Contribution of Oil Layer Mechanism to the Hydrocarbon Emissions from Spark-Ignition Engines

Abstract

One mechanism by which some fuel may escape the main combustion process in spark-ignition engines and be exhausted as unburned hydrocarbons is by absorption in the lubricating oil on the cylinder liner. The importance of this mechanism is, however, uncertain. Modeling studies suggest that the process can take place within an engine cycle, and that the amount of fuel absorbed in the oil layer is significant. Investigations in combustion bombs, and engines operated without lubricant indicate that this contribution is substantial. However, several researchers have reported no significant effect, when using combinations of fuels and oils with very different solubility characteristics.This study comprise the results from several experiments and modeling efforts. A ten component synthetic fuel, and different lubricants formulated to study the effect of base stock and viscosity were tested. The predicted variation in oil layer thickness caused by viscosity differences, were of the order of 20-40%. Steady state engine tests were carried out at three different coolant temperatures using a single cylinder engine with the combustion chamber of a production engine. A piston with reduced ring-pack crevices, and a cylinder head gasket designed to line up flush with the liner, were used to minimize the combustion chamber crevice volumes, and thus focus on HC derived from the oil layer mechanism. In addition to total HC, speciated engine-out emissions were recorded for the cold operating condition.Critical unknown physical parameters for the fuel components and lubricants were experimentally determined. Head space gas chromatography was used to measure solubilities, and Nuclear Magnetic Resonance was used to establish diffusion rates. Numerical modeling was employed to estimate the oil layer thickness, and the absorption rates for the individual fuel components. The estimated differences in absorption rate or "source strength" for the different lubricants, as caused primarily by different oil layer thickness, and fuel-oil solubility were of the order of 20%. The speciated emissions showed some sensitivity to oil layer thickness at low load and speed, with thicker oil layers producing more of the heavy, i.e. more soluble, fuel components. At higher loads and speeds speciated emissions showed very little to none sensitivity to oil layer thickness. For total engine-out HC, i.e. fuel and non-fuel HC, no significant effect of oil layer thickness, or calculated source strength, was observed at any of the operating conditions, contrary to what would be expected if fuel-oil absorption was contributing significantly to engine-out HC. Thus these results do not support oil layer mechanism as a significant source of unburnt hydrocarbons in the tested engine.

Effect of different lubricating oil on the adsorption/desorption process of fuel in oil layer in an SI engine

“…The other important fact is being temperature and molecular weight. The dynamic viscosity at various temperatures for different lubricating oils are shown in Figure a calculated using Vogel correlation given by Equation .…”

“…Here, it is assumed that there is no transverse flow across the oil film. Equation proposed by Wilke and Chang is used for calculating diffusion coefficient. where T , MW O and η O are temperature, molecular weight and dynamic viscosity of lubricating oil, respectively.…”

“…The constants A and B depend on the lubricants and are calculated from the experimental data . The calculated values of A and B for a set of different oils with gasoline as a fuel are summarised in Table .…”

Development of Phenomenological Models for Engine-Out Hydrocarbon Emissions from an SI DI Engine within a 0D Two-Zone Combustion Chamber Description

“…The Henry constant of the surrogate mixture is then calculated as a weighted average of the Henry constants of the component on the basis of the molar fractions. Multiple literature sources [26,35,56,68] report an exponential trend of the Henry constant with oil temperature. For this reason, the calculated 𝑘 B values for the fuel used in this work at discrete temperatures have been fitted with an exponential function over the oil temperature T 3,+ in °C.…”