Catalyst and Exhaust System: Overview: Notes

The catalytic converter and exhaust systems work together to control the release of harmful engine exhaust emissions into the atmosphere. The engine exhaust gas consists mainly of nitrogen (N), carbon dioxide (CO₂ ) and water vapor (H₂ O). However, it also contains carbon monoxide (CO), oxides of nitrogen (NO_x ), hydrogen (H), and various unburned hydrocarbons (HCs). CO, NO_x , and HCs are major air pollutants, and their emission into the atmosphere must be controlled.

The exhaust system generally consists of an exhaust manifold, front exhaust pipe, front heated oxygen sensor (HO2S), rear exhaust pipe, rear HO2S, a muffler and an exhaust tailpipe. The catalytic converter is installed between the front and rear exhaust pipes. Catalytic converter efficiency is monitored by the on board diagnostic (OBD) system strategy in the powertrain control module (PCM). For specific OBD catalyst monitor information, refer to the CATALYST EFFICIENCY MONITOR .

The number of HO2S used in the exhaust stream and the location of these sensors depend on the vehicle emission certification level (LEV, ULEV, PZEV). The hybrid vehicle is a partial zero emission vehicle (PZEV) equipped with 2 separate HO2Ss that provide input to the PCM. The first sensor in the exhaust stream before the catalyst (HO2S11) is used for primary fuel control and the last sensor after the catalyst (HO2S12) is used to monitor the light-off catalyst.

Fig 1: HO2S Location

Courtesy of FORD MOTOR CO.