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Common Rail Systems Chapter 30. DSL - 131. OBJECTIVES. Identify common rail (CR) diesel fuel systems. Identify some of the diesel engines currently using common rail diesel fuel injection. Outline the fuel subsystems in a typical CR system.
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Common Rail SystemsChapter 30 DSL - 131
OBJECTIVES • Identify common rail (CR) diesel fuel systems. • Identify some of the diesel engines currently using common rail diesel fuel injection. • Outline the fuel subsystems in a typical CR system. • Trace fuel flow routing from tank to injector on common rail, diesel-fueled engines. • Describe the electronic management circuit components used in common rail fuel systems. • Describe the operation of the inline and radial piston pumps used to achieve sufficient flow to produce rail and injection pressures in a typical CR system. • Understand how rail pressures are managed in electronically managed, common rail diesel fuel systems. • Outline the operation of an electrohydraulic injector. • Identify some of the characteristics of different OEM common rail diesel fuel-injection systems.
INTRODUCTION • Electronically controlled, common rail (CR) diesel fuel injection systems were introduced on small-bore and automobile diesel engines in the late 1990s. • These systems were manufactured by Robert Bosch and Delphi Lucas, and both had similar operating principles. • Early systems had about 20,000 psi rail pressure. • Newer versions may exceed 35,000 psi. • According to the EPA, by 2010 nearly all major diesel OEMs is using a CR system on at least one of their engine families to meet emission/performance standards.
What is CR? • CR only refers to systems where fuel injection pressure is maintained at the fuel rail that directly feeds the injectors. • Beginning in 2008, second generation CR systems were developed where rail pressures are amplified at the injector prior to injection were introduced. • CRs and/or amplified CRs do not use oil pressure or mechanical action to amplify the injection pressure (see chapter 31).
Function of the Rail • During this presentation the term rail describes the supply manifold or gallery that directly feeds all of the fuel injectors. • Similar to gasoline fuel injection. • Major differences: • CR operates at higher pressures (in some cases above 35,000 psi) • CR precisely manages rail pressure within a wide range of values. • Similarities • Managing and switching injectors. • Injector units are similar, especially when considering DI (gasoline Direct Injection).
CAUTION • Do not confuse post-2007 and post-2010 engines with their earlier versions, even if they are nominally the same series. • In most cases, the fuel system, engine displacement, and emissions control apparatus are completely different.
ADVANTAGES OF CRDIESEL FUEL SYSTEMS • CR systems are simpler mechanically than earlier systems leading to higher reliability. • Electronics have much more control over fueling for better combustion control. • Specific improvements: • Lower emissions • Improved fuel economy • Lower engine noise levels • Optimization of cylinder pressures • Made possible by: • Improved metallurgy • Development of electrohydraulic injectors w/faster opening/closing events at nozzle.
CR SUBSYSTEMS AND COMPONENTS • Fuel subsystem • Stores and supplies fuel to the high-pressure rail pump. • High-pressure pump • Engine driven pump capable of producing pressures of up to 35,000 psi or more. • May be of either radial piston or multiple cylinder inline design. • ECM-controlled rail pressure control valve • A linear proportioning solenoid with built in spool valve • Is an ECM output.
CR SUBSYSTEMS ANDCOMPONENTS (Cont.) • A V-Ref supplied rail pressure sensor • Signals “actual rail pressure to the ECM continuously. • Enables “closed loop” capabilities. • CR • Stores fuel at injection pressure. • AKA high pressure accumulator. • EHIs (Electronic Hydraulic Injector) • Act as ECM switched hydraulic valves. • Injects fuel at close to rail pressure values
CR Special Features • Reasons for CR acceptance: • Ability to achieve high injection pressures independent of engine speed. • Ability to control droplet size for best performance under all conditions. • Ability to switch EHIs at high speed. • Currently up to 7 “events” during a single power stroke. • Overall better management of fuel for: • Improvedfuel economy. • Reduced emissions • Improved response to power change requests. • Improved cold starting. • Finely atomized fuel at cranking and cold idle conditions.
CR Management Electronics • CR is a full authority system. • Fuel quantity determined by ECM • ECM monitors: • Ambient conditions • Chassis conditions • Engine conditions • ECM can determine electronic and hydromechanical faults. • ECM may initiate: • Torque limitation • Power de-rate • Limp home mode • ECM can interface with other chassis and powertrain components.
CR MANAGEMENT ELECTRONICS • Input Circuit • Sensor circuit inputs to the ECM to include: • Electronic signal transducers • Electronic signal amplifiers • Fluid pressure transducers. • Analog Inputs • Include • Mass airflow • Engine fluid and intake air temps • Battery voltage • Converted to digital values by the A/D converter in the ECM • Some analog signals are converted to digital by the sensor.
CR MANAGEMENT ELECTRONICS • Digital Inputs • Includes • On-off switching signals • Digital sensor signals (RPM and component position) • Bypass the A/D unit • Signal Conditioning • Reduce voltage to a specific range • Filter out spikes and lows • Amplify to system signal levels. • Reduces noise.
CR MANAGEMENT ELECTRONICS • Processing Cycle • Compares input signals to operational maps (algorithms) • Limited editing may be available to OEM or technician. • May include: • Failure strategies • Calibration data • Power trim/derate programming • Post 2008 • Log faults to EEPROM • Communicate with J1939 data bus (allows communication with chassis electronic systems.
CR MANAGEMENT ELECTRONICS • Fueling Algorithm • Primary function is closed loop cycle between desired fuel pressure and actual fuel pressure. • Desired fuel pressure – what the ECM computes that it needs. • Actual fuel pressure – what the transducer actually measures at the fuel rail. • Fueling concerns. • Minimizing emissions • After 2010 may include: • EGR – exhaust gas recirculation • Catalytic converters • DPF - diesel particulate filters • SCR – selective catalytic reduction systems
CR MANAGEMENT ELECTRONICS • Output Circuit • Rail Pressure Management Control (RPMC) • PWN (pulse width modulated) output to control valve • Fuel directed to fuel rail or return as necessary to control pressure. • NOTE: This valve is spring loaded and will default to the bypass condition in the event of electrical failure, shutting down the engine. • Injector Drivers • PWM signal to EHIs • Usually spiked to 100vdc for solenoid injectors, sometimes less to piezoelectric units. • Current draw usually lower to piezoelectric units.
CR FUEL ROUTING CIRCUIT • The CR fuel routing system is modular so, for purposes of study, we can divide it up as follows: • Fuel subsystem • Items 1-5 • Transfer or Presupply pump may be • Electric and located in fuel tank • Mechanical on the engine • Supplies fuel to high pressure pump. • Fuel filter • OEM responsibility • Nominally 8 microns. • Should have water separator
CR FUEL ROUTING CIRCUIT • High-pressure pump • Item 6 • May be 3 cylinder radial pump (Bosch & Denso) • May be inline piston as used by Caterpillar. • All pumps are capable of maintaining injection pressure at rated engine speed and load. • Unneeded fuel is returned to the tank via rail pressure control valve.
CR Fuel Routing Circuit. • Rail pressure control valve. • Solenoid is electrical • Slow response to maintain/increase pressure • Spring return is mechanical • Quickly compensate for pressure fluctuations/reduce pressure.
Caterpillar CR High-Pressure Pump and the Solenoids that Act as Rail Pressure Control Valves • Similar in operation to the Bosch unit. • When energized, closes the valve pressurizing the rail. • When de-energized spring pressure opens valve sending fuel to the low pressure return circuit.
CR FUEL ROUTING CIRCUIT • Pressure accumulator or rail • Item 8 • Acts as an accumulator and dampens pulses.
CR FUEL ROUTING CIRCUIT • Electrohydraulic injectors • Item 9
CAUTION • Never attempt to prime a CR fuel system equipped with an inline gear pump using shop air pressures applied anywhere in the fuel subsystem: Pressurized air can blow out the transfer pump seals. • If the transfer pump seals fail, the pump must be replaced.
Rail Pressure Sensor • May be • Piezoelectric (most common) • Capacitance • Wheatstone bridge. • Uses a 5 vdc reference. • Analog output
Flow Limiter • In the event an injector is stuck open, the valve will close stopping fuel flow to the injector.
Location of Quill Tube Recess on a Cylindrical Caterpillar EHI
Rail, High-Pressure Pipe, and Quill Tube Disassembled and Assembled on the Engine
WARNING • Never crack the high-pressure pipe nuts attempting to bleed them. It could prove to be dangerous and most OEMs prefer CR high-pressure lines to be single-use devices. (They yield to deform on initial torque to form a better seal.) • CR fuel systems are designed to self-prime.
ELECTROHYDRAULIC INJECTORS • Two general types of Electrohydraulic injectors (EHIs) are used, differentiated by the type of actuator used: • First generation EHIs used a solenoid control valve • Some more recent types use piezoelectric actuators.
ELECTROHYDRAULIC INJECTORS (Cont.) • We use a Bosch EHI for purposes of the description here. • An EHI can be subdivided as follows: • Nozzle assembly • Hydraulic servo-system • Actuator valve
https://www.youtube.com/watch?v=zbOzX_OFDIg • https://www.youtube.com/watch?v=cIkMtnd3LGQ
CAUTION • Never attempt to locate an engine miss by cracking open the high-pressure lines that feed the EHIs. • Use OEM technical literature to troubleshoot engine as well as fuel system malfunctions.
Denso EHI Five-Phase Multipulse Injection Cycle Represented Graphically
https://www.youtube.com/watch?v=ftchx1TDNJo • http://www.dailymotion.com/video/xkxog1_diesel-piezo-injector-pulse-n-pressure_auto
PHASES OF INJECTION • On common rail systems, multiple injection events can occur during one power stroke. • Pilot injection • Small amount of fuel injected just prior to main injection. • Improves combustion efficiency • Reduces noise • More even torque production (smoother pressure curve) • Main injection • Multiple pulses into the established flame front. • Essentially nibbles at power stroke rather than gulp. • Dosing • AKA afterburn. • Small amount of fuel injected after combustion has ended. • Does not burn in the combustion chamber/cylinder. • Provides temporary rich mixture to rhodium catalyst to reduce NOx emissions. • Some fuel may be transmitted back to the intake via the EGR system to provide some “pilot” injection effect.
CR INJECTOR CALIBRATION PROGRAMMING • Most CR fuel systems require their ECMs to be programmed with a fuel flow code specific to each EHI. • The fuel flow code is the injection quantity calibration data. • Caterpillar uses the term E-trim, and Denso uses the term quick response (QR) code. • Matches fuel flow characteristics so that all function similarly.
CAUTION • Injection quantity calibration data or E-trim must be programmed to the ECM every time an EHI is changed out. • This may require entering a numeric code, located on the injector, into the injector calibration field using an electronic service tool (EST). • Alternatively, it may require connection to the OEM data hub to download a calibration file to the engine ECM. • Failure to reprogram E-trim can result in unbalanced engine fueling.
ADAPTIVE TRIM • Most CR management electronics complement injector quantity calibration with adaptive trim. • Evaluates EHI performance at hourly intervals and “corrects” fuel flow programming. • AKA “a-trim”
Location of the QR Code on a Denso EHIUsed on a Hino Engine • Codes are used to calibrate the ECM to evenly match cylinder performance.
Caterpillar High-Pressure Pump TimingPin Locked into Position • Some systems require the high pressure pump to be timed to the engine. • This is to make sure that the pressure pulses align with the operating pulses of the injector units and maintain smooth pressure cycles at the rail.
Diagnosing CR System Problems • Until fairly recently, troubleshooting EHIs was usually a matter of using OEM software to identify a malfunctioning component, then replacing the defective unit. • The number of CR fuel systems in service has resulted in an extensive repair infrastructure.
Diagnostic Routines • It should be emphasized that the three requirements of accurately diagnosing CR problems are: • Data bus connection hardware • OEM diagnostic software • Access to the OEM service information system (SIS) • As of 2013, the introduction of mandatory HD-OBD may provide some generic testing codes.
Easy to Access Location of the EHIs on a Post-2010 Cummins ISB Engine
WARNING • When working with CR fuel systems, never crack the high-pressure pipe nuts attempting to bleed or troubleshoot problems. • It could prove to be dangerous and result in costly system malfunctions. • Note that most (but not all) OEMs prefer CR high-pressure lines to be single-use devices. • (They yield to deform to shape on initial torque.) • CR fuel systems are designed to self-prime, so there is no reason to crack the pipe nuts after initial torque.
SUMMARY • Common rail (CR) diesel fuel systems are currently used on a wide range of engines that extend from small- to large-bore highway diesels. • CR diesel fuel systems have full authority engine management capability and are networked to J1939 data buses. • The fuel subsystems that supply CR fuel systems have few functional differences when compared with other current fuel supply circuits. • The electronic controls on a typical CR system consist of an input circuit, processing hardware, and actuator circuit. • The primary outputs that manage fueling on a CR system are the rail pressure control valve and the switching of the EHI actuators. • Bosch CR systems use either radial piston high-pressure pumps or inline piston pumps.
SUMMARY (Cont.) • The Caterpillar CR system on C7 and C9 engines uses an inline piston, high-pressure pump. • Rail pressures on CR diesel fuel systems are managed by the ECM using a rail pressure control valve. This applies flow restriction to fuel discharged by the high-pressure pump and options it either to the rail or to the return circuit. • The rail pressure control valve is a linear proportioning solenoid with an integral spool valve. It options fuel from the high-pressure pump outlet either to the rail or to the fuel return circuit. • Actual rail pressures are signaled to the ECM by the rail pressure sensor. • Rail pressure sensors use a piezo-resistive operating principle.