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: 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
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 D’Injection (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, Bosch—perhaps due to some pressure by Daimler-Benz—acquired the UNIJET technology initially developed by the efforts of Fiat and Elasis (a Fiat subsidiary) for further development and production [Stumpp 1996]. Bosch’s 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 speed—especially 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. . 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.
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.saechsischesindustriemuseum.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
: The components of a common rail fuel injection system in clude 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
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 the fuel pulses delivered by the pump and the fuel-injection cycles in common rail systems. However, studies with a fuel injection system for light vehicles has shown that the progressive reduction in the accumulator volume from 20 to 3 cm3 has no impact on the amplitude of these pressure fluctuations and little negative impact on injector performance [Baratta 2008][Catania 2012]. The highpressure control capability of the system in these studies resulted from the
synergic action of both the system high-pressure hydraulic capacitance and the pressure control device. Although the duty cycle of either the pressure control valve (PCV) or the fuel metering valve at the pump inlet (FMV) depended on the rail size, the high-pressure control system was capable of keeping the pressure level adequately close to the nominal value for the range of accumulator volumes studied. This finding has been applied to the design of newer generation common rail systems for passenger cars which use smaller rail volumes then in the past. This finding also opens the door to the possibility of removing the rail entirely from the high-pressure circuit. In fact, such a system concept, referred to as Common Feeding, has been developed [Catania 2012]. It uses a small hydraulic accumulator volume integrated in the pump which is then connected directly to the injector feed lines. The pressure sensor, PCV and FMV are also pump integrated. The resulting injection system has low hydraulic inertia that gives rise to fast dynamic response during transients and reduced production costs. Furthermore, this system matches the requirements of easy installation on the engine. It should be noted that a minimum accumulation volume is required in the highpressure circuit to avoid an excessive decrease in the pressure level during the injection event. Effective monitoring of the pressure in the high pressure circuit also requires a minimum volume to ensure pressure control system stability. The minimum volume for these functions is about one order of magnitude lower than the standard rail volume [Catania 2012].
Baratta, M., et al., 2008. “Hydraulic Circuit Design Rules to Remove t he Dependence of the Injected Fuel Amount on Dwell Time in Multijet CR Systems”, ASME Trans.,Journal of Fluids Engineering, 130(12), 121104-1-121104-13, doi:10.1115/1.2969443
Bianchi, G., et al., 2005. “Numerical Investigation of Critical Issues in Multiple -Injection Strategy Operated by a New C.R. Fast- Actuation Solenoid Injector”, SAE Technical Paper 2005-01-1236, doi:10.4271/2005-01-1236 Catania, A.E., A. Ferrari, 2012. “Development and Performance Assessment of the New Generation CF Fuel Injection System for Diesel Passenger Cars”, Applied Energy, 91(1), 483-495, doi:10.1016/j.apenergy.2011.08.047 Catania, A.E., et al., 2008. “Experimental investigation of dynamic effects on multiple -injection common rail system performance”, Journal of Engineering for Gas Turbines and Power, 130(3), 032806-1-032806-13, doi:10.1115/1.2835353
: There are several approaches to control the pressure in the common rail. One early approach method was to supply more fuel than is needed to the common rail and use a pressure control valve to spill the excess fuel back to the fuel tank. A more preferred approach is to meter the fuel at the high pressure pump in order to minimize the amount of fuel pressurized to the rail pressure. A variety of fuel metering can be used for the later. Some practical common rail implementations utilize both approaches with the control strategy depending on the engine operating conditions.
Introduction
Pressure Control Valve
Pump Metering
Practical Rail Pressure Control
Production common rail fuel systems are equipped with a closed-loop high pressure control-system that stabilizes the rail pressure within a relatively small margin to the nominal value specified by the electronic control unit for a given engine operating condition. The pump maintains the rail pressure by continuously delivering fuel to the common rail. This pressure is monitored by a pressure sensor and the difference between the nominal rail pressure value and the measured one is the input signal for the controller. In control terminology , the rail pressure is the system output while the position of the actuator used to control the rail pressure is the system input .
There are a number of approaches to control the pressure in the common rail. One way is to supply more fuel than is needed to the common rail and use a high pressure regulator—commonly referred to as a pressure control valve—in the high-pressure circuit to spill the excess fuel back to the fuel tank. In this approach, the pressure control valve position is the control system input. While this approach was used exclusively in some early fuel injection systems such as those with Bosch CP1 pumps (Figure 1 and Figure 2), poor efficiency and an excessively high fuel return temperatures can result. Another approach is to meter the fuel at the high pressure pump to ensure that only the amount of fuel required by the injectors is supplied to the common rail. A number of pump metering approaches are possible. One common approach is to meter the fuel drawn into the pump (inlet metering) with some type of inlet metering valve (IMV)—sometimes also referred to simply as a fuel metering valve (FMV). Another approach is to allow the pump to draw in an uncontrolled amount of fuel and meter the pump’s discharge flow (outlet metering) with a valve such as an outlet metering valve (OMV). Another means is to vary the effective displacement of the high pressure pump. By carefully controlling the amount of fuel entering the pump and avoiding compression of excess fuel to high pressure, the fuel injection system hydraulic efficiency can be improved and generation of excessively high fuel temperatures can be avoided. It should be noted, however, that metering the fuel at the injection pump may not avoid the need for a high pressure regulator. A pressure regulator can still be used to provide some trimming of the rail pressure.
A pressure control valve (PCV) for controlling rail pressure can be located at one rail extremity (pump-external PCV), Figure 1, or at the pump outlet (pumpintegrated PCV), Figure 2. The pump-external PCV leads to lower pump
manufacturing costs but the proximity of the regulator to the injectors can introduce additional disturbances in injector dynamics. In the pump-integrated PCV solution, the fuel throttled by the control valve joins the leakage flow from the pumping chambers as well as the fuel flowing in the pump’s cooling and lubrication circuits. This combined flow is discharged from the pump to return to the fuel tank.
. Common Rail Diesel Fuel Injection System with Pressure Control Valve located on the Rail (Source: Bosch)
. Bosch CP1 Pump with Integrated Pressure Control Valve (Source: Bosch)
Rail pressure control with a PCV is inherently fast because of the proximity of the system input (PCV) and system output (rail pressure sensor). In other words, the system does not include the delay resulting from fuel passing through the high pressure pump as would be the case for some of the pump metering approaches.
