Modeling the Evaporative Emissions of Oil-Fuel Mixtures

Motor vehicle hydrocarbon evaporative emissions are a crucial part of emissions regulations, and increasingly-stringent regulations stipulate essentially zero fuel-based hydrocarbon evaporative emissions. In port fuel injected engines, there is the potential for accumulation of PCV effluent in the intake system under certain vehicle operating conditions. The majority of this effluent is oil, but a percentage has been shown to be fuel. The percentage of fuel in this oil-fuel mixture in the intake is at a minimum equivalent to the fuel dilution level of the crankcase oil, and at times can be higher due to other sources of fuel, and fuel vapor, in the intake. This accumulation of liquid oil-fuel mixture can be a contributor of hydrocarbon evaporative emissions migrating out of the air induction system when subjected to transient temperatures while the engine is off. This paper describes a transient, multi-domain physical model in Modelica^® to describe the evaporative emissions of fuel-oil mixtures during a simulated diurnal Sealed Housing Evaporative Determination (SHED) test. Following a description of the SHED test and the model, analytic results are shown and validated against the experimental data over a wide range of operating conditions. An analytic design of experiments (DOE) was conducted and compared with available experimental results for a range of liquid compositions and amounts, surface areas, and transient temperature profiles. The model proved to be very effective at predicting both the magnitude and trend-wise behavior of the evaporative emissions of fuel-oil mixtures over the range of conditions simulated.