Tài liệu đào tạo cố vấn dịch vụ của mercedes

công hay đang thất bại thì cũng đang thay rằng những đúc kết nào đó, của một người nào đó phù hợp với mình ở những thời điểm khác nhau. Nếu bạn tâm đắc với một châm ngôn nào, thì nó cũng thường là kim chỉ lam cho những hãy động tiếp theo của mình. Hi vọng, những châm ngôn hay sẽ luôn giúp bạn thành công hơn và nếu có thất bại thì cùng luôn tiến về phía trước, đững dậy sau sai lầm. Vì sự thành công luôn đích đến những quãng đường để đế thì sẽ trải qua rất nhiều vực thẳm

Cars • Maintenance Technician (MT)

Engine Compustion Basic Training Working Document Participant Document

A repeat order for this document or supplementary delivery can not be offered.

These documents are determined to be used in training programmes only and are not touched by current modification.

Printed in Germany

ã 2004 Copyright AutomotiveTraining GmbH

This work including all its details is protected by copyright Reproduction, copying, revision, translation, distribution,

micro filming and data storing and/or use in electronic systems (including data banks and online services) of this

publication, wholly or in part, is only allowed with our previous written permission and with source credit.

Page Title

1 Foreword

2 Combustible mixture

6 Mixture formation in gasoline engines

20 Ignition system at gasoline engines

34 Exhaust-gas treatment on gasoline engines

40 Mixture formation in diesel engines

54 Exhaust-gas treatment on diesel engines

58 Supercharging

66 Power and torque

68 Exhaust system

Main objective

The Maintenance Technician should be able to carry out inspections and

simple repairs competent and conscientiously with the provided instruments

The Training modules should be carried out practically in conjunction with the

workshop information system WIS Alongside the basics of the automobile

technology background information from the WIS is also being included

The objective of these modules is the intermediation of basic physical

knowl-edge The MT should know terms and units, which he or she uses on a daily

basis in the field of work on the vehicle

Target group of these Modules are ”Trainees” who have not absolved a

tech-nical apprenticeship and for previous professionals

The participants can name and explain all components for the fuel-mixture

generation and exhaust gas treatment They can obtain and use information

for the disassembly and assembly of an engine

Basics motor combustion

No matter if the combustion takes place according to the diesel or the line principle there has to take place mixture formation, compression, ignitionand combustion This learning unit introduces into the combustion processand its involved systems It shows alternatives of the power raise of combus-tion engines and clarifies questions to the formation of exhaust gases and itstreatment

gaso-These subjects are being treated separately according to the combustionprinciples of gasoline and diesel engines

Combustible mixture

For a complete combustion of the fuel a certain amount of oxygen resp air is

required The blend of fuel and air is called mixture

Now, first the fuel

Fuel

Fuels and their placing in the vehicle have to fulfil certain requirements

regarding the suitability of combustion and the transport:

• Simple and reliable formation of a combustible fuel-air mixture

which ignites very easily and fast and combusts fast and without

any residues

• Low weight and little space for the energy unit and the fuel

con-tainer

• Fail-safe transport

• Immediate readiness for operation

• Comfortable and possibly safe placing and transport in the

vehi-cle

Types of fuels

There are solid, liquid and gaseous fuels Nowadays, however, only liquid and

limited gaseous fuels are significant The best way to fulfil the

above-mentioned requirements is by using liquid fuels

Common liquid fuels are gasoline and diesel They consist of different carbons of the same origin (crude oil) Hydrocarbons are compounds of car-bon (C) and hydrogen (H2) The manufacturing process starts with the distilla-tion of the crude oil in its components By further procedures the componentsare being refined to gasoline and diesel

hydro-Besides, there are more fuels on the basis of regenerative (reusable) energies.The following so-called alternative fuels are partly in use resp partly in dis-cussion:

• Methane, produced partly of animal by-products,

• Methanol, produced of wood-biomass,

• Ethanol, produced of sugar-cane biomass,

• Vegetable oil, produced of rape-biomass,

• Hydrogen, produced of nuclear energy or solar energy

The use of alternative fuels requires conversions on the engine, vehicle andthe infra structure of the fuel distribution In the near future their use is onlyexpected in certain areas In medium-term, methanol seams to be the mostpromising alternative whereas the chances for hydrogen technology might begiven only for long-term

The higher the octane number the more knocking resistant and thereforehigh-graded is the fuel.

On petrol stations, the octane number is declared by the letters ROZ ROZmeans Research Octane Number and equals to the international standardisa-tion Diesel fuel needs to have a high ignition quality in contrast to gasolinefuel As a reminder: The diesel principle is based on self-ignition

The standard reference number for the ignition quality of diesel is the cetane

number Beyond it, diesel fuel has got one more character Diesel is getting

viscid at low temperatures and therefore becomes useless From about –24°C onwards the inspissation starts

The table below shows the octane and cetane numbers for common fuels

Premium gasoline 95Premium high grade gasoline 98-100

Characters of fuels

The main character why fuels are being used for combustion at all is that they

have saved chemically bonded energy The chemical energy the combustion

engine converts into mechanical energy (force)

The chemical energy of the fuel is being declared with the so-called specific

calorific value in joule per kilogram (J/kg)

Besides, fuels own certain characters concerning the readiness to ignite

themselves even under high pressure and temperatures This readiness is

called ignition quality The dimensional figure for the ignition quality is the

octane number (gasoline engine) and the cetane number (diesel engine).

Gasoline fuel (petrol) should own a low ignition quality in order to prevent the

rest of the mixture from igniting automatically As a reminder: The gasoline

engine ignites controlled by external ignition

In this way the octane number characterises the safety of petrol against the

appearance of undesired self-ignition Undesired self-ignitions are also called

knocking combustion The knocking of the engine is feared because it can

come to destruction on the engine The knocking noise mostly appears at low

rotational speed and full load (accelerating knocking) or at high rotational

speed (high-speed knocking)

Air demand

The air demand is the amount of air, which is needed for the complete

com-bustion of a fuel It is also called stoichiometric air demand

Concerning the motor combustion in general one differs more or less from the

stoichiometric mixture ratio The ratio of the actual air mass mL to the

stoich-iometric air mass is called air ratio λ.

3 different mixture conditions are being differed in the motor operation:

• A mixture with air deficiency has got a λ < 1 and is called a “rich”

mixture

• A mixture with a stoichiometric mixture ratio has got a λ = 1

(ratio: fuel to air = 1 to 14,8) and is called stoichiometric mixture

• A mixture with excess air has got a λ > 1 and is called “lean”

mixture

Nowadays, gasoline engines are almost exclusively equipped with a regulated

catalytic converter Therefore it is necessary to operate them with nearly a

stoichiometric mixture

Diesel engines are always being operated with excess air to avoid too much

soot production

!

Mixture formation in gasoline engines

The treatment of gasoline fuels is being proceeded outside the cylinder with

the help of a carburettor (here not dealt with because not the state of

tech-nology anymore) by injection into the intake manifold or by direct injection

into the combustion chamber

The pictures show the intake-manifold injection and the direct injection

The following tasks must be fulfilled concerning the mixture formation and the

air-fuel-mixture metering:

• Formation of a gaseous fuel-air mixture of delicately distributed

fuel

• Exact dose of the fuel for the desired air ratio

• Fuel metering of the mixture amount by throttle devices to adjust

the power

132

4

132

4

The mixture forming system is not only involved in the optimisation of the

combustion within the stoichiometric air ratio but also in further, essential

operating values:

• Make possible a quick start

• Make possible a stable idle

• Operate economically in the part-load range

• React quickly upon load changes e.g acceleration

• To deal with the fuel dose in a way to maintain the harmful

ex-haust-gas emissions possibly low in all conditions of operation

• To enrich the mixture with fuel to reach the maximum power

• To operate independently of the atmospheric condition

Mixture composition

The mixture composition has got a large influence on the important operating

values The fuel-air-mixture ratio or the air ratio is describing it

The air ratio area in which gasoline engines can be operated is being

deter-mined by the rich and lean misfire limit resp by according ignition limits

Air ratio area of the gasoline engine: 0,5-0.7 < λ < 1,3-1,7

Basically the mixture ratio depends on:

• the temperature of the engine,

• the engine speed,

• engine load (load)

Therefore, the following operational conditions of the engine require a richmixture:

• cold start,

• warm-up,

• acceleration (full load),

• high performance (full load)

Full load is the operational condition in which it is being driven with the tle valve completely opened This is independent from the rotational speed.When the engine is warm and the engine load is low (so-called part load) it isbeing driven economically with a lean mixture

throt-Mixture-formation facility at gasoline engines –

intake manifold injection

During the injection in the gasoline engine fuel is being pushed through a

pump and injected either individually in front of each cylinder (multi point fuel

injection) or centrally in the area of the highest air speed (single-point

injec-tion) by injection valves At the single-point injection the injection of the fuel

happens by only one injection valve (single point) and at the multi point fuel

injection it happens by one injection valve per cylinder (multi point)

In order to adapt the fuel flow to the airflow according to the desired air ratiocertain control systems are required They are called mixture formation facili-ties They have to be able to measure the air quantity or air mass as well ascontrol an allocation of the fuel quantity according to the measured air quan-tity The injection valves count to further system components of the mixture-formation systems

You can imagine the operation-dependant mixture formation as follows: Thedriver demands a certain power of the engine by the position of the accelera-tor The accelerator is connected to the throttle valve of the engine Thethrottle valve controls the load condition in the mixture-formation facility Ifmore power is required, the throttle valve will open wider Thereby a biggerairflow can get into the combustion chambers The air quantity is being meas-ured A larger quantity of fuel can be injected with the measured, larger airquantity This is how the engine gets more power

How exact the mixture-formation facilities operate we will discuss later on

Mechanic controlled, continuous multi point fuel injection (K-Jetronic)

At the so-called „K-Jetronic“ of the Bosch company the fuel quantity is being

injected continuously and dosed by the measurement of the sucked-in air

quantity

AT_07.3_0009

By the opening of the throttle valve the sensor plate is also being opened.

This works because of the intake vacuum: When the throttle valve is open theengine is able to suck-in a large quantity of air and therefore it takes thesensor plate far into the air funnel

The excursion of the sensor plate is in a strong context with the air quantity

According to the excursion of the sensor plate, square slots are being

changed in their cross section by the control plunger in the fuel distributor.Thereby, the fuel quantity is being dosed to the nozzles The quantity ofsquare slots equals to the quantity of cylinders in the engine The controlplunger is connected with the sensor plate by a linkage The differential-pressure valve cares for a constant pressure above the metering slot The

injection valves spray the fuel even at low flow rates very subtle and open at

a fuel pressure of about 3,3

An electric pump (fuel pump) supplies the system with a pressure of about 5,7 bar, which is being held constantly by the pressure regulator A pressure

store holds the pressure constant for a certain time even after the engine has

been shut down This is needed to ease the starting of the hot engine

The basic adjustment of the air ratio happens by an idle mixture screw at idlestatus and virtually stays constant in the whole part-load area For full loadthe mixture enrichment is being made by altering the outline of the air funnel.For the cold start (cold-start enrichment = rich mixture) serves a special cold-start valve For the warm-up there is being used a warm-up regulator (control-pressure regulator) and an auxiliary-air device The warm-up regulator adjuststemperature dependent the control pressure, which acts on the controlplunger and regulates the excess fuel during the warm-up by increasing ordecreasing the control pressure

Electro-mechanic controlled, continuous multi point fuel injection

(KE-Jetronic)

The development of K-Jetronic to KE-Jetronic happened by an electronic trunk

offer (basic system as K-Jetronic) Thereby, intelligent functions as e.g

λ-regulation can be realised on λ= 1 for the cleaning of the catalytic converter

or a warm-up enrichment The advantage of the KE-Jetronic to purely

elec-tronic systems is the ability of limp-home because the basic system continues

to operate mechanically during a black out of the electronic

Electronic controlled multi point fuel injection (L-Jetronic)

By the progress in the semiconductor technology the electronic control of theinjection quantity has been made possible Therefore there is a multitude ofcorrection possibilities for an improvement of efficiency, tractability andexhaust-gas emissions

At the L-Jetronic the air quantity which is being inducted by the engine ismeasured directly and the fuel quantity which is to be injected is being de-termined

AT_07.3_0012

AT_07.3_0010

For the measurement of the air quantity there is a rectangular sensor plate in

the intake manifold of the engine The sensor plate is being held by the airflow

against the return force of a spiral spring in a certain angle position The

position of the sensor plate corresponds to a certain air quantity according to

the size of the cross section A potentiometer measures the position and

enters the data in an electronic control unit By the direct airflow measuring,

production tolerances and the wear of the engines are being compensated

The control unit determines by means of the air quantity, the engine speed,

the temperature of the coolant and the oil, the air pressure, the course of

acceleration and the overrun with the help of program maps programmed in

advance the fuel quantity which has to be injected

AT_07.3_0011

AT_07.3_0013

The fuel is being delivered from the tank over an electric pump and a fuel

filter The fuel filter keeps back impurities and has to be changed within the

maintenance intervals

All injection valves of an engine are being selected by the control unit with

impulses at the same time Therefore they inject at different phases of the

operating cycle of the individual cylinders

In order to provide approximately same conditions for all cylinders, the

injec-tion quantity is being divided into 2 halves and injected twice per power cycle

of a cylinder (intermittent group injection) The mixture formation is being

improved by the forward shift of one part of the fuel in front of the closed

intake valve, which is caused thereby

The control unit determines the injected quantity by the pulse duration of the

valves The release of the control impulses happens from the distributor

con-tact points in the ignition distributor

Here is also integrated a cold-start device For a safe cold start serves a very

subtle atomisable cold-start valve

A contact set on the throttle valve shuts off the fuel supply completely in the

overrun above the engine idle speed and besides that controls the full-load

enrichment

Electronic controlled injection (LH-Jetronic)

The basic system equals to the L-Jetronic The development shows among

other things a change of the air-quantity measurement Instead of the

meas-urement of the air quantity with a sensor plate the air mass is being measured

with a hot-wire air-mass meter and since 1987 with the hot-film air-mass

meter

A thin heated platinum wire resp a platinum-film resistor is being cooled bythe air-mass flow Thereby its electric resistance changes In a amplifier com-ponent this change is being registered and the current raised by the wire resp.the film resistor And that is so far that a constant temperature rules in thehot wire resp in the film resistor The current needed for that is a measure forthe intake air mass

A further development is the cold-start enrichment It is integrated in thecontrol unit and is being reached by the enlargement of the duration of injec-tion or the injection frequency Thereby the cold-start valve and the thermo-time switch do not apply

Motronic engine control (ME)

Motronic owns a single control unit where the whole engine control can take

place electronically

There are different types of Motronic

The table below gives you an overview of the Motronic variants and its ation in the different Mercedes engines:

ME 1.0 119

120

129/140210/129140

September1995 Launch ME 2.7.1 275

285

215/220230/240 October 2002

March 1997 Launch ME 2.8 112

113

163/170202/203208/209210/211215/220230

June 2000 relievesME 2.0

ME 2.1 104

111

129/140170/202208/210

August 1996 Launch

ME 2.7 137 220/215 January 2000 Launch

ME 2.8.1 112/113

170/203209/211215/220230

March resp

October 2001

like ME 2.8, butwith AMGspecificextents

Following functions comprises a modern Motronic generation:

• Electronic controlled fuel-injection installation with an air-mass

meter

• Electronic controlled ignition system

• Idle speed regulation: Because of turning on consumers (cooling

fan, air-conditioner compressor, etc.) during the idle, the idle

speed sinks and varies and the Motronic regulates against it

With it a stable idle speed can be reached

• Lambda-regulation

• Knock control: On each cylinder there is a knock sensor

(micro-phone) which recognises a combustion knock As soon as a

knocking occurs the sensor reports it to the Motronic

There-upon, the moment of ignition is being adjusted to “retard” If then

the knocking stays away the moment of ignition will be adjusted

gradually in the direction “advance”

• Electronic pressure-charging regulation (only in connection with a

• Cruise control device regulation

• The opening speed of the throttle valve is being adapted to thedriving style of the driver (ME 2.8)

• Pressure sensor in the intake manifold for the recognition f thealtitude (the air pressure sinks with the altitude) (ME 2.8)

Besides, the Motronic supports the control units of the other vehicle systems.Therefore, there is e.g a connection with the control unit of the automaticgearbox While switching a rotating speed reduction will set in, in order topreserve the gearbox and to increase the comfort The anti-slip regulation(ASR) permits the connection with the ABS-control unit to increase the drivingsafety Further connections exist at vehicles with ESP between the controlunit of dynamics of vehicular operation and the Motronic Therefore controlleddrive and brake actions can be done

Block diagram of the ME-Motronic

Control unit

Basic coordination of the programmap

Start controlPost-start phase, full load, accelerationenrichment

Overrun fuel cutoffEngine-speed limitationLambda closed-loop controlIdle-speed control

Tank-ventilation systemExhaust-gas recirculationTorque guide

Crude-oil functionCruise controlLoad-changes regulationSecondary-air injectionEOBD II

CAN-bus-system

Engine speed / induction-type pulse-generator

Ignition TDC of the first cylinder / Hall generator

Air mass / air-mass meter

Position of the throttle valve /Throttle valve potentionmeterEngine temperature / NTC -engine

Exhaust-gas oxygen before cat conv / lambda sensor I

Intake-manifold pressure / pressure sensor

Differential pressure / pressure sensor

Exhaust-gas oxygen after cat conv / lambda sensor II

Ignition TDC of the first cylinder / Hall generator

Position of accelerator / accelerator potentiometer

Main relais

Fuel-pump relais /fuel pumpInjection valvesETC-servomotorTank-ventilation valveExhaust-gas recirculation-valve

Shutoff valveHeating lambda sensor IHeating lamda sensor IISecondary-air valveSecondary-air pump

• Statified-charge operation: Different ers of the fuel-air mixture are being for-mated by swirl movement Thereby, amixture of different air ratio is layered inthe combustion chamber In the statified-charge operation the mixture only in thearea of the spark plug has to be ignitable.

lay-In the other part of the combustionchamber there is only fresh A/F mixtureand residual exhaust gas without non-combusted fuel In the idle and part-loadrange results therefore an overall verylean mixture (fuel-air mixture approx

1:40) The petrol consumption is alsobeing reduced But because it is notdriven at λ = 1 the use of a new catalytic-converter technology is necessary

AT_07.3_0015.1

Gasoline direct injection

This is a relatively new procedure of the inner mixture formation, which has

been introduced by Mitsubishi in 1997

At direct injection systems the fuel is being injected directly into the

combus-tion chamber by electromagnetic operated injeccombus-tion valves An injeccombus-tion valve

is assigned to each cylinder The mixture formation takes place within the

cylinder

During the operation the engine only takes in air and no longer the fuel-air

mixture as with conventional injection systems There is an advantage of the

new system: In the intake manifold no fuel will be condensed anymore and

cause high exhaust-gas values

By the external mixture formation the fuel-air mixture exists generally

ho-mogenous (subtle and even mixed) in the stoichiometric ratio in the whole

combustion chamber Whereas the mixture formation in the combustion

chamber allows two completely different modes of operations:

• Homogeneous operation: As with the ternal mixture formation a homogeneousmixture exists in the whole combustionchamber with the homogeneous opera-tion The whole fresh air in the combus-tion chamber takes part in the combus-tion Therefore this mode of operation isbeing used with full load

ex-AT_07.3_0015.2

1 What is meant by mixture formation at combustion engines? 4 What is meant by the mixture ratio of the fuel-air mixture? The ratio of

a Mixture of gasoline and premium gasoline a fuel to air

b Formation of fuel-air mixture b diesel fuel to gasoline fuel and air

c Formation of air ratio and octane number c basic gasoline to additives and air

2 What does gasoline and diesel consist of? 5 What means ignition quality? How is the ignition quality called at gasoline

and diesel engines?

3 What are alternative fuels? Name 3! 6 What means λ = 1?

7 Which statements are correct? 10 Which modes of operation requires a rich fuel-air mixture?

a Gasoline fuels should posses a low ignition quality a Cold-start, warm up, acceleration, high performance

b Gasoline fuels should posses a high ignition quantity b Only at high performance

c Diesel fuels should posses a low ignition quality c Only at cold-start and high performance

d Diesel fuels should posses a high ignition quality

8 Which statements are correct? 11 The mixture ratio of fuel to air depends on

a A rich mixture possesses a λ > 1 a Torque, temperature and load of the engine

b A rich mixture possesses a λ < 1 b Torque, load and piston weight of the engine

9 When is the throttle valve of the engine completely opened? Does this

depend on the torque?

12 How many injection valves are there at the single-point intake-manifoldinjection on a 4-cylinder engine?

13 Is the fuel quantity being adapted to the air quantity and the other way

round?

15 What are the differences between a Motronic and a K-Jeronic?

14 Describe a mixture formation unit of your choice! 16 By which dimension “knows” the control in the hot-wire air-mass meter

which air mass has been taken in?

Ignition system at gasoline engines

The job of the ignition is to ignite the compressed air-fuel mixture at the right

time and thereby induce the combustion The right time is called ignition

point In the gasoline engine this happens by an electric spark between the

electrode and the spark plug An assumption for a perfect operation of the

engine and the catalytic converter is an ignition operating properly under all

conditions Misfiring lead to a bad engine performance, high consumption and

to damage on the catalytic converter due to overheating By the afterburning

of the non-combusted mixture in the catalytic converter it overheats

For the generation of the ignition spark generally high-voltage ignition-systems

have succeeded They consist of the spark plug(s) and the actual ignition

system to generate the needed high voltage

Moment of ignition

The moment in which the mixture has to be ignited essentially depends on:

• Rotational speed

The combustion time of the mixture at a constant air-fuel mixture

is steady Approx 2 milli seconds pass from the moment of tion until the complete combustion If the engine speed in-creases, the time for the power cycle will decrease Therefore, athigh rotational speed there has to be ignited earlier

igni-• Load

At low loads the mixture is growing lean, the residual exhaust gasportion increases and the filling decreases This impact causes ahigher ignition lag and a lower rate of combustion in the mixture

so that there has to be ignited earlier

For these reasons there is an adjusting device in the ignition system, whichregulates the ignition point, depending on the load and the rational speed

AT_15_0012

It is usual to relate the ignition point on the position of the crankshaft to the

top dead centre (TDC) It is declared as an angle in degree before TDC There

are 2 adjustment variants to be differed:

• Retard adjustment: adjusting of the ignition angel in direction of

the TDC,

• Advance adjustment: adjusting in the TDC in the opposite direction

The choice of the ignition point influences on various ignition processes and

knocking is a non-controlled combustion in the engine It is being

produced by an impulsive combustion of mixture parts, which

have not been reached by the flame front In this case the

igni-tion point lies too far in the direcigni-tion “advance” Knocking

opera-tion leads to a raise in temperature and a steep raise of the

pres-sure in the combustion chamber Prespres-sure pulsation result from

it, which overlay the regular pressure characteristic Damage can

occur on cylinder head, cylinder head gasket, bearings and

pis-tons There is a difference between the acceleration knock (at

low rotational speed and high loads) and high-speed knock (high

rotational speed and high loads)

1 Ignition point at the right time

2 Ignition point too early

3 Ignition point too late

Spark plug

The spark plug is screwed into every combustion chamber and has got the

task to bring in the ignition energy to the combustion chamber An electric

spark between the electrodes introduces the combustion to the cylinder

charge

The insulator, consisting of a ceramic material, is being inserted gas-tight into

the plug casing with gaskets

Usual electrode gaps are 0,6 to 0,9 mm A bigger electrode gap activates a

bigger mixture volume, requires a higher ignition voltage and is very exacting

to the ignition system and the insulation A smaller electrode gap causes

danger of combustion misses by to low activated volume

The spark plug determines together with other components of the engine

decisively the function of the engine It has to make possible a secure

cold-start as well as to always guarantee a non-intermittent path

1 Insulator

2 Electrode gap

12

The requirements for spark plug are enormous:

• Electric: Isolation of high voltages over 30 KV

• Mechanic: Pressure peaks up to over 100 bar inclusively

guaran-teeing gas tightness

• Chemical load: The plug is exposed to chemical reactions of the

combustion at high temperatures Aggressive residues lead,

cir-cumstances permitting, to deposits, which can alter the

charac-ters

• Thermic load: Extreme changes of temperature: Combustion until

2800°C, charge cycle at 60-100°C For a secure function of a spark

plug there have to be maintained 2 limit temperatures during the

operation

• Lower limit temperature ∼ 500°C: The insulator nose has to

be-come so hot that carbonaceous residues burn down and with it

electric shunts are being prevented

• Upper limit temperature ∼ 900°C: The temperature should not be

exceeded to prevent auto-ignitions Otherwise, the mixture does

not ignite by the controlled introduced ignition spark but by the

high temperature of over 900°C

The operating temperature of a spark plug sets itself as equilibrium ture between the heat absorption and heat dissipation Heat absorption takesplace by the combustion temperatures Heat dissipation is being done to thefresh A/F mixture and over the centre electrode, the insulator nose and theplug casing over the cylinder head to the coolant

tempera-The heat supply depends on the engine construction Engines with high cific performance have generally higher temperatures in the combustionchamber The maintenance of the limit temperatures is being reached byaccording variation of the insulator nose

spe-The identification for the thermic loading capacity of a plug is the heat range: The construction types of spark plugs differ not only in the heat value (long,

short insulator nose) but also in the applied electrode material, the shapesand the numbers After the shapes and the number of the electrodes it isbeing differed:

• Front electrode

AT_15_0014

• Low heat range identification numbers

A short insulator nose and therefore a smaller

heat absorbing surface, quick heat

dissipa-tion, low resistance against sooting, high

re-sistance against auto-ignitions (“cold plug”)

AT_15_0013.2

• Side electrode

AT_15_0015

• High heat range identification numbers

Long insulator nose and therefore a big heat

absorbing surface, slow heat dissipation, high

resistance against sooting, low resistance

against auto-ignitions (“hot plug”)

AT_15_0013.1

• Surface-gap spark plugwithout ground electrode

AT_15_0016

Ignition system

The energy needed for the ignition is being provided by the ignition system

and allocated at the ignition point of the spark plug of the respective cylinder

The high voltage needed for the ignition is being created only at the ignition

point The electric energy needed for that is being taken from a temporary

storage According to that kind of energy storage it is being differed in:

• Coil ignition,

• Capacitor-discharge ignition

Coil ignition

The breaker-triggered coil ignition, which is being presented here, is the

sim-plest version of an ignition system in which all functions are being realised

The elements are:

• Ignition coil as an inductive energy storage,

• Ignition distributor with a mechanic ignition contact breaker

(release of the high voltage generation at the ignition point),

ad-vance mechanism (adjusting of the ignition point by the

cen-trifugal and the vacuum advance mechanism), distributor finger

(distribution of the high-voltage pulse on the ignition cable of

each cylinder) and ignition capacitor.

Circuit diagram and function of the elements

The ignition energy source for the coil ignition is the ignition coil It stores

the energy in the magnetic field and delivers it at the right time to the ignition

as a high-voltage pulse (ignition pulse) over high-tension ignition cables to theaccording spark plug The storage bases on an induction procedure (magneticfield with coil) The ignition coil therefore is called “inductive storage”

The ignition coil consists of 2 wire coils winded one upon the other, which arereciprocal insulated: The primary winding with few windings out of a thickcopper wire and the secondary winding with much windings out of a thincopper wire The primary and the secondary winding enclose the iron core.This iron core has got the job to intensify the magnetic field and with it thestorage energy

Before the ignition starts the way between the electrodes of the spark plug iscompletely non-conductive The ignition pulse is being linked up over theignition cables to the central electrode and causes there a steep rise of volt-age If a certain voltage (ignition voltage > 15000 volt) is being reached theway will be getting conductive and the spark can flash over Directly after thatthe voltage will sink steeply to the firing voltage This firing voltage is suffi-cient to preserve the spark Thereby the mixture can still be ignited in case ithas not happened yet by the high ignition voltage.

AT_15_0018

The one end of the primary winding is connected with the plus terminal of the

battery over the ignition switch The other end lies upon the interrupter on the

earth The ignition capacitor is switched parallel to the interrupter It is driven

mechanically in a way that an interruption happens when an ignition should

take place The secondary winding lies with the one end on the earth, as well

The other end is connected over the ignition distributor with the central

elec-trode of the spark plug

At closed ignition switch the primary winding connected with the plus terminal

of the battery Is the break contact being closed current flows – the primary

current It rises up to an end value the so-called “no-load current” In

conven-tional coil ignition systems with a mechanic interrupter this no-load current is

limited to 3 or 4 ampere

At the ignition point the interrupter opens the primary electric circuit and

interrupts it thereby In this moment the magnetic field breaks down and

induces a voltage in the primary and secondary electric circuit

Important for the formation of the ignition spark is the level of the secondary

voltage It is all the higher,

• the faster the magnetic field breaks down,

• the bigger the winding ratio of the primary and secondary coil and

• the more the primary current

Due to the applied winding ratios of 1:100 there is being created a high

volt-age on the secondary side

1 short-time spark head,

2 spark tail as a post-discharge with a slightly undulatingfiring voltage

3 Spark voltage line

As soon as the delivered energy from the storage comes below a minimum

amount the ignition spark breaks down Hereby the spark path becomes

non-conductive again

By opening the interrupter a voltage is also being induced in the primary

winding of the ignition coil, which is 300-400 volt

Without any precaution the voltage on the break contact would cause a strong

spark This would lead to ignition-energy consumption, strong contact erosion

and high contact resistances on the contact areas The ignition capacitor

avoids such negative appearances It is switched electrically parallel to the

interrupter In the moment of the primary current interruption it takes the

electric charge and creates with it a secondary path to the opening break

contact It is being charged on the induced peak voltage A certain time is

needed for that In the meantime the contact areas lie so far apart that no

spark can flash over

The lifting speed of the breaker lever at a sparking rate below 3000 / min is

so low that despite the capacitor a weak spark formation still takes place

That means at frequent urban cycle the wear on the break contacts is high

and they have to be replaced more often

Further development of the coil ignition

The developments of the conventional coil ignition are:

• breaker-triggered transistor coil ignition (TSZ-k),

• breakerless-triggered transistor coil ignition (TSZ)

Developments have become necessary because the conventional coil ignitionhad the following weak points:

• wear of the break contacts,

• partly a too less energy observation of the ignition coil

With the TSZ-k now a transistor took the place of the interrupter as a circuitbreaker for the primary current The interruption of the control current on thebasis of the transistor, the mechanic interrupter in the distributor takes over.I.e the transistor acts here as a switch with the positions “on / off” If thebreak contact is closed the control current will flow and the transistor iselectric conductive (position “on”) Herewith the primary current can flow inorder to charge the ignition coil If the break contact opens the control cur-rent will be interrupted and the transistor will become electric conductive(position “off”) Thereby the primary current in the ignition circuit becomesinterrupted and the generation of the high voltage in the secondary circuitgets introduced With the transistor up to 9 ampere can be switched (higherenergy observation of the ignition coil)!

At the TSZ the mechanic interrupter for the control current of the transistor is

being replaced by electric switches Both elements are being placed in the

distributor By the omission of the mechanic interrupter the ignition point

stays constant over the operating time

Otherwise, the TSZ-k and the TSZ are constructively identical with

conven-tional coil ignitions

Spark-advance mechanism at coil ignitions

The ignition angle represents an important dimension for the control of thecombustion in gasoline engines

Due to the heavy change of the ignition point and combustion time, depending

on the mixture composition, the ignition point must be adapted accordingly.Hereby, different adjusting systems for the rotational speed and load adjust-ment are available

AT_15_0020.1

AT_15_0019

• Centrifugal advance mechanism

The centrifugal advance mechanism isinstalled in the ignition distributor andhas got the job to adjust the ignitionpoint to “advance” with increasing ro-tational speed

On the carrying plate connected withthe drive shaft two flyweights arestored pivoted They are being broughtmore and more off their neutral posi-tion by the centrifugal force againstthe force of the extension spring at in-creasing engine speed The driversadjust the breaker cam, stored pivoted

in the drive shaft, relatively to the driveshaft

AT_15_0020.2

• Vacuum control: The vacuum control adjusts the ignition point

dependent on the intake-manifold vacuum After a physical law a

vacuum is being created by high flow velocity of the air in the

in-take manifold At part-load the gasoline engine is being operated

with a less ignitable, slowly combustible mixture and therefore it

has to be ignited earlier

As a measure for the load the vacuum in the intake manifold is

being taken The advance of the ignition point, dependent on the

vacuum, is integrated in the ignition distributor and operates

ad-ditively to the centrifugal advance depending on the rotational

speed The vacuum directly behind the throttle valve acts on the

membrane

This is being displaced against the action of the spring at

part-load and twists the breaker plate against the rotational direction

of the breaker cam over the vacuum advance arm The spring is

preloaded in a way that there happens no adjustment at full load

By stops on the vacuum advance arm the adjustment range is

being limited

AT_15_0021

Electronic ignition systems – capacitor-discharge ignitions (CDI)

A further step in development was the integration of the electronic in ignitionsystems and the change to capacitor-discharge ignition systems These areidentified by the storage of the ignition energy happening in a capacitor It can

be breaker-triggered or breakerless-triggered The CDI consists of a ignitiontrigger box and an ignition transformer The ignition trigger box contains acharger, the capacitor as well as the needed electronic The charger is avoltage converter, which converts the battery voltage into the much highercharge voltage of the capacitor The capacitor is being charged to 300-400volt The switch of the charge currents (up to 100 ampere) happens by athyristor (electronic switch) The ignition transformer transforms the dis-charge current outflowing of the storage capacitor to the needed high voltagefor the generation of the ignition spark

The advantages of the CDI in contrast to the coil ignition are:

• higher ignition voltages

• extensive insensibility against the dirty spark plugs

H I N T S F O R M A I T E N A N C E

Spark advance at electronic ignition systems

Ignition systems with flyweight and vacuum controlled ignition-angel

advance-systems can only realise simple adjusting characteristics depending on the

rotational speed and the load Thereby different operational conditions and

mixture compositions are being seized only insufficient

With the application of electronic ignition systems it is possible to ignite the

fuel-air mixture at all operational conditions of the engine in each case

opti-mal

At the place of the mechanic centrifugal and vacuum advance mechanism

steps an ignition map laid down in the control unit With the help of the input

variables rotational speed / crankshaft position load (intake manifold vacuum)

and further parameters (e.g engine temperature, data of the knock sensor

etc.) the control unit assigns the respectively optimal ignition point to each

point of operation

for ignition systems

The usual operations on modern, electronic ignition systems without amechanic distributor are limited to the exchange of spark plugs accord-ing to the maintenance intervals (because every susceptible mechanic isequipped with static distribution and fixed adjusted ignition points exworks) At conventional systems additional operations on the adjust-ment of the ignition point, renewing of the distributor contact points andexchange of the capacitor take place

Before the exchange of the spark plugs it is necessary to clean the areasaround the spark plugs and then to pull off the ignition cables It has to

be ensured that the ignition cables are being plugged on the same placeafterwards A sticker can do this or there is already a cylinder mark onthe ignition cable The spark plugs can be screwed out with a spark plugkey It has to be paid attention on the exact heat range of the spark plugfor the vehicle Subsequently, the screwing in of the new spark plugsand the plugging on of the ignition cable takes place Watch out for thetightening torque (see workshop information system WIS)

1 Why is misfiring to prevented? 4 By what means the ignition point is determined?

2 What is the ignition point?

3 Which statements are correct? 5 By what means can it be altered?

a The temperature of the insulator nose should not exceed 900°C

b The temperature of the insulator nose should be lower than 500°C

c The temperature of the insulator nose should be between 500 and

900°C

6 What is knocking? Furthermore, differ between the acceleration knock

and the high-speed knock! 8. Which disadvantage have got distributor contact points?

7 Describe the elements and the function of a coil ignition! 9 What has been altered therefore?

!