Effect of Different Fuels on the Unburned HC Emission from SI Engine: A Theoretical Study 2016-28-0148
Unburned hydrocarbon (HC) emission results because part of the fuel inducted into the engine escapes combustion. HC emission is dependent on many mechanisms such as adsorption and desorption of fuel in oil layer, flame quenching, fuel escaping into crevices and accumulation of fuel in engine deposits, etc. Out of these, the oil layer adsorption/desorption mechanism contributes to about 25-30% of total engine-out HC emissions. In this work adsorption/desorption mechanism is studied considering the engine to be fueled with different fuels. The fuel adsorbed/desorbed in lubricating oil is modeled with a one-dimensional partial differential equation (PDE). One dimension PDE is chosen as the transverse flow across the oil film is considered to be negligible. The PDE is solved using finite difference explicit scheme in which the space derivatives and the time derivatives are approximated with a second order central difference method and forward Euler method, respectively. The unburned fuel concentration at the oil-gas interface is predicted using a function derived from Henry’s law. It is found that the main factors affecting the amount of fuel diffused into oil layer are the properties Henry’s constant and diffusion coefficient of fuel in the oil. The results are presented for five different fuels - isooctane, toluene, benzene, propane and methane, and two lubricants - SAE10W30 and SAE15W60. Input parameters being considered are mean oil temperature, equivalence ratio, intake manifold pressure and engine speed. The simulation results show that the gaseous fuels such as methane and propane release lesser HC due to oil layer adsorption/desorption, in comparison to liquid fuels such as isooctane, toluene and benzene.