Analytical Scaling Model for Hydrocarbon Emissions From Fuel Absorption in Oil Layers in Spark Ignition Engines

ABSTRACT

Cyclic absorption and desorption of fuel by the lubricating oil layer on the cylinder wall has been suggested as a significant source of unburned hydrocarbon emissions from spark ignition engines. A simple analytical model describing this dynamic process as unsteady one-dimensional cyclic absorption and desorption of a dilute amount of gas in a thin liquid layer on an impervious wall has been developed.

A general solution for a periodic variation in fuel concentration in the gas phase is obtained, and three distinct limits for the net absorption of hydrocarbons are shown to exist depending on the relative time scales for the hydrocarbon concentration forcing function, the diffusion in the liquid and in the gas phase. A general criterion for when gas-phase convection species transfer resistance must be taken into account is also derived.

Focus was given to the relative importance of the gas-phase and liquid-phase diffusion rates, and the scaling of the hydrocarbon absorption phenomena with realistic engine operating conditions. Therefore, physical properties, such as oil layer temperature, oil layer thickness, solubility, and transport coefficients were estimated by empirical correlations relating them to the engine operating condition rather than as constant inputs.

Calculated results show that under typical conditions, the rate of absorption expressed as a percentage of the total charge is primarily a function of fuel component solubility and engine operating temperature. The absolute rate of absorption scales linearly with load, and the sensitivity to speed depends on the operating temperature. At low temperatures, where diffusion and convection species transfer are controlling, the rate of absorption and desorption decreases with increasing engine speed. At normal operating temperatures however, oil layer thickness rather than species transfer is controlling and the process becomes nearly independent of engine speed.

Key Words:

• Combustion in practical systems
• internal combustion engines
• environmental combustion
• HCs and CO