PIV examination of spray-enhanced swirl flow for combustion stabilization in a spray-guided stratified-charge direct-injection spark-ignition engine

Practical implementation of spray-guided stratified-charge direct-injection spark-ignition engines can be inhibited by combustion instability, in particular the occurrence of misfire and partial burns. Performance testing in an all-metal spray-guided stratified-charge direct-injection spark-ignition engine shows that increasing the engine speed from 1000 to 2000 r/min can cause a deterioration of the combustion stability for operation without intake-generated swirl. Introducing swirl to the in-cylinder air charge motion maintains combustion stability while the speed is increased. To gain understanding how swirl reduces cycle-to-cycle variability of the flow, two-dimensional Particle Image Velocimetry (PIV) measurements were made in a horizontal swirl plane near the top of the piston bowl and in a central vertical tumble plane. Tests with and without injection were conducted at 1000 and 2000 r/min for operation both with and without swirl. The results demonstrate that the swirl creates flow patterns in each cycle that are more similar to the ensemble-averaged cycle, and with decreased variability. Furthermore, the fuel injection causes a redistribution of angular momentum resulting from spray–swirl interaction. The gas-phase swirl flow is redistributed by the spray to create a very repeatable vortex with enhanced angular momentum close to the spray centerline. This decreases the cycle-to-cycle variability of the flow patterns. Quantified changes in the stability of the flow patterns with swirl and engine speed are consistent with the combustion-instability trends.