LV37 petrol fuel systems (3)

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Student Workbook

LV37 Petrol Fuel Systems (3)

kap all phase 2 & 3 6/11/03 11:36 am Page 15

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and Repair

MODULE LV37 PETROL FUEL SYSTEMS (3)

Electronic Control Principles: 9

Electronic Fuel Injection: 12

Page Page

…

Catalytic Converters 40 Exhaust Gas Recirculation (EGR): 50

Introduction

Introducing the fuel in an efficient manner is of paramount importance on a

modern vehicle Carburetion will always be a bit ‘hit and miss’ no matter how advanced such systems become With drivers demanding better fuel

economy and better performance, and governments demanding fewer harmful emissions, only electronic control of the fuelling of an engine can meet all

requirements

Air/Fuel Ratio

14.7:1

A four-stroke petrol engine running on 95 RON unleaded fuel will run at its

most efficient with an air/fuel ratio of 14.7:1 (that is 14.7 parts air to every 1

part fuel) This is because with such a mixture strength, all the fuel will be

burnt and all the oxygen in the air will be burnt If you burn all the fuel then

you are wasting none, and if you burn all the oxygen you will create maximum combustion pressure that will translate into maximum torque (if all other

conditions are also ideal such as ignition timing)

This magical mixture strength is often referred to as ‘stoichiometric’ This is a term borrowed from chemists – they will refer to a reaction as stoichiometric if during the course of that reaction all the constituents are fully consumed

It should be noted however that a stoichiometric mixture is rarely of any use to

an engine This is because of their natural inefficiency

Your average 4-stroke petrol engine is around 25% efficient This means that for every four gallons of fuel that you put in, only one gallon turns the flywheel! This inefficiency is mainly due to heat loss – it is worth noting that a petrol

engine is a heat engine – it converts heat energy through the burning of the fuel and air into kinetic energy (the rotation of the flywheel) Inevitably a good deal of the heat energy created escapes through the exhaust pipe, into the

cooling system etc

Another area of inefficiency is in the mixing of the fuel and air Oxygen is the only constituent gas within air that is of any use to us in this instance as it

supports combustion Air is only 21% oxygen (78% nitrogen and 1% others)

So only roughly one fifth of all the air drawn into the engine during the

induction stroke is of any use to us

The difficulty that we have is ensuring that the particles of fuel are mixed

efficiently with the air to ensure that we make the best possible use of that

21% oxygen If we cannot do that, not all the oxygen will burn and we have

an inefficient engine We use numerous ways to ensure that thorough mixing does take place but they can all be related to two techniques: creating

turbulence (or swirl) in the induced air and introducing the fuel in as finely an atomised state as possible

Electronic fuel injection helps us to achieve that fine atomisation

In spite of all the electronics our engines are still inefficient If we cannot mix the fuel and air well enough to ensure that all the oxygen burns, all we can do

is ensure that we supply more fuel that is actually needed That way we

increase the likelihood of the oxygen burning

A rich mixture helps us to burn all the oxygen and therefore create maximum torque

Inevitably, a rich mixture increases the emissions produced by the engine and also has a serious effect on the fuel economy Therefore, if we are looking for good economy and low emissions we need to run the engine using as little

fuel as possible That way there is an abundance of oxygen and that will help

us to ensure that we burn all the fuel

A lean mixture helps us to burn all the fuel and therefore reduce emissions

and improve fuel economy

Emission Gases

N 2

O 2 HC

N 2

CO 2

H 2 O

Perfect combustion - stoichiometric

Under perfect conditions, when petrol is burnt in air at a mixture strength of 14.7:1, nitrogen (N2), carbon dioxide (CO2) and water (H2O) are produced

But as we have seen, perfection is a holy grail In reality, our engine produces the following gases:

• carbon monoxide (CO)

• hydrocarbons (HC)

• oxygen (O2)

• carbon dioxide (CO2)

• oxides of nitrogen (Nox)

CO is produced by incomplete combustion of the fuel, which is in turn caused

by a lack of sufficient oxygen at the time of combustion The temperatures

around the cylinder walls are low, leading to “quenching” meaning that the

temperature is too low for combustion to occur The fuel left unburnt in these quenching zones is then exhausted during the exhaust cycle

• raw gas remaining near the walls of the cylinder after burning, and

exhausted during cycle (refer to the previous quenching zones

diagram)

• low compression during coasting or deceleration leads to incomplete combustion of fuel resulting in raw HC gas in the exhaust

It can be seen in the diagram that HC is also directly affected by mixture

strength

Oxides of nitrogen (Nox)

Nox is produced by the nitrogen and oxygen in the air of the air/fuel mixture, which combine if the temperature inside the combustion chamber rises above about 1,800°C (3,300°F)

There is also a strong connection between ignition timing and the production

of Nox This is because advancing or retarding the ignition timing changes

the maximum temperature reached in the combustion chamber

The above shows the relationship between air/fuel ratio and the three main

emission gases

Electronic Control Principles

Input signal

Output signal

Electronic control systems consist of sensors (information gatherers), an ECM (Electronic Control Module – the decision maker) and actuators (to carry out actions) It is a three-way process – the sensors gather the information and the ECM receives this information The ECM processes the information and makes a decision based on what it has been taught That decision is

translated into an action by an actuator and something happens based on that action

Electronic control systems are based on what is probably the most adaptable, intelligent thing on the face of the planet Us!

Think about how we interact with the environment – we sense something

(sight, sound, smell, taste, touch) using our sensors (eyes, ears, nose,

tongue, nerve endings) and the information that we sense is sent to our brain (ECM) for processing Our brain (ECM) makes a decision based on that

information and controls our actuators to suit the situation (our muscles)

We pick something up, sense that it’s too hot to touch, and drop it

It should be noted that we are discussing the fundamentals of electronic

control here This same basic principle can be applied to any electronic

control system – EFI, ABS, cruise control etc

Electronic Control Unit

sensors

Throttle position

Engine speed

Battery voltage

Injector

In order for the ECM (ECU) to be able to process information, it requires a

power supply This is often supplied via a relay (EFI main relay – see diagram above) This relay is ignition controlled Most modern vehicles are configured

The above shows a variation on this theme: The ignition switch provides only

a signal to the ECM (ECU) and the ECM then grounds the relay in order to

control its own supply This type of configuration is found on any vehicle

where continued control functions are required for a short time after the driver turns off the ignition An example would be any vehicle equipped with a

stepper motor type idle speed control valve These valves have to be stepped out after the engine is switched off to enable the engine to breathe properly the next time it is cranked Some vehicles spin the fuel lift pump at high

speed for a few seconds after the ignition is turned off in order to prime the

fuel circuit ready for the next time the engine is cranked

Sensor power supply circuit

ECU

Micro processor

Voltage stabilization circuit (5 volt)

ECU battery supply voltage

Sensor supply voltage – 5 volts Sensor voltage via micro processor

(temperature sensors etc.)

ECU

Micro processor

Voltage stabilization circuit (5 volt)

ECU battery supply voltage

Sensor supply voltage – 5 volts Sensor voltage via micro processor

(temperature sensors etc.)

The above shows the sensor power supply circuit

Any passive sensor (a sensor that experiences a change in resistance in

proportion to changing conditions, rather than one that generates its own

voltage) must be supplied with a very closely controlled voltage If we

supplied unconditioned battery voltage to such a sensor, its signal voltage

would fluctuate not only in accordance with changing sensed conditions but also in accordance with battery voltage

Battery voltage is applied to the ECM and the ECM supplies 5 volts to each passive sensor An integrated circuit (IC) is used to achieve this effect, but it

is primarily a Zener diode

This 5 volts is also used by the ECM’s microprocessor circuit to enable the

ECM to fulfil its responsibilities

Potential problems

Should the circuit open mid-harness, some or all of the sensors that depend

on this 5 volts will fail At best, the engine will run badly, at worst not at all

Should the circuit short to ground, we will have the same problem with the

sensors, but in addition to this, the ECM microprocessor will receive no

voltage A very good indication of this is that the check engine warning light (MIL or malfunction indicator light) will no longer illuminate when the ignition switch is turned on (this facility normally acts as a bulb check when all is well) The reason for this is that it is the responsibility of the ECM to ground the

warning lamp when the ignition is turned on to carry out this bulb check; it

cannot do this if the ECM microprocessor is effectively dead

Electronic Fuel Injection (EFI)

Supply the correct air / fuel ratio to the cylinders in accordance with engine conditions

Supply the correct air / fuel ratio to the cylinders in accordance with engine conditions

EFI is a system where engine conditions are sensed electronically An ECM decides how much fuel is needed for these sensed conditions and introduces that amount through the activation of the fuel injectors The key sensor is

airflow If the engine is to receive the correct air/fuel ratio for the current

conditions, the ECM needs to know in very accurate terms how much air is

flowing into the engine at any given time This way, the ECM can inject the relevant quantity of fuel to achieve the necessary air/fuel ratio for those

conditions All other sensors can be considered secondary to air flow – they simply enable the ECM to make a far more informed decision (and therefore accurate one)

Fuel supply

If the injectors when opened are to inject fuel, they need to be supplied with fuel The diagram above shows the circuit that achieves this

Fuel is lifted out of the tank by the fuel pump (housed within the tank) and

pressure fed to the fuel rail on the engine via the fuel filter Pressed into the fuel rail are the injectors, when the ECM opens these injectors, the fuel is

injected at the back of the intake valves

The pulsation damper does what its name suggests it does – it dampens any pulsations that are in the fuel created by the fuel pump

It does this by reacting directly to the fluctuations in fuel pressure felt under its diaphragm If the pressure fluctuations are high the damper flexes up,

increasing the total volume of the fuel rail and therefore dropping the pressure within If the pressure fluctuation is low then the diaphragm flexes down,

decreasing the total volume of the fuel rail and therefore increasing the

pressure within If these pressure fluctuations were not compensated for, the fuel injection quantity would vary in relation to these, and the engine would be imbalanced

Some manufacturers fit a pin to the damper assembly that you can access

externally This pin will protrude under the influence of the fuel pressure and

is a very quick and effective way of checking for the presence of pressure

when carrying out diagnosis

Fuel pressure regulator

High pressure

in fuel rail

Return to fuel tank

Manifold vacuum

Manifold vacuum

Manifold vacuum

Spring

Diaphragm

The purpose of the fuel pressure regulator is to compensate for any pressure differential variations experienced across the fuel injector due to intake

manifold pressure changes

For example, if the engine is running at light load – engine speed 2000 rpm partial throttle, the pressure in the manifold will be low When the injector

opens, this low pressure will help draw in the fuel, creating an over fuelling

situation The regulator reduces the fuel pressure behind the injector (in the rail) under these conditions so that the difference in pressure across the

injector is constant The diagram shows how this assembly detects manifold pressure The manifold pressure either helps or hinders the lifting of the

diaphragm and therefore influences the volume of fuel that is able to return to the tank This directly affects rail pressure If the manifold hose should

become detached, the engine over fuels (atmospheric pressure is acting on the regulator continually, the equivalent of WOT – wide open throttle)

It should be noted that most modern engines no longer use this type of

regulator and use a fuel returnless system Pressure in the rail is kept at a

constant through the use of a pressure regulator valve housed inside the fuel tank A manifold pressure sensor measures any fluctuations in manifold

pressure and the ECM compensates for these variations in its fuel injection duration calculation This way, no fuel is returned to the tank from a hot

engine and this helps to reduce evaporative emissions from the tank

considerably (a smaller charcoal canister can be fitted)

Fuel pump

The fuel pump is electrically driven and is housed inside the fuel tank to

reduce noise There is no risk of fire from any sparks that might be created at the commutator and brushes of this motor, as the pump is always immersed in fuel (even if you run out) so there is no oxygen present Even if the tank was

to rupture and all of the fuel was to drain, there would still be no fire as the

environment would be super-rich and would still not support combustion The check valve is to retain residual fuel pressure in the line to aid starting If this check valve fails, poor starting will result

Fuel filter

The fuel filter is provided to ensure that any impurities in the fuel are filtered out They are often a non-serviceable item i.e fitted for life but manufacturers’ instructions should be referred to

Air induction system

Engine load is dictated by the position of the throttle butterfly – i.e driver demands

Air flow is sensed by the air flow sensor – e.g air flow meter

Air drawn into intake system Air cleaner

Engine load is dictated by the position of the throttle butterfly – i.e driver demands

Air flow is sensed by the air flow sensor – e.g air flow meter

Air drawn into intake system Air cleaner

Engine load is dictated by the position of the throttle butterfly – i.e driver demands

Air flow is sensed by the air flow sensor – e.g air flow meter

Air drawn into intake system Air cleaner

The air induction system consists of all components between the air cleaner element and the intake ports The main players are the air flow meter (if

fitted) the throttle body and the manifold/induction trunking

Their positions can be seen in the diagram The operation of the throttle body and the air flow meter are covered in detail later in this book

Airflow is measured either directly through the use of an air flow meter or

indirectly through the use of a manifold pressure sensor

Vane type airflow meter

Throttle position switch / sensor usually located on the side of the throttle body assembly

The position of the vane plate is influenced directly by the airflow volume, as the deflecting force will be proportionate to this Mounted on the same axis as the vane plate is a potentiometer (mechanically varied resistance – see

throttle position sensor and its value is therefore changed by the rotation of

the vane If we supply the potentiometer with a closely regulated voltage (5 volts) the voltage returned by the potentiometer to the ECM will alter

proportionately in accordance to the vane’s position

An example of a potentiometer that most of you will identify with is a Scalextric hand controller!

The damping chamber prevents natural turbulence in the airflow (created by the internal pumping effect of the cylinders) from setting up a frequency and producing a hysteresis effect This would make the engine hunt badly

Signal terminal Supply voltage terminal Supply voltage terminal

Earth terminal Earth terminal

ECU

Electrical circuit for vane type airflow meter

An oscilloscope is ideal for checking the serviceability of such a sensor as any worn parts of the potentiometer track will create dips and spikes in the signal voltage Use of an ohmmeter is limited to checking only the sensor and not the circuitry

Hot wire type airflow meter

A platinum wire is suspended in the flow of air and a voltage applied to it by the ECM The resulting current flow heats the wire A thermistor is used to sense the temperature of the wire and when the target temperature is reached the voltage is maintained The flow of air across the wire cools it, and the

ECM has to apply a higher voltage to achieve its target wire temperature The degree of cooling created by the air flow is in direct proportion to the amount

of air flow, and the voltage required to achieve the target temperature is

therefore also in direct proportion to this The ECM interprets the voltage as airflow volume

On a cold day the cooling effect will be greater for a given amount of air flow, therefore the required voltage will be higher The ECM interprets this as an increase in airflow and will increase fuel injection volume correspondingly

This is just what we want as cooler air is denser and therefore each cylinder will receive a greater amount of oxygen per intake stroke

This sensor is therefore a true air mass sensor

The best way to test this sensor is to monitor the change in voltage as you

Manifold pressure sensor

voltage An integrated circuit called a piezoelectric element is mounted on a small diaphragm This diaphragm is influenced directly by the pressure in the manifold As the diaphragm flexes, the resistance of the piezoelectric IC

alters We apply 5 volts to the IC and the voltage from it alters in proportion to the degree of flex on the diaphragm The lower the pressure, the greater the flex the higher the resistance the lower the signal voltage the shorter the fuel injection duration required

Digital type

The operation of this sensor is identical to the analogue type with one

exception; the signal is converted into a digital signal within the sensor and

supplied to the ECM as such (the analogue sensor’s signal is converted to

digital inside the ECM) Both types of sensor can be checked using a scope and a source of vacuum such as a Mityvac pump The digital type can also

be checked using frequency

explanation of duty cycle

This sensor is very accurate but is rather expensive Vortices (turbulence) are created in the air by presenting to it a vortex generator (a plastic post) These vortices strike a small mirror and cause it to vibrate When the mirror is in a particular position it reflects a light (provided by an LED) straight onto a

phototransistor, switching it on The phototransistor will switch at a frequency dictated by the speed of mirror vibration, which in turn is directly influenced by the frequency of the vortices These vortices become progressively more

frequent as the airflow volume increases The ECM measures this frequency and interprets it as airflow volume Check this signal with an oscilloscope

The biggest enemy of this sensor is dirt Foreign bodies can adhere to the

intake side of the sensor during servicing of the air filter and these tend to

disturb the airflow and create vortices of their own! The engine will run badly The scope pattern will be non-uniform

Exercise 2

Serviceable signals will look very similar to the signals in Exercise 1 Draw a

diagram showing how the signal will look if affected by foreign matter:

Engine Speed and Position Sensors

In order for the ECM to inject into the correct cylinder at the correct time, it

must be able to sense the position of the engine With any increase in engine speed we have less time to introduce the fuel via the injector (the intake

valves are open for a far shorter period) but the reaction speed of the injectors remains constant If the ECM has a means of sensing engine speed then it can compensate for this by providing current to open the injector slightly

earlier The above diagram shows a typical AC inductive type speed sensor

This is the fundamental underlying principle of electrical generation

It can be seen that the sensor consists of a coil of wire wrapped around a

magnet (inductive pickup) placed close to a toothed wheel (the rotor or

reluctor)

The rotor (toothed wheel) is connected to the crankshaft and therefore rotates

as the engine rotates When the rotor tip passes near to the coil, an AC

current is produced which is sensed by the ECM When the air gap is large, there is little influence on the magnetic flux around the pickup and therefore signal voltage is low When the rotor tip approaches the pickup, the tip

increases the strength of the magnetic flux and produces a positive voltage from the pickup When the rotor tip leaves the pickup, the tip reduces the

strength of the magnetic flux and produces a negative voltage

The rotation of the rotor near to the pickup therefore causes an AC current to

be produced

Exercise 3

Plot the AC signal on the diagram on page 24

The rotor on the diagram on page 24 has 4 teeth on it This indicates that it will generate 4 AC pulse per revolution of the crankshaft If we programme the ECM with this information, it can calculate engine speed by simply

counting the pulses

Example:

100 pulses received every second

4 pulses per revolution

25 revolutions per second (100 divided by 4)

1500 revolutions per minute (rpm), (25 multiplied by 60 seconds)

In reality the crankshaft rotor has more than 4 teeth The more teeth that it

has, the more accurate the ECM’s speed calculation will be at any given point

Exercise 4

Complete the circuitry for the speed sensor shown on page 24:

Engine speed and position sensing

36-2 rotor The minus 2 part of the rotor provides positional information

(normally TDC) However, as the engine is 4 stroke, this could indicate TDC

compression No1 cylinder or TDC valve overlap For correct control of the

fuel injectors clarification of this is important The camshaft position sensor provides definitive positional information because it is positioned on the

camshaft, which only rotates once per engine cycle

Cam position sensor

Camshaft

Rotor tooth

Sensor

Camshaft

Rotor tooth

Camshaft

Rotor tooth

To confirm TDC compression, the ECM monitors the 36-2 crank sensor as

well as the cam position sensor When it receives the minus 2 flat line and the single pulse from the cam sensor it knows exactly where the engine is within the 4 stroke cycle This ensures that fuel is injected into the correct branch of the manifold on the induction stroke