Aside from aerosols produced during the combustion of fossil fuels, the oil mist vented through the crankcase breather of the engine is considered as a threat to the environment or, in case of closed ventilation systems, to the functionality of the engine. In the past, these “blow-by” aerosols have been investigated mainly from the perspective of emitted oil mass. This study instead focuses on sources and reduction of fine aerosols in the size range of about 0.2-5 μm, where number concentrations are of equal importance.
The investigation is conducted on a commercial truck diesel engine; aerosols are sampled with an optical particle counter at various locations along the blow-by path, in the region of the cylinder head before and after the oil aerosol separation system. The contribution of the turbocharger to the total aerosol load is found to be 24% by number and 21% by mass. The air compressor adds 8%-20% concerning number and mass only depending on the engine load.
The influences of the engine load and engine speed are investigated across the engine performance map, whereby the load turns out to be more relevant. By keeping the oil temperature constant across the entire range, it also presents that the oil temperature plays an important - but not the only - role for the emission level. Based on the relatively constant shape of particle size distributions across a wide range of engine loads and speeds, one can conclude that the mechanisms of droplet generation remain largely unchanged.
As expected, the passive pre-separator does not have a noticeable removal effect in the size of interest, while the active main separator removes 99% of the total mass down to 0.2 μm, and between 78% and 98% of the total number.
