Introduction to On Board Diagnostics (II)

Functionality of PowertrainControl Module (PCM) inOBD IIThe PCM obtains the mass air flow from the MAF sensor and obtains the massfuel required to keep the air/fuel ratio equal to stoichiometry (14.7) from lookuptables. The inputs to the lookup table is MAP, Engine RPM, Coolant temperature,and EGR, all of which are readily available by computation, or lookup table.The value of the speed density product R a *d a is given by:R a = (Engine RPM/60) * ( Engine displacement/2)* volumetric efficiency - EGR volume flow rated a = M a / R a , where M a is the mass of air, and R a is the volume at in take air temperature T.Tables of d a , the density of air measured versus temperature are available in lookup tables.Engine displacement and volumetric efficiency are engine design parameters, which areconstant. Lookup tables with inputs: Engine RPM, MAP, T, and EGR give directly themass flow rate of air, which is product R a *d a . This is used as input into another lookuptable that gives the duty cycle of the fuel injector, which gives the amount of fuel required tokeep the A/F mixture at stoichiometry. This lookup is performed by the PCM to complywith OBD II regulation mandated by CARB and EPA for controlling emissions..