Fuel/air mixing process and combustion in an optical direct-injection engine

Abstract

Fuel/air mixing is analysed on an optical gasoline direct-injection engine. The fuel concentration field is measured using a laser-induced flourescence technique. Three conditions, which vary injection timing while keeping constant the ignition timing, are investigated. A qualitative correlation is made between the local fuel/air ratio near the spark plug with ignition and combustion in the chamber. It is found that the mean concentration at the ignition point is high when the delay between injection and ignition is short. In some cycles, liquid fuel covers the electrodes resulting in frequent misfires. The best operating point is achieved with near-stoichiometric conditions at the ignition point, and a moderate concentration cyclic variability. On the contrary, early injection timing gives lean conditions and large cyclic variations at the ignition point. Moreover, the mixture is more dilute in the chamber, and combustion is slow and unstable. Cycle to cycle correlation between fuel concentration near the spark gap and combustion initiation shows that locally lean conditions give very slow combustion.