HC from Lubricating Oil Film

Fuel hydrocarbons are absorbed in the oil film present on the cylinder walls during intake and compression strokes, which get desorbed back into the burned gases during combustion and expansion. On combustion, the partial pressure of fuel in the burned gases becomes nearly zero and the concentration gradient makes the fuel to be desorbed from oil and diffuse back into the burned gases. The desorbed fuel vapours from oil film are oxidized depending upon the temperature, pressure and composition of the burned gases.
The maximum amount of fuel that can be dissolved per unit volume of oil is given by:

(2.33)

where, n_foisthe number of moles of fuel absorbed in oil, n_o is number of moles of oil per unit volume, X_fc mole fraction of fuel in the combustion chamber gases close to the oil film, P is instantaneous cylinder pressure, H is theHenry’s constant. Mole fraction of fuel vapours in oil,

(2.34)

As n_fo << n_fo, x_fcan be approximated as n_fo/ n_o.   
Henry’s constant is a measure of fugacity of the fuel components (solute) in liquid phase or inverse of its solubility. Henry’s constant increases with temperature and decreases exponentially with increase in molecular weight of the solute (in present case the fuel). At 400 K, Henry’s constant for n-hexane, iso-octane and ethyl-benzene is 200, 120 and 45 kPa, respectively. A larger fraction of the heavier fuel components would be absorbed in the oil as they have a smaller value of H.  Taking average cylinder pressure under compression stroke as 0.5 MPa and typical oil film temperature equal to 400 K, the mole fraction of fuel vapour absorbed in oil film at equilibrium for the stoichiometric mixture of isooctane and air would be about 0.07
.The lubricant oil film thickness is a strong function of oil viscosity and hence the oil temperature. It also varies with engine speed. The oil film thickness on the cylinder wall varies between 1 and 10 μm. At temperature of 400 K, the diffusion time to reach equilibrium for fuel vapour absorption in oil film of 1 μm thick ness is about 10^-3 seconds and for a 10 μm thick film it would equal to 10^-1 seconds. For an engine speed of 3000 rpm, intake and compression strokes together would take 2x10^-2 seconds. Thus, for oil films of 1 to 2 μm thickness state of equilibrium in fuel vapour absorption would be achieved under engine conditions.
The absorption and desorption of fuel in the oil film and its contribution to the HC emissions involves several processes. Some of the absorbed hydrocarbon vapours in oil are carried to the crankcase where these are desorbed.
 It has been seen that when lubricating oil was added to fuel or to the engine cylinder or deposited on the piston crown, the exhaust hydrocarbons increased in proportion to the oil added.  The increased exhaust HC from the engine were identified as unburned fuel and partially oxidized fuel species and not the unburned oil or oil oxidation species.
Potential HC contribution of the engine oil film depends on the solubility of fuel in the engine oil and the amount of engine oil present in the combustion chamber.