Chapter 19 - Common Rail

8/20/2019 Chapter 19 - Common Rail

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Chapter 19 - Common Rail

High Pressure FuelInjection Systems

Diesel Engine Technology For Automotive Technicians

Understanding & Servicing Contemporary Clean DieselTechnology


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Title, Edition Author(s)

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.2

What is Common Rail?

Common rail fuel systems represent the most recenttechnological achievements in clean diesel engine fuelinjection technology.


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Conventional Fuel System

Disadvantage

Camshaft actuated fuel systems plunger velocities are low at slowengine speeds. Poor pressurization and atomization results.

Timing and rate control are fixed in camshaft geometry


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Common rail fuel systems pressurize fuel independently ofengine speed.

CR pressurizes fuel

independently of enginespeed. Availability of high fuelpressure over all engine

speeds = Loweremissions, improved power and fuel economy !


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A variety of subcategories of common rail fuel systems exist. Generally, common rail refers to a fuel rail system which

supplies fuel at injection pressure to all injectors

This chapter!


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Common Rail – Features

High-pressure common rail supplies fuel at injection pressure toinjectors

ECM

Rail Pressure Sensor

Injectors

High-Pressure Fuel Pump

Fuel PressureRegulator

SolenoidsHigh-Pressure Fuel Rail


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Common Rail Advantages – Rate Shaping

The latest high pressure common rail systems are capable of differentinjection events in one combustion cycle.


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Common Rail Advantages - Injection Rate Shaping

The quantity of fuel injected per degree of crank angle rotation is referred to as the fuelrate.

Injection rate affects emissions, performance, fuel economy and even engine noise levels. A specific injection rate will correspond to any given engine speed and load condition as

well as operating condition (i.e. oil, fuel and air temperature, boost pressure etc..)


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Common Rail - Components

All common rail systems have the following parts in

common: Injectors High pressure pump Fuel rail Relief valve Rail pressure sensor Fuel pressure regulator


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Common Rail - Components

This 3-cylinder CR diesel used in the MB Smart car achieves4.2L/110km combined fuel mileage. (56-mpg US)

High-Pressure Fuel

Rail

InjectorsFuel Supply from

High Pressure Pump

Fuel Rail PressureSensor

ReturnFuel

Relief Valve


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CR Injectors

High pressure common rail is a recent innovation. The technology did not exist to switch fuel injection on and off at

pressures exceeding 20K-psi at the speeds required for combustion


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CR Injectors - Types

ElectromagneticSolenoid

Piezo-Electric

Actuator

These DMAX injectors

represent the two typesof CR injectors.

1. Electromagneticsolenoids

2. Piezoelectric inlineactuators


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CR Injector Comparisons


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CR Injectors – Solenoid Type

Solenoid injectors use anelectromagnet to control

injection events.


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Solenoid CR Injection

Injection events are

triggered electricallybut the main actuationforces are hydraulic

The solenoid armatureis a pilot needle valvewhich controls the flowof fuel out of apressure chamber

above the needlevalve.


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Solenoid CR InjectionSequence

Important note – highpressure fuel (yellow) holdsthe nozzle valve against its

seat.

Pressure above the needle valve isslightly greater than below so

injection cannot take place


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1. The injector at rest

High pressure fuel is atthe bottom and top of the

needle valve but noinjection takes place sincethe larger surface area ontop of the needle valvehold the nozzle against its

seat.


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2. Injection

The solenoid is energized and liftsthe pilot needle or check ball from its

seat above the needle valve.

Fuel pressure above the needle valvedrops and fuel pressure below thevalve forcing the nozzle from its seat.

Fuel cannot fill the chamber abovethe needle valve quickly since thereis a restriction in the passagewaybetween the rail pressure fuel and

pressure chamber

Restriction

Fuel Return


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3. End of Injection

The solenoid is de-energized and the

check ball return to its seat.

Fuel at rail pressure builds-up quicklyabove the needle valve forcing it

against it’s seat thus ending injection


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Adaptive Cylinder Balance

This 2004 6.6L DMAX engine is adjusting fuel rate at idle toeach injector to enable smoother engine operation


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Pilot Injection

Note pilot injection factors are variable for CR engines. UnlikeHEUI and other fuel systems where pilot injection ismechanical, CR engines it is achieved electronically –microseconds in duration! (2004 6.6L DMAX)


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Piezo-electric Actuated Injectors Piezo technology enables

the fastest switchingtime for injectorscurrently allowing up to

seven injection events inone injection sequence.

The mass of the inline

actuators is less thansolenoid actuatedinjectors

Solenoid

Inline Piezo

Actuator


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Piezo-electric Actuated Injectors

Comparisons of injector response time (top solenoid,bottom piezo-electric actuation.


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Piezo-electric Actuated Injectors

Piezo-electric injectors can achieve lower emission reduction withimproved rate shaping and multiple injection events.


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Piezo-electric Principles

Piezo crystals expand when current is applied to a stack of crystalsand contract when the currents polarity is reversed.

Note the change in current direction and the shape of the crystals

Piezo Mineral Crystal


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Piezo-electric Principles

Stacks of piezo crystals deform when energized with electrical current.

Piezo crystals respond much faster than electromagnetic solenoids


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Piezo-electric CR Injection Sequence

Piezoelectric actuators operate a servo valve to changehydraulic pressure acting on the needle valve holding thenozzle closed


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2008MY Ford 6.4L Powerstroke Diesel uses piezo electricactuated CR injectors manufactured by Siemens


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The Ford nozzle has aconventional type ofnozzle.


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Like solenoid type CRinjectors, in a piezo injector

the nozzle needle valve isheld in place by hydraulicforce of pressurized fuel.

A mushroom shaped valvecontrols the exit of fuelpressure above a controlpiston holding the needlevalve on its seat.


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High pressure fuel forces the

control piston against theneedle valve


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Applying current to the piezo

crystals forces the mushroomvalve downward releasingpressure above the controlpiston to the lower pressure fuelinlet.

Fuel pressure below the needlevalve force the valve upwardsand injection begins

ExitingFuel


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Injection Ending - Current Reversal

The injection event can end quickly by changing the direction of current

flow through the stack of piezo crystals. Without pressure from the piezo crystals, hydraulic force closes the

mushroom valve and pressure returns to the top of the control piston


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Hydraulically Amplified CR Injectors

Current heavy-duty diesel are using hydraulically amplified CR injectors.

These injectors use an internal amplifier/intensifier piston similar in principle toHEUI injectors


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Co-Axial Nozzle CR Injectors

Two rows of spray orifices are used in a co-axial nozzle toachieve more sophisticated rate shaping of injection


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CR Injector Failure

CR injectors are very sensitive to dirt.

The smallest particle of dirt causing a solenoid or mushroom valve to stay opencould allow the injector to continuously inject fuel.

This overheated cylinder which damaged valves was caused by an injector “stuck open”.


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CR Injectors – Lines

Injectors

Fuel ReturnLines

High PressureLines

The injector and lines on the 2006 6.6L DMAX engine are all locatedoutside the scalloped valve covers to prevent fuel leaks into the engine


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These Cummins B series injectors use a quill tube to connect an external injector linewith the injector.

These tubes must be carefully tightened and the high-pressure connection cleanedduring assembly to prevent leakage!

Quill Tube

HighPressure

Connection


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This early 6.6l DMAX has line fitting clamps to prevent looseningand leakage of lines.


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CR Injector Lines

Never crack-open a high pressure line when running an engine.Cylinder misfires cannot be detected and high pressure fuel canpenetrate the skin

RailPressureSensor

Fuel Rail

HighPressure

Lines


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Ford Duratorq I-4 CR Engine

The hugely popular European Ford TDCi 2.0L Duratorqengine (Ford Galaxy van shown)


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CR Injector Trim Codes

Characteristics of CR injectorscause fuel delivery variations over

a wide range of operation(pressure & actuation time)


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CR Injector Trim Codes

Trim codes

Trim codes or injection quality adjustment (IQA) codes for each injectorare entered in the ECM module and used to compensate for deliveryquantity variations

High Pressure Pumps


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High Pressure Pumps

High pressure pumps take low transfer fuel pressure and pressurize the fuel tothe for injection.

Hi h P P


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High Pressure Pumps

A Bosch CP-3 Pump is commonly used in automotive applications

Radial Plungers

Fuel PressureRegulator

Low PressureGear Type

Transfer Pump

Fuel Return

High PressureOutlet

Inlet/Outlet

Check Valve


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High Pressure Pumps

Bosch CP-3 High Pressure Pump

FuelReturn

FuelInlet

Fuel Pressure

Regulator


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High Pressure Pumps

Bosch CP-3High

Pressure

Pump

Location of the high-pressure pump on a 2007 6.6L DMAX

i h


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High Pressure Pumps

Rear geartrain mounted pump of the 6.7L Cummins diesel

High PressurePump

Hi h P P


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High Pressure Pumps

Location of the high pressure pump on the 2008MYPowerstroke

High Pressure Pumps


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High Pressure Pumps

Radial plunger pumps use three lobed cams to driven by the engine to pressurize fuel. Check valves in the end of the plunger pistons operate to control flow of fuel in and

out of the pumping chambers

High Pressure Pumps Pressure Regulation


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High Pressure Pumps – Pressure Regulation

A fuel pressure regulating valve controls fuel pressure using a PWM signal sent from theECM.

With no signal, all pressurized fuel is sent to the fuel rail. Increasing the PWM signal lowers fuel pressure to the rail

High Pressure Pumps Pressure Regulation


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High Pressure Pumps – Pressure Regulation

Note the duty-cycle reportedfor the fuel pressure regulator

(35%)

Fuel temperature is calculatedthrough ECM measurement of

the resistance of the pressureregulator coil.

High Pressure Pumps – Inlet Volume Regulation


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High Pressure Pumps – Inlet Volume Regulation

To reduce parasitic power loss some pumps such as used on thePowerstroke regulate the amount of fuel admitted into the pump usinga volume control regulator.

Fuel Rail


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Fuel RailFuel Rail

Fuel RailPressure Sensor

Fuel RailInlet

The fuel rail is common to several cylinders.

Internal orifices dampen pressure pulses

Fuel Rail


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©2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.58

Fuel Rail

This Ford Dura-torque diesel

uses a circular shaped fuelrail (Delphi) for packagingreasons

Fuel Rail

P P t ti P Li iti V l


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Pressure Protection – Pressure Limiting Valve

To protect the high pressure system from over pressurization apressure limiting valve is used in the high pressure section.

Pressure Protection


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Pressure Protection

The function block on this DMAX contains the pressureliing valve.

Chapter 19 - Common Rail

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