LV23 petrol fuel systems (2)

LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2) LV23 petrol fuel systems (2)

kap all phase & 6/11/03 11:36 am Page 13 Student Workbook LV23 Petrol Fuel Systems (2) LV23/SWB Student Workbook for Technical Certificates in Light Vehicle Maintenance and Repair MODULE LV23 PETROL FUEL SYSTEMS (2) Contents Page Page … Introduction Carburettor Adjustments: First throttle valve opening Second throttle valve opening Secondary touch angle Kick-up Fast idle Unloader Choke breaker Automatic choke Idle mixture adjusting screw Accelerating pump Float level 10 10 12 13 14 Idle Mixture Adjustment: Using a CO meter 15 15 Four Gas Analyser: Adjustment without a CO meter Air filter Fuel filter Carburettor diagnosis Progress check Exercise 17 19 20 20 21 22 23 Vapour Lock 25 Percolation 27 Copyright © Automotive Skills Limited 2003 All Rights Reserved Icing 28 Principles of Combustion and Exhaust Emissions 29 Combustion Process: Oxides of nitrogen Hydrocarbons Carbon monoxide Carbon dioxide Exercise Emission standards 31 33 34 36 36 38 40 Emission Control Systems: Catalytic converters Oxygen sensor Throttle positioner system Hot idle compensation system Choke breaker Choke opener system Auxiliary acceleration pump system 41 41 42 43 45 46 47 48 Dash Pot System: Deceleration fuel cut off system Cold mixture heater Hot engine starting compensation system Progress check 50 51 52 LV23: Petrol Fuel Systems (2) Issue 1 53 55 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue Introduction Within Phase Petrol Fuel Systems LV10, explanations were given on the operations of both carburettor and injection based fuel systems The components were identified and their operations explained for various different types of carburettors Within Phase Petrol Fuel Systems LV23 we will look at the routine maintenance requirements for carburettor based fuel systems Explanations will be given on how to diagnose various faults associated with carburettors, together with an overview of the combustion process When explanations are given on how to adjust the various parts of the carburettor, a single type of carburettor will be used There are numerous types of carburettors used by the various manufacturers, so before carrying out maintenance or adjustments the manufacturers’ manuals must be consulted Fig Carburettor Adjustments The carburettor, as with other components, needs to be checked and adjusted at regular intervals Incorrect adjustment can lead to high fuel consumption, incorrect emissions and general incorrect engine operation If the carburettor is incorrectly adjusted for a long period of time then serious engine malfunction may occur, including burnt valves and pistons Conventional carburettors have externally mounted adjustment screws, which allow the adjustment of high and low idle speeds as well as the air/fuel mixture Other adjustments can be carried out by slightly bending the various levers, or by stripping off components from the carburettor and following the manufacturers’ adjustment procedures Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue The following diagrams outline the various adjustments needed A single type of twin choke carburettor has been used to outline the adjustments The procedure will change depending on the type of carburettor to be adjusted, i.e constant depression carburettors, constant choke carburettors and variable venturi carburettors, although the types of adjustments will be fairly generic First throttle valve opening Fully open the first throttle valve Visually check the first throttle valve angle Opening angle 90 degrees Adjust the first throttle valve opening by bending the throttle lever stopper Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue Second throttle valve opening First open the first throttle valve Fully open the second throttle valve lever Visually check the throttle valve opening angle Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue Opening angle 90 degrees Adjust the second throttle valve opening by bending the throttle lever stopper Secondary touch angle Open the first throttle valve until the throttle valve lever part A touches part B At this time, check the first throttle valve opening angle using the manufacturers’ recommended gauge Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue Adjust by bending part A Kick-up Open the first throttle valve until the kick arm slightly opens the second throttle valve Check the clearance between the second throttle valve and the body Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue Adjust part A to leave a kick-up clearance of 0.04 – 0.16 mm Fast idle Fully close the choke valve by turning the coil housing Check the throttle valve clearance between the throttle valve and the bore Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue Adjust by turning the fast idle adjusting screw leaving a clearance of 0.1 mm Unloader Fully close the choke valve by turning the coil housing Fully open the first throttle valve Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue Emission Control Systems Catalytic converters CO2 H2O N2 CO HC NOx O2 To reduce the emissions discharged from a motor vehicle, the manufacturers first tried to get the air/fuel ratio as near to the stoichiometric as possible In reality, even if this were achieved modern vehicles would not reach the stringent emission regulations set down by the Government To lower these emissions further, catalytic converters were introduced into the exhaust systems A catalyst is a substance that increases the rate of a chemical reaction without being used up itself Noble metals are used within the catalytic converter to enable this chemical reaction to take place The types of metal used are: rhodium, palladium, platinum and iridium These metals are applied to a ceramic baffle material, called substrates These substrates supply a large surface area with which the exhaust gas can contact the noble metals When the exhaust gases pass through the catalytic converter they pass over the noble metals, and this causes a reaction called catalyst oxidation Catalyst oxidation adds O2 to the molecular structure of the carbon monoxide, hydrocarbons and oxides of nitrogen Modern catalytic converters that carry this process are called three-way catalysts, and they help to reduce all the pollutants produced by the combustion process The only problems associated with this type of catalytic converter, is that its operating temperature needs to exceed 400°C, and the air/fuel mixture entering the combustion chamber needs to be close to the stoichiometric fuel ratio Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 41 Below is a diagram showing the gases that enter the catalytic converter, and the gases that leave the catalytic converter NOx + CO N2 + CO2 NOx + HC N2 + CO2 + H2O O2 + CO CO O2 + HC H2O + CO2 Oxygen sensor For the catalytic converter to operate correctly the air/fuel mixture entering the combustion chamber must be near the stoichiometric fuel ratio To enable this to be monitored an oxygen sensor is fitted into the exhaust system This oxygen sensor sends a signal to the engine control unit in the form of a voltage to inform the engine control unit of the oxygen content contained within the exhaust gas From the oxygen content contained, the engine control unit is able to assess the air/fuel ratio entering the combustion chamber Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 42 If the oxygen sensor judges that the oxygen content is low, then the engine control unit views the air/fuel ratio as being rich In this condition a high voltage is sent from the oxygen sensor to the engine control unit If the oxygen content is high, then a low voltage is sent to the engine control unit The engine control unit views this as a lean air/fuel ratio With a carburettor based engine the air/fuel ratio is altered by the engine control unit, by admitting more or less air through the air bleeder The air/fuel mixture is normally kept slightly richer than the theoretical ideal or stoichiometric fuel mixture Throttle positioner system VTV TP Diaphragm When the driver removes his or her foot from the accelerator, the throttle butterfly or valve closes shut completely This sudden closing of the valve causes a vacuum to be created within the inlet manifold This vacuum draws the fuel that has adhered to the walls of the inlet manifold into the combustion chamber, causing the engine to run rich In conjunction with the engine running rich the closing of the throttle valve causes the compression pressure to fall inducing incomplete combustion and engine misfire To compensate for this a throttle positioner is fitted The throttle positioner opens the throttle valve slightly during deceleration, causing the air/fuel mixture that enters the combustion chamber to burn completely Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 43 When the vehicle is being driven normally no vacuum is applied to throttle positioner, so the diaphragm is pushed to the left via a spring This in turn pushes the adjusting screw to the left When the vehicle is decelerating the lever linked to the throttle valve makes contact with the adjusting screw stopping the throttle valve from closing fully Over a short period of time vacuum from the throttle positioner port acts on the throttle positioner diaphragm causing the throttle valve to close gradually Condition TP Port Vacuum TP Diaphragm Throttle Valve Idling Intake manifold vacuum Pulled by intake manifold vacuum Idling position Cruising Nearly atmospheric pressure Pushed out by diaphragm spring High-speed position Deceleration Intake manifold vacuum Pulled by intake manifold vacuum Opens slightly, then closes slowly to the idling position Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 44 Hot idle compensation system When a motor vehicle is driven in hot conditions, the temperature within the engine compartment will rise As the temperature rises, the petrol within the carburettor will begin to boil, causing the petrol to give off vapour This petrol vapour will be discharged through the main nozzle causing the air/fuel mixture entering the engine to become over rich This will cause the engine to idle erratically or stall To eliminate this problem a hot idle compensation system is fitted to the carburettor or the air cleaner This system also assists in reducing the hydrocarbon and carbon monoxide levels leaving the exhaust system There are different types of systems fitted to carburettor engines, but within this phase an explanation will be given on the type actually fitted to the carburettor Cold Hot The carburettor based system uses a bimetal strip to open and close the hot idle compensation valve allowing air to bypass the main nozzle It does this by sensing the temperature of the incoming air, opening the valve when the temperature is hot, and closing it when the temperature is cold Intake Air Temp Thermo Valve HIC System Cold (below 55°C) Closed Off Hot (above 75°C) Open On (air volume is controlled by the HIC valve) Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 45 Choke breaker Hot Cold For the engine to operate correctly, the choke butterfly must open gradually as the engine begins to warm up This is done using a bimetal strip or a heated coil within the automatic choke The choke breaker assists this process, as it opens the butterfly to a given angle as engine coolant reaches a given temperature When the engine is cold, normally just after the engine has started, vacuum is applied to the left hand chamber of the choke breaker, acting upon diaphragm A This vacuum sucks on diaphragm A, drawing the connecting rod to the right, opening the choke valve the first step As the engine coolant reaches the correct temperature, the thermostatic vacuum sensing valve opens This allows vacuum from the inlet manifold to act on diaphragm B drawing it to the right As the diaphragm moves to the right so does the connecting rod opening the choke valve to the second stage Coolant temp TVSV Diaphragm A Below 7°C Open (L-J) Pulled Not pulled Slight Above 17°C Open (K-L) Pulled Pulled Much Copyright © Automotive Skills Limited 2003 All Rights Reserved Diaphragm B Choke link pull LV23: Petrol Fuel Systems (2) Issue 46 Choke opener system The choke breaker system as described opens the choke butterfly through two stages as the engine begins to warm up The choke opener ensures that the choke butterfly opens fully when the engine reaches approximately 60°C This temperature will vary depending on which manufacturer designed the carburettor to which it is fitted When the choke has been opened fully this system releases the fast idle cam allowing the engine to idle at the correct speed Cold Just after the vehicle has started when the engine is cold atmospheric pressure is applied to the choke opener diaphragm The thermostatic vacuum-switching valve controls the atmospheric pressure diverted to the choke opener Atmospheric pressure enters the valve from the air filter side of the choke butterfly, travelling through the thermostatic vacuum-switching valve to the choke opener As the diaphragm is not being drawn to the left by vacuum, spring pressure keeps the choke closed Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 47 Hot When the engine temperature rises, the thermostatic switching valve moves, connecting the inlet manifold to the choke opener The vacuum generated within the manifold acts on the diaphragm within the choke opener, drawing it to the left This operation pulls the control rod to the left opening the choke butterfly As the choke is pulled to the left, the fast idle cam is released allowing the engine to return to the correct idle speed Coolant temp TVSV Diaphragm Choke Valves Fast Idle Cam Engine RPM Below 50°C Open (J-M) Released by spring tension Closed by automatic choke Set at 1st or 2nd step High Above 68°C Open (K-M) Pulled by manifold vacuum Open Released to 3rd step Low Auxiliary acceleration pump system In order for a motor vehicle to meet the needs of the driver, it must be able to accelerate smoothly even if the vehicle is cold When the engine is hot an acceleration pump is sufficient to enable the vehicle to accelerate smoothly without hesitation When the vehicle is cold, this pump is not able to deliver enough fuel to give the desired results For this reason an auxiliary acceleration pump is fitted, to support the main acceleration pump Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 48 Cruising When the engine is cold, the thermostatic vacuum-switching valve allows vacuum to be applied to the auxiliary acceleration pump chamber, from the inlet manifold This vacuum is applied to chamber A drawing the diaphragm to the right As the diaphragm moves to the right, fuel is sucked into chamber B Accelerating When the driver presses the accelerator pedal during acceleration the vacuum within the inlet manifold drops This lowering of pressure reduces the amount of force acting on the diaphragm, allowing the spring pressure to draw the diaphragm to the left As the diaphragm moves to the left, fuel is forced out of chamber B through the accelerator pump nozzle into the venturi This action enables the driver to accelerate smoothly when the vehicle is cold When the engine reaches a given temperature set by the manufacturer the vacuumswitching valve closes stopping the pump from operating Engine Operation TVSV Engine Intake Vacuum Diaphragm in AAP Fuel Below 50°C Open (K-N) Constant rpm High Pulled by vacuum Drawn into AAP chamber Acceleration Low Returned by spring tension Forced into acceleration nozzle Above 68°C Closed (K-N) No operation Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 49 Dash Pot System When the driver releases the accelerator pedal during deceleration, the throttle valve snaps shut This sudden closing of the throttle valve causes an increase in vacuum within the inlet manifold The increased vacuum causes fuel that has adhered to the walls of the inlet manifold to be drawn into the engine causing it to run rich As this is happening the compression pressure drops due to the deceleration, causing misfiring and unstable combustion Due to this process the hydrocarbon and carbon monoxide levels increase The dash pot system shown above is fitted to overcome this problem by stopping the throttle valve from snapping shut This will assist the complete combustion of the air/fuel mixture entering the combustion chamber Deceleration Cruising When the vehicle is cruising the diaphragm is pushed to the right via spring pressure As the vehicle decelerates vacuum is applied to the diaphragm through the VTV valve, causing the throttle valve to close slowly Engine Operation TP Port DP Diaphragm Throttle Valve Idling Intake manifold vacuum Pulled by intake manifold vacuum At idling position Cruising Near atmospheric pressure Pushed out by diaphragm spring Medium or highspeed position deceleration Intake manifold vacuum Pulled by intake manifold vacuum Copyright © Automotive Skills Limited 2003 All Rights Reserved Opens slightly, then slowly closed to idling position LV23: Petrol Fuel Systems (2) Issue 50 Deceleration fuel cut off system Various systems have been used to eliminate the problems associated with decelerating the vehicle The deceleration fuel cut off system is included within this category, as it stops fuel entering the engine through the slow running circuit during deceleration This eliminates afterburning within the exhaust silencer When the driver releases the throttle pedal the throttle valve closes, in this position the throttle switch is turned off The engine control unit is able to sense that the switch is in the off position, and in conjunction with an rpm signal it is able to determine that the vehicle is decelerating When the engine control unit has reached this decision it operates the fuel cut off solenoid valve closing the slow running circuit of the carburettor Engine speed Throttle Position Switch ECU Fuel cut-off Solenoid Valve Slow Circuit In Carburetor Less than 1900rpm On On On Open On On On Open Off Off Off Closed Greater than 2300rpm Off Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 51 Cold mixture heater When the engine is first started, fuel entering the inlet manifold adheres to the sidewalls This increases the emissions and reduces the driveability of the vehicle To compensate for this cold mixture heaters are fitted into the inlet manifold These heaters heat the inlet manifold during the warming of the engine assisting in the evaporation of the air/fuel mixture The engine control unit controls the heaters fitted to the inlet manifold When the engine control unit receives a signal from the alternator it is able to determine that the engine is running The engine temperature is fed to the engine control unit in the form of a signal from the coolant temperature sensor If the coolant temperature is below the manufacturers’ given limit then the CMH relay are turned on, causing the heaters within the inlet manifolds to heat up IG Switch Engine Coolant Temp Thermo Switch ECU CMH Relay CHM Off Not running - - - Off Off On Not running - - Off Off Off On On On On (heated) Off Off Off Off Running Below 43°C Above 55°C Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 52 Hot engine starting compensation system When diagnosing faults with a vehicle fitted with a carburettor based fuel system, understanding the type of carburettor fitted is important One common problem with carburettors is the amount of time needed to start the vehicle after it had been stationary for a short period of time The reason for this being that when the vehicle is moving forward cool air is either drawn in by the cooling fan, or comes in naturally as the vehicle moves forward This keeps the engine bay and the carburettor relatively cool When the vehicle is switched off after a journey the engine is going to be hot, this heat is transferred into the engine bay heating the fuel within the carburettor As the fuel is heated vapour is given off causing the vehicle to run rich during starting The hot engine starting compensation system reduces this causing the vehicle to start more quickly Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 53 When the driver turns the ignition switch off, the power supplied to the outer vent control valve is also switched off, opening the passage between the float chamber and the bi-metal vacuum switching valve If the engine bay temperature rises above the manufacturers’ specified limit, then the bi-metal vacuum switching valve opens, allowing vaporised fuel to pass down to the charcoal canister As the charcoal canister now collects the vaporised fuel it does not enter the inlet manifold When the engine is cranked power is supplied to the outer vent control valve, closing the passage between the float chamber and the bi-metal vacuum switching valve The fuel that has been collected by the charcoal canister, is drawn back into the engine through the purge port, due the reduction of increased vacuum generated by the engine Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 54 Progress check Answer the following questions: What three pollutants are emitted from the exhaust due to the engine combustion process? Give an explanation on the term stoichiometric fuel ratio What proportion of the earth’s atmosphere is made of nitrogen and oxygen? Why is the hot idle compensation system fitted to the carburettor? Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 55 ... Reserved LV23: Petrol Fuel Systems (2) Issue Introduction Within Phase Petrol Fuel Systems LV10, explanations were given on the operations of both carburettor and injection based fuel systems. .. process? Draw a picture of the fuel filter showing the flow of fuel through it: Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 22 Exercise For the... up CO HC Air fuel ratio too rich Idling CO HC Air fuel ratio rich or at theoretical level Copyright © Automotive Skills Limited 2003 All Rights Reserved LV23: Petrol Fuel Systems (2) Issue 38