Modulation schemes for on-board diagnostic exhaust system

摘要

The invention includes an on-board diagnostic system for monitoring exhaust gas from an automotive combustion engine and includes a sample port connected to the exhaust piping, preferably downstream from the catalytic converter. The exhaust port leads to a sample cell to which an infrared spectrometry system is operably connected to measure the concentration of hydrocarbons in the sample cell. An air pump is connected to the exhaust port and communicates with the sample cell. The air pump is operated to continuously purge the sample cell by pushing small amounts of clean air through the sample cell into the exhaust.