Oil aerosols formed in the crankcase of reciprocating engines are a significant source of particulate matter and may lead to engine component degradation when used in closed crankcase ventilation (CV) systems or constitute a significant emission source in open CV systems. In modern CV systems, filtration or inertial separation is used to collect oil particles. These devices are highly efficient over a range of flow rates and temperatures; however, some system conditions result in a decrease in efficiency with increasing temperature in a manner not predicted by theory.
Experiments were performed on a bench system that introduced atomized oil particles into a clean, pre-heated, air stream that was conveyed isothermally through a heated duct to an oven containing the removal device, either a coalescing filter or an inertial separator, followed by another heated section and an exit duct. The aerosol was sampled using identical upstream and downstream isokinetic sampling systems and characterized using a Dekati Electrical Low-Pressure Impactor (ELPI) that measured particle concentration and size in the range from 0.043 to 8.46μm aerodynamic diameter.
Neither single fiber efficiency (SFE) filtration theory nor physical arguments describing the performance of the inertial separator predict a significant dependence total and fractional efficiency and most penetrating particle size (MPPS) on temperature in the range explored here, 25 to 115°C, but in many cases our measurements show MPPS shifting to smaller size and total efficiency decreasing with increasing temperature. These observations are consistent with the hypothesis that, at device temperature, the particles being processed are smaller than the particles being measured, at room temperature. Particles shrink by evaporation as they pass through the heated sections, but cool and grow by condensation as the sample cools in the sampling lines or in the exit duct.
Reported carbon number distributions of typical lubricating oils and evaporation kinetics indicate that droplets in the size range investigated may lose more than half of their mass under temperature conditions, up to 115°C, found in typical crankcase removal devices under engine high-load conditions. These droplets are in a dynamic balance with their vapors and may change during sampling and measurement. Great care must be taken in design of systems used to characterize devices intended to remove volatile droplets and interpretation of measurements made, as the size of the particles measured may not be the same as the size of particles passing through the device.