Soot emissions from direct-injection engines are sensitive to the fuel-air mixing process, and may vary between combustion cycles due to turbulence and injector variability. Conventional exhaust emissions measurements cannot resolve inter- or intra-cycle variations in particle emissions, which can be important during transient engine operations where a few cycles can disproportionately affect the total exhaust soot. The Fast Exhaust Nephelometer (FEN) is introduced here to use light scattering to measure particulate matter concentration and size near the exhaust port of an engine with a time resolution of better than one millisecond. The FEN operates at atmospheric pressure, sampling near the engine exhaust port and uses a laser diode to illuminate a small measurement volume. The scattered light is focused on two amplified photodiodes.
Proof-of-concept tests were conducted on a heavy-duty single-cylinder research engine using a Westport high-pressure direct-injection (HPDI) natural gas fuel system. For this engine, the particulate emissions are dominated by soot at high loads, as they would be for a conventional diesel engine. When tested on the diluted exhaust, the FEN shows a close linear correlation with a commercial light-scattering instrument (DustTrakTM DRX 8533). Undiluted PM measurements close to the exhaust port show a spike (several times the average) after the exhaust valve opens; the signal then drops to a plateau for the remainder of the cycle. The magnitudes of the peak and the plateau vary by a factor of two or more from cycle to cycle, depending on the engine operating mode. Analysis of the ratio of the forward-to-backward light scattering signal indicates the emission of larger particles soon after the opening of the exhaust valve. On average, the diameter of the freshly emitted soot aggregates sampled after the exhaust valve is smaller than the diluted soot. Particle coagulation in the exhaust pipes and surge tank may explain this. Using the ratio of signals at two angles, the mass concentration at the exhaust port can be adjusted according to the Rayleigh-Debye-Gans (RDG) light scattering theory, and brought closer to the average concentrations in the diluted exhaust.