Sampling and conditioning of engine blow-by aerosols for representative measurements by optical particle counter
N. Nowak, K. Scheiber, J. Pfeil, J. Meyer, A. Dittler, T. Koch , G. Kasper
Journal of Aerosol Science, October 2020, Volume 148, 105612, https://doi.org/10.1016/j.jaerosci.2020.105612
Particle size distributions (PSDs) of engine blow-by aerosol are commonly obtained with an optical particle counter (OPC), but representative measurements are challenging due to high number concentrations, elevated temperatures, presence of saturated vapor, as well as unsteady flow conditions. A comprehensive study of blow-by aerosol sampling/conditioning and OPC performance in such an environment was therefore conducted, based on a commercial OPC (Palas Promo 2000).
Single (1:9) and double-stage (1:80) sampling/dilution/conditioning systems were devised and their transfer functions characterized in the OPC sensor range of 0.3–17 μm. With (mostly inertial) particle losses >90% at 10 μm, correcting for them was crucial for reliable PSD measurements. The effect of saturated oil vapor on droplet growth during cool-down and dilution of the sample flow was investigated between 80 and 120 °C, based on actual vapor concentration data. Without dilution, droplet growth became significant above 100 °C while diluting 1:20 with 20 °C air suppressed growth.
Tests of over-all sampling and dilution strategy with engine blow-by aerosol gave excellent reproducibility and good agreement (after loss corrections) with reference data regarding PSD in the 0.3–10 μm range as well as total mass. The actual engine blow-by PSD was significantly broader though, exceeding the OPC range on both ends. Number-to-mass conversion of OPC data under-reported total mass by 10%–20% (depending on sensor range) compared to weighed filter samples, provided OPC contamination with oil deposits was avoided. A re-calibration procedure is proposed to deal adequately also with pressure pulsations resulting from engine operation.