Planar laser-induced fluorescence imaging of crevice hydrocarbon emissions in a spark-ignited engine

Abstract

In recent years, concerns over the impact of internal combustion engine hydrocarbon emissions on the environment have prompted tighter regulation on allowable emission lebels. While much work has been done on reducing hydrocarbon emissions after they have entered the exhaust stream, less direct monitoring of the emission sources has been performed. An optically accessible four-stroke internal combustion engine was used to investigate how fuel composition and engine operating conditions affect hydrocarbon emissions. Various crevices, ranging in size from 1 to 2 mm, were simulated by drilling holes into a flat wall built into the head of the engine. Emissions from the residual fuel ejected by these crevices were directly monitored using planar laser-induced fluorescence (PLIF) from iso-octane/n-heptane fuel blends doped with 3-pentanone. The fluorescence was imaged at various times during the engine cycle and found to be extremely dependent on crevice size, engine load, and fuel reactivity, Under most normal load conditions, the largest crevice showed evidence of significant flame penetration, while flame penetration into the smaller crevices was found to vary with engine load. The results for the quench diameter were in good agreement with a simple crevice flame-quenching model. Fuels with lower octane ratings were shown to enhance flame penetration due to their increased reactivity.