Using CFD to model the distortion of pollutant concentration signal at exhaust lines

The present study introduces a method for generating the distortion of species concentration signals within exhaust lines, employing Computational Fluid Dynamics (CFD) modelling instead of experimental techniques. Understanding distortion is important when e.g. tailpipe pollutant signals need to be correlated to engine operation patterns and related correction models need to be built. The approach was initially applied on a typical exhaust line geometry of a car, demonstrating the ability of CFD modelling to simulate the CO₂ signal distortion from engine out to tailpipe, and was validated against laboratory measurements. Comparisons for variable operating points, between simulations and experiments, presented a good agreement with a deviation of up to 0.04 s in the time required for the signal to reach 50 % of its final output value (t₅₀), and of up to 0.1 s in the rise time (t_90-10). The methodology was applied to a vehicular exhaust line comprising tubing and buffer volumes, simulating catalysts or mufflers. This investigation aimed to identify the parameters that influence the distortion of CO₂, revealing that inlet exhaust gas velocity can significantly impact CO₂ distortion. Inlet temperature variations of ± 50 K produce a negligible deviation in t₅₀ of ± 0.01 s for low inlet velocities and of ± 0.001 s for high inlet velocities. CFD is a useful tool to characterize pollutants signal generation within exhaust line configurations not limited to cars. Such a technique can be used to any mobile or stationary combustion system to study species concentration distortions caused by the individual components.