N. Nowak, T. Sinn, K. Scheiber, C. Straube, J. Pfeil, J. Meyer, T. Koch, G. Kasper, A. Dittler

Aerosol Science and Technology, https://doi.org/10.1080/02786826.2021.1976720

The crankcase of combustion engines is an undesirable source of oil vapor and oily aerosols. The roles of nucleation and growth by vapor condensation in their formation (with and without pre-existing nuclei) were investigated both experimentally and by one-dimensional simulation under engine-like conditions using actual engine oil as well as hexadecane. Initial equilibrium saturation levels (at T_sat = 100°C to 130°C) were known and well defined; cooling took place in laminar tube flow at a defined rate.

Nucleation and growth of pre-existing nuclei lead to separate peaks around 0.25 ± 0.1 μm and 0.7 ± 0.2 μm respectively, that agree remarkably well with actual engine data. The position of the latter peak was quite insensitive to primary nuclei size in the range of 0.04 to 0.2 μm. Increasing either the vapor concentration (via a more volatile oil or higher T_sat) or decreasing the cooling rate lead to slightly larger nucleation peaks (0.15 –> 0.35 μm) but significantly lower peak concentrations (10x). Although counter-intuitive at first, this is explainable by shifts in supersaturation profile during cool-down.

Experiment and simulation limit spontaneous nucleation to pre-existing nuclei concentrations below about 2 to 5x10⁵ cm⁻³ at engine oil temperatures ≥100°C. Significant vapor losses to system walls during cool-down are an important factor in an actual engine environment as well as in the lab and must be included for realistic simulations. Losses affect actual number concentrations more than peak location.