Introduction to On Board Diagnostics (II)

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Fundamentals of PowertrainControl strategies & OBD IIDiagnosticsFuel System Monitoring: : For fuel control strategies multipoint pulsed fuel injection system sis assumed. The powertrain control strategy is to provide thecorrect Air/Fuel ratio under all operating conditions, except during cold-start. The systems involved in this control are fuel metering, fuel fpump, ignitiontiming, fuel injectors, injector pulse width, and lambda control. . The PCM determines the required injector pulse width to maintain Air/Fuel ratio withinthe lambda control window (0.93 to 1.07). The PCM adds correction n factors to injectior pulse width to increase fuel injection during cold start, and wide wopen throttle, in closed-loop loop operation. During deceleration, PCM closes fuel injection. Ignition timing affects emissions. Excessive spark advance willcause engine knock. consequently fuel system monitoring is done by using predetermined data map with optimal fuel required for each load (MAPvalue) and engine RPM point. The amount of fuel is determined by the duty cycle of the injector pulse width.The lambda closed-loop loop control system provides feedback to the PCM on the necessary y correction to the preset data points. The corrected informationis stored in the PCM’s memory so that the next time that operation point is reached, less lcorrection of the Air/Fuel ratio will be required. If the PCMcorrection passes a predetermined threshold, it indicates a faulty fuel system, that some component in the fuel supply system is outside of its operatingrange. Some possibilities are defective fuel pressure regulator, contaminated fuel injectors, defective manifold absolute pressure (MAP) sensor, intake airsystem leakage, or exhaust system leakage. All electronic components are checked for circuit continuity, rated current, rated voltage, vand rationalparameter values within limits of operation. These include fuel pump, ignition circuit, injection solenoids, engine RPM sensor, and MAP sensor. If thefuel correction exceeds the limit, either in absolute value or in update rate, the fuel system is deemed faulty and a fault code is stored and MIL isilluminated. Since fuel system has a major impact on emissions, its diagnostics are crucial to control emissions and consequently y to OBD II.The legal OBD II requirements are: The diagnostic system shall monitor mthe fuel delivery system for its ability to provide compliance with emissionstandards.Diagnostic technique: Deviations of the stoichiometric ratio which last for a longer time are stored within the adaptive mixture controller. If thesevalues exceed defined limits, components of the fuel system are deemed faulty. MIL is illuminated at that time. Fuel system diagnostics are described inmore detail in a later section.
Evaporative System Monitoring: Hydro Carbons (HC) in the form of fuel vapors escaping from the e vehicle, primarily from thefuel tank are required to be monitored to reduce emissions as legislated by EPA and required by OBD II. There are two principalcauses of fuel vapor in the fuel tank: increasing ambient temperature and return of unused hot fuel from the engine. Theevaporative control system consists of a vapor ventilation line that exits the fuel tank and enters fuel vapor canister. The canisterconsists of an active charcoal element which absorbs the vapor and aallows only air to escape to the atmosphere. Only a certainvolume of fuel vapor can be contained by the canister. The vapors s in the canister must therefore be purged into the engine andburned by the engine so that the canister can continue to store vapors when they are generated.To accomplish this another purge line leads from the char coal canister cto the intake manifold. Included in this line is thecanister purge solenoid valve. The layout of a typical evaporative ve emission control system is described in a later section.During engine operation vacuum in the intake manifold causes flow w through the charcoal canister because the canister ventopening at the charcoal filter end is at atmospheric pressure. The Tcanister purge valve meters the amount of flow from thecanister. The amount of fuel vapor in the canister and therefore, , contained in the flow stream, is not known. Therefore it iscritical that the lambda control system is operating and adjusting the fuel requirement as the vapors are being purged. Purgevapors could otherwise result in upto 30% increase in Air/Fuel mixture richness in the engine. Purge control valve is situated inthe pipe line that connects the intake manifold of the engine to the charcoal canister.Control of the purge valve must allow for two criteria:• There must be enough vapor flow so that charcoal canister does not nbecome saturated and leak fuel vapors into theatmosphere.• Purge flow must generally occur under lambda closed-loop loop control so that the effect of the purge vapors on A/F ratio canbe detected and the fuel metering corrected.When the PCM commands the purge valve to meter vapor from the canister, it requests a duty cycle (ratio of ON time to OFFtime). This allows the amount of vapor flow to be regulated depending on the engine operating conditions. When lambdacontrol is not operating, during cold-start, only low duty-cycles and therefore, small amount of purge vapors, are allowed intothe intake manifold. Under deceleration fuel cut off, the purge valve is closed entirely to minimize the possibility of unburnedHCs in the exhaust.The OBD II diagnostic system shall control the air flow of the complete cevaporative system. In addition , the diagnostic systemshall also monitor the complete evaporative system for the emission ion of HC vapor into the atmosphere by performing a pressureor vacuum check of the complete evaporative system. From time to time, manufacturers may occasionally turn off theevaporative purge system in order to carry out a check.The following is the procedure: At idle position, the canister purge pvalve is activated, and the lambda controller is monitored forits reaction.A pressure sensor in the fuel tank would provide a pressure profile pwhich will determine if a leak existed in the system.For leak detection of the evaporative system, a valve installed led at the atmospheric side of the canister which is the output to theactive carbon filter is shut off and the canister pressure is decreased to about -1.5KPa. . The complete system is turned off and thepressure within the canister is monitored for variation with time. The pressure gradient, together with other parameters like theamount of fuel, will indicate possible leaks. If a leak is detected ted the MIL is illuminated. The complete test suite is moreelaborate and is described in detail in a later section.
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OBD II monitors more components and
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Sensors and ActuatorsKnock sensorEm
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Secondary air valvesSensors and Act
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Functionality of PowertrainControl
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Introduction to On Board Diagnostics (II)

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