet Pipe Effects. The occurrence of high amplitude/low frequency

    pressure waves during the injection event represents one of the most important

    challenges in reducing the dwell time between multiple injections. Reducing the

    amplitude of these oscillations is an important objective of fuel injection system

    designers. A significant attenuation of pressure oscillations can be achieved by

    selecting the appropriate dimensions for the injector inlet pipe[Bianchi

    2005][Catania 2008].

  • The energy stored in pressure waves induced by injection events with the same

    injection duration and rail pressure remains almost constant when the

    geometrical parameters of the injector supply pipes are modified. Hence, owing

    to the fact that the energy stored in a sinusoidal pressure-wave train increases

    with the square of both its amplitude and frequency, hydraulic layout

    modifications leading to increased pressure-oscillation amplitudes should yield

    reduced frequencies and vice versa [Baratta 2008].

    Since the frequency of the pressure waves is strictly related to the geometric

    features of the high-pressure circuit, the focus is on designing the circuit in order

    to maximize the frequency of the waves. Physical modeling systematically shows

    that this frequency increases with the injector inlet pipe aspect ratio, that is the

    ratio of the length to the internal diameter, and this is confirmed by experiments.

    Modulating pressure-wave oscillations in this way is considered an active

    damping strategy.

    Alternatively, the introduction of orifices at the rail to pipe connections or inside

    the injector can be used. This is considered a passive damping strategy. For a

    particular injection duration and rail pressure, an orifice will generally decrease

    the injected fuel quantity when compared to a hydraulic layout without an orifice.

    The relative reduction is variable, but typically is less than 10%. An orifice will

    also reduce injection system hydraulic efficiency.

    Rail Volume Effects. A relatively large volume accumulator has traditionally

    been considered fundamental to dampen the pressure fluctuations caused by...

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