Legal requirements

Californian exhaust emissions laws require the monitoring of combustion misfiring, as the latter leads to an increase in noxious emissions and to TWC damage. A distinction must be made between emission relevant misfiring after start-up and during the journey and misfiring which is damaging to the TWC.

Emission relevant faults exist if the limits of the FTP emissions test have been exceeded 1.5 times.

Fig 1: Identifying Emission Gas Ratios During Misfire

Courtesy of PORSCHE OF NORTH AMERICA, INC.

A misfire rate of slightly above 2 % is sufficient in the example shown here to reach a high HC value and so to exceed the threshold.

Misfiring damaging to the TWC is present if the temperature of the TWC reaches over 1000° C.

Fig 2: Identifying Function Of Load/Speed Range Percentages

Courtesy of PORSCHE OF NORTH AMERICA, INC.

The drawing shows what misfire rate (in %) - as a function of the load/speed range - allows the temperature of the TWC to climb to approx. 1000 °C.