engine performance info... · the ignition system must create a spark, or current flow, acr oss...

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Engine PerformanceTo have an efficient running engine, there must be the

correct amount of fuel mixed with the correct amount

of air. These must be present in a sealed container and

shocked by the right amount of heat, at the correct

time. With total efficiency, the engine would burn all

the fuel it receives and would release extremely low

amounts of pollutants in its exhaust. Although total

efficiency is not possible at this time, late-model

engines emit very low amounts of pollutants, thanks to

emission control devices.

To have highly efficient engines, the basic engine needs

highly efficient ignition, fuel, and emission control sys-

tems. Although there are many different designs of

these systems, the designs are similar in operation.

Engine performance systems are some of the most tech-

nologically advanced systems found on modern vehi-

cles. They all rely on a network of input sensors and

output devices and tie directly into the electronic

engine control system. Mastery of these systems is nec-

essary for an automotive technician.

The material in Section 4 matches the content areas of

the ASE certification test on engine performance and

sections of the certification test on engine repair.

Chapter 21 ■ IgnitionSystemsChapter 22 ■ Ignition SystemServiceChapter 23 ■ Fuel SystemsChapter 24 ■ Fuel SystemDiagnosis and ServiceChapter 25 ■ CarburetorsChapter 26 ■ CarburetorDiagnosis and ServiceChapter 27 ■ Fuel InjectionChapter 28 ■ Fuel InjectionDiagnosis and ServiceChapter 29 ■ EmissionControl SystemsChapter 30 ■ EmissionControl Diagnosis and ServiceChapter 31 ■ On-BoardDiagnostic SystemsChapter 32 ■ On-BoardDiagnostic System Diagnosisand Service

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21

One of the requirements for an efficient engine is the cor-rect amount of heat shock, delivered at the right time.This requirement is the responsibility of the ignition sys-tem. The ignition system supplies properly timed, high-voltage surges to the spark plugs. These voltage surgescause combustion inside the cylinder.

The ignition system must create a spark, or currentflow, across each pair of spark plug electrodes at the prop-er instant, under all engine operating conditions. Thismay sound relatively simple, but when one considers thenumber of spark plug firings required and the extremevariation in engine operating conditions, it is easy tounderstand why ignition systems are so complex.

If a six-cylinder engine is running at 4,000 revolutionsper minute (rpm), the ignition system must supply 12,000sparks per minute because the ignition system must fire3 spark plugs per revolution. These plug firings must alsooccur at the correct time and generate the correct amountof heat. If the ignition system fails to do these things, fueleconomy, engine performance, and emission levels will beadversely affected.

PURPOSE OF THE IGNITIONSYSTEMFor each cylinder in an engine, the ignition system hasthree main jobs. First, it must generate an electricalspark that has enough heat to ignite the air/fuel mixture

in the combustion chamber. Secondly, it must maintainthat spark long enough to allow for the combustion ofall the air and fuel in the cylinders. Lastly, it must deliv-er the spark to each cylinder so combustion can beginat the right time during the compression stroke of eachcylinder.

When the combustion process is completed, a veryhigh pressure is exerted against the top of the piston. Thispressure pushes the piston down on its power stroke. Thispressure is the force that gives the engine power. For anengine to produce the maximum amount of power it can,the maximum pressure from combustion should be pre-sent when the piston is at 10 to 23 degrees after top deadcenter (ATDC). Because combustion of the air/fuel mix-ture within a cylinder takes a short period of time, usu-ally measured in thousandths of a second (milliseconds),the combustion process must begin before the piston ison its power stroke. Therefore, the delivery of the sparkmust be timed to arrive at some point before the pistonreaches top dead center.

Determining how much before TDC the spark shouldbegin gets complicated because of the fact that as thespeed of the piston as it moves from its compressionstroke to its power stroke increases, the time needed forcombustion stays about the same. This means the sparkshould be delivered earlier as the engine’s speed increas-es (Figure 21–1). However, as the engine has to providemore power to do more work, the load on the crankshaft

O B J E C T I V E S◆ Describe the three major functions of an ignition system. ◆ Name the operating conditions of an enginethat affect ignition timing. ◆ Name the two major electrical circuits used in ignition systems and their com-mon components. ◆ Describe the operation of ignition coils, spark plugs, and ignition cables. ◆ Explain howhigh voltage is induced in the coil secondary winding. ◆ Describe the various types of spark timing systems,including electronic switching systems and their related engine position sensors. ◆ Explain the basic opera-tion of a computer-controlled ignition system. ◆ Explain how the fuel injection system may rely on compo-nents of the ignition system. ◆ Describe the various types of spark timing systems, including electronicswitching systems and their related engine position sensors. ◆ Describe the operation of distributor-basedignition systems. ◆ Describe the operation of distributorless ignition systems.

IGNITION SYSTEMS

Figure 21–1 Ignition must begin earlier as engine speedincreases. Courtesy of Ford Motor Company

tends to slow down the acceleration of the piston and thespark should be somewhat delayed.

Figuring out when the spark should begin gets morecomplicated due to the fact that the rate of combustionvaries according to certain factors. Higher compressionpressures tend to speed up combustion. Higher octanegasolines ignite less easily and require more burning time.

Increased vaporization and turbulence tend to decreasecombustion times. Other factors, including intake airtemperature, humidity, and barometric pressure, alsoaffect combustion. Because of all of these complications,delivering the spark at the right time is a difficult task.

There are basically two types of ignition found ontoday’s vehicles: distributor ignition (BDI) and electronicignition (EI) or distributorless ignition systems (DIS).The various designs and operation of these two types arediscussed in this chapter.

IGNITION TIMINGIgnition timing refers to the precise time spark occurs.Ignition timing is specified by referring to the position ofthe #1 piston relation to crankshaft rotation. Ignition tim-ing reference timing marks can be located on engine partsand on a pulley or flywheel to indicate the position of the#1 piston. Vehicle manufacturers specify initial or baseignition timing.

When the marks are aligned at TDC, or 0, the pistonin cylinder #1 is at TDC of its compression stroke. Addi-tional numbers on a scale indicate the number of degreesof crankshaft rotation before TDC (BTDC) or after TDC(ATDC). In a majority of engines, the initial timing isspecified at a point between TDC and 20 degrees BTDC.

If optimum engine performance is to be maintained,the ignition timing of the engine must change as the oper-ating conditions of the engine change. Ignition systemsallow for these necessary changes in many ways; these arecovered in greater detail later in this chapter. All the dif-ferent operating conditions affect the speed of the engineand the load on the engine. All ignition timing changesare made in response to these primary factors.

Engine RPMAt higher rpms, the crankshaft turns through moredegrees in a given period of time. If combustion is to becompleted by 10 degrees ATDC, ignition timing mustoccur sooner or be advanced.

However, air/fuel mixture turbulence (swirling)increases with rpm. This causes the mixture inside thecylinder to turn faster. Increased turbulence requires thatignition must occur slightly later or be slightly retarded.

These two factors must be balanced for best engineperformance. Therefore, while the ignition timing mustbe advanced as engine speed increases, the amount ofadvance must be decreased some to compensate for theincreased turbulence.

Engine LoadThe load on an engine is related to the work it must do.Driving up hills or pulling extra weight increases engineload. Under load there is resistance on the crankshaft,therefore the pistons have a harder time moving through

534 S E C T I O N 4 • E n g i n e P e r f o r m a n c e

their strokes. This is evident by the low measured vacu-um during heavy loads.

Under light loads and with the throttle plate(s) par-tially opened, a high vacuum exists in the intake mani-fold. The amount of air/fuel mixture drawn into themanifold and cylinders is small. On compression, thisthin mixture produces less combustion pressure and com-bustion time is slow. To complete combustion by 10degrees ATDC, ignition timing must be advanced.

Under heavy loads, when the throttle is opened fully,a larger mass of air/fuel mixture can be drawn in, and thevacuum in the manifold is low. High combustion pressureand rapid burning results. In such a case, the ignition tim-ing must be retarded to prevent complete burning fromoccurring before 10 degrees ATDC.

Firing OrderUp to this point, the primary focus of discussion has beenignition timing as it relates to any one cylinder. However,the function of the ignition system extends beyond timingthe arrival of a spark to a single cylinder. It must performthis task for each cylinder of the engine in a specificsequence.

Each cylinder of an engine must produce power oncein every 720 degrees of crankshaft rotation. Each cylin-der must have a power stroke at its own appropriate timeduring the rotation. To make this possible, the pistons androds are arranged in a precise fashion. This is called theengine’s firing order. The firing order is arranged toreduce rocking and imbalance problems. Because thepotential for this rocking is determined by the design andconstruction of the engine, the firing order varies fromengine to engine. Vehicle manufacturers simplify cylinderidentification by numbering each cylinder (Figure 21–2).

Regardless of the particular firing order used, the num-ber 1 cylinder always starts the firing order, with the restof the cylinders following in a fixed sequence.

The ignition system must be able to monitor the rota-tion of the crankshaft and the relative position of each pis-ton to determine which piston is on its compressionstroke. It must also be able to deliver a high-voltage surgeto each cylinder at the proper time during its compres-sion stroke. How the ignition system does these thingsdepends on the design of the system.

BASIC CIRCUITRYAll ignition systems consist of two interconnected elec-trical circuits: a primary (low voltage) circuit and a sec-ondary (high voltage) circuit (Figure 21–3).

Depending on the exact type of ignition system, com-ponents in the primary circuit include the following.

BatteryIgnition switchBallast resistor or resistance wire (some systems)Starting by-pass (some systems)Ignition coil primary windingTriggering deviceSwitching device or control moduleThe secondary circuit includes these components.Ignition coil secondary windingDistributor cap and rotor (some systems)Ignition (spark plugs) cablesSpark plugs

Primary Circuit OperationWhen the ignition switch is on, current from the batteryflows through the ignition switch and primary circuitresistor to the primary winding of the ignition coil. Fromhere it passes through some type of switching device andback to ground. The switching device can be electroni-cally or mechanically controlled by the triggering device.The current flow in the ignition coil’s primary windingcreates a magnetic field. The switching device or controlmodule interrupts this current flow at predeterminedtimes. When it does, the magnetic field in the primarywinding collapses. This collapse generates a high-voltagesurge in the secondary winding of the ignition coil. Thesecondary circuit of the system begins at this point.

Some ignition systems have a ballast resistor con-nected in series between the ignition switch and the coilpositive terminal. This resistor supplies the correctamount of voltage and current to the coil. In some igni-tion systems, a calibrated resistance wire is used in placeof the ballast resistor. Today, many ignition systems are

C H A P T E R 2 1 • I g n i t i o n S y s t e m s 535

Figure 21–2 Examples of typical firing orders.

4

1 3

42

563 4

12

27

56 3 4

81

3 2

5 3 1

6 4 2

1

7 5 3 1

8 6 4 2

Figure 21–3 Ignition systems have a primary and secondary (high voltage) circuit. Courtesy of Mitsubishi Motor Sales of America,Inc.


not equipped with the resistor. These systems supply 12volts directly to the coil.

There are also some ignition systems that do notrequire a ballast resistor. For instance, some control unitsdirectly regulate the current flow through the primary ofthe coil. Hall-effect and optical systems do not require bal-last resistors either. The signal voltage is not changed bythe speed of the distributor because it is in an inductivemagnetic signal generating system.

Secondary Circuit OperationThe secondary circuit carries high voltage to the sparkplugs. The exact manner in which the secondary circuitdelivers these high voltage surges depends on the systemdesign. Until 1984 all ignition systems used some type ofdistributor to accomplish this job. However, in an effortto reduce emissions, improve fuel economy, and boostcomponent reliability, most auto manufacturers are nowusing distributorless or Electronic Ignition (EI) systems.

Figure 21–4 Typical spark distribution system. Courtesy of American Honda Motor Co., Inc.

In a Distributor Ignition (DI) system, high-voltagefrom the secondary winding passes through an ignitioncable running from the coil to the distributor. Thedistributor then distributes the high voltage to the indi-vidual spark plugs through a set of ignition cables (Figure21–4). The cables are arranged in the distributor capaccording to the firing order of the engine. A rotor whichis driven by the distributor shaft rotates and completesthe electrical path from the secondary winding of the coilto the individual spark plugs. The distributor delivers thespark to match the compression stroke of the piston. Thedistributor assembly may also have the capability ofadvancing or retarding ignition timing.

The distributor cap is mounted on top of the distrib-utor assembly and an alignment notch in the cap fits overa matching lug on the housing. Therefore the cap canonly be installed in one position, which assures the cor-rect firing sequence.

The rotor is positioned on top of the distributor shaft,and a projection inside the rotor fits into a slot in theshaft. This allows the rotor to only be installed in oneposition. A metal strip on the top of the rotor makes con-tact with the center distributor cap terminal, and the outerend of the strip rotates past the cap terminals as it rotates

(Figure 21–5). This action completes the circuit betweenthe ignition coil and the individual spark plugs accordingto the firing order.

EI systems have no distributor; spark distribution iscontrolled by an electronic control unit and/or the vehi-


Figure 21–5 Relationship of a rotor and distributor cap.Courtesy of American Honda Motor Co., Inc.

Figure 21–6 An electronic ignition system for a 6-cylinder engine. Courtesy of Ford Motor Company

cle’s computer (Figure 21–6). Instead of a single ignitioncoil for all cylinders, each cylinder may have its own igni-tion coil, or two cylinders may share one coil. The coilsare wired directly to the spark plug they control. An igni-tion control module, tied into the vehicle’s computer con-trol system, controls the firing order and the spark timingand advance. This module is typically located under thecoil assembly.

A specific amount of energy is available in a secondaryignition circuit. In a secondary ignition circuit, the ener-gy is normally produced in the form of voltage requiredto start firing the spark plug and then a certain amountof current flow across the spark plug electrodes to main-tain the spark. Distributorless ignition systems are capa-

ble of producing much higher energy than conventionalignition systems.

Since distributor ignition and electronic ignition sys-tems are both firing spark plugs with approximately thesame air gap across the electrodes, the voltage requiredto start firing the spark plugs in both systems is similar.If the additional energy in the EI systems is not releasedin the form of voltage, it will be released in the form ofcurrent flow. This results in longer spark plug firing times.The average firing time across the spark plug electrodesin an EI system is 1.5 milliseconds compared to approx-imately 1 millisecond in a DI system. This extra time mayseem insignificant, but it is very important. Current emis-sion standards demand leaner air-fuel ratios, and this


additional spark duration on EI systems helps to preventcylinder misfiring with leaner air-fuel ratios. This is whymost car manufacturers have equipped their engines withEI systems.

IGNITION COMPONENTSAll ignition systems share a number of common compo-nents. Some, such as the battery and ignition switch, per-form simple functions. The battery supplies low-voltagecurrent to the ignition primary circuit. The current flowswhen the ignition switch is in the start or run position.Full-battery voltage is always present at the ignitionswitch, as if it were directly connected to the battery.

Ignition CoilsTo generate a spark to begin combustion, the ignition sys-tem must deliver high voltage to the spark plugs. Becausethe amount of voltage required to bridge the gap of thespark plug varies with the operating conditions, most late-model vehicles can easily supply 30,000 to 60,000 voltsto force a spark across the air gap. Since the battery deliv-ers 12 volts, a method of stepping up the voltage must beused. Multiplying battery voltage is the job of a coil.

The ignition coil is a pulse transformer. It transformsbattery voltage into short bursts of high voltage. As ex-plained previously, when a wire is moved through a mag-netic field, voltage is induced in the wire. The inverse ofthis principle is also true—when a magnetic field movesacross a wire, voltage is induced in the wire.

If a wire is bent into loops forming a coil and a mag-netic field is passed through the coil, an equal amount ofvoltage is generated in each loop of wire. The more loopsof wire in the coil, the greater the total voltage induced.

Also, the faster the magnetic field moves through thecoil, the higher the voltage induced in the coil. If thespeed of the magnetic field is doubled, the voltage out-put doubles.

An ignition coil uses these principles and has two coilsof wire wrapped around an iron core. An iron or steel coreis used because it has low inductive reluctance. In otherwords, iron freely expands or strengthens the magneticfield around the windings. The first, or primary, coil isnormally composed of 100 to 200 turns of 20-gauge wire.This coil of wire conducts battery current. When a cur-rent is passing through the primary coil, it magnetizes theiron core. The strength of the magnet depends directly onthe number of wire loops and the amount of current flow-ing through those loops. The secondary coil of wires mayconsist of 15,000 to 25,000, or more, turns of very finecopper wire.

Because of the effects of counter EMF on the currentflowing through the primary winding, it takes some timefor the coil to become fully magnetized or saturated.Therefore current flows in the primary winding for some

time between firings of the spark plugs. The period oftime during which there is primary current flow is calleddwell. The length of the dwell period is important.

When current flows through a conductor, it willimmediately reach its maximum value as allowed by theresistance in the circuit. If a conductor is wound into acoil, maximum current will not be immediately present.As the magnetic field begins to form as the current beginsto flow, the magnetic lines of force of one part of the wind-ing pass over another part of the winding. This tends tocause an opposition to current flow. This occurrence iscalled reactance. Reactance causes a temporary resis-tance to current flow and delays the flow of current fromreaching its maximum value. When maximum currentflow is present in a winding, the winding is said to be sat-urated and the strength of its magnetic field will also beat a maximum.

Saturation can only occur if the dwell period is longenough to allow for maximum current flow through theprimary windings. A less than saturated coil will not beable to produce the voltage it was designed to produce.If the energy from the coil is too low, the spark plugs maynot fire long enough or not fire at all.

When the primary coil circuit is suddenly opened, themagnetic field instantly collapses. The sudden collapsingof the magnetic field produces a very high voltage in thesecondary windings. This high voltage is used to push cur-rent across the gap of the spark plug. Figure 21–7 showsthe coil’s primary and secondary circuits in basic terms.


Figure 21–7 Current passing through the coil’s primarywinding creates magnetic lines of force that cut across andinduce voltage in the secondary windings.

Figure 21–9 Typical E-core ignition coil. Courtesy of FordMotor Company

IGNITION COIL CONSTRUCTION A laminatedsoft iron core is positioned at the center of the ignitioncoil and an insulated primary winding is wound aroundthe core. The two ends of the primary winding are con-nected to the primary terminals on top of the coil (Fig-ure 21–8). These terminals are usually identified withpositive and negative symbols. Enamel-type insulationprevents the primary windings from touching each other.Paper insulation is also placed between the layers ofwindings.

Secondary coil windings are on the inside of the pri-mary winding. A similar insulation method is used on thesecondary and primary windings. The ends of the sec-ondary winding are usually connected to one of the pri-mary terminals and to the high-tension terminal in the coiltower. When the windings and core assembly is mount-ed in the coil container, the core rests on a ceramic insu-lating cup in the bottom of the container. Metal sheathingis placed around the outside of the coil windings in thecontainer. This sheathing concentrates the magnetic fieldon the outside of the windings. A sealing washer is posi-tioned between the tower and the container to seal theunit. The coil assembly is filled with oil through the screwhole in the high-tension terminal. The unit is sealed to pro-tect the windings and to keep the oil inside. The oil helpsto cool the coil.

Most newer coils are not oil-cooled; they are air-cooledinstead. These coils are constructed in much the sameway, except the core is constructed from laminated sheetsof iron. These sheets are shaped like the letter “E” andthe primary and secondary windings are wound aroundthe center of the E-core (Figure 21–9).

SECONDARY VOLTAGE The typical amount of sec-ondary coil voltage required to jump the spark plug gapis 10,000 volts. Most coils have at least 25,000 volts avail-able from the secondary. The difference between the

required voltage and the maximum available voltage isreferred to as secondary reverse voltage. This reserve volt-age is necessary to compensate for high cylinder pres-sures and increased secondary resistances as the sparkplug gap increases through use. The maximum availablevoltage must always exceed the required firing voltage orignition misfire will occur. If there is an insufficientamount of voltage available to push current across thegap, the spark plug will not fire.

Since DI and EI systems are both firing spark plugswith approximately the same air gaps, the amount of volt-age required to fire the spark is nearly the same in bothsystems. However, EI systems have higher voltagereserves. If the additional voltage in an EI system is notused to start the spark across the plug’s gap, it is used tomaintain the spark for a longer period of time.

The number of ignition coils used in an ignition sys-tem varies with the type of ignition system found on avehicle. In most ignition systems with a distributor, onlyone ignition coil is used. The high voltage of the secondarywinding is directed, by the distributor, to the variousspark plugs in the system. Therefore, there is one sec-ondary circuit with a continually changing path.

While distributor systems have a single secondary cir-cuit with a continually changing path, distributorless sys-tems have several secondary circuits, each with anunchanging path.

Spark PlugsEvery type of ignition system uses spark plugs. The sparkplugs provide the crucial air gap across which the highvoltage from the coil flows in the form of an arc. The threemain parts of a spark plug are the steel core, the ceram-ic core, or insulator, which acts as a heat conductor; anda pair of electrodes, one insulated in the core and the


Primary(−) terminal

Secondary winding22,000 windings

Primarymagnetic field

Primary winding200 turns

Primary(+) terminal

High tension terminal

Soft iron core

Figure 21–8 Ignition coil design. Courtesy of Daimler-Chrysler Corporation

“E” CORE COIL HAS A HIGHER ENERGY TRANSFER DUE TO THE LAMINATIONS PROVIDINGCLOSED MAGNETIC PATH

CALLED “E” COREBECAUSE OF THE SHAPE OF THE LAMINATIONSMAKING UP THE CORE

Figure 21–10 Components of a typical spark plug.

other grounded on the shell. The shell holds the ceramiccore and electrodes in a gas-tight assembly and has thethreads needed for plug installation in the engine (Figure21–10). An ignition cable connects the secondary to thetop of the plug. Current flows through the center of theplug and arcs from the tip of the center (or side) electrodeto the ground electrode. The resulting spark ignites theair/fuel mixture in the combustion chamber. Most auto-motive spark plugs also have a resistor between the topterminal and the center electrode. This resistor reducesradio frequency interference (RFI), which prevents noiseon stereo equipment. Voltage peaks from RFI could alsointerfere with, or damage, on-board computers. There-fore, when resistor-type spark plugs are used as originalequipment, replacement spark plugs must also be resis-tor-type. The resistor, like all other resistances in the sec-ondary, increases the voltage needed to jump the gap ofthe spark plug.

Spark plugs come in many different sizes and designsto accommodate different engines. To fit properly, sparkplugs must be of the proper size and reach. Another designfactor that determines the usefulness of a spark plug for aspecific application is its heat range. The desired heatrange depends on the design of the engine and on the typeof driving conditions the vehicle is subject to.

A terminal post on top of the center electrode is thepoint of contact for the spark plug cable. The center elec-trode, commonly made of a copper alloy, is surroundedby a ceramic insulator and a copper and glass seal is locat-ed between the electrode and the insulator. These sealsprevent combustion gases from leaking out of the cylin-der. Ribs on the insulator increase the distance betweenthe terminal and the shell to help prevent electric arcingon the outside of the insulator. The steel spark plug shell

is crimped over the insulation and a ground electrode, onthe lower end of the shell, is positioned directly below thecenter electrode. There is an air gap between these twoelectrodes, and the width of this air gap is specified by theauto manufacturer. Some spark plugs have platinum-tipped electrodes (Figure 21–11), that greatly extend thelife of the plug.

A spark plug socket may be placed over a hex-shapedarea near the top of the shell for plug removal and instal-lation. Threads on the lower end of the shell allow theplug to be threaded into the engine’s cylinder head.

Size. Automotive spark plugs are available in either 14-or 18-millimeter diameters. All 18-millimeter plugs fea-ture tapered seats that match similar seats in the cylinderhead and need no gaskets. The 14-millimeter variety canhave either a flat seat that requires a gasket or a taperedseat that does not. The latter is the most commonly used.All spark plugs have a hex-shaped shell that accommo-dates a socket wrench for installation and removal. The14-millimeter, tapered seat plugs have shells with a 5⁄8-inch hex; 14-millimeter gasketed and 18-millimetertapered seat plugs have shells with a 13⁄16-inch hex.

Reach. One of the most important design characteristicsof spark plugs is the reach (Figure 21–12). This refers to


Figure 21–11 A platinum-tipped spark plug.

Figure 21–12 Spark plug reach: long versus short.

Figure 21–13 Spark plug heat range: hot versus cold.

the length of the shell from the contact surface at the seatto the bottom of the shell, including both threaded andnonthreaded sections. Reach is crucial. The plug’s air gapmust be properly placed in the combustion chamber soit can produce the correct amount of heat. Installing plugswith too short a reach means the electrodes are in a pock-et and the arc is not able to adequately ignite the air/fuelmixture. If the reach is too long, the exposed plug threadscan get so hot they will ignite the air/fuel mixture at thewrong time, causing preignition. Preignition is a termused to describe abnormal combustion, which is causedby something other than the heat of the spark.

Heat Range. When the engine is running, most of theplug’s heat is concentrated on the center electrode. Heatis quickly dissipated from the ground electrode becauseit is threaded into the cylinder head. The spark plug heatpath is from the center electrode through the insulatorinto the shell and to the cylinder head, where the heat isabsorbed by engine coolant circulating through the cylin-der head. Spark plug heat range is determined by thedepth of the insulator before it contacts the shell. Forexample, in a cold-range spark plug, the depth of the insu-lator is short before it contacts the shell. This cold-typespark plug has a short heat path, which provides coolerelectrode operation (Figure 21–13).

In a hot spark plug, the insulator depth is increasedbefore it makes contact with the shell. This provides alonger heat path and increases electrode temperature. Aspark plug needs to retain enough heat to clean itselfbetween firings, but not be so hot that it damages itselfor causes premature ignition of the air/fuel mixture in thecylinder.

If an engine is driven continually at low speeds, thespark plugs may become carbon fouled. Under this con-dition, a hotter range spark plug may be required. Severehigh-speed driving over an extended time period mayrequire a colder range spark plug to prevent electrodeburning from excessive combustion chamber heat.

The heat range is indicated by a code imprinted on theside of the plug, usually on the porcelain insulator.

Spark Plug Air Gap. The correct spark plug air gap isessential to achieve optimum engine performance andlong plug life. A gap that is too wide requires higher volt-age to jump the gap. If the required voltage is greater thanwhat is available, the result is misfiring. Misfiring resultsfrom the inability of the ignition to jump the gap or theinability to maintain the spark. On the other hand, a gapthat is too narrow requires lower voltages, which leads torough idle and prematurely burned electrodes, due tohigher current flow. Always set the gap according to themanufacturer’s specifications.

Ignition CablesSpark plug cables, or ignition cables, make up the sec-ondary wiring. These cables carry the high voltage fromthe distributor or the multiple coils to the spark plugs. Thecables are not solid wire; instead they contain fiber coresthat act as resistors in the secondary circuit (Figure21–14). They cut down on radio and television interfer-ence, increase firing voltages, and reduce spark plug wearby decreasing current. Insulated boots on the ends of thecables strengthen the connections as well as prevent dustand water infiltration and voltage loss.

Some engines have spark plug cable heat shields(Figure 21–15) pressed into the cylinder head. Theseshields surround each spark plug boot and spark plug.They protect the spark plug boot from damage due to theextreme heat generated by the nearby exhaust manifold.

SPARK TIMING SYSTEMSTo better understand the operation of current ignitionsystems, it is helpful to first review how older, fullymechanical distributor systems worked.

S H O P T A L KWhen spark plugs must be replaced, follow therecommendations for plug type as specified bythe engine and plug manufacturers. These are

recommendations only. However, until you knowmore about the engine and its operating conditions

than the manufacturers do, follow theirrecommendations. ■


Glass and cotton braid

Insulation

Core

Jacket

• Hypalon — normal• Silicon — high temperature

Figure 21–14 Spark plug cable construction. Courtesyof DaimlerChrysler Corporation

Figure 21–16 Typical early solid-state ignition system. Courtesy of DaimlerChrysler Corporation

Figure 21–15 Spark plug boot heat shields. Courtesy ofDaimlerChrysler Corporation

Breaker Point IgnitionBreaker point ignition systems were used on vehicles formore than 60 years, but were abandoned many years agoas engineers looked for ways to decrease emissions andincrease fuel efficiency.

The distributor assembly acted as a mechanical switchto turn the primary circuit on and off. The distributor’sshaft, cam, breaker points, and condenser performed thisfunction. Contact points are used as the primary circuittriggering and switching device. The breaker point assem-bly, which was mounted on the breaker plate inside thedistributor, consisted of a fixed contact, movable contact,movable arm, rubbing block, pivot, and spring. The fixed

contact was grounded through the distributor housing,and the movable contact was connected to the negativeterminal of the coil’s primary winding. As the cam turnedby the camshaft, the movable arm opened and closed,which opened and closed the primary circuit in the coil.When the points were closed, primary current flowattempted to saturate the coil. When the points opened,primary current stopped and the magnetic field collapsed,causing high voltage to be induced in the secondary. Thefiring of the plug was the result of opening the points.

Because voltage was still present at the movable armwhen the breaker arms opened, current could continueto arc across the open point gap, which could damagethe points. To prevent this, a condenser was attached tothe movable arm. In this way, the voltage at the movablearm was retained by the condenser instead of arcingacross the gap.

The distributor also mechanically adjusted the timethe spark arrived at the cylinder through the use of twomechanisms: the centrifugal advance and the vacuumadvance units. This improved engine performance, fuelefficiency, and emission levels.

As its name implies, the distributor mechanically dis-tributed the spark so that it arrived at the right time duringthe compression stroke of each cylinder. The distributor’sshaft, rotor, and cap performed this function.

Solid-State IgnitionFrom the fully mechanical breaker point system, ignitiontechnology progressed to basic electronic or solid-stateignitions (Figure 21–16). Breaker points were replaced


Spark plug wire

Sparkplug

Distributor capand rotor

Ignitionswitch Dual

ballastresistor

Ignition coil

Secondarywindings

Primarywindings

ECUopensprimary

Electroniccontrolunit

Reluctor

Pickupcoil

Distributor

Voltage signalfrom pick-up

High voltage surge

with electronic triggering and switching devices. The elec-tronic switching components are normally inside a sepa-rate housing known as an electronic control unit (ECU)or control module. The original (solid state) electronicignitions still relied on mechanical and vacuum advancemechanisms in the distributor.

As technology advanced, many manufacturers ex-panded the ability of the ignition control modules. Forexample, by tying a manifold vacuum sensor into the igni-tion module circuitry, the module could now detect whenthe engine was under heavy load and retard the timingautomatically. Similar add-on sensors and circuits weredesigned to control spark knock, start-up emissions, andaltitude compensation.

Computer-Controlled IgnitionComputer-controlled ignition systems offer continuousspark timing control through a network of engine sensorsand a central microprocessor. Based on the inputs it re-ceives, the central microprocessor or computer makesdecisions regarding spark timing and sends signals to theignition module to fire the spark plugs according to thoseinputs and the programs in its memory.

SWITCHING SYSTEMSElectronic ignition systems control the primary circuitwith an NPN transistor. The transistor’s emitter is con-nected to ground. The collector is connected to the neg-ative (–) terminal of the coil. When the triggering devicesupplies a small amount of current to the base of theswitching transistor, the collector and emitter allow cur-rent to build up in the coil primary circuit. When the cur-rent to the base is interrupted by the switching device, thecollector and emitter interrupt the coil primary current.An example of how this works is shown in Figure 21–17,which is a simplified diagram of an electronic ignitionsystem.

Engine Position SensorsThe time when the primary circuit must be opened andclosed is related to the position of the pistons and the

crankshaft. Therefore, the position of the crankshaft isused to control the flow of current to the base of theswitching transistor.

A number of different types of sensors are used tomonitor the position of the crankshaft and control theflow of current to the base of the transistor. These engineposition sensors and generators serve as triggeringdevices and include magnetic pulse generators, metaldetection sensors, Hall-effect sensors, and photoelectric(optical) sensors.

The mounting location of these sensors depends onthe design of the ignition system. All four types of sen-sors can be mounted in the distributor, which is turnedby the camshaft.

Magnetic pulse generators and Hall-effect sensors canalso be located on the crankshaft. These sensors are alsocommonly used on EI systems.

Magnetic Pulse GeneratorBasically, a magnetic pulse generator consists of twoparts: a timing disc and a pick-up coil. The timing discmay also be called a reluctor, trigger wheel, pulse ring,armature, or timing core. The pick-up coil, which con-sists of a length of wire wound around a weak permanentmagnet, may also be called a stator, sensor, or pole piece.Depending on the type of ignition system used, the tim-ing disc may be mounted on the distributor shaft, at therear of the crankshaft (Figure 21–18), or on the crank-shaft vibration damper (Figure 21–19).

The magnetic pulse or PM generator operates on basicelectromagnetic principles. Remember that a voltage canonly be induced when a conductor moves through a mag-netic field. The magnetic field is provided by the pick-upunit and the rotating timing disc provides the movementthrough the magnetic field needed to induce voltage.

As the disc teeth approach the pick-up coil, they repelthe magnetic field, forcing it to concentrate around thepick-up coil (Figure 21–20A). Once the tooth passes bythe pick-up coil, the magnetic field is free to expand orunconcentrate (Figure 21–20B), until the next tooth onthe disc approaches. Approaching teeth concentrate the


Figure 21–17 When the triggering device supplies a small amount of current to the transistor’s base, the primary coilcircuit is closed and current flows.

Figure 21–20 (A) Wide gap produces a weak magnet-ic signal (B) Narrow gap produces a strong magnetic field.Courtesy of DaimlerChrysler Corporation

Since there is no movement or change in the field, volt-age at this precise moment drops to zero. At this point,the switching device inside the ignition module reacts tothe zero voltage signal by turning the ignition’s primarycircuit current off. As explained earlier, this forces themagnetic field in the primary coil to collapse, discharg-ing a secondary voltage to the distributor or directly tothe spark plug.

As soon as the tooth rotates past the pick-up coil, themagnetic field expands again and another voltage signalis induced. The only difference is that the polarity of thecharge is reversed. Negative becomes positive or positivebecomes negative. Upon sensing this change in voltage,the switching device turns the primary circuit back on andthe process begins all over.

The slotted disc is mounted on the crankshaft, vibra-tion damper, or distributor shaft in a very precise man-ner. When the disc teeth align with the pick-up coil, thiscorresponds to the exact time certain pistons are near-ing TDC. This means the zero voltage signal needed totrigger the secondary circuit occurs at precisely the cor-rect time.


Figure 21–18 Magnetic pulse generator positioned tosense flywheel rotation. Courtesy of DaimlerChrysler Corporation

Figure 21–19 Crankshaft position sensor. Courtesy ofFord Motor Company

magnetic lines of force, while passing teeth allow them toexpand. This pulsation of the magnetic field causes thelines of magnetic force to cut across the winding in thepick-up coil, inducing a small amount of AC voltage thatis sent to the switching device in the primary circuit.

When a disc tooth is directly in line with the pick-upcoil, the magnetic field is not expanding or contracting.

Strongermagneticfield

Narrowair gap

Weakmagneticfield

Wideair gap

(A)

(B)

Figure 21–21 Magnetic pulse generator (pick-up coil)used on early Chrysler electronic ignition systems.

The pick-up coil might have only one pole as shownas Figure 21–21. Other magnetic pulse generators havepick-up coils with two or more poles.

Metal Detection SensorsMetal detection sensors are found on early electronic igni-tion systems. They work much like a magnetic pulse gen-erator with one major difference.

A trigger wheel is pressed over the distributor shaftand a pick-up coil detects the passing of the trigger teethas the distributor shaft rotates. However, unlike a mag-netic pulse generator, the pick-up coil of a metal detec-tion sensor does not have a permanent magnet. Instead,the pick-up coil is an electromagnet. A low level of cur-rent is supplied to the coil by an electronic control unit,inducing a weak magnetic field around the coil. As thereluctor on the distributor shaft rotates, the trigger teethpass very close to the coil (Figure 21–22). As the teethpass in and out of the coil’s magnetic field, the magnet-ic field builds and collapses, producing a correspondingchange in the coil’s voltage. The voltage changes aremonitored by the control unit to determine crankshaftposition.

Hall-Effect SensorThe Hall-effect sensor or switch is the most commonlyused engine position sensor. There are several good rea-sons for this. Unlike a magnetic pulse generator, the Hall-effect sensor produces an accurate voltage signalthroughout the entire rpm range of the engine. Further-more, a Hall-effect switch produces a square wave signalthat is more compatible with the digital signals requiredby on-board computers.

Functionally, a Hall switch performs the same tasksas a magnetic pulse generator. But the Hall switch’smethod of generating voltage is quite unique. It is based

on the Hall-effect principle, which states: If a current isallowed to flow through a thin conducting material, andthat material is exposed to a magnetic field, voltage isproduced.

The heart of the Hall generator is a thin semiconduc-tor layer (Hall layer) derived from a gallium arsenate crys-tal. Attached to it are two terminals-one positive and theother negative-that are used to provide the source currentfor the Hall transformation.

Directly across from this semiconductor element is apermanent magnet. It is positioned so that its lines of fluxbisect the Hall layer at right angles to the direction of cur-rent flow. Two additional terminals, located on either sideof the Hall layer, form the signal output circuit.

When a moving metallic shutter blocks the magneticfield from reaching the Hall layer or element, the Hall-effect switch produces a voltage signal. When the shut-ter blade moves and allows the magnetic field to expandand reach the Hall element, the Hall-effect switch doesnot generate a voltage signal (Figure 21–23).

The Hall switch is described as being “on” any timethe Hall layer is exposed to a magnetic field and a Hallvoltage is being produced (Figure 21–24). However,before this signal voltage can be of any use, it has to bemodified. After leaving the Hall layer, the signal is rout-ed to an amplifier where it is strengthened and invertedso the signal reads high when it is actually coming in lowand vice versa. Once it has been inverted, the signal goesthrough a pulse-shaping device called the Schmitt triggerwhere it is turned into a clean square wave signal. Afterconditioning, the signal is sent to the base of a switchingtransistor that is designed to turn on and off in responseto the signals generated by the Hall switch assembly.

The shutter wheel is the last major component of theHall switch. The shutter wheel consists of a series ofalternating windows and vanes that pass between theHall layer and magnet. The shutter wheel may be part of


Figure 21–22 In a metal detecting sensor, the revolv-ing trigger wheel teeth alter the magnetic field produced bythe electromagnet in the pick-up coil.

Figure 21–23 Blade and magnetic field action in aHall-effect switch.

the distributor rotor (Figure 21–25) or be separate fromthe rotor.

The points where the shutter vane begins to enter andbegins to leave the air gap are directly related to primarycircuit control. As the leading edge of a vane enters the

air gap, the magnetic field is deflected away from the Halllayer; Hall voltage decreases. When that happens, themodified Hall output signal increases abruptly and turnson the switching transistor. Once the transistor is turnedon, the primary circuit closes and the coil’s energy stor-age cycle begins.

Primary current continues to flow as long as the vaneis in the air gap. As the vane starts to leave the gap, how-ever, the reforming Hall voltage signal prompts a paral-lel decline in the modified output signal. When the outputsignal goes low, the bias of the transistor changes. Primarycurrent flow stops.

In summary, the ignition module supplies current tothe coil’s primary winding as long as the shutter wheel’svane is in the air gap. As soon as the shutter wheel movesaway and the Hall voltage is produced, the control unitstops primary circuit current, high secondary voltage isinduced, and ignition occurs.

In addition to ignition control, a Hall switch can alsobe used to generate precise rpm signals (by determining


Magneticlines

MagnetHallelement

Blade out of Hall effect switchlow voltage signal

MagnetHallelement

Magneticlines

Blade in Hall effect switchhigh voltage signal

Figure 21–24 Operation of a Hall-effect switch. Courtesy of Ford Motor Company

Rotor

Shutter blade

Hall effectswitch unit

Figure 21–25 Hall effect switch mounted inside adistributor. Courtesy of DaimlerChrysler Corporation

Figure 21–27 Typical centrifugal advance mechanism.

Figure 21–26 Distributor with optical-type pick-ups.Courtesy of DaimlerChrysler Corporation

the frequency at which the voltage rises and falls) and pro-vide the sync pulse for sequential fuel ignition operation.

Photoelectric SensorA fourth type of crankshaft position sensor is the photo-electric sensor. The parts of this sensor include a light-emitting diode (LED), a light-sensitive phototransistor(photo cell), and a slotted disc called a light beam inter-rupter (Figure 21–26).

The slotted disc is attached to the distributor shaft.The LED and the photo cell are situated over and underthe disc opposite each other. As the slotted disc rotatesbetween the LED and photo cell, light from the LEDshines through the slots. The intermittent flashes of lightare translated into voltage pulses by the photo cell. Whenthe voltage signal occurs, the control unit turns on the pri-mary system. When the disc interrupts the light and thevoltage signal ceases, the control unit turns the primarysystem off, causing the magnetic field in the coil to col-lapse and sending a surge of voltage to a spark plug.

The photoelectric sensor sends a very reliable signalto the control unit, especially at low engine speeds.These units have been primarily used on Chrysler andMitsubishi engines. Some Nissan and General Motorsproducts have used them as well.

Timing AdvanceAs stated, early solid-state ignition systems changed thetiming mechanically. At idle, the firing of the spark plugusually occurs just before the piston reaches top deadcenter. At higher engine speeds, however, the spark mustbe delivered to the cylinder much earlier in the cycle toachieve maximum power from the air/fuel mixturebecause the engine is moving through the cycle morequickly. To change the timing of the spark in relationto rpm, the centrifugal advance mechanism was used(Figure 21–27).

CENTRIFUGAL ADVANCE This mechanism consistsof a set of pivoted weights and springs connected to thedistributor shaft and a distributor armature assembly.During idle speeds, the springs keep the weights in placeand the armature and distributor shaft rotate as oneassembly. When speed increases, centrifugal force caus-es the weights to slowly move out against the tension ofthe springs. This allows the armature assembly to moveahead in relation to the distributor shaft rotation. Theignition’s triggering device is mounted to the armatureassembly. Therefore, as the assembly moves ahead, igni-tion timing becomes more advanced.

VACUUM ADVANCE During part-throttle engineoperation, high vacuum is present in the intake manifold.To get the most power and the best fuel economy fromthe engine, the plugs must fire even earlier during thecompression stroke than is provided by a centrifugaladvance mechanism.

The heart of the vacuum advance mechanism (Figure21–28) is the spring-loaded diaphragm, which fits insidea metal housing and connects to a movable plate on whichthe pick-up coil is mounted. Vacuum is applied to one sideof the diaphragm in the housing chamber, while the otherside of the diaphragm is open to the atmosphere. Anyincrease in vacuum allows atmospheric pressure to pushthe diaphragm. In turn, this causes the movable plate torotate. The more vacuum present on one side of thediaphragm, the more atmospheric pressure is able tocause a change in timing. The rotation of the movableplate moves the pick-up coil so the armature develops asignal earlier. These units are also equipped with a springthat returns the timing back to basic as the vacuumdecreases.


Rotor

LEDs

Disc

Photodiodes

The vacuum advance unit is bolted to the side of thedistributor housing. On some models, the vacuum hosefor the vacuum advance is connected directly to theintake manifold, whereas on other models this hose isconnected to a fitting right above the throttle plates. Thediaphragm is pushed inward toward the distributorhousing by a spring positioned between the diaphragmand the end of the sealed chamber. An arm is attachedto the side of the diaphragm next to the distributor hous-ing, and the inner end of the arm is mounted over a pinon the pickup plate.

If the engine is operating at a moderate cruisingspeed, high intake manifold vacuum is applied to thevacuum advance diaphragm, thereby advancing the tim-ing. When the engine is operating at a moderate cruis-ing speed with a partially opened throttle, the amountsof cylinder air intake, compression pressure, and com-pression temperature are low. Under these conditions,the air/fuel mixture does not burn as fast and the addi-tional spark advance provides more burn time, whichincreases fuel economy and engine performance.

If the engine is operating at or near wide-open throt-tle, the amounts of cylinder air intake, compression pres-sure, and compression temperature are increased, whichcauses faster burning of the air/fuel mixture. During theseoperating conditions, the spark advance must be retard-ed to prevent engine detonation. Intake manifold vacu-um is very low at wide throttle opening. Therefore, littlevacuum is present to work against the spring in the vac-uum advance unit. When this action occurs, the pick-upcoil is moved back toward the base timing position andthe amount of spark advance is reduced.

If the vacuum advance hose is connected above thethrottle plates, the vacuum advance does not provide anyspark advance when the engine is idling. When the vac-uum advance hose is connected directly into the intake

manifold, the vacuum advance is fully advanced at idlespeed, which provides lower engine temperatures.

DistributorThe reluctor and distributor shaft assembly rotates onbushings in the aluminum distributor housing. A roll pinextends through a retainer and the distributor shaft toretain the shaft in the distributor. Another roll pin retainsthe drive gear to the lower end of the shaft. On manyengines, this drive gear is meshed with the camshaft gearto drive the distributor. Some distributors have an offsetslot on the end of the distributor shaft that meshes witha matching slot in a gear driven from the camshaft. Thegear size is designed to drive the distributor at the samespeed as the camshaft, which rotates at one-half the speedof the crankshaft in a four-stroke cycle engine.

If the distributor has advance mechanisms, the cen-trifugal advance is sometimes mounted under the pickupplate, and the vacuum advance is positioned on the sideof the distributor. Most engines manufactured in recentyears have computer-controlled spark advance, and thedistributor advance mechanisms are eliminated.

DISTRIBUTOR IGNITION SYSTEMOPERATIONThe primary circuit of a distributor ignition (DI) systemis controlled electronically by one of the sensors justdescribed and an electronic control unit (module) thatcontains some type of switching device.

Primary CircuitWhen the ignition switch is in the ON position, currentfrom the battery flows through the ignition switch andprimary circuit resistor to the primary winding of the igni-tion coil. From there it passes through some type ofswitching device and back to ground. The switching de-vice is controlled by the triggering device. The currentflow in the ignition coil’s primary winding creates a mag-netic field. The switching device or control module inter-rupts this current flow at predetermined times. When itdoes, the magnetic field in the primary winding collaps-es. This collapse generates a high-voltage surge in the sec-ondary winding of the ignition coil. The secondary circuitof the system begins at this point and as a result the sparkplug fires.

Once the plug stops firing, the transistor closes the pri-mary coil circuit. The length of time the transistor allowscurrent flow in the primary ignition circuit is determinedby the electronic circuitry in the control module.

Some systems used a dual ballast resistor. The ceram-ic ballast resistor assembly is mounted on the fire wall andhas a ballast resistor for primary current flow and an aux-iliary resistor for the control module. The ballast resistorhas a 0.5-ohm resistance that maintains a constant pri-


Pickup coilassemblyPickup

platerotation

Distributorrotation

ReluctorVacuumhose

Vacuumadvanceunit

Figure 21–28 Typical vacuum advance unit operation.Courtesy of DaimlerChrysler Corporation

mary current. The auxiliary ballast resistor uses a 5-ohmresistance to limit voltage to the electronic control unit.

DI System DesignThrough the years there have been many different designsof DI systems. All operate in the basically the same waybut are configured differently. The systems described inthis section represent the different designs used by man-ufacturers. These designs are based on the location of theelectronic control module (unit) (ECU) and or the typeof triggering device used.

DI Systems with External Ignition Module. Ford MotorCompany used two generations of Dura-Spark ignitionsystems (Figure 21–29). The second design (Dura-SparkII) was based on the first (Dura-Spark I). The Dura-SparkII had the ECU mounted away from the distributor, typ-ically on a fender wall. The distributor was fitted with cen-trifugal and vacuum advance assemblies. The negativeprimary coil terminal is referred to as a distributor elec-tronic control (dec) or tachometer (tach) terminal. Thisterminal is connected to the ignition module.

The distributor pick-up coil is connected through twowires to the module. A wire is also connected from thedistributor housing to the module. This wire supplies aground connection from the module to the pick-up plate;therefore, the module does not need to be grounded at itsmounting.

An armature is mounted on the distributor shaft witha roll pin. The armature has a high point for each cylin-der of the engine. Rivets are used to attach the pick-upcoil to the plate, which indicates that the pick-up gap isnot adjustable.

A unique feature of this ignition system is the designof the distributor. The cap is a two-piece unit. An adapter,the lower portion of the cap, is positioned on top of thedistributor housing. Its upper diameter is larger than thelower. This increased diameter allows for a larger distrib-utor cap. The larger diameter cap places the spark plugterminals farther apart. This helps to prevent cross firing.

DI Systems with Module Mounted on the Distributor.A thick film integrated (TFI) ignition system has theECU mounted on the distributor. This ignition systemfrom Ford uses an epoxy (E) core ignition coil. The wind-ings of the coil are set in epoxy and are surrounded byan iron core. Neither a ballast resistor nor resistor wireare used to connect the ignition switch to the coil. Thenegative primary coil terminal is referred to as a tach ter-minal. This terminal is connected to the module. A wireis connected from the ignition switch start terminal tothe module. The module-to-pick-up terminals extendthrough the distributor housing, and three pick-up leadwires are connected to these terminals (Figure 21–30).Heat-dissipating grease must be placed on the back ofthe module to prevent overheating of the module. Con-


Figure 21–29 A typical DI system. Courtesy of Ford Motor Company

ventional centrifugal and vacuum advances are used inthe TFI distributor.

DI Systems with Internal Ignition Module. Perhaps thebest example of a DI system with the ignition moduleinside the distributor is the GM High Energy Ignition(HEI) system (Figure 21–31). Some HEI units also con-tain the ignition coil, others have the coil remotely mount-ed away from the distributor. Some early HEI designshave centrifugal and vacuum advance units, while othersutilize electronic spark timing.

The pick-up coil surrounds the distributor shaft, anda flat magnetic plate is bolted between the pick-up coiland the pole piece. A timer core that has one high point

for each engine cylinder is attached to the distributorshaft. The number of timer core high points matches thenumber of teeth on the pole piece. This design allows thetimer core high points to be aligned with the pole pieceteeth at the same time. The module is mounted to thebreaker plate and is set in heat-dissipating grease. Acapacitor is connected from the module voltage supplyterminal to ground on the distributor housing.

In some HEI designs, the coil is mounted in the topof the distributor cap; other designs have externallymounted coils. The coil battery terminal is connecteddirectly to the ignition switch, and the coil tachometer(tach) terminal is connected to the module (Figure21–32). HEI coils are basically E core coils and rely onthe surrounding air to dissipate the coil’s heat.

A wire also extends from the coil’s battery terminal tothe module. In systems with an internal ignition coil, aground wire is connected between the frame of the coiland the distributor housing. This lead is used to dissipateany voltage induced in the coil’s frame.

When the ignition switch is on and the distributorshaft is not turning, the module opens the primary igni-tion circuit. As the engine is cranked and the timer corehigh points approach alignment with the pole piece teeth,a positive voltage is induced in the pick-up coil. This volt-age signal causes the module to close the primary circuit,and current begins to flow through the primary windings,causing a magnetic field to form around them.

At the instant of alignment between the timer corehigh points and pole piece teeth, the pick-up coil voltagedrops to zero. As these high points move out of alignment,


(TFI only) Statorassembly

Shaftassembly(without TFI)

Retainerassembly

Base assembly

Diaphragm

TFImodule

(TFI only)

Figure 21–30 TFI distributor assembly. Courtesy of FordMotor Company

Figure 21–31 The control module mounted inside aGM HEI distributor.

Ignition wire(battery feed)terminal

Latch (4)

Connector

Connect tachometer tothis terminal 88–98Connect tachometer todiagnosis connectorterminals 6 and G.

Figure 21–32 HEI distributor terminal identification.Courtesy of General Motors Corporation—Oldsmobile Motor Division

a negative voltage is induced in the pick-up coil. This volt-age signal to the module causes the module to open theprimary circuit. When this action occurs, the magneticfield collapses across the ignition coil windings, and theinduced secondary voltage forces current through the sec-ondary circuit and across the spark plug gap.

HEI modules have a variable dwell feature, whichcloses the primary circuit sooner as engine speed increas-es. In an eight-cylinder distributor, there are 45 degreesof distributor shaft rotation between the timer core highpoints. At idle speed, the module closes the primary cir-cuit for 15 degrees and opens the circuit for 30 degrees.If the engine is operating at high speed, the module mayclose the primary circuit for 32 degrees and open the cir-cuit for 13 degrees. This dwell increase which occurs withspeed increase allows for better coil saturation at higherspeeds. Since dwell is a function of the module, there isno dwell adjustment.

Computer-Controlled DI SystemsSpark timing on these systems is controlled by a computerthat continuously adjusts ignition timing to obtain opti-mum combustion. The computer monitors the engineoperating parameters with various sensors. Based on thisinput, the computer signals an ignition module to col-lapse the primary circuit, allowing the secondary circuitto fire the spark plugs.

Timing control is selected by the computer’s program.During engine starting, computer control is by-passedand the mechanical setting of the distributor controlsspark timing. Once the engine is started and running,spark timing is controlled by the computer. This schemeor strategy allows the engine to start regardless ofwhether the electronic control system is functioningproperly or not.

The goal of computerized spark timing is to producemaximum engine power, top fuel efficiency, and mini-mum emissions levels during all types of operating con-ditions. The computer does this by continuously adjustingignition timing. The computer determines the best sparktiming based on certain engine operating conditions suchas crankshaft position, engine speed, throttle position,engine coolant temperature, and initial and operatingmanifold or barometric pressure. Once the computerreceives input from these and other sensors, it comparesthe existing operating conditions to information perma-nently stored or programmed into its memory. The com-puter matches the existing conditions to a set of conditionsstored in its memory, determines proper timing setting,and sends a signal to the ignition module to fire the plugs.

The computer continuously monitors existing condi-tions, adjusting timing to match what its memory tells itis the ideal setting for those conditions. It can do this veryquickly, making thousands of decisions in a single second.

The control computer typically has the following types ofinformation permanently programmed into it.

Speed-related spark advance. As engine speed in-creases to a particular point, there is a need for moreadvanced timing. As the engine slows, the timingshould be retarded or have less advance. The com-puter bases speed-related spark advance decisions onengine speed and signals from the TP sensor.Load-related spark advance. This is used to improvepower and fuel economy during acceleration andheavy load conditions. The computer defines the loadand the ideal spark advance by processing informa-tion from the TP sensor, MAP, and engine speed sen-sors. Typically, the more load on an engine, the lessspark advance is needed.Warm-up spark advance. This is used when the engineis cold, because a greater amount of advance isrequired while the engine warms up.Special spark advance. This is used to improve fueleconomy during steady driving conditions. Duringconstant speed and load conditions, the engine will bemore efficient with much advance timing.Spark advance due to barometric pressure. This isused when barometric pressure exceeds a preset cal-ibrated value.All of this information is looked at by the computer

to determine the ideal spark timing for all conditions. Thecalibrated or programmed information in the computeris contained in what is called software look-up tables.

Ignition timing can also work in conjunction with theelectronic fuel control system to provide emission con-trol, optimum fuel economy, and improved driveability.They are all dependent on spark advance. Some exam-ples of computer-controlled DI systems follow.

Chrysler’s Dual Pick-up System. This system has twoHall-effect switches in the distributor when the engine isequipped with port fuel injection. In some units, the pick-up unit used for ignition triggering is located above thepick-up plate in the distributor and is referred to as thereference pick-up. The second pick-up unit is positionedbelow the plate. A ring with two notches is attached tothe distributor shaft and rotates through the lower pick-up unit. This lower pick-up is called the synchronizer(SYNC) pickup.

In other designs, the two pick-up units are mountedbelow the pick-up plate and one set of blades rotatesthrough both Hall-effect units (Figure 21–33). The shut-ter blade representing number one cylinder has a largeopening in the center of the blade. When this blade rotatesthrough the SYNC pick-up, a different signal is producedcompared to the other blades. This number one blade sig-nal informs the PCM when to activate the injectors.


Figure 21–33 A distributor assembly with two Hall-effect switches below the pick-up plate. Courtesy of Daimler-Chrysler Corporation

There are two Hall-effect pick-ups in distributors usedwith Chrysler port fuel injection, whereas the distributorsused with throttle body injection have a single pick-up.When the SYNC pickup signal is received by the PCMevery one-half distributor shaft revolution, or once everycrankshaft revolution, the PCM grounds two injectors toinject fuel into two cylinders.

Distributors with Optical-Type Pick-ups. The 3.0 L V-6engine available in some Chrysler products has a distrib-utor fitted with an optical pick-up assembly with twolight-emitting diodes and two photo diodes. A thin plateattached to the distributor shaft rotates between theLEDs above the plate and the photo diodes below theplate. This plate contains six equally spaced slots, whichrotate directly below the inner LED and photo diode.

The inner LED and photo diode act as the referencepick-up. As in Hall-effect-pickup systems, the referencepick-up in the optical distributor provides a crankshaftposition and speed signal to the PCM. When the ignitionswitch is on, the PCM supplies voltage to the optical pick-up, which causes the LEDs to emit light. If a solid part ofthe plate is under the reference LED, this light does notshine on the photo diode. Under this condition, the photodiode does not conduct current, and the reference volt-age signal to the PCM is 5 V. As a reference slot movesunder the LED, the light shines on the photo diode. Thediode then conducts current and the reference voltagesignal to the PCM is 0 V.

The outer LED, photo diode, and row of slots performa function similar to that of the SYNC pick-up in a dis-tributor with Hall-effect pick-ups. The outer row of slotsis closely spaced, and the width between each slot repre-sents 2 degrees of crankshaft rotation. On the outer rowthere is one area where the slots are missing. When thisblank area rotates under the LED, a different SYNC volt-age signal is produced, which informs the PCM regard-ing the number one piston position. The PCM uses this

signal for injector control. As the outer row of slots rotatesunder the outer LED, the SYNC voltage signal to thePCM cycles from 0 V to 5 V. The reference pick-up sig-nal informs the PCM when each piston is a specific num-ber of degrees before TDC on the compression stroke.

When this signal is received, the PCM scans the inputsand calculates the spark advance required by the engine.The SYNC sensor signals always keep the PCM informedof the exact position of the crankshaft. The PCM opensthe primary ignition circuit and fires the next spark plugin the firing order to provide the calculated spark advance.

GM’s HEI with EST. A seven- (Figure 21–34) or eight-terminal module is used in some General Motors dis-tributors with computer-controlled spark advance andfuel injection. Two of the module terminals are connect-ed to the coil primary terminals, and two other moduleterminals are connected to the pick-up coil. The otherfour module terminals are connected through a four-wireharness to the PCM. These four wires are identified as by-pass, electronic spark timing (EST), ground, and refer-ence wires.

When the engine is starting, the pick-up coil signalgoes directly to the module. Immediately after the enginestarts, the PCM sends a 5-V signal through the by-passwire to the module. This signal is converted to a digitalsignal and causes the module circuit to switch, whichforces the pick-up signal to travel through the referencewire to the PCM. Crankshaft position and speed infor-mation are obtained from the pick-up signal to the PCM.

The PCM scans the input sensors and then sends a sig-nal on the EST wire to the module. This signal commandsthe module to open the primary circuit and fire the nextspark plug at the right instant to provide the precise sparkadvance indicated by the input signals.

Honda’s DI System with EST. Honda, as well as othermanufacturers, also fit the ignition module inside the dis-


Vent

Cap insulating ring

Insulating rotor

Hall effect andfuel sync connectors

Housing

Shutter blade

Insulatedcoil contact

Hall effectswitch unit

Figure 21–34 A seven-terminal GM HEI/EST ignitionmodule. Courtesy of General Motors Corporation—Chevrolet MotorDivision

1 DISTRIBUTOR MODULE2 IGNITION COIL TERMINALS3 EST TERMINALS4 PICK-UP COIL TERMINALS

tributor (Figure 21–35). The distributor is also fitted witha Hall-effect switch that is directly connected to the con-trol module. The only external electrical connections forthe module are from the PCM and to the ignition coil.The PCM controls the activity of the module which, inturn, controls the dwell of the primary and therefore theignition timing.

Ford’s TFI-IV System. In some DI systems with com-puter-controlled spark advance, the module is mountedaway from the distributor or mounted to it. Ford MotorCompany’s TFI-IV system is such a system. This systemis similar to the TFI-IV system that relied on a distribu-tor fitted with mechanical and vacuum advance units.

The distributor contains the Profile Ignition Pickup(PIP), an octane rod, and a Hall-effect vane switch stator.

During the period of time the Hall-effect device is turned“on” and “off”, a digital voltage pulse is produced. Thepulse is used by EEC-IV electronics for crankshaft posi-tion and the calculation of the required ignition timing.Ignition timing required for a particular operating condi-tion is determined by inputs from various sensors that arecorrelated with values in the computer’s memory.

The PIP signal is an indication of crankshaft positionand engine speed. This signal is fed into the TFI-IV mod-ule and to the PCM. This signal, along with many oth-ers, allows the PCM to accurately calculate ignitiontiming needs for the conditions. The PCM produces a sig-nal “Spout” and sends it to the TFI-IV module. The mod-ule compares the spout signal to the PIP signal thencontrols the activity of the ignition coil and the firing ofthe spark plugs.


Figure 21–35 Honda’s distributor with a built-in control module. Courtesy of American Honda Motor Co., Inc.

ELECTRONIC IGNITION SYSTEMOPERATIONIn Electronic Ignition (EI) systems (Figure 21–36), eachcylinder may have its own ignition coil, or two cylindersmay share one coil. The coils are wired directly to thespark plug they control. An ignition control module, tiedinto the vehicle’s computer control system, controls thefiring order and the spark timing and advance.

In many EI systems, a crank sensor located at the frontof the crankshaft is used to trigger the ignition system.When a distributor is used in the ignition system, the dis-tributor drive gear, shaft, and bushings are subject towear. Worn distributor components cause erratic ignitiontiming and spark advance, which results in reduced econ-omy and performance plus increased exhaust emissions.Since the distributor is eliminated in EI systems, ignitiontiming remains more stable over the life of the engine,which means improved economy and performance withreduced emissions.

There are many advantages of a distributorless igni-tion system over one that uses a distributor. Here are someof the more important ones:

Fewer moving parts, therefore less friction and wear.Flexibility in mounting location. This is importantbecause of today’s smaller engine compartments.Less required maintenance; there is no rotor or dis-tributor cap to service.Reduced radio frequency interference because thereis no rotor to cap gap.Elimination of a common cause of ignition misfire, thebuildup of water and ozone/nitric acid in the distrib-utor cap.

Elimination of mechanical timing adjustments.Places no mechanical load on the engine in order tooperate.Increased available time for coil saturation.Increased time between firings, which allows the coilto cool more.

Basic ComponentsThe computer, ignition module, and position sensorscombine to control spark timing and advance. The com-puter collects and processes information to determinethe ideal amount of spark advance for the operating con-ditions. The ignition module uses crank/cam sensordata to control the timing of the primary circuit in thecoils (Figure 21–37). Remember that there is more thanone coil in a distributorless ignition system. The igni-tion module synchronizes the coils’ firing sequence inrelation to crankshaft position and firing order of theengine. Therefore, the ignition module takes the placeof the distributor.

Primary current is controlled by transistors in the con-trol module. There is one switching transistor for eachignition coil in the system. The transistors complete theground circuit for the primary, thereby allowing for adwell period. When primary current flow is interrupted,secondary voltage is induced in the coil and the coil’sspark plug(s) fire. The timing and sequencing of ignitioncoil action is determined by the control module and inputfrom a triggering device.

The control module is also responsible for limiting thedwell time. In EI systems there is time between plug fir-ings to saturate the coil. Achieving maximum currentflow through the coil is great if the system needs the highvoltage that may be available. However if the high volt-age is not needed, the high current is not needed and theheat it produces is not desired. Therefore, the controlmodule is programmed to only allow total coil saturationwhen the very high voltage is needed or the need for it isanticipated.

The ignition module also adjusts spark timing below400 rpm (for starting) and when the vehicle’s controlcomputer by-pass circuit becomes open or grounded.Depending on the exact DIS system, the ignition coils canbe serviced as a complete unit or separately. The coilassembly is typically called a coil pack (Figure 21–38)and is comprised of two or more individual coils.

On those DIS systems that use one coil per spark plug,the electronic ignition module determines when eachspark plug should fire and controls the on/off time ofeach plug’s coil.

The systems with a coil for every two spark plugs alsouse an electronic ignition module, but they use the wastespark method of spark distribution. Each end of the coil’s


Figure 21–36 A supercharged Buick 3800 V-6 with anelectronic ignition system. Courtesy of General Motors Corpora-tion—Buick Motor Division

Figure 21–37 An EI system with two crankshaft position sensors and one camshaft position sensor. Courtesy of GeneralMotors Corporation—Cadillac Motor Car Division


Ign. controlRef/cam loB+

Fuse Ignition switch

Battery

PCM

Knock sensor

ICM with coils

Camshaft position sensorwith sprocket

Crankshaft position sensorswith reluctor ring

Spark plugs

24X1/2X cam4X refBypass

+ –

secondary winding is attached to a spark plug. Each coilis connected to a pair of spark plugs in cylinders whosepistons rise and fall together. When the field collapses inthe coil, voltage is sent to both spark plugs that areattached to the coil. In all V-6s, the paired cylinders are1 and 4, 2 and 5, and 3 and 6 (or 4 and 1 and 3 and 2 on4-cylinder engines). With this arrangement, one cylinderof each pair is on its compression stroke while the other

is on the exhaust stroke. Both cylinders get spark simul-taneously, but only one spark generates power, while theother is wasted out the exhaust. During the next revolu-tion, the roles are reversed.

Due to the way the secondary coils are wired, whenthe induced voltage cuts across the primary and sec-ondary windings of the coil, one plug fires in the normaldirection—positive center electrode to negative side elec-

Figure 21–39 Polarity of spark plugs in an EI system.Courtesy of Ford Motor Company

trode—and the other plug fires just the reverse side tocenter electrode (Figure 21–39). As shown in Figure21–40, both plugs fire simultaneously, completing theseries circuit. Each plug always fires the same way onboth the exhaust and compression strokes.

The coil is able to overcome the increased voltagerequirements caused by reversed polarity and still fire two

plugs simultaneously because each coil is capable of pro-ducing up to 100,000 volts. There is very little resistanceacross the plug gap on exhaust, so the plug requires verylittle voltage to fire, thereby providing its mate (the plugthat is on compression) with plenty of available voltage.

Figure 21–41 shows a waste spark system in whichthe coils are mounted directly over the spark plugs so nowiring between the coils and plugs is necessary. On othersystems, the coil packs are mounted remote from thespark plugs. High-tension secondary wires carry high-voltage current from the coils to the plugs.

Some EI systems use the waste spark method of fir-ing but only have one secondary wire coming off eachignition coil. In these systems (Figure 21–42), one spark


Figure 21–38 A coil pack for a distributorless ignitionsystem.

Figure 21–40 The manner in which two spark plugsare fired by a single ignition coil in EI system circuits.

Bolt/screw-elek ign mdl

Bolt/screw—Ign coil hsg

Coil Asm—Ign

Cover–Elek ign mdl

Module Asm–Elek ign

Bolt/screw–Ign coil hsg

Harness Asm–Elec ign mdl wrg

Coil Asm–Ign

Housing Asm–Ign coil

Cover–Ign coil hsg

Connector–Splg

Boot–Splg

Retainer–Splg boot

Spacer–Ign coil

Contact–Ign coilSeal–Ign coil term

Housing Asm–Ign coil

Figure 21–41 A cableless EI system. Courtesy of GeneralMotors Corporation—Oldsmobile Division

Figure 21–42 A six-cylinder engine with three coils and three spark plug wires. Reprinted with permission

plug is connected directly to the ignition coil and the com-panion spark plug is connected to the coil by a high-ten-sion cable.

Other EI systems have one coil for every spark plug.The coil mounts directly to the spark plug and the assem-bly (Figure 21–43) is called a “coil-on-plug” assembly.

A few DIS systems have one coil per cylinder with twospark plugs per cylinder. During starting only one plug isfired. Once the engine is running the other plug also fires.One spark plug is located on the intake side of the com-bustion chamber while the other is located on the exhaustside. Two coil packs are used, one for the plugs on theintake side and the other for the plugs on the exhaust

side. These systems are called dual plug systems (Figure21–44). During dual plug operation, the two coil packsare synchronized so each cylinder’s two plugs fire at thesame time. The coils fire two spark plugs at the sametime. Therefore, on a four-cylinder engine, four sparkplugs are fired at a time: two during the compressionstroke of the cylinder and two during the exhaust strokeof another cylinder.

EI System OperationFrom a general operating standpoint, most distributor-less ignition systems are similar. However, there are vari-ations in the way different distributorless systems obtain


Figure 21–43 A coil-on-plug assembly. Courtesy of FordMotor Company

a timing reference in regard to crankshaft and camshaftposition.

Some engines use separate Hall-effect sensors to mon-itor crankshaft and camshaft position for the control ofignition and fuel injection firing orders. The crankshaftpulley has interrupter rings that are equal in number tohalf of the cylinders of the engine (Figure 21–45). Theresultant signal informs the PCM as to when to fire theplugs. The camshaft sensor helps the computer determinewhen the number one piston is at TDC on the compres-sion stroke.

Defining the different types of EI systems used by man-ufacturers focuses on the location and type of sensorsused. There are other differences, such as the constructionof the coil pack, wherein some are a sealed assembly andothers have individually mounted ignition coils. Some EIsystems have a camshaft sensor mounted in the openingwhere the distributor was mounted. The camshaft sensorring has one notch and produces a leading edge and trail-ing edge signal once per camshaft revolution. These sys-tems also use a crankshaft sensor. Both the camshaft andcrankshaft sensors are Hall-effect sensors.

Locating the cam sensor in the opening previouslyoccupied by the distributor merely takes advantage of thebore and gear that was already present. Seeing that thedistributor was driven at camshaft speed, driving acamshaft position sensor by the same mechanism justmade sense (Figure 21–46). This modification reallymade sense when older engine designs were modified fordistributorless ignition.

As the crankshaft rotates and the interrupter passesin and out of the Hall-effect switch, the switch turns themodule reference voltage on and off. The three signals areidentical and the control module can not distinguishwhich of these signals to assign to a particular coil. Thesignal from the cam sensor gives the module the infor-mation it needs to assign the signals from the crankshaftsensors to the appropriate coils (Figure 21–47). Thecamshaft sensor synchronizes the crankshaft sensor sig-nals with the position of the number one cylinder. Fromthere the module can energize the coils according to thefiring order of the engine. Once the engine has started,the camshaft signal serves no purpose.


Figure 21–44 A dual plug system for a four-cylinder engine. Courtesy of Ford Motor Company

Figure 21–45 A crankshaft pulley for a six-cylinderengine has three interrupter rings.

Figure 21–46 A camshaft sensor assembly designed tofit into the distributor bore. Courtesy of Ford Motor Company

Some systems have the camshaft sensor mounted inthe front of the timing chain cover (Figure 21–48). Amagnet on the camshaft gear rotates past the inner endof the camshaft sensor and produces a signal for eachcamshaft revolution. GM’s 3.8 L non-turbocharged SFIV-6 engine has a firing order of 1-6-5-4-3-2. Spark plugs

1-4, 6-3, and 5-2 are paired together on the coil assem-bly. When a trailing edge camshaft sensor signal isreceived during initial starting, the coil module preparesto fire the coil connected to spark plugs 5-2. After thecamshaft sensor signal is received, the next trailing edgecrankshaft sensor signal turns on the primary circuit ofthe 5-2 coil, and the next leading edge crankshaft sensorsignal informs the coil module to open the primary cir-cuit of the 5-2 coil (Figure 21–49). When this coil fires,one of these cylinders is always on the compression strokeand the other cylinder is on the exhaust stroke. After the5-2 coil firing, the coil modul

engine performance info... · the ignition system must create a spark, or current flow, acr oss...

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