Characterization of High-Tumble Flow Effects on Early Injection for a Lean-Burn Gasoline Engine

The influence of early induction stroke direct injection on late-cycle flows was investigated for a lean-burn, high-tumble, gasoline engine. The engine features side-mounted injection and was operated at a moderate load (8.5 bar brake mean effective pressure) and engine speed (2000 revolutions per minute) condition representative of a significant portion of the duty cycle for a hybridized powertrain system. Thermodynamic engine tests were used to evaluate cam phasing, injection schedule, and ignition timing such that an optimal balance of acceptable fuel economy, combustion stability, and engine-out nitrogen oxide (NOx) emissions was achieved. A single cylinder of the 4-cylinder thermodynamic engine was outfitted with an endoscope that enabled direct imaging of the spark discharge and early flame development. Tejhese measurements were complemented by optical-engine tests used to directly measure the impact of fuel injection dynamics on late-cycle flows via a combination of particle image velocimetry and diffuse back-illuminated extinction imaging techniques. From the optical imaging measurements, different interaction modes between the tumble vortex and liquid fuel spray were identified that were found to persist late into the compression stroke and directly influence velocity fields in the vicinity of the spark plug at the time of ignition. These modal interactions strongly depended on injection timing, with advanced injection timings relative to the optimal found to destructively interfere with the tumble vortex, which in turn compromised late-cycle spark plug flow development and was attributed to the significant deterioration in combustion stability during the thermodynamic engine tests. These results highlight the critical importance of spray targeting with respect to tumble vortex location.