Liquid fuel transport into the cylinder in spark ignition engines

Abstract

Liquid fuel inflow into the cylinder is considered an important source of exhaust hydrocarbon (HC) emissions from automotive spark ignition engines particularly during the start-up and subsequent warm up period. As fuel does not readily vaporize under cold engine conditions, significant quantities enter the cylinder in liquid form, where they would be stored in various locations (i.e. combustion chamber crevices, the oil layer, or the cylinder head surface), not fully participate in the subsequent combustion process, and significantly contribute to engine out HC emissions. This study identifies and quantifies the mechanisms via which liquid fuel is transported into the cylinder of a firing spark ignition engine during a simulated start-up and warm up process. In a firing, transparent single cylinder flow visualization engine a Phase Doppler Particle Analyzer (POPA) and a Planar Laser Induced Fluorescence (PLIF) technique were used to analyze in-cylinder liquid fuel droplets. From the measurements four major mechanisms of liquid fuel transport into the cylinder were identified using open and closed valve port fuel injection timing. The spatial (resulting from geometric features of the intake port) as well as the temporal variations (due to both engine warm up and cycle position) of the measured fuel droplet characteristics in the vicinity of the intake valve were analyzed. A procedure based on the PDP A and PLIF measurements was developed and validated to estimate the volume of liquid fuel entering the cylinder as a function of time during warm-up. The procedure was applied to compare the volume of liquid fuel entering the cylinder using open and closed valve injection. The dependence of the fuel transport mechanisms on important engine operating variables additionally was analyzed. Considered here were the effects of injection timing, fuel volatility, intake valve timing, injector type, and spray targeting in the intake port. The measurements are complemented by a one dimensional fuel droplet evaporation model to describe liquid fuel evaporation in the cylinder. The model was used to assess the amounts of fuel evaporation in the cylinder as well as of liquid fuel impingement on the cylinder liner and piston during engine warm-up.