Introduction to On Board Diagnostics (II)

Powertrain and EmissionControls in PassengerVehicles (contd(contd)The powertrain functions are described to show how the PCM controls the emissions while delivering thetorque to the vehicle requested by the driver.Throttle & Intake Manifold: The Throttle Body assembly is an air valve. It regulates the air flow into theengine and thereby contributes to the control of engine speed and d power. IACV(idle air control valve )provides additional air flow during starting of the engine and during idle.IACV bypasses the throttle to provide additional air to compensate for the loads during closedthrottle. EGR (exhaust gas recirculation)provides exhaust gases to the intake manifold. This has the effect ofreducing oxygen content in the enginecylinder. This in turn reduces the combustion temperature of the cylinder flame. This has theimportant effect of reducing the NOx (Oxides of nitrogen) emissions which is regulated by the EPA.Intake manifold is the main air passage from the throttle valve to the engine cylinders. The amountof air through the intake manifold to the cylinder is the same for feach cylinder on each intake stroke.Then each cylinder requires an amount of fuel determined by the density of the air in the cylinder.MAP sensor is used to compute the density of the air in the intake manifold. Barometric absolutepressure is used to compute the EGR flow. The Manifold vacuum is the difference between thesetwo pressures which is measured. The required fuel is in direct proportion to this air mass whichis controlled by the PCM to maintain the exact stoichiometric ratio (14.7) of air/fuel that gives theminimum HC emissions and meet EPA regulations.