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kap all phase & 6/11/03 11:36 am Page 15 Student Workbook LV37 Petrol Fuel Systems (3) LV37/SWB Student Workbook for Technical Certificates in Light Vehicle Maintenance and Repair MODULE LV37 PETROL FUEL SYSTEMS (3) Contents Page Page … Introduction Air/Fuel Ratio Emission Gases: Hydrocarbons Oxides of nitrogen (Nox) Electronic Control Principles: Power supply Sensor power supply circuit Potential problems 10 11 11 Electronic Fuel Injection: Fuel supply Fuel pressure regulator Fuel pump Fuel filter Air induction system Sensors Vane type airflow meter Hot wire type air flow meter Manifold pressure sensor Analogue type Digital type Exercise Exercise 12 13 14 15 16 16 17 17 19 20 20 21 21 23 Engine Speed and Position Sensors: Principle of operation Exercise Exercise Engine speed an position sensing Cam position sensor Exercise 24 24 25 25 26 26 27 Hall Effect Sensors: Exercise Throttle position sensors Exercise 28 29 30 31 Temperature Sensing: Exercise Why sense temperature? 32 33 34 Knock Sensing: Air/fuel mixture control Potentiometer 35 36 37 Oxygen/Lambda Sensors: Operation 38 39 (Cont.) -1Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Page Page … Catalytic Converters 40 Fuel Injectors: Resistance checks of injectors Injector control signal Injection duration 41 42 43 44 Injector Circuitry: Simultaneous injection Group injection Sequential injection Single point injection 45 45 45 46 47 Idle Speed Control: Control signal 48 49 Engine Speed Limiting 50 Exhaust Gas Recirculation (EGR): EGR system Pressure charging 50 51 51 Turbocharger 52 Intercooling 53 Blowers 53 Circuitry – MAP Sensed and Air Flow Sensed EFI: Maintenance 54 55 Gas Analysis 56 -2Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue 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 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 -3Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue 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 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 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 -4Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Emission Gases N2 N2 O2 CO2 HC H 2O 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 -5Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue CO 16 14 12 10 10 12 14 16 18 Air / fuel ratio A rich mixture can also contribute to the production of CO Hydrocarbons HC is raw unburnt petrol emitted from the automobile It comes from the following sources: • 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 -6Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue CO HC 16 14 12 10 10 12 14 16 18 Air / fuel ratio 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 -7Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue The above shows the relationship between air/fuel ratio and the three main emission gases -8Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue 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 Engine load sensor Throttle position Temperature sensors Engine speed Battery voltage Input signals Electronic Control Unit Output signals Injector The above shows its application to EFI -9Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Resistance checks of injectors .- Ω Injectors are commonly used with either a high resistance of 13 - 18 ohms or a lower resistance injector of 1.5 - 3.0 ohms A ballast resistor may be fitted in series with a low resistance injector The resistor limits the current flow in the circuit when the circuit is switched on Initially when the circuit is switched on the ballast resistor resistance is low, when the resistor heats up, the resistor resistance becomes higher, lowering the current flow Later systems can normally limit the current flow through the injector circuit (also refer to injector signals) A lower resistance injector will initially allow a higher electrical current flow through the injector circuit, which will allow the injector to open more quickly A lower resistance injector will also reduce the induced voltage generated during the injector operation The reduction in EMF will allow the injector to open and close more quickly giving precise fuel injection control -42Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Injector control signal Supply voltage (relay) ECU Induced voltage Battery voltage Battery voltage Zero voltage Injector “ON” time usually measured in milli-seconds The control signal “ON” time (injection duration) will alter dependant on the ECM input signals – engine temperature / engine load / throttle position etc An injector control signal waveform is illustrated above When the injector is inoperative, the supply voltage is available When the injector is operated, the supply voltage reduces to virtually zero volts The earth circuit voltage may not reduce to completely zero, as the earth path will usually have a slight resistance When the injector circuit is switched off, the voltage will rise The period from when the injector opens until the period when the injector closes is referred to as the injection duration Notice from the injector waveform, that the closure of the injector induces a high voltage into the circuit, typically 40 60 volts The induced voltage can be used to diagnose dirty/sticking injector solenoids The voltage will then reduce to the supply voltage again -43Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Injection duration Engine at idle speed – short injection duration Engine cold – long injection duration Engine under high load - long injection duration Engine deceleration – very short injection duration The length of the injection duration will depend on the ECU input signals When the engine is cold, the engine requires more fuel and so the injector duration is long (approximately - 15 milliseconds) As the engine warms up and therefore requires less fuel, the injection duration shortens When the engine is at idle and at the correct operating temperature, the injection duration is typically 2.0 - 3.0 milliseconds During high load (wide open throttle, acceleration) more fuel is required and the injection duration lengthens, dependent on the ECU input signals During vehicle deceleration (overrun situations), fuel is not required and therefore under certain conditions (engine speed above 1500 rpm and the throttle closed) the injection duration will be very short or with some EFI systems, disappear completely (only the supply voltage is available) These waveforms and duration figures are only guidelines They will differ from system to system and therefore always refer to vehicle specific information before checking any signals etc -44Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Injector Circuitry Simultaneous injection Supply voltage (relay) ECM The circuit illustrates a simultaneous injection system All of the injectors are open at the same time (simultaneous) The injectors are opened twice per engine cycle and therefore half the required fuel is injected at each injector opening The injectors are not normally timed to the engine cycle The injectors are controlled by switching the earth circuit Group injection Supply voltage (relay) ECM The circuit illustrates a grouped injection system The system may also be referred to as a semi-sequential injection system The groups may be opened together (twice per engine cycle) or the groups may be opened in sequence to the opening of the intake valves (intake valve closed) Opening the injectors to the timing of the intake valves will usually provide lower emissions, increase in fuel economy and an increase in engine power The injector control signal should be checked at both of the ECM control terminals The control signals should be identical -45Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Sequential injection Supply voltage (relay) ECM The circuit illustrates a sequential injection system The injectors are opened in sequence, which is timed to the firing order of the engine The injectors are opened only once per engine cycle The injector-opening period (injection duration) is longer than that of the simultaneous system as the injector are only opened once per engine cycle The ECM requires information on the engine’s exact position (firing order) in order to operate the sequential injector operation The engine speed/position signal will only contain information on the crankshaft position, not the cylinder firing order The cylinder recognition sensor signal provides the ECM information on the exact engine position and therefore the firing order of the engine The ECM can therefore provide the injector control signal to the correct cylinder at the correct time If the cylinder recognition sensor signal is not available, the ECM will usually operate the injectors in a simultaneous mode -46Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Single point injection Supply voltage (relay) ECM The title refers to the type of injection system, only one injector supplying the cylinders with fuel The fuel injector was controlled in accordance with engine conditions The injector could be opened with each cylinder cycle (four control signals for a cylinder engine) or when fuel demand was high, the fuel injector could be multi pulsed to provide sufficient fuel for the high engine demand Although not used by vehicle manufacturers now, these systems were very common during the 90’s on small engine vehicles Problems encountered were similar to those of carburettors, emission levels were higher than multi point fuel systems and the systems were subject to temperature changes (hot and cold) Emission regulations could not be met by these types of fuel systems and were phased out by the end of the 90’s -47Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Idle Speed Control Supply voltage (relay) Engine speed ECU Idle speed control valve alters position to influence the quantity of air that can bypass the throttle butterfly The above diagram indicates the idle speed control valve altering the engine idle speed by varying the air bypassing the throttle butterfly The ECM controls the position of the idle speed control valve The ECM stores the engine idle speed information in its memory; this is often referred to as the engine idle target speed The ECM receives an engine speed sensor signal When the throttle is in the idle position (the ECM receives an idle position signal from the throttle position sensor) the ECM controls the engine idle speed to the target speed Note: The ECM may vary the target speed dependent on additional sensor signals i.e engine temperature The idle speed control actuator illustrated is a rotary type idle speed control valve The valve is supplied with battery voltage from the system relay The ECM controls the position of the valve by switching the actuator earth circuit ON/OFF (similar to other actuators such as the injector) Most idle speed control actuators operate in a similar manner although the control signals will vary from system to system -48Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue The control signal A B Off On Cycle B x 100 Duty ratio = A + B Duty cycle/ratio is a method of controlling current flow digitally An ECM can only affect this type of control By switching current on and off very rapidly and controlling the relationship between ‘current on’ and ‘current off’ time, an increased or reduced current flow can be achieved For example: If the on time is longer than the off time, a large amount of current will flow, but if the off time is longer than the on time, only a small amount of current will flow The relationship between on and off time is expressed as a percentage The higher the value, the longer the on time comparatively Idle speed control valves are commonly controlled through the use of a duty cycle signal Exercise 10 Calculate the duty ratio for the following time values: On time milliseconds Off time milliseconds Your answer: Will the idle speed control valve be opening or closing? -49Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Engine Speed Limiting Engine rpm An engine can be protected from over-revving very simply The ECM is taught a pre-determined maximum permissible speed and when this is reached, it cuts fuel injection The engine speed sensor provides information on current engine speed Exhaust Gas Recirculation (EGR) Oxides of nitrogen emissions (Nox) are produced in very large quantities once combustion temperatures exceed 1800°C Manufacturers employ many methods to ensure that this temperature is not exceeded and EGR is one of them By allowing a controlled amount of exhaust gas to re-circulate back to the combustion chamber, the next combustion process is far cooler This is because the exhaust gas contains water vapour, which brings about a cooling effect, and the presence of the gas means lower combustion pressures (less cylinder space for air and fuel) -50Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue EGR system It can be seen from the diagram that the ECM controls the EGR valve The ECM decides when it is acceptable to recirculate (not when the driver is demanding power) and also when Nox is most likely At this point, it controls the lifting of the valve with the use of a duty cycle signal and solenoid Pressure charging To obtain more power from an engine you have to increase the torque produced, the engine speed or preferably both Torque is a product of the force acting on the piston crown (the combustion pressure) and the crankshaft throw In order to increase the force acting on the piston crown it is necessary to increase the combustion pressure (or the size of the piston crown) This entails filling your cylinders as efficiently as possible If you can force-feed them so that they actually receive more fuel and air than there is theoretical space for, then you are supercharging your engine and high combustion pressures will result There are many ways to supercharge an engine and the use of a turbocharger is one -51Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Turbocharger The turbocharger is an air compressor driven by gases The gases generated by the engine turn the turbine which is mounted on a common shaft linked to the compressor This forces air into the engine under pressure When the engine is put into overrun conditions i.e with throttle shut the turbocharger will create a large back pressure back from the throttle This would slow the turbocharger and reduce efficiency To prevent this, a dump valve is fitted that allows this excess pressure to vent to the atmosphere (an atmospheric dump valve) or to recirculate back around the turbocharger compressor (a recirculating dump valve) Should the engine be driven so hard that there is risk of over pressurisation occurring, the ECM will open the waste gate, which allows further exhaust gas pressure to bypass the turbine An inlet manifold pressure sensor enables it to detect dangerous amounts of boost Engine coolant Intake air temperature temperature sensor sensor Knock sensor Engine speed Engine load sensor (Air flow / MAP) Input signals Electronic Control Unit Output signals Turbo waste gate solenoid It can be seen from the diagram that many signals are taken into account by the ECM for control of the waste gate actuator -52Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Intercooling The process of turbocharging heats the air considerably The fact that you are compressing it, combined with the fact that the exhaust gas is running so close to the induced air makes this inevitable As we have seen previously, hot air is thin which means there is less oxygen for the engine to burn which would result in a reduction in achievable power An intercooler cools this compressed air before it enters the cylinder There are two main types of intercooler – an air-to-air charge cooler and an air to liquid charge cooler They are both heat exchangers, but one uses air to cool the induction process and the other uses coolant Blowers A blower is another method of pressure charging the engine The main difference between a blower and a turbocharger is the method employed to turn it Whereas exhaust gases drive the turbocharger, the blower is mechanically driven – normally by a belt but occasionally by gears -53Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Circuitry – MAP sensed and air flow sensed EFI Throttle position sensor MAP sensor Engine Cylinder Engine Intake speed recognition coolant air temp temp sensor sensor sensor Knock sensor Main relay Warning lamp Fuel pump relay Engine management ECU Fuel pump M Injectors Idle speed control EVAP solenoid Coil pack Engine coolant temp sensor Idle mixture adjuster Lambda sensor Fuel pump relay EFI main relay Air flow sensor M EFI ECU Fuel pump Injectors Idle speed control Throttle position switch Engine speed signal Typical fuel injection wiring diagram -54Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Maintenance EFI systems require very little routine maintenance The air cleaner element should be replaced regularly, as should the fuel filter Always refer to manufacturers’ instructions -55Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue Gas Analysis The picture shows a quality gas analyser This analyser is capable of showing readings for all five critical emission gases: • carbon monoxide (CO) • hydrocarbons (HC) • oxygen (O2) • carbon dioxide (CO2) • oxides of nitrogen (Nox) The tables below show typical values for a current engine at running temperature Typical gas reading with engine at cruise speed CO % 0.1 max HC ppm 50 max CO2 15.0 O2 0.5 max Lambda 0.97 – 1.03 Typical gas reading with engine at idle speed CO % 0.1 max HC ppm 75 max CO2 14.5 O2 1.0 max Lambda 0.97 – 1.03 -56Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue ... Reserved LV37: Petrol Fuel Systems (3) Issue Exercise Draw the circuit for a Hall effect sensor: -29Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue... Reserved LV37: Petrol Fuel Systems (3) Issue 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. .. the fuel created by the fuel pump -13Copyright © Automotive Skills Limited 2003 All Rights Reserved LV37: Petrol Fuel Systems (3) Issue It does this by reacting directly to the fluctuations in fuel