Fast Exhaust-Runner Soot Measurements in a Diesel Optical Engine

A novel exhaust-runner soot diagnostics has been developed and tested in a skip-fired diesel optical engine. Crank-angle-resolved soot emissions are measured during the cylinder blowdown and exhaust processes by multi-pass extinction of coherent, visible (635 nm) light as it passes through an optical exhaust runner. To evaluate diagnostic accuracy, comparisons are made of soot volume fraction measured by extinction in the optical exhaust runner and by a conventional smoke meter. The diagnostic exhibits excellent sensitivity with soot volume fraction detection limits of better than 0.2 parts per billion (ppb). The experiments also employed an engine exhaust particle sizer (EEPS) to characterize particle size distributions from the skip-fired optical engine.

The diagnostic has been employed in this work to assess skip-fired cycle variations in soot emissions for two- and six-hole production diesel fuel injectors as oxygen concentration (i.e., dilution) is varied. As oxygen concentration is reduced for the two-hole tip, both mean soot volume fraction and cycle variations increase, with the coefficient of variation (COV) of extinction as high as 46%. At low intake oxygen (O₂) concentrations, soot apparently leaves the cylinder relatively early in the exhaust process. As O₂ concentration increases and soot volume fraction declines, smaller soot particles (as measured by the EEPS) leave the cylinder later in the exhaust stroke. Trends for the six-hole injector are similar, but cycle variations are significantly lower. The reduction in the cyclic variation of soot volume fraction as the number of orifices increases from two to six provides evidence that soot formed in a burning jet is a relatively stochastic event for which the COV is inversely proportional to the square of the number of distinct combustion plumes. The persistence of 27% exhaust soot COV for the six-hole tip at 14% intake O₂ suggests that improved control and optimization of the spray formation and combustion event could significantly reduce the average engine-out PM emissions.