common rail fuel injection

Common Rail Fuel Injection

This is a preview of the paper, limited to some initial content. Full access requires

DieselNet subscription.

Please log in to view the complete version of this paper.

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


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


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


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


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 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 high-

pressure 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 high-

pressure 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].

References

Baratta, M., et al., 2008. Hydraulic Circuit Design Rules to Remove the 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

Abstract: 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

Introduction

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 regulatorcommonly referred to as a pressure control valvein 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 pumps 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.

Pressure Control Valve

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 (pump-

integrated 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 pumps cooling and

lubrication circuits. This combined flow is discharged from the pump to return to

the fuel tank.

Figure 1. Common Rail Diesel Fuel Injection System with Pressure Control

Valve located on the Rail

(Source: Bosch)

Figure 2. 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.

common rail fuel injection

Documents