Tunable infrared lasers enable monitoring the exhaust gas transients of internal combustion engines. Combustion in the cylinders of an engine leads to the formation of hot gaseous products, which are expelled through the exhaust pipe. The time transients of the expelled gas composition and temperature inside the pipe inform about the engine performance and its impact on air pollution. It is thus crucial to monitor such transients in-operando and in real-time, a task that requires non-invasive sampling on a time scale much shorter than the crank shaft revolution, which lasts tens of milliseconds. This challenge has been successfully tackled by researchers from Purdue University and the Ford Motor Company, who have developed an optical setup measuring the transients of NO, CO, CO₂ mole fractions and temperature with a 200 microsecond resolution. In this setup, which has been tested on a light-duty truck with a 8-cylinder engine, collinear beams from four mid-infrared tunable cascade lasers are collected by an infrared detector after crossing the engine-out exhaust pipe through windows. By scanning the laser wavelengths across suitable spectral regions in a time-multiplexed way, optical absorption bands characteristic of the three gases are recorded by the detector. Spectrum analysis ultimately yields the gas temperature and mole fractions with a few % accuracy. This operation is repeated with a 5 kHz frequency to record well-defined transients induced by individual cylinder firing and exhaust events.

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