Introduction to On Board Diagnostics (II)

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Combustion and Rotational dynamics: (Figures 1 to 3)The Engine provides the mechanical power to the vehicle. The engine cylinders perform the combustion of air/fuelmixture at stoichiometric ratio (14.7). The Crankshaft assembly and flywheel house the Crank angle sensor which sensesthe position of the Top Dead center (TDC) of the cylinder and provides the necessary ignition spark at the correct crankangle between the reference point on the flywheel and the horizontal centerline of crank shaft. The amount of fuel neededfor the combustion in the engine cylinder is a direct function of the throttle position and the mass of air through the intakemanifold which is controlled by the driver’s accelerator pedal. This mass of air is measured with the Mass Air Flow(MAF) sensor. The correct air mass is computed by compensating for fthe intake air temperature which is measured by theintake air temperature sensor. The Manifold Absolute Pressure (MAP) sensor measures the intake manifold pressurewhich is also used to measure the amount of air going into the cylinder as a second method to determine the amount offuel that should be sent to the fuel injection nozzles for spraying into the cylinder. This is to ensure that accurate amountof fuel is used in the cylinder to achieve fuel economy as well as to reduce emissions by efficient combustion. An EngineSpeed sensor is needed to provide an input to PCM to compute ignition timing. Engine speed is measured by enginespeed sensor similar to crankshaft position sensor. Another variable able which must be measured for engine control is thethrottle angle or the throttle valve position which is measured by the Throttle Angle Sensor.The throttle plate is mechanically linked to the accelerator pedal which is operated by the driver. When the pedal ispressed the throttle plate rotates and allows more air to pass through tthe intake manifold. The angle of rotation of throttleplate is measured by the throttle angle sensor. This can be used to measure the mass of air going into the cylinder.Knock is caused by a rapid rise in cylinder pressure during combustion caused by high manifold pressure (MAP) andexcessive spark advance. It is important to detect knock and avoid excessive knock to avoid damage to the engine. Knockis detected by the Knock sensor.During engine off condition, the fuel stored in the fuel system tends to evaporate intothe atmosphere. To reduce these HC emissions, they are collected by a charcoal filter in a canister.The collected fuel is released into the fuel intake through a purge solenoid valve controlled by thePCM periodically.
Powertrain and EmissionControls in PassengerVehicles (contd(contd)Automatic Transmission:The Automatic transmission uses a hydraulic or fluid coupling to transmit engine power to the wheels.Efficient transmission of engine output to the automatic transmission ssion input shaft is performed througha transmission lockup clutch similar to a standard pressure-plate plate clutch placed inside the torqueconverter (the fluid coupling used as a torque amplifier). In order to smoothly engage the lockup clutchthe hydraulic fluid pressure is adjusted by controlling the output current applied to the lockup solenoidvalves.Automatic transmission is controlled by inputs from the vehicle speed sensor and throttle position sensorwhich senses the vehicle load. The automatic gear shift points, the point at which the lockupclutch is activated, and the clutch’s hydraulic pressure level are acontrolled by the PCM. The optimalshifts and lockup operations are carried out using a solenoid valve to open and close the hydrauliccircuit, primed by the hydraulic pump.The transmission’s input- shaft speed is monitored during shifting by the speed sensor after the ON/OFFsignal is output from the shift solenoid valves. The shifting process pis adjusted by the hydraulic pressureof the clutch so that the clutch is smoothly engaged. The engine torque is controlled in synchronism withthe shift to reduce impact due to shift. During cruise, the lockup clutch is engaged and is disengagedduring shifts, which improves fuel economy and emissions.
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Introduction to On Board Diagnostics (II)

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