Using PIV Measurements to Determine the Role of the In-Cylinder Flow Field for Stratified DISI Engine Combustion 2014-01-1237

In a companion study [1], experimental observations in a stratified-charge DISI engine operated with late injection of E70 led to the formation of two hypotheses: (1) For highly stratified spray-guided combustion, the heat-release rate of the main combustion phase is primarily controlled by mixing rates and turbulence level associated with fuel-jet penetration. (2) During the main combustion phase, the role of the in-cylinder flow field generated by the intake and compression strokes is primarily its stochastic disturbance of the mixing and flow associated with the fuel jets, thereby causing cycle-to-cycle variations of the spray-guided stratified combustion. Here, these hypotheses are tested. An optical engine was operated skip fired at 1000 and 2000 rpm, and exhibited the same combustion properties observed in the steady-state all-metal engine tests. High-speed particle image velocimetry (PIV) and spray imaging are used to quantify the intake-generated in-cylinder flow momentum, the spray induced momentum, and the resulting liquid spray variability.

The PIV measurements reveal that the spatially-averaged gas-flow speed (momentum) without injection at 2000 rpm is twice that of 1000 rpm. In contrast, just after injection the gas flow spatial average speed at 2000 rpm is only 24% higher due to the dominance of spray momentum. This is comparable to the 16% increase of the measured ensemble-averaged heat-release rate (in kW). The cyclic variability of the in-cylinder flow speed prior to injection is measured to be considerably higher at 2000 rpm compared to 1000 rpm. Though the injected liquid spray reduced the flow-speed cyclic-variability after injection, the higher variability did persist. The spray imaging reveals that the increased flow-speed variability at 2000 rpm causes increased variability of the spray jet trajectory, jet coalescence, and spray rotation from cycle to cycle. This work supports both the hypotheses that motivated this study.