Experimental Study of Lube Oil Characteristics in the PCV System and Effects on Engine Oil Consumption

Abstract

Engine oil consumption is an important source of hydrocarbon and particulate emissions in modern automobile engines. Great efforts are being made in recent years to minimize the impact of oil consumption on engine emissions. Research engineers continue to study the sources and driving mechanisms of oil consumption. Of these mechanisms, the contribution from the Positive Crankcase Ventilation (PCV) system is the least investigated. However, recent studies have shown that the blowby contribution to oil consumption could be significant under certain conditions. The PCV system refers to the system that vents the blowby gases loaded with oil back to the engine air intake system. It includes oil separators, air breather connecting the intake with the crankcase, and a flow-regulating valve. In a closed crankcase ventilation system, the oil-loaded blowby and crankcase gases are fed back into the air intake. Blowby gases are formed during engine operation by leakage of unburdened and compressed air-fuel mixture from the combustion chamber past the engine piston and ring pack into the crankcase. The oil that is circulated back into the intake through the PCV system could leave deposits in the intake manifold, and thus both the quantity and physical characteristics of the re-circulated oil are of great interest. This study analyzes the PCV blowby-oil consumption mechanisms and examines the main oil sources and oil characteristics by combining oil-consumption with in-cylinder measurements. A sulfur-tracer method was used along with a gravimetric method to measure the blowby oil consumption dependence on oil level, coolant outlet temperature, operating speed and load in a production spark ignition engine. Liquid oil distribution on the liner and piston was also studied using a Laser Induced Fluorescence (LIF) technique. In addition, in-cylinder variables such as the liner temperature and cylinder pressure that affect the oil evaporation and blowby flow rates were also measured. The blowby oil consumption map showed an increase in oil consumption with load and speed. Further analysis showed that the blowby flow was mainly dependent on the load of the engine, whereas the oil concentration in the blowby did not show this strong relationship. Whether the blowby gases pick up more oil in the ring-pack region than oil in the crankcase was thus carefully analyzed. Supplementally, a strong relationship was observed between oil consumption and sump oil level, showing that oil in the crankcase is an important source of oil in the blowby. More experiments were run at different coolant outlet temperatures to study the blowby oil consumption aimed at identifying the sources of oil in the blowby. The results show that oil evaporation is dependent on the liner temperatures that increase with load and speed. These data provided additional information to distinguish among the possible sources of blowby oil consumption. Additional experimentation was carried out to estimate drop-size distribution of oil suspended in the ventilated gases, which showed that entrainment of small oil droplets in the PCV flow varied with both speed and load.