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ELECTRONIC ENGINE MANAGEMENT AND EMISSIONS

As greater emphasis is placed on automotive technology ,due to public demand for better vehicle performance, economy and reliability, and both government and public demands for less pollutant emissions from motor vehicles, it has become increasingly more apparent that automotive engineers and technicians require up-to-date and accurate information to enable motor vehicles to be maintained to the required standards. This INTER-JECT site provides the information required for the maintenance of engine management systems and continues to follow the standards set in this volume one, with slightly more emphasis on diagnostic procedures and the "KNOW HOW".

The information provided in this covers 80 percent of new passenger cars sold throughout Australia, which are equipped with computer controlled fuel injection engines. Imported luxury and exotic vehicles have been also included on this site but some of the vehicles do not have data listing due to the lack of available information at the time of writing. The philosophy of this INTER-JECT Site has been continued on into this information of data. We assume that the mechanic is familiar with the basic principles of electronics and emissions systems, and is competent in the use of multimeters and the installation and use of pressure gauges and flow meters. This free Information continues on from volume one by providing an outline of each system, the diagnostic procedure to be followed by links, and the specific data required to test individual components of each system. Please understand that some of the date are specific short cuts and trade secrets and password protected for INTER-JECT members who have the right INTER-JECT diagnostic equipment to perform this task.
Modern electronic engine management systems have proven to be very reliable, and system faults
can usually be traced to simple items such as blown fuses or faulty wiring connectors.

ELECTRONIC ENGINE MANAGEMENT AND EMISSIONS INTRODUCTION

For some years now, pollution has been a problem in Australia, and in effort to reduce the level of pollutants emitted to the atmosphere, both Federal and State governments have passed legislation to impose restrictions on the source of pollutants. This text describes the various components used on most of the passenger cars and vehicles sold and used solely in Australia. These components were designed by the motor vehicle manufacturers in an effort to restrict the pollutants emitted by the motor vehicle and to keep the motor vehicle and to keep the pollutants within the limits imposed by relevant legislation in the various States of Australia.

Since 1970, progressively tighter emission control requirements on passenger car and vehicles using passenger car engines have been imposed . These requirements "Design Rules ", apply only to the vehicles built after the date on which the design rule takes effect.

Because of varying State regulations, some States may impose more stringent regulations than the design rules require, due to climatic conditions, population and other factors. However, all States must at least meet the design rule requirements which have been imposed as follows; July 1970 Draft regulations prohibiting the escape of crankcase gases to the atmosphere on all vehicles built on or after July 1st, 1970.

January 1972 Design rule 26 limiting the carbon monoxide content of exhaust gas at idle to 4.5% by volume for passenger cars manufactured on or after January 1st. 1972.

January 1974 Design rule 27 requiring new passenger cars manufactured on or after January 1st, 1974 to pass a test comprising four cycles with limits on carbon monoxide and hydrocarbons emitted .

January 1975 Draft regulation requiring the installation of evaporative-emission control systems on passenger cars and derivatives built on or after January 1st, 1975.

July 1976 DESIGN RULE 27A applying to passenger cars and derivatives built on or after July 1st, 1976 imposing a limit on carbon monoxide, hydrocarbons and oxides of nitrogen emitted during an extended test cycle. Design rules 27B and 27C were introduced in January 1982 and January 1983 respectively. These involved alterations to the testing methods and administrative practices of Design rule 27A.

February 1986 Design rule 37 applying to passenger cars and derivatives built on or after February 1st, 1986 imposes that every vehicle shall be designed and constructed to operate on unleaded petrol.

The following text and some illustrations by | links |describe the various emission control components and how they operate. These components are utilised by the vehicle manufacturers in an effort of combat the problem of automobile pollution, created by the use of fossil fuels and lubricants.

VEHICLE EMISSIONS                                                                                         
There are four main forms of pollutants emitted by motor vehicles . These are; Carbon monoxide [CO] created by the incomplete combustion of the fuel, due to insufficient oxygen [or air].
(That's why Diesel vehicle smoke, smoke you can see!)

HYDROCARBON [HC] can be roughly classified into two types, that emitted in the form of unburnt fuel and that emitted in a different form, created by the thermal decomposition of the fuel. Oxides of Nitrogen [Nox] is also formed as part of the overall combustion process. They increase as the combustion temperatures increase. When combined with hydrocarbons, in the presence of sunlight they form a photochemical smog.
Lead [Pb] a petrol additive in the form of Tetraethyl Lead, retains its properties either in the form of fuel evaporation or in the form of exhaust gas.

SOURCES OF POLLUTION                                                                                 
Pre-July 1st, 1976 There are three main sources of pollution; The crankcase emits approximately 20% of a vehicle's total HC and CO emissions created by exhaust gas blow-by in the cylinders. The fuel system accounts for about 18% of a vehicle's HC emissions. These emissions are created by fuel evaporation. The exhaust system accounts for approximately 62% of a vehicle's HC, CO and lead emission and is the major source of Nox emissions.

Post July 1st, 1976 The exhaust system accounts for almost all of the HC,CO,NOx and lead emissions. Post February 1st, 1986 The exhaust system accounts for almost all of the HC,CO and Nox emission.

POSITIVE CRANKCASE VENTILATION (PCV) SYSTEM
All conventional automotive internal combustion engine produce "blow-by" gasses. When the engine is operating, pressure on the compression stroke forces some of the fuel/air mixture past the piston rings and into the crankcase. With a new "tight" engine, this amount of blow-by is insignificant; however, as rings wear and tolerances increase, more of the unburned fuel is forced into the crankcase. Here, blow-by gasses combine with the unburnt gases which enter on the power stroke to contaminate motor oil.

Sludge and deposits are eventually result in high smoke emissions and causing serious engine and environment damages.

Until recently, in most cars, crankcase blow-by gases were removed by a road draft tube. Attached to the engine crankcase, the tube drew the gases out of the crankcase by creating an airstream within the tube as the car moved. The gases were vented directly into the atmosphere, with fresh air entering the Crankcase through an opening in the oil filler cap. The road draft tube was only partially effective. It did not operate efficiently at speeds less than 30 kilometres per hour, or at all when the car was standing with the engine idling. The growing concern over air pollution pointed to the need for finding some way to dispose of the blow-by gases, other than permitting them to escape into the atmosphere.

To counter the pollutants emitted by the crankcase, a system was developed which draws the blow-by gases from the crankcase, through the inlet manifold into the combustion chamber, where they are burned with air/ fuel mixture. This system is called positive crankcase ventilation [PCV]. The PCV system utilises a regulating, one-way valve, or a metering orifice, which serves two purposes. The first is to regulate the flow of the polluted air from the crankcase to the inlet manifold and the second is to prevent backfire reaching the crankcase.

The regulating [ PCV] Valve is installed between the crankcase and the intake manifold to control air flow through the crankcase under all engine operating conditions. It does this either by restricting or increasing the flow. When the engine is idling or at low speed with a light load, the intake manifold vacuum is high. The spring-loaded plunger in the valve is drawn to a position restricting air flow. But as engine speed increases, manifold vacuum falls off and the spring moves the plunger to a position permitting greater air flow. Should backfire occur, back pressure instantly closes the valve to prevent flame from reaching the crankcase. NOTE: Some engines have a metering orifice in place of a regulating valve, which is located inside a fitting on the intake manifold .

EVAPORATIVE EMISSION CONTROL SYSTEM                                           

This system controls the escape of hydrocarbons [in the form of fuel vapour] from the fuel tank. To achieve this, the fuel tank is sealed to the atmosphere by means of a non-venting fuel filler cap, which although non-venting, but some are equipped with a relief valve to relieve any pressure or vacuum build-up in the fuel tank. A hose, connected at one end to a venting pipe located on the top of the fuel tank or fuel tank filler neck, and connected at the other end to a charcoal canister draws the fuel vapours from the fuel tank to the charcoal canister. The canister Utilises activated charcoal granules to absorb and store the vapours until the vapours are drawn out and burned by the engine during normal operation. When the engine is operating at above idle speed, the purge control valve [located in the top of the canister] is opened by a vacuum signal from the throttle body. This action allows air to be drawn through the filter, at the base of the canister, and through the charcoal granules, purging the vapours from the charcoal granules and drawing the vapours into the inlet manifold to be burned in the combustion process.

| Trade Tip | ** Because the air, which is drawn through the filter via the hole in the base of the canister, is usually heavily contaminated with road dust, the filter may become blocked or restricted. This component is normally totally overlooked by all automotive service technicians when it comes to finding a performance or fuel problem.

Certain vehicles incorporate a bimetallic vacuum switching valve [BVSV] in the canister purge hose preventing canister purging action until a specific engine coolant temperature has been attained. Thereafter, purging action is controlled by the vacuum signal operating the canister at engine speeds greater then idle.

EXHAUST GAS RECIRCULATION [EGR] SYSTEM.                                       

The EGR system recycles a small amount of exhaust gas back into the inlet manifold, which in turn dilutes the incoming air/fuel mixture and lowers combustion temperatures. This system is used as a means of controlling Nox [oxides of nitrogen ] emission. The EGR system consists of a vacuum operated metering [EGR] valve and an EGR modulator valve. Exhaust gases are fed via the EGR valve to the modulator valve where they act on the valve diaphragm, closing the valve to the atmosphere. The modulator valve then directs vacuum to the EGR VALVE, which opens and directs exhaust gases to the intake manifold .

ELECTRONIC ENGINE MANAGEMENT

With the requirement for all modern passenger cars and vehicles with passenger car engines to operate on unleaded petrol [ULP] and meet the current emission control requirements, manufacturers are utilising electronics to provide efficient engine management. Although various systems and methods are used for electronic engine management, they are all basically the same in that they all use electronic fuel injection and electronic ignition systems. The typical electronic engine management system uses sensors to monitor various engine parameters i.e. air flow, coolant temperature, throttle position etc. The sensor signals are input to the Electronic Control Unit [ECU] where they are processed and used by the ECU to control fuel injection duration, ignition timing and idle speed for optimum engine performance and lower exhaust emissions .

ELECTRONIC CONTROL UNIT [ECU]                                                             

The Electronic Control Unit [ECU] is the heart of modern vehicle electronic management systems.
It monitors various sensor outputs and uses the data to control;
a. Electronic Fuel injection [EFI],
b. Electronic Spark Advance [ESA],
and c. Idle Speed .

The ECU detects and stores in its memory various sensor malfunctions. By shorting two specified terminals on a; check connector; or by rotating a diagnosis mode selector knob, any stored malfunction is recalled from the computer memory and output as a coded message. Dependent upon vehicle type, the code can be interpreted by either ;
a. Counting the number of times red and green light emitting diodes [LEDs] illuminate on the ECU, b. Counting the number of times the; check engine; light illuminates on the vehicle dash,
or c. Counting the number of oscillations of an analogue voltmeter when the voltmeter is connected across specified terminals of the; "check connector."
The ECU also supplies a back-up circuit providing minimum driveability when there is, a computer failure [sometimes called the limp-home mode].

OXYGEN [O2] SENSOR:                                                                                       

With further advancement toward total control of vehicle emissions and fuel economy, vehicle manufacturers made a device which measures the oxygen content of the engine exhaust gases. This device, commonly known as the Oxygen Sensor, is mounted on the exhaust system, upstream of the catalytic converter, with the sensing part of the device protruding into the exhaust gas mainstream. [ Link to Picture ], illustrates the various components of a typical oxygen sensor. The sensor's sensing bulb is made of ceramic material [e.g. alumina, zirconia, titania etc.] and coated with a thin layer of a micro-porous noble metal [e.g. platinum] both on the inside and the outside of the sensing bulb. The outer surface area of the sensing bulb [which is exposed to the exhaust gases] is coated with a porous ceramic layer to hinder erosion of the noble metal layer by solid particles in the exhaust and to guard against contamination caused by combustion residues.

The operation of the oxygen sensor is based on the fact that the ceramic material in the sensor bulb becomes oxygen-ion conductive after reaching a temperature in excess of 300C. When the operating temperature is reached, the oxygen acting on the outside of the sensing bulb [exhaust gases] and the oxygen acting on the inside of the sensing bulb [via the air vent in the outer protective cover] differ ,a voltage is generated between the two interfaces due to the special properties of the materials used.

When the engine is operating with the ideal air/fuel ratio, the oxygen sensor generates approximately 400 millivolts which is transmitted to the EFI computer verifying that the air /fuel ratio is correct. if the oxygen content of the exhaust gases drops of ,the voltage output of the oxygen sensor increases, signalling to the computer that a rich mixture is being delivered to the engine .Instantaneously the computer reduces the duration of injection, thus reducing the fuel delivery and leaning off the air/ fuel ratio. Conversely, as the voltage drops below the ideal, the computer lengthens the duration of injection thereby enriching the air/ fuel ratio.

There are many types of oxygen sensors used by automotive manufactures; the heated sensor, and the non-heated sensor, reverse polarity sensors exclusive for Honda and so on. The heated sensor is generally utilized when vehicle design requires the oxygen sensor to be installed away from the exhaust manifold, to where exhaust temperatures are cooler. A secondary means of heating is required to heat and maintain the sensor's ceramic material at the desired temperature. However, some manufacturers are currently using the heated oxygen sensor to stabilize and enhance the sensors performance, even though the sensor is located in or near the exhaust manifold.

The non-heated oxygen sensor is used by most manufacturers and is generally located in or near to the exhaust manifold where the exhaust gas temperatures are sufficiently high enough to operate the sensor.

KNOCK SENSOR                                                                                                   

As vehicle manufacturers strive for better performance and efficiency from their engines a problem has evolved in the form of knocking combustion. This is caused primarily by bad fuel with high kerosene contend with very low octane rating an increase in compression ratio and ignition timing, but also applies to turbo-charger engines, due to the increased volume of air and fuel forced into the cylinders. To overcome this knocking combustion [detonation] a device called the Knock Sensor is used. The knock sensor is mounted directly on the cylinder head or engine block  | link to picture | and incorporates an active element that is made of piezo-ceramic material, which when activated by engine knocks or vibrations, converts the
knock /vibrations to electrical signals. The electrical signals are transmitted to the ECU, which retards the ignition timing to alleviate the detonation.

AUXILIARY AIR VALVE                                                                                     

Some vehicle manufacturers are using auxiliary air valves |  link to picture  | in conjunction with the EFI system to improve engine idle during the engine warm-up period. (Mainly on older models.)

The auxiliary air valve consists of a housing, which encases a sliding valve, a bimetallic strip, a heater element and electrical connections | link to picture |.

When a cold engine is started, the EFI computer supplies additional fuel to the engine. This action would cause the engine to run rough and /or stall while the engine is at normal idle speed. The auxiliary air valve is used to overcome the rough idle or stalling problem by allowing additional air to by -pass the throttle valve and flow into the inlet manifold. The additional air promotes better combustion and increases the engine idle speed.

When the engine is started, an electric current flows through the heater element in the auxiliary air valve causing the element to heat up. Because the heater element is wound around the bimetallic strip, heat is transferred, by conduction, to the bimetallic strip, causing the bimetallic strip to deflect in accordance with the heat. In so doing, the bimetallic strip causes the sliding valve to rotate to block off the port. After several minutes of engine operation the air valve port is blocked off and air ceases to flow |  link to picture  |. By this time the engine has warmed up sufficiently to enable the EFI computer to revert to normal operation.

AIR FLOW METER                                                                                                

To enable electronic fuel injection [EFI] system's to function correctly accurate metering of fuel in accordance with the mass of air inducted or forced into the cylinders is necessary. It is paramount at all cost, to keep carbon away behind inlet valves!! Air filters must be clean! Otherwise this fools the airflow meter, and the air flow meter fools the ECU. On hot wire sample port air-mass meters, backed in dust particles on the element become a heat shield, degrading the correct air sensing ability, which causes a flat spot. This fools the ECU of inhibiting the acceleration enrichment that in some case the engine backfires into the intake manifold. This problem is only fixed by submersing the hot wire element into INTER-JECTRON. A new calibration setting must be done after the element is clean to match up the air velocity losses of the engine as described in the data-pool section "Air flow meter".

This air sensing is achieved by a number of various air flow meters, (MAP sensors is a manifold absolute pressure sensing device to change this pressure variation to electronic signals for the ECU to process, see D-Jetronic.) which measures the flow rate of air used by the engine and signals the computer. The higher the rate of air flow, the higher the voltage signal to the EFI computer, and vice versa. The computer uses these signals to regulate the duration of the injection of fuel into each cylinder, thereby maintaining the air/fuel ratio as close as practicable to the stoichiometric ratio of 14:1.

The air flow meter uses one of three methods to measure the air flow rate of the engine; the flap type meter, [ link to picture ] which utilizes a sensor flap attached to a potentiometer; the hot-wire type flow meter, [ link to picture ] where the computer measures the voltage required to keep the platinum wire in a heated state, and the vortex type flow meter, which uses ultrasonics to measure air disturbance in the flow meter. There are some other type of air flow meters as well with enhancements, hot film, sample port, etc. In the flap type air flow meter [ link to picture ], air flowing through the bore of the meter, acts against the sensor flap, forcing the flap to move and open the bore of the meter to a point where the air flows without restriction. When the flap moves, it also moves the wiper arm of a potentiometer, which changes the signal voltage to the EFI computer. The computer then alters the duration of injection in proportion to the volume of air flow. The hot-wire air flow meter utilizes an extremely thin platinum wire located in the air stream path within the bore of the air flow meter [ link to picture ]. When the engine is started, a current is supplied to the air flow meter to heat the platinum wire. With the engine at idle, the heating current is approximately 500 milli-amperes [mA]. When the engine is accelerated, the air flow over the hot wire increases, cooling the wire and dropping the resistance. The computer immediately corrects this situation by increasing the heating current, returning the hot wire to its original temperature. At the same instant the computer increases the duration of injection in proportion to the air mass passing through the air flow meter. These changes take place within a few milliseconds and before the increased air mass reaches the cylinders. The vortex type air flow meter [ link to picture ] uses ultrasonic waves to measure the air mass flowing through the bore of the air flow meter into the engine.

To enable accurate calibration of the air mass to be achieved, the air passing through the air flow meter must be stabilized i.e. vortices, swirling and eddies must be removed from the flow of air before it enters the air flow meter. This is accomplished by a honeycomb air flow regulator, positioned at the inlet of the air flow meter . A vortex generating rod, placed in the regulated air flow stream, creates asymmetrically but regular vortices downstream of the rod [ see picture ]. The created vortices modulate [change] an ultrasonic wave, which is transmitted across the air flow at a constant frequency by the transmitter. The modulated ultrasonic waves are detected as a rich or lean condition by the receiver and converted into pulses by the modulator. The pulses are then analysed by the ECU. If a rich or lean condition has been detected, the ECU alters the fuel injection duration accordingly, i.e. pulse frequency increases due to increased vortices being generated by a larger air flow across the vortex generating rod.

INTAKE AIR TEMPERATURE SENSOR                                                          

The quantity of air necessary for efficient combustion is dependent on the temperature of the inlet air. i.e. with the same throttle opening the volumetric efficiency of the cylinders drops as the temperature of the inlet air rises. To counteract the effect of varying inlet air temperatures, an air temperature sensor, is installed in the inlet duct of the air flow sensor [ see picture ]. As the inlet air temperature rises the resistance of the air temperature sensor decreases, decreasing the voltage drop across the air temperature sensor. The voltage drop is sensed by the ECU, which alters the fuel injection accordingly. The intake air temperature sensor is constructed using a negative temperature coefficient [NTC] semi-conductor type resistor. Unlike normal resistor characteristics, the NTC types resistance decreases with increased temperature. The NTC resistor is housed in thin plastic which serves to;
a. insulate the resistor from the air flow meter housing temperature,
b. Seal the resistor from the air flow, and
c. Enable the resistor to react quickly to changes in air flow temperature.

COOLANT TEMPERATURE SENSOR                                                           

During the engine warm-up phase, the engine requires considerable fuel enrichment to maintain normal idle speed. As the engine warms up, the enrichment required decreases gradually to zero when normal engine operating temperature is reached . The fuel enrichment is controlled via the ECU in accordance with the signal it receives from the coolant temperature sensor. The coolant temperature sensor signal is continually monitored during normal engine operations and is used by the ECU to modify the fuel injection duration and ignition timing. The coolant temperature sensor is constructed using a negative temperature coefficient [NTC] semi-conductor type resistor
[ link to picture ]. Unlike normal resistor characteristic, the NTC type resistance decreases with increased temperature .

FUEL SYSTEM                                                                                                     

Currently there are two methods of delivering fuel, under pressure, to the injectors. One method utilizes a single high pressure fuel pump, and the other method utilizes two fuel pumps, a low pressure pump [usually located in the fuel tank] and a high pressure pump. The most commonly used pump [high or low pressure] is the electrically driven roller-cell type. With this type of fuel pump, both the electric motor and the pump share the same housing and operate while surrounded by fuel within the housing [ link to picture ]. In so doing, the fuel serves to cool the electric motor, lubricate the bearings and prevent the pump seals from drying out.

The roller-cell pump consists of a cylindrical chamber in which the motor driven rotor disc is positioned off centre, toward the chamber's fuel outlet port [ link to picture ]. The rotor has equally spaced recesses machined into its circumference, in which metal rollers are positioned and move freely. When the rotor disc is rotating, centrifugal force moves the rollers out and against the inner wall of the chamber effectively acting as circulating seals. When the rotor disc is rotating, fuel flow through the inlet port into a low pressure area created between two of the metal rollers. As the second roller moves past the inlet port the fuel is trapped between the two rollers and carried around by the rotor disc to be displaced through the outlet port in the pump chamber. The fuel pump delivers fuel to the fuel rail via a fuel filter and pulsation damper. The fuel rail, or distributor pipe, distributes fuel to each injector and its volume is large enough to prevent variations in pressure affecting the injected quantity of fuel. A fuel pressure regulator is installed at the outlet of the fuel rail [ see picture ]. The pressure regulator is a diaphragm controlled overflow type that maintains a constant fuel pressure in the fuel rail of approximately from 200 to 250 KPA, irrespective of engine speed. When fuel pressure exceeds the regulators preset value, a valve controlled by the diaphragm is forced open, allowing excess fuel to flow back to the fuel tank [ see picture ]. The spring chamber of the regulator is connected to a vacuum port on the intake manifold, which results in the fuel pressure being dependent on the absolute pressure in the manifold [ see picture ]. Therefore, irrespective of the throttle valve position, the pressure drop across the injectors will be constant. The fuel injectors are solenoid operated valves. Battery voltage is applied, either directly or through a solenoid resistor, to the injector whenever the ignition switch is turned to START or ON. The injectors operation is controlled by the ECU which completes their electrical circuit to chassis earth. The injector consist of a valve body and a needle valve. The valve body contains the solenoid winding and a guide for the needle valve
[ see picture ]. The needle valve, with fitted solenoid armature, has a specially ground pintle that atomizes the fuel as it is inducted into the cylinder. When there is no earth applied to the solenoid, the needle valve is pressed against its seat on the valve outlet by a helical spring and fuel cannot be injected. When an earth is supplied to the injector, by the ECU, a magnetic field is generated in the solenoid winding which lifts the needle valve approximately 0.1 mm and allows fuel to be injected through a calibrated annular orifice.

THE INJECTOR valves have special holders which have an internal rubber moulding. This method of mounting the injectors prevents the formation of fuel vapour bubbles by insulating the injectors from heat, and also ensures that the injector is not subjected to excessive vibration.

IDLE SPEED CONTROL                                                                                       

Idle speed control is achieved by by-passing air around the throttle valve . The amount of by-pass air required to maintain a constant idle speed is controlled by the ECU which monitors engine loading caused by the air conditioner cycling or being turned ON or OFF, electrical loading [rear window heater, blower fan, headlights etc.] and power steering loads. The idle speed control [ISC] valve is usually mounted on the throttle body and although the valves, method of operation differs on some vehicles, the principle is similar in that idle speed is controlled by altering the amount of by-pass air around the throttle, e.g. if idle rpm is too low, more air is by-passed around the throttle valve to increase idle rpm.   [ See picture ]

IGNITION SYSTEM                                                                                             

The ignition system in the electronically managed engine control system, consists of a distributor, coil/coil pack and igniter or power transistor. The ECU monitors crankshaft position, engine speed, engine load, engine coolant temperature etc. and sends a trigger signal to the igniter or power transistor at precisely the right instant, ensuring optimum ignition timing. The trigger signal, received by the igniter or power transistor, switches the current flow through the primary winding of the coil ON and OFF which induces a high current flow in the secondary winding of the coil. The resultant high tension voltage is distributed to the spark plugs in engine firing order.

DASH POT                                                                                                              

The dash pot is a mechanical device that delays the closing of the throttle valve during deceleration. The dash pot consists of a spring loaded diaphragm and plunger. An orifice in the dash pot regulates the air bleed from the dash pot diaphragm chamber when the throttle valve begins to close and makes contact with the plunger [ see picture ].

THERMOSTATIC VACUUM SWITCH                                                               

This may be known as a thermal vacuum switch [TVS], ported vacuum switch [ PVS], coolant temperature override [CTO], thermal ignition control [TIC], thermal vacuum switching valve [TVSV], or thermal vacuum valve [TVV]. The purpose of these valves is to provide a coolant temperature override for the ignition timing advance and the EGR control system. Various valves are illustrated in [ this picture ]. When engine coolant reaches a predetermined temperature, the thermal vacuum switch opens, allowing manifold vacuum to be directed to the EGR VALVE, the distributor and various other vacuum operated devices, which are required to operate only when the engine is normal operating temperature.

CATALYTIC CONVERTER                                                                                    

The reason for installing a catalytic converter in the exhaust system , [ see picture ]. is to reduce the hydrocarbon [HC] and carbon monoxide [CO] emission produced by the combustion of fuel [petrol] in the engine. The catalytic converter chemically changes the HC and CO to H2O [water] and CO2 [carbon dioxide] respectively. To change the HC and CO into harmless materials, the converter requires catalytic elements such as platinum and palladium to start an oxidation or burning reaction in the catalytic converter. As oxygen is essential for the oxidation process, additional oxygen may be required in the exhaust system. This can be achieved by either adding air to the exhaust system via an air pump or pulse air system or by leaning off the air/fuel mixture at the inlet manifold. On most vehicles now, an oxygen sensor is installed in the exhaust system, to ensure that there is sufficient oxygen in the exhaust gases to accomplish the oxidation process. The sensor transmits a signal to a computer which, in turn, either leans off the air/fuel mixture at the inlet manifold or injects air into the exhaust gases at the exhaust manifold. In the case of a three-way converter, air is injected into the centre chamber to oxidize the rear element. In most cases through, vehicle manufactures have designed their carburettor controlled vehicle to run on leaner mixtures or to reduce the flow of fuel when coasting at or below a specified number of rpm. In the case of oxides of nitrogen [Nox] the oxidation catalytic reaction has no effect. A separate reaction called reduction is utilized. The reduction process differs from the oxidation process in that it removes the oxygen from the Nox and produces harmless nitrogen [N2] and CO2 by chemically promoting the switch of oxygen from the Nox to the CO compound. The elements rhodium and platinum are used as reduction catalysts. Because the oxidation and the reduction reactions oppose each other, it is hard for both to occur at the same time in the same place . However , an oxidation and a reduction catalyst can be combined in the same converter, but a second oxidation catalyst is required for complete emission control. An oxidation/reduction catalyst, is often called a two-stage catalyst, a three-way catalyst [because it works on all three major pollutants], or a hybrid catalyst . The hybrid catalyst and a second oxidation catalyst can be installed in opposite ends of the same converter housing or in two separate converters. Catalytic converters consists of a diffuser, a shell, monolithic elements and stainless steel mesh
[ see picture ]. The shell is made from stainless steel to avoid rust due to the high operating temperatures which may range from 482-871;C. The honeycomb type monolithic elements are constructed from a ceramic material, to which an alumina [aluminium oxide ] wash is applied to form a substrate which will withstand high temperatures and provide a catalyst support.
[ see picture of a core meltdown due to rich mixture ].  The substrate is then located with a thin layer of catalytic material such as platinum, palladium or rhodium and platinum. The monolithic elements are quite brittle, so to prevent the elements from being damaged by shock or severe jolts, the elements are placed in a stainless steel mesh which acts as a cushion. The mesh also protects the elements from thermal shock caused by temperature extremes, and keeps the elements properly located within the shell. The diffuser is installed at the inlet of the converter to ensure that the exhaust gases flow uniformly over the entire area of the elements. If the diffuser was not used, the exhaust gases would flow only through the central portion of the elements.

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