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Single-cycle exhaust soot measurement from internal combustion engines

University of British Columbia

The black carbon particulate matter (soot) emissions from internal combustion engines have negative health and climate impacts. PM emissions are typically characterized with modest temporal resolutions; however, in-cylinder investigations have demonstrated significant variability and the importance of individual cycles. Detecting such variations in the exhaust requires measurements close to the exhaust valve, which are not possible with the current sensors. Here, a methodology for characterizing the cycle-specific PM concentration at the exhaust-port of a single-cylinder research engine is developed using a light-scattering sensor, the Fast Exhaust Nephelometer (FEN). The FEN light scattering is converted to soot mass concentration (Cₘ) and mass-mean mobility diameter (dm,g) using an inversion algorithm based on the Rayleigh-Debye-Gans model for fractal aggregates (RDGFA). The model incorporates the external mixing hypothesis (EMH) to correlate the diameter of primary particles with the aggregates. The inversion parameters are obtained from Transmission Electron Microscopy (TEM) and literature, resulting in Cₘ and dm,g that are within ±10% of the reference methods. The results could vary by ±40% due to uncertainties in the RDGFA parameters; however, by incorporating the EMH morphology model, the variations are reduced to within ~ ±25% of the reference measurements. The response time of the FEN, determined from a “skip-fired” scheme by disabling the fuel injection, is on average 55 ms. This is well below the engine cycle period (~100 ms) for the considered engine speeds. A cycle-specific PM mass averaging method was developed based on the characteristics of the exhaust-port signals. Using this cycle-resolved method, it is shown that the cycle-to-cycle coefficient of variation of Cₘ is 40%, while the in-cylinder gross indicated mean effective pressure (GIMEP) varies by 2%. Despite their different ranges of variation, the cycle-specific Cₘ and GIMEP are negatively correlated with R² ~ 0.2-0.7, where cycles with low GIMEP emit more soot. The physical causes of this association deserve further investigation, but are expected to be caused by local fuel-air mixing effects. The methods and findings of this work can further our understanding of the engine variability under transient conditions, and assist the interpretation of the in-cylinder variations observed in optical engine experiments.

Text

2020-12-24

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/76910

Mechanical Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/