m12 hald6891 06 se ch12 - the eyethe-eye.eu/public/worldtracker.org/college books/automotive...

After studying Chapter 12, the reader will be able to:

1. Prepare for ASE Engine Performance (A8) certification test content

area “C” (Fuel, Air Induction, and Exhaust Systems Diagnosis and

Repair).

2. Explain how a diesel engine works.

API Gravity (p. 173)

Diesel Oxidation Catalyst (DOC) (p. 170)

Differential Pressure Sensor (DPS) (p. 171)

Direct Injection (DI) (p. 161)

Glow Plug (p. 167)

Heat of Compression ( p. 161)

High-Pressure Common Rail (HPCR) (p. 164)

Hydraulic Electronic Unit Injection (HEUI) (p. 164)

CHAPTER 12

DIESEL ENGINE

OPERATION AND

DIAGNOSIS

DIESEL ENGINE

OPERATION AND

DIAGNOSIS

3. Describe the difference between direct injection (DI) and indirect

injection (IDI) diesel engines.

4. List the parts of the typical diesel engine fuel system.

5. Explain how glow plugs work.

6. List the advantages and disadvantages of a diesel engine.

7. Describe how diesel fuel is rated and tested.

Indirect Injection (IDI) (p. 161)

Injection Pump (p. 161)

Lift Pump (p. 163)

Opacity (p. 175)

Pop Tester (p. 174)

Regeneration (p. 171)

Water-Fuel Separator (p. 163)

OBJECTIVES

KEY TERMS

# 101145 C t PH OH CHET A H ld P N 160 K/PMS DESIGN SERVICES OF

M12_HALD6891_06_SE_CH12.QXD 6/11/08 4:35 AM 60

Diesel Engine Operation and Diagnosis 161


INJECTOR

EXHAUSTVALVE

AIRINTAKEVALVE

FUEL INJECTIONPUMP

INJECTOR

TRANSFERPUMP

FUELTANK

RETURN LINE

SUPPLY LINE

INLET LINE

INJECTORLINE

FIGURE 12-2 A typical injector-pump-type automotive diesel fuelinjection system.

FIGURE 12-1 Diesel combustion occurs when fuel is injected into thehot, highly compressed air in the cylinder.

DIESEL ENGINES

In 1892, a German engineer named Rudolf Diesel perfectedthe compression-ignition engine that bears his name. Thediesel engine uses heat created by compression to ignite thefuel, so it requires no spark ignition system.

The diesel engine requires compression ratios of 16:1 andhigher. Incoming air is compressed until its temperaturereaches about 1000°F (540°C). This is called heat of com-pression. As the piston reaches the top of its compressionstroke, fuel is injected into the cylinder, where it is ignited bythe hot air. See Figure 12-1.

As the fuel burns, it expands and produces power. Becauseof the very high compression and torque output of a dieselengine, it is made heavier and stronger than the same sizegasoline-powered engine.

A common diesel engine uses a fuel system precisioninjection pump and individual fuel injectors. The pump deliv-ers fuel to the injectors at a high pressure and at timed inter-vals. Each injector sprays fuel into the combustion chamber atthe precise moment required for efficient combustion. SeeFigure 12-2.

In a diesel engine, air is not controlled by a throttle as ina gasoline engine. Instead, the amount of fuel injected is var-ied to control power and speed. The air–fuel mixture of adiesel can vary from as lean as 85:1 at idle to as rich as 20:1at full load. This higher air–fuel ratio and the increased com-pression pressures make the diesel more fuel-efficient than agasoline engine in part because diesel engines do not sufferfrom throttling losses. Throttling losses involve the powerneeded in a gasoline engine to draw air past a closed or par-tially closed throttle.

In a gasoline engine, the speed and power are controlledby the throttle valve, which controls the amount of air enter-ing the engine. Adding more fuel to the cylinders of a gasoline

engine without adding more air (oxygen) will not increase thespeed or power of the engine. In a diesel engine, speed andpower are not controlled by the amount of air entering thecylinders because the engine air intake is always wide open.Therefore, the engine always has enough oxygen to burn thefuel in the cylinder and will increase speed (and power) whenadditional fuel is supplied.

Diesel engines are built in both two-stroke and four-stroke versions. The most common two-stroke diesels werethe truck and industrial engines made by Detroit Diesel. Inthese engines, air intake is through ports in the cylinder wall.Exhaust is through poppet valves in the head. A blowerpushes air into the air box surrounding liner ports to supplyair for combustion and to blow the exhaust gases out of theexhaust valves.

Indirect and Direct Injection

In an indirect injection (abbreviated IDI) diesel engine, fuelis injected into a small prechamber, which is connected to thecylinder by a narrow opening. The initial combustion takesplace in this prechamber. This has the effect of slowing the rateof combustion, which tends to reduce noise. See Figure 12-3.

All indirect diesel injection engines require the use of aglow plug.

In a direct injection (abbreviated DI) diesel engine, fuelis injected directly into the cylinder. The piston incorporates adepression where initial combustion takes place. Direct injec-tion diesel engines are generally more efficient than indirectinjection engines, but have a tendency to produce greateramounts of noise. See Figure 12-4.

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162 CHAPTER 12


INTAKEVALVE

PISTON

PRECHAMBER

FUELINJECTOR

GLOWPLUG

FIGURE 12-3 An indirect injection diesel engine uses a prechamberand a glow plug.

PISTON

INTAKEVALVE

FUELINJECTOR

CYLINDERHEAD

FIGURE 12-4 A direct injection diesel engine injects the fuel directlyinto the combustion chamber. Many designs do not use a glow plug.

While some direct injection diesel engines use glow plugsto help cold starting and to reduce emissions, many directinjection diesel engines do not use glow plugs.

Diesel Fuel Ignition

Ignition occurs in a diesel engine by injecting fuel into theair charge, which has been heated by compression to a

temperature greater than the ignition point of the fuel orabout 1000°F (538°C). The chemical reaction of burningthe fuel liberates heat, which causes the gases to expand,forcing the piston to rotate the crankshaft. A four-strokediesel engine requires two rotations of the crankshaft tocomplete one cycle. On the intake stroke, the piston passesTDC, the intake valve(s) open, the fresh air is admitted intothe cylinder, and the exhaust valve is still open for a few de-grees to allow all of the exhaust gases to escape. On thecompression stroke, after the piston passes BDC, the intakevalve closes and the piston travels up to TDC (completionof the first crankshaft rotation). On the power stroke, thepiston nears TDC on the compression stroke, the diesel fuelis injected by the injectors, and the fuel starts to burn, fur-ther heating the gases in the cylinder. During this powerstroke, the piston passes TDC and the expanding gasesforce the piston down, rotating the crankshaft. On the ex-haust stroke, as the piston passes BDC, the exhaust valvesopen and the exhaust gases start to flow out of the cylinder.This continues as the piston travels up to TDC, pumpingthe spent gases out of the cylinder. At TDC, the secondcrankshaft rotation is complete.

THREE PHASES

OF COMBUSTION

There are three distinct phases or parts to the combustion in adiesel engine.

1. Ignition delay. Near the end of the compression stroke,fuel injection begins, but ignition does not begin imme-diately. This period is called delay.

2. Rapid combustion. This phase of combustion occurswhen the fuel first starts to burn, creating a sudden risein cylinder pressure. It is this rise in combustion cham-ber pressure that causes the characteristic diesel engineknock.

3. Controlled combustion. After the rapid combustionoccurs, the rest of the fuel in the combustion chamberbegins to burn and injection continues. This is an areanear the injector that contains fuel surrounded by air.This fuel burns as it mixes with the air.

DIESEL ENGINE

CONSTRUCTION

Diesel engines must be constructed heavier than gasoline en-gines because of the tremendous pressures that are created inthe cylinders during operation. The torque output of a dieselengine is often double or more than the same size gasolinepowered engines. See the comparison chart.




FUEL TANK AND LIFT PUMP

A fuel tank used on a vehicle equipped with a diesel enginediffers from the one used with a gasoline engine in severalways, including:

■ A larger filler neck for diesel fuel. Gasoline filler necks aresmaller for the unleaded gasoline nozzle.

■ No evaporative emission control devices or charcoal (car-bon) canister. Diesel fuel is not as volatile as gasoline and,

FIGURE 12-5 The common rail on a Cummins diesel engine. A high-pressure pump (up to 30,000 PSI) is used to supply diesel fuel to this commonrail, which has cubes running to each injector. Note the thick cylinder walls andheavy-duty construction.

FIGURE 12-6 A rod/piston assembly from a 5.9-liter Cummins dieselengine used in a Dodge pickup truck.

System orComponent Diesel Engine Gasoline Engine

Block Cast iron and heavySee Figure 12-5.

Cast iron or aluminumand as light as possible

Cylinder head

Cast iron or aluminum

Cast iron or aluminum

Compressionratio

17:1 to 25:1 8:1 to 12:1

Peak enginespeed

2,000 to 2,500 RPM

5,000 to 8,000 RPM

Pistons andconnectingrods

Aluminum with com-bustion pockets andheavy-duty rods See Figure 12-6.

Aluminum, usually flattop or with valve reliefbut no combustionpockets

therefore, diesel vehicles do not have evaporative emis-sion control devices.

The diesel fuel is drawn from the fuel tank by a lift pumpand delivers the fuel to the injection pump. Between the fueltank and the lift pump is a water-fuel separator. Water isheavier than diesel fuel and sinks to the bottom of the separa-tor. Part of normal routine maintenance on a vehicle equippedwith a diesel engine is to drain the water from the water-fuelseparator. A float is usually used inside the separator, whichis connected to a warning light on the dash that lights if thewater reaches a level where it needs to be drained.

NOTE: Water can cause corrosive damage as well as wear to diesel engine

parts because water is not a good lubricant.Water cannot be atomized by a

diesel fuel injector nozzle and will often “blow out” the nozzle tip.

Many diesel engines also use a fuel temperature sensor.The computer uses this information to adjust fuel deliverybased on the density of the fuel. See Figure 12-7.

Injection Pump

A diesel engine injection pump is used to increase the pressureof the diesel fuel from very low values from the lift pump tothe extremely high pressures needed for injection.

Injection pumps are usually driven by the camshaft at thefront of the engine. As the injection pump shaft rotates, the


164 CHAPTER 12

# 101145 Cust: PH-OH-CHET Au: Halderman Pg. No. 164 K/PMS DESIGN SERVICES OF

CARLISLE

FIGURE 12-7 Using an ice bath to test the fuel temperature sensor.

FIGURE 12-8 A typical distributor-type diesel injection pump show-ing the pump, lines, and fuel filter.

diesel fuel is fed from a fill port to a high-pressure chamber. Ifa distributor-type injection pump is used, the fuel is forced outof the injection port to the correct injector nozzle through thehigh-pressure line. See Figure 12-8.

NOTE: Because of the very tight tolerances in a diesel engine, the small-

est amount of dirt can cause excessive damage to the engine and to the fuel

injection system.

Distributor Injection Pump

A distributor diesel injection pump is a high-pressure pumpassembly with lines leading to each individual injector. Thehigh-pressure lines between the distributor and the injectorsmust be the exact same length to ensure proper injection tim-ing. The injection pump itself creates the injection advanceneeded for engine speeds above idle and the fuel is dischargedinto the lines. The high-pressure fuel causes the injectors toopen. Due to the internal friction of the lines, there is a slightdelay before fuel pressure opens the injector nozzle. SeeFigure 12-9.

NOTE: The lines expand some during an injection event.This is how tim-

ing checks are performed. The pulsing of the injector line is picked up by a

probe used to detect the injection event similar to a timing light used to

detect a spark on a gasoline engine.

High-Pressure Common Rail

Newer diesel engines use a fuel delivery system referred to asa high-pressure common rail (HPCR) design. Diesel fuelunder high pressure, over 20,000 PSI (138,000 kPa), is appliedto the injectors, which are opened by a solenoid controlled bythe computer. Because the injectors are computer controlled,the combustion process can be precisely controlled to providemaximum engine efficiency with the lowest possible noise andexhaust emissions. See Figure 12-10 on page 166.

HEUI SYSTEM

Ford 7.3- and 6.0-liter diesels use a system Ford calls aHydraulic Electronic Unit Injection system, or HEUI sys-tem. The components that replace the traditional mechanicalinjection pump include a high-pressure oil pump and reservoir,pressure regulator for the oil, and passages in the cylinder headfor flow of fuel to the injectors.

Fuel is drawn from the tank by the tandem fuel pump,which circulates fuel at low pressure through the fuelfilter/water separator/fuel heater bowl and then fuel is di-rected back to the fuel pump where fuel is pumped at highpressure into the cylinder head fuel galleries. The injectors,which are hydraulically actuated by the oil pressure from thehigh-pressure oil pump, are then fired by the power train con-trol module (PCM). The control system for the fuel injectorsis the PCM and the injectors are fired based on various inputsreceived by the PCM. See Figure 12-11 on page 166.

HEUI injectors rely on O-rings to keep fuel and oil frommixing or escaping, causing performance problems or enginedamage. HEUI injectors use five O-rings. The three external O-rings should be replaced with updated O-rings if they fail.The two internal O-rings are not replaceable and if these fail,




LIFTPUMP

FUELFILTER

FUEL INJECTION PUMP

RETURNLINE

EACH OF THE HIGH PRESSURE LINES MUST BE OF EQUAL LENGTH

FUEL TANK

INJECTIONTIMINGSTEPPERMOTOR

PIVOTADVANCEPISTON

ADVANCE RETARD

FUELLEVELSENSOR

INJECTOR

FIGURE 12-9 A schematic of a Stanadyne diesel fuel injection pump assembly showing all of the related components.

the injector or injectors must be replaced. The most commonsymptoms of injector O-ring trouble include:

■ Oil getting in the fuel■ The fuel filter element turning black■ Long cranking times before starting■ Sluggish performance■ Reduction in power■ Increased oil consumption often accompanies O-ring

problems or any fault that lets fuel in the oil

DIESEL INJECTOR NOZZLES

Diesel injector nozzles are spring-loaded closed valves thatspray fuel directly into the combustion chamber or precom-

bustion chamber. Injector nozzles are threaded into thecylinder head, one for each cylinder, and are replaceable asan assembly.

The top of the injector nozzle has many holes to deliveran atomized spray of diesel fuel into the cylinder. Parts of adiesel injector nozzle include:

■ Heat shield. This is the outer shell of the injector nozzleand has external threads where it seals in the cylinder head.

■ Injector body. This is the inner part of the nozzle andcontains the injector needle valve and spring, and threadsinto the outer heat shield.

■ Diesel injector needle valve. This precision machinedvalve and the tip of the needle seal against the injectorbody when it is closed. When the valve is open, diesel fuelis sprayed into the combustion chamber. This passage is


166 CHAPTER 12


FILTERWITH WATERSEPARATORAND INTEGRATEDHAND PUMP

HIGH PRESSURE PUMP

TANKELECTRONICCONTROLMODULE

SENSORS ACTUATORS

HIGH PRESSURE

PRESSURELIMITING VALVE

RAIL PRESSURESENSOR COMMON RAIL

(RIGHT BANK)

COMMON RAIL(LEFT BANK)

LOW PRESSURE

FIGURE 12-10 Overview of a computer-controlled high-pressure common rail V-8 diesel engine.

FIGURE 12-11 A HEUI injector from a Ford PowerStroke diesel engine.The grooves indicate the location of the O-rings.

Ford 7.3- and 6.0-liter diesel engines pump unfiltered oilfrom the sump to the high-pressure oil pump and thento the injectors. This means that not changing oil regu-larly can contribute to accumulation of dirt in the en-gine and will subject the fuel injectors to wear andpotential damage as particles suspended in the oil getforced into the injectors.

Tech TipCHANGE OIL

REGULARLY IN

A FORD DIESEL

ENGINE



If a gasoline-powered vehicle runs out of gasoline, it is aninconvenience and a possible additional expense to getsome gasoline. However, if a vehicle equipped with adiesel engine runs out of fuel, it can be a major concern.

Besides adding diesel fuel to the tank, the otherproblem is getting all of the air out of the pump, lines,and injectors so the engine will operate correctly.

The procedure usually involves cranking the en-gine long enough to get liquid diesel fuel back into thesystem, but at the same time keeping cranking timeshort enough to avoid overheating the starter. Consultservice information for the exact service procedure ifthe diesel engine is run out of fuel.

NOTE: Some diesel engines such as the General Motors Duramax V-8 are equipped with a priming pump located under the hood on topof the fuel filter. Pushing down and releasing the priming pump with avent valve open will purge any trapped air from the system. Always fol-low the vehicle manufacturer’s instructions.

TechTipNEVER ALLOW A

DIESEL ENGINE TO

RUN OUT OF FUEL

FIGURE 12-12 Typical computer-controlled diesel engine fuel injectors.

controlled by a solenoid on diesel engines equipped withcomputer-controlled injection.

■ Injector pressure chamber. The pressure chamber is amachined cavity in the injector body around the tip of theinjector needle. Injection pump pressure forces fuel intothis chamber, forcing the needle valve open.

DIESEL INJECTOR

NOZZLE OPERATION

The electric solenoid attached to the injector nozzle is com-puter controlled and opens to allow fuel to flow into the injec-tor pressure chamber. See Figure 12-12.

The diesel injector nozzle is mechanically opened by thehigh-pressure fuel delivered to the nozzle by the injectorpump. The fuel flows down through a fuel passage in the in-jector body and into the pressure chamber. The high fuel pres-sure in the pressure chamber forces the needle valve upward,compressing the needle valve return spring and forcing the


needle valve open. When the needle valve opens, diesel fuelis discharged into the combustion chamber in a hollow conespray pattern.

Any fuel that leaks past the needle valve returns to thefuel tank through a return passage and line.

GLOW PLUGS

Glow plugs are always used in diesel engines equipped witha precombustion chamber and may be used in direct injectiondiesel engines to aid starting. A glow plug is a heating ele-ment that uses 12 volts from the battery and aids in the start-ing of a cold engine. As the temperature of the glow plugincreases, the resistance of the heating element inside in-creases, thereby reducing the current in amperes needed bythe glow plugs.

Most glow plugs used in newer vehicles are controlled bythe power train control module (PCM), which monitorscoolant temperature and intake air temperature. The glowplugs are turned on or pulsed on or off depending on the tem-perature of the engine. The PCM will also keep the glow plugturned on after the engine starts to reduce white exhaustsmoke (unburned fuel) and to improve idle quality after start-ing. See Figure 12-13.

The “wait to start” lamp will light when the engine andthe outside temperature is low to allow time for the glow plugsto get hot. The “wait to start” lamp will not come on when theglow plugs are operating after the engine starts.

NOTE: The glow plugs are removed to test cylinder compression using a

special high-pressure reading gauge.



GLOWPLUG 2

GLOWPLUG 4

GLOWPLUG 6

GLOWPLUG 8

GLOWPLUG 1

GLOWPLUG 3

GLOWPLUG 5

GLOWPLUG 7

G101

GLOWPLUGRELAY

GLOWPLUG/INTAKEHEATER RELAYASSEMBLY

FUSIBLELINK

INTAKEAIR (IA)HEATER

GLOWPLUG

RELAYCONTROL

ENGINECONTROLMODULE(ECM)

POWERDISTRIBUTION

FUSIBLELINK

3

INTAKEAIR (IA)HEATERRELAY

FUSIBLELINK3

BATTERYFUSE175 A

FUSEHOLDER

HOT AT ALL TIMES HOT IN RUN AND START

FUEL HEATERFUSE15 A

FUSEBLOCK–UNDERHOOD

GLOWPLUGSIGNAL

52

IA HEATERRELAYCONTROL

78C1 C1

INTAKEHEATERDIAG 1

29 C2

INTAKEHEATERDIAG 2

ENGINECONTROLMODULE

62

FUSIBLELINK

FIGURE 12-13 A schematic of a typical glow plug circuit. Notice that the relay for the glow plug and intake air heater are both computer controlled.

Frequently Asked Question

WHAT ARE DIESEL ENGINE ADVANTAGES AND DISADVANTAGES?

A diesel engine has several advantages compared to asimilar size gasoline-powered engine including:

1. More torque output2. Greater fuel economy3. Long service life

A diesel engine has several disadvantages comparedto a similar size gasoline-powered engine including:

1. Engine noise, especially when cold and/or atidle speed

2. Exhaust smell3. Cold weather startability4. A vacuum pump is needed to supply the vacuum needs of

the heat, ventilation, and air conditioning system5. Heavier than a gasoline engine (See Figure 12-14.)6. Fuel availability7. Usually turbocharged adding to the number of parts and

increasing the cost of the engine. See Figure 12-15.

168 CHAPTER 12




ENGINE-DRIVEN

VACUUM PUMP

Because a diesel engine is unthrottled, it creates very littlevacuum in the intake manifold. Several engine and vehiclecomponents operate using vacuum, such as the exhaust gasrecirculation (EGR) valve and the heating and ventilationblend and air doors. Most diesels used in cars and light trucksare equipped with an engine-driven vacuum pump to supplythe vacuum for these components.

HEATED INTAKE AIR

Some diesels, such as the General Motors 6.6-liter DuramaxV-8, use an electrical heater wire to warm the intake air to helpin cold weather starting and running. See Figure 12-16.

ACCELERATOR PEDAL

POSITION SENSOR

Some light truck diesel engines are equipped with an elec-tronic throttle to control the amount of fuel injected into theengine. Because a diesel engine does not use a throttle in the airintake, the only way to control engine speed is by controlling

FrequentlyAsked Question

WHAT IS THE BIG DEAL FOR THENEED TO CONTROL VERY SMALLSOOT PARTICLES?

For many years soot or particulate matter (PM) wasthought to be less of a health concern than exhaustemissions from gasoline engines. It was felt that the sootcould simply fall to the ground without causing any no-ticeable harm to people or the environment. However,it was discovered that the small soot particulates whenbreathed in are not expelled from the lungs like largerparticles but instead get trapped in the deep areas of thelungs where they accumulate.

FIGURE 12-16 A wire wound electrical heater is used to warm theintake air on some diesel engines.

FIGURE 12-14 Roller lifter from a GM Duramax 6.6-liter V-8 dieselengine. Notice the size of this lifter compared to a roller lifter used in a gasolineengine.

FIGURE 12-15 All light diesels built in the last ten years are turbocharged with most equipped with variable vane turbochargers.


170 CHAPTER 12


25% 50%

PERCENTAGE THROTTLE OPENING

APP SENSOR

75% 100%

5V

4V

3V

2V

1V

0

APP #2

APP #1

APP #3

FIGURE 12-17 A typical accelerator pedal position (APP) sensor usesthree different sensors in one package with each creating a different voltage asthe accelerator is moved.

the amount of fuel being injected into the cylinders. Instead ofa mechanical link from the accelerator pedal to the diesel in-jection pump, a throttle-by-wire system uses an acceleratorpedal position sensor. To ensure safety, it consists of three sep-arate sensors that change in voltage as the accelerator pedal isdepressed. See Figure 12-17.

The computer checks for errors by comparing the voltageoutput of each of the three sensors inside the APP and comparesthem to what they should be if there are no faults. If an error isdetected, the engine and vehicle speed are often reduced.

SOOT OR PARTICULATE

MATTER

Soot particles may come directly from the exhaust tailpipe orthey can also form when emissions of nitrogen oxide and var-ious sulfur oxides chemically react with other pollutants sus-pended in the atmosphere. Such reactions result in theformation of ground-level ozone, commonly known as smog.Smog is the most visible form of what is generally referred toas particulate matter. Particulate matter refers to tiny particlesof solid or semisolid material suspended in the atmosphere.This includes particles between 0.1 micron and 50 microns indiameter. The heavier particles, larger than 50 microns, typi-cally tend to settle out quickly due to gravity. Particulates aregenerally categorized as follows:

■ TSP, Total Suspended Particulate—Refers to all parti-cles between 0.1 and 50 microns. Up until 1987, the EPAstandard for particulates was based on levels of TSP.

■ PM10—Particulate matter of 10 microns or less (approx-imately 1/6 the diameter of a human hair). EPA has astandard for particles based on levels of PM10.

■ PM2.5—Particulate matter of 2.5 microns or less (ap-proximately 1/20 the diameter of a human hair), alsocalled “fine” particles. In July 1997, the EPA approved astandard for PM2.5.

In general, soot particles produced by diesel combustionfall into the categories of fine, that is, less than 2.5 microns andultrafine, less than 0.1 micron. Ultrafine particles make upabout 80% to 95% of soot.

DIESEL OXIDATION

CATALYST (DOC)

Diesel oxidation catalyst (DOC) consists of a flow-throughhoneycomb-style substrate structure that is washcoated with alayer of catalyst materials, similar to those used in a gasoline en-gine catalytic converter. These materials include the precious met-als platinum and palladium, as well as other base metalscatalysts. Catalysts chemically react with exhaust gas to convertharmful nitrogen oxide into nitrogen dioxide, and to oxidize ab-sorbed hydrocarbons. The chemical reaction acts as a combustorfor the unburned fuel that is characteristic of diesel compressionignition. The main function of the DOC is to start a regenerationevent by converting the fuel-rich exhaust gases to heat.

The DOC also reduces carbon monoxide, hydrocarbons,plus odor-causing compounds such as aldehydes and sulfur,and the soluble organic fraction of particulate matter. Duringa regeneration event, the Catalyst System Efficiency test willrun. The engine control module (ECM) monitors this effi-ciency of the DOC by determining if the exhaust gas temper-ature sensor (EGT Sensor 1) reaches a predeterminedtemperature during a regeneration event.

DIESEL EXHAUST

PARTICULATE FILTER (DPF)

The heated exhaust gas from the DOC flows into the dieselparticulate filter (DPF), which captures diesel exhaust gasparticulates (soot) to prevent them from being released intothe atmosphere. This is done by forcing the exhaust through aporous cell which has a silicon carbide substrate withhoneycomb-cell-type channels that trap the soot. The chan-nels are washcoated with catalyst materials similar to those inthe DOC filter. The main difference between the DPF and atypical catalyst filter is that the entrance to every other cellchannel in the DPF substrate is blocked at one end. So insteadof flowing directly through the channels, the exhaust gas isforced through the porous walls of the blocked channels andexits through the adjacent open-ended channels. This type offilter is also referred to as a “wall-flow” filter.

Soot particulates in the gas remain trapped on the DPFchannel walls where, over time, the trapped particulate mat-ter will begin to clog the filter. The filter must therefore be




FIGURE 12-18 A diesel exhaust particulate filter on a Cummins 6.7-liter diesel engine.

purged periodically to remove accumulated soot particles.The process of purging soot from the DPF is described asregeneration. See Figure 12-18.

Exhaust Gas TemperatureSensors

There are two exhaust gas temperature sensors that functionin much the same way as engine temperature sensors. EGTSensor 1 is positioned between the DOC and the DPF whereit can measure the temperature of the exhaust gas entering theDPF. EGT Sensor 2 measures the temperature of the exhaustgas stream immediately after it exits the DPF.

The engine control module (ECM) monitors the signalsfrom the EGT sensors as part of its calibrations to control DPFregeneration. The ECM supplies biased 5-volts to the signal cir-cuit and a ground on the low reference circuit to EGT Sensor1. When the EGT Sensor 1 is cold, the sensor resistance is high.As the temperature increases, the sensor resistance decreases.With high sensor resistance, the ECM detects a high voltage onthe signal circuit. With lower sensor resistance, the ECMdetects a lower voltage on the signal circuit. Proper exhaust gastemperatures at the inlet of the DPF are crucial for properoperation and for starting the regeneration process. Too high atemperature at the DPF will cause the DPF substrate to melt orcrack. Regeneration will be terminated at temperatures above1470°F (800°C). With too low a temperature, self-regenera-tion will not fully complete the soot-burning process.

DPF Differential Pressure Sensor (DPS)

The DPF differential pressure sensor (DPS) has two pres-sure sample lines:

■ One line is attached before the DPF, labeled P1■ The other is located after the DPF, labeled P2

The exact location of the DPS varies by vehicle modeltype (medium duty, pickup or van). By measuring P1 exhaustsupply pressure from the DOC, and P2, post DPF pressure, theECM can determine differential pressure, also referred to as“delta” pressure, across the DPF. Data from the DPF differen-tial pressure sensor is used by the ECM to calibrate for control-ling DPF exhaust system operation. See Figure 12-19.

Diesel Particulate FilterRegeneration

Soot particulates in the gas remain trapped on the DPF chan-nel walls where, over time, the buildup of trapped particulatematter will begin to clog the filter. The filter must therefore be

FIGURE 12-19 A differential pressure sensor showing the two hosesfrom the diesel exhaust particulate filter.


WHAT IS AN EXHAUST AIR COOLER?

An exhaust air cooler is simply a length of tubing with anarrower center section that acts as a venturi. Thecooler is attached to the tailpipe with a bracket that pro-vides a gap between the two. As hot exhaust rushes pastthe gap, the venturi effect draws surrounding air intothe cooler and reduces the exhaust temperature. Thecooler significantly lowers exhaust temperature at thetailpipe from a potential 788° to 806°F (420° to 430°C)to approximately 520°F (270°C).


172 CHAPTER 12


purged periodically to remove accumulated soot particles. Theprocess of purging soot from the DPF by incineration is de-scribed as regeneration. When the temperature of the exhaustgas is increased sufficiently, the heat incinerates the soot par-ticles trapped in the filter, leaving only residual ash from theengine’s combustion of lubrication oil. The filter is effectivelyrenewed.

The primary reason for soot removal is to prevent thebuildup of exhaust back pressure. Excessive back pressure in-creases fuel consumption, reduces power output, and can po-tentially cause engine damage. There are a number ofoperational factors that can trigger the diesel engine controlmodule to initiate a DPF regeneration sequence. The ECMmonitors:

■ Distance since last DPF regeneration■ Fuel used since last DPF regeneration■ Engine run time since last DPF regeneration■ Exhaust differential pressure across the DPF

DPF Regeneration Process

A number of engine components are required to functiontogether for the regeneration process to be performed. ECMcontrols that impact DPF regeneration include late post-injections, engine speed, and adjusting fuel pressure. Addinglate post-injection pulses provides the engine with additionalfuel to be oxidized in the DOC which increases exhaust tem-peratures entering the DPF to about 900°F (500°C) and higher.The intake air valve acts as a restrictor that reduces air entry tothe engine which increases engine operating temperature. Theintake air heater may also be activated to warm intake airduring regeneration.

The variable vane turbocharger also plays a role in achiev-ing regeneration temperatures by reducing or increasing boostdepending on engine load.

Types of DPF Regeneration

DPF regeneration can be initiated in a number of ways, de-pending on the vehicle application and operating circum-stances. The two main regeneration types are:

■ Passive■ Active

Passive Regeneration. During normal vehicle operationwhen driving conditions produce sufficient load and exhausttemperatures, passive DPF regeneration may occur. Thispassive regeneration occurs without input from the ECM orthe driver. A passive regeneration may typically occur whilethe vehicle is being driven at highway speed or towing atrailer.

Active Regeneration. Active regeneration is commandedby the ECM when it determines that the DPF requiresit to remove excess soot buildup and conditions for filterregeneration have been met. Active regeneration is usuallynot noticeable to the driver. The vehicle needs to be driven atspeeds above 30 mph for approximately 20 to 30 minutes tocomplete a full regeneration. During regeneration, the ex-haust gases reach temperatures above 1000°F (550°C). If a re-generation event is interrupted for any reason, it will continuewhere it left off (including the next drive cycle) when the con-ditions are met for regeneration. Active regeneration is for themost part transparent to the customer. There are times whenregeneration is required, but the operating conditions do notmeet the ECM’s requirements, such as on a delivery vehiclethat is driven on frequent short trips or subjected to extendedidling conditions. In such cases, the ECM turns on a “regen-eration required” indicator to notify the vehicle operator thatthe filter requires cleaning.

DPF Service Regeneration

Another active regeneration method, the “DPF ServiceRegeneration” is a useful tool for the dealership technician.The procedure would typically be used to clean the DPFwhen vehicle operating conditions did not allow the DPF toregenerate normally while the vehicle is driven. A service re-generation procedure can also be run in order to clean theDPF when there is an unknown amount of soot present. Thismight result from engine or engine control errors caused by aCharge Air Cooler leak or low compression. In these cases, aDTC P2463 would normally set, and the DPF would have 80grams or less of accumulated soot. If over 100 grams of sootare present, P244B sets and a service light comes on to warnthe driver.


WILL THE POST-INJECTIONPULSES REDUCE FUELECONOMY?

Maybe. Due to the added fuel injection pulses and latefuel injection timing, an increase in fuel consumptionmay be noticed on the Driver Information Center (DIC)during the regeneration time period. A drop in overallfuel economy should not be noticeable.




Conditions for Running a DPF Service Regeneration

A service regeneration cannot be initiated if there are activediagnostic trouble codes (DTCs) present. Other conditionsthat the ECM checks are as follows:

■ The battery voltage is greater than 10 volts.■ The engine speed is between 600 and 1,250 RPM.■ The brake pedal is in the released position.■ The accelerator pedal is in the released position.■ The transmission must be in park or neutral.■ The engine coolant temperature (ECT) is between 158°F

(70°C) and 239°F (115°C).■ The vehicle’s fuel tank level must be between 15% and

85% capacity. For safety, refueling should never be per-formed during the regeneration process.

■ The exhaust gas temperature (EGT Sensors 1 and 2) mustbe less than 752°F (400°C).

CAUTION: To avoid extremely elevated exhaust temperatures, inspect

the exhaust cooler vent located at the tailpipe and remove any debris or mud

that would impede its operation.

1. DO NOT connect any shop exhaust removal hoses to the vehicle’s

tailpipe.

2. Park the vehicle outdoors and keep people, other vehicles, and

combustible material a safe distance away from the vehicle during

Service Regeneration.

3. Do not leave the vehicle unattended during Service Regeneration.

WARNING: Tailpipe outlet exhaust temperature will be greater than

572°F (300°C) during service regeneration.To help prevent personal injury

or property damage from fire or burns, keep vehicle exhaust away from

any object and people.

ASH LOADING

Regeneration will not burn off ash. Only the particulate mat-ter (PM) is burned off during regeneration. Ash is a noncom-bustible by-product from normal oil consumption. Ashaccumulation in the DPF will eventually cause a restriction inthe particulate filter. To service an ash loaded DPF, the DPFwill need to be removed from the vehicle and cleaned or re-placed. Low ash content engine oil (API CJ-4) is required forvehicles with the DPF system. The CJ-4 rated oil is limited to1% ash content.

DIESEL EXHAUST

SMOKE DIAGNOSIS

While some exhaust smoke is considered normal operation formany diesel engines, especially older units, the cause of exces-sive exhaust smoke should be diagnosed and repaired.

Black Smoke

Black exhaust smoke is caused by incomplete combustion be-cause of a lack of air or a fault in the injection system thatcould cause an excessive amount of fuel in the cylinders.Items that should be checked include the following:

■ Check the fuel specific gravity (API gravity).■ Perform an injector balance test to locate faulty injectors

using a scan tool.■ Check for proper operation of the engine coolant temper-

ature (ECT) sensor.■ Check for proper operation of the fuel rail pressure (FRP)

sensor.■ Check for restrictions in the intake or turbocharger.■ Check to see if the engine is using oil.

White Smoke

White exhaust smoke occurs most often during cold enginestarts because the smoke is usually condensed fuel droplets.White exhaust smoke is also an indication of cylinder misfireon a warm engine. The most common causes of white exhaustsmoke include:

■ Inoperative glow plugs■ Low engine compression■ Incorrect injector spray pattern■ A coolant leak into the combustion chamber

Gray or Blue Smoke

Blue exhaust smoke is usually due to oil consumption causedby worn piston rings, scored cylinder walls, or defective valvestem seals. Gray or blue smoke can also be caused by a defec-tive injector(s).

SCAN TOOL DIAGNOSIS

Diesel engines since the late 1980s have been computer con-trolled and are equipped with sensors and activators to controlfunctions that were previously mechanically controlled. Alllight truck diesels since 1996 have also adhered to on-boarddiagnostic systems (second generation [OBD-II]). The use of a


174 CHAPTER 12


FIGURE 12-20 A scan tool is used to retrieve diagnostic troublecodes and to perform injector balance tests.

FIGURE 12-21 A compression gauge designed for the highercompression rate of a diesel engine should be used when checking thecompression.scan tool to check for diagnostic trouble codes (DTCs) and to

monitor engine operation is one of the first diagnostic steps.See Figure 12-20.

COMPRESSION TESTING

A compression test is fundamental for determining themechanical condition of a diesel engine. Worn piston rings cancause low power and excessive exhaust smoke. A dieselengine should produce at least 300 PSI (2,068 kPa) of com-pression pressure and all cylinders should be within 50 PSI(345 kPa) of each other. See Figure 12-21.

GLOW PLUG RESISTANCE

BALANCE TEST

Glow plugs increase in resistance as their temperature in-creases. All glow plugs should have about the same resist-ance when checked with an ohmmeter. A similar test of theresistance of the glow plugs can be used to detect a weakcylinder. This test is particularly helpful on a diesel enginethat is not computer controlled. To test for even cylinder bal-ance using glow plug resistance, perform the following on awarm engine.

1. Unplug, measure, and record the resistance of all of theglow plugs.

2. With the wires still removed from the glow plugs, startthe engine.

3. Allow the engine to run for several minutes to allowthe combustion inside the cylinder to warm the glowplugs.

4. Measure the plugs and record the resistance of all of theglow plugs.

5. The resistance of all of the glow plugs should be higherthan at the beginning of the test. A glow plug that is in acylinder that is not firing correctly will not increase in re-sistance as much as the others.

6. Another test is to measure exhaust manifold tempera-ture at each exhaust port. Misfiring cylinders will runcold. This can be done with a contact or noncontactthermometer.

INJECTOR POP TESTING

A pop tester is a device used for checking a diesel injectornozzle for proper spray pattern. The handle is depressed and pop off pressure is displayed on the gauge. See Figure 12-22.

The spray pattern should be a hollow cone. This will varydepending on design. The nozzle should also be tested forleakage—dripping of the nozzle while under pressure. If thespray pattern is not correct, cleaning, repairing, or replacing ofthe injector nozzle may be necessary.




If diesel fuel is found on the engine, a high-pressure leakcould be present. When checking for a high-pressureleak, wear protective clothing including safety glasses andface shield plus gloves and long-sleeved shirt. Then use apiece of cardboard to locate the high-pressure leak. Whena diesel is running, the pressure in the common rail andinjector tubes can reach over 20,000 PSI. At these pres-sures the diesel fuel is atomized and cannot be seen butcan penetrate the skin and cause personal injury. A leakwill be shown as a dark area on the cardboard. When aleak is found, shut off the engine and locate the exact location of the leak without the engine running.

CAUTION: Sometimes a leak can actually cut through the card-board, so use extreme care.

Tech TipALWAYS USE

CARDBOARD TO

CHECK FOR HIGH-

PRESSURE LEAKS

DIESEL EMISSION TESTING

The most commonly used diesel exhaust emission test used instate or local testing programs is called the opacity test. Opac-ity means the percentage of light that is blocked by theexhaust smoke.

■ A 0% opacity means that the exhaust has no visible smokeand does not block light from a beam projected throughthe exhaust smoke.

■ A 100% opacity means that the exhaust is so dark that itcompletely blocks light from a beam projected through theexhaust smoke.

■ A 50% opacity means that the exhaust blocks half of the light from a beam projected through the exhaustsmoke.

Snap Acceleration Test

In a snap acceleration test, the vehicle is held stationary withwheel chocks and brakes released as the engine is rapidly

accelerated to high idle with the transmission in neutral whilesmoke emissions are measured. This test is conducted a mini-mum of six times and the three most consistent measurementsare averaged together for a final score.

Rolling Acceleration Test

Vehicles with a manual transmission are rapidly accelerated inlow gear from an idle speed to a maximum governed RPMwhile the smoke emissions are measured.

Stall Acceleration Test

Vehicles with automatic transmissions are held in a stationaryposition with the parking brake and service brakes appliedwhile the transmission is placed in “drive.” The accelerator isdepressed and held momentarily while smoke emissions aremeasured.

The standards for diesels vary according to the type ofvehicle and other factors, but usually include a 40% opacityor less.

FIGURE 12-22 A typical pop tester used to check the spray patternof a diesel engine injector.


176 CHAPTER 12


Summary

1. A diesel engine uses heat of compression to ignite thediesel fuel when it is injected into the compressed air inthe combustion chamber.

2. There are two basic designs of combustion chambersused in diesel engines. Indirect injection (IDI) uses a pre-combustion chamber whereas a direct injection (DI)occurs directly into the combustion chamber.

3. The three phases of diesel combustion include:a. Ignition delayb. Rapid combustionc. Controlled combustion

4. The typical diesel engine fuel system consists of the fueltank, lift pump, water-fuel separator, and fuel filter.

5. The engine-driven injection pump supplies high-pressurediesel fuel to the injectors.

6. The two most common types of fuel injection used inautomotive diesel engines are:a. Distributor-type injection pumpb. Common rail design where all of the injectors are fed

from the same fuel supply from a rail under high pressure

7. Injector nozzles are either opened by the high-pressurepulse from the distributor pump or electrically by thecomputer on a common rail design.

8. Glow plugs are used to help start a cold diesel engine andhelp prevent excessive white smoke during warm-up.

9. The higher the cetane rating of diesel fuel, the moreeasily the fuel is ignited.

10. Most automotive diesel engines are designed to operateon grade #2 diesel fuel in moderate weather conditions.

11. The API specific gravity of diesel fuel should be 30 to 39with a typical reading of 35 for #2 diesel fuel.

12. Diesel engines can be tested using a scan tool, as well asmeasuring the glow plug resistance or compression read-ing to determine a weak or nonfunctioning cylinder.

FIGURE 12-23 The letters on the side of this injector on a Cummins6.7-liter diesel indicate the calibration number for the injector.

In the past, it was common practice to switch diesel fuelinjectors from one cylinder to another when diagnosinga dead cylinder problem. However, most high-pressurecommon rail systems used in new diesels use preciselycalibrated injectors that should not be mixed up duringservice. Each injector has its own calibration number.See Figure 12-23.

Tech TipDO NOT SWITCH

INJECTORS

M12_HALD6891_06_SE_CH12.QXD 6/10/08 2:55 AM 6



1. What is the difference between direct injection and indirectinjection?

2. What are the three phases of diesel ignition?

3. What are the two most commonly used types of automotivediesel injection systems?

4. Why are glow plugs kept working after the engine starts?

5. What is the advantage of using diesel fuel with a high cetanerating?

6. How is the specific gravity of diesel fuel tested?

Review Questions

1. How is diesel fuel ignited in a warm diesel engine?

a. Glow plugs

b. Heat of compression

c. Spark plugs

d. Distributorless ignition system

2. Which type of diesel injection produces less noise?

a. Indirect injection (IDI)

b. Common rail

c. Direct injection

d. Distributor injection

3. Which diesel injection system requires the use of a glow plug?

a. Indirect injection (IDI)

b. High-pressure common rail

c. Direct injection

d. Distributor injection

4. The three phases of diesel ignition include ________.

a. Glow plug ignition, fast burn, slow burn

b. Slow burn, fast burn, slow burn

c. Ignition delay, rapid combustion, controlled combustion

d. Glow plug ignition, ignition delay, controlled combustion

5. What fuel system component is used in a vehicle equipped witha diesel engine that is not usually used on the same vehiclewhen it is equipped with a gasoline engine?

a. Fuel filter

b. Fuel supply line

c. Fuel return line

d. Water-fuel separator

6. The diesel injection pump is usually driven by a ________.

a. Gear off the camshaft

b. Belt off the crankshaft

c. Shaft drive off of the crankshaft

d. Chain drive off of the camshaft

7. Which diesel system supplies high-pressure diesel fuel to all ofthe injectors all of the time?

a. Distributor

b. Inline

c. High-pressure common rail

d. Rotary

8. Glow plugs should have high resistance when ________ andlower resistance when ________.

a. Cold/warm

b. Warm/cold

c. Wet/dry

d. Dry/wet

9. Technician A says that glow plugs are used to help start a dieselengine and are shut off as soon as the engine starts. Techni-cian B says that the glow plugs are turned off as soon as a flameis detected in the combustion chamber. Which technician iscorrect?

a. Technician A only

b. Technician B only

c. Both Technicians A and B

d. Neither Technician A nor B

10. What part should be removed to test cylinder compression on adiesel engine?

a. An injector

b. An intake valve rocker arm and stud

c. An exhaust valve

d. A glow plug

Chapter Quiz


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