Simultaneous Measurement of Multiparameter of Diesel Engine Exhaust Based on Mid-Infrared Laser Absorption Spectroscopy

Exhaust parameters of an engine carry vital information that can help to understand its transient duty cycles. We developed a mid-infrared wavelength modulation spectroscopic sensor for simultaneous measurement of exhaust temperature, pressure, and nitric oxide (NO) and nitrogen dioxide (NO2) concentrations by using two quantum cascade lasers (QCLs) for the first time. The dual-channel signals were demodulated and normalized respectively with a specially designed digital lock-in amplifier (DLIA), achieving calibration-free measurements to adapt to the harsh and variable working conditions of diesel engines. Two-line thermometry was performed using a newly selected spectral line pair of NO2 (1629.85 and 1630.33 cm
$^{-1}$
). Simultaneous measurement of the quadruple parameter of diesel exhaust, i.e., temperature, pressure, and NO and NO2 concentrations, was first achieved using only three spectral lines and a developed multiparameter inversion algorithm. The feasibility of the sensor was demonstrated in field tests in an in-service vehicle. Exhaust temperature, pressure, and NO and NO2 concentrations were obtained continuously with 0.1 s time intervals, which clearly show these parameters in the transition and steady stages of the engine, respectively. The method and results can be used to evaluate the engine operation performance and combustion diagnostics.