Effect of spray targeting and energy-assisted ignition on in-flame flow fields in an optical compression ignition engine

The present study applies high-speed Mie-scattering spray imaging and flame image velocimetry (HS-FIV) to a jet fuel flame in a small-bore optical compression-ignition engine. The integration of sustainable aviation fuel (SAF) into the jet fuel supply may introduce challenges for stable engine operation due to low fuel reactivity. To resolve this issue, the engine is equipped with an ignition assistant plug providing additional heat into the compressed air. The impact of ignition assistant on the combustion of low reactivity fuel varies with injector tip protrusion, requiring detailed optical analysis to evaluate how the spray targeting changes relative to the piston bowl and thermal distributions. An optical engine is operated on a blend of 40% SAF and 60% F-24, a conventional jet fuel with mil-spec additives. The injector tip protrusion was varied between 1.5 and 4.5 mm below the cylinder head and for each protrusion, the high-speed movies were obtained with 30 cycles for sprays and 100 cycles for flames to address uncertainty concerns. The spray images were post processed via image binarisation to compute the liquid penetration length. An ensemble averaging method was applied to the FIV-derived flow fields to show the in-flame flow structure development while a spatial filtering approach was used for flow turbulence and combustion stability analysis. The spray image results exhibited decreased liquid penetration length and higher vaporisation for deeper injector tip protrusion, indicating higher temperature within the piston bowl. When the ignition assistant plug was activated, the FIV results showed similar overall flow structures and flow magnitude distribution to the plug off condition despite more advanced combustion phasing. However, lower cyclic variation was measured for the plug on condition, indicating more stable combustion as a key benefit of the active energy assistance. Regarding the injector tip protrusion variation, the shortest depth of 1.5 mm showed more retarded combustion phasing and lower peak pressure than those of 3.0 mm despite higher wall bounced-off flow magnitude. Enhanced vaporisation of the tested fuel blend as the injector tip was positioned deeper into the bowl was a likely cause of this observed trend. However, as the injector tip was protruded further to 4.5 mm, the combustion phasing was also more retarded and peak pressure was lower than those of 3.0 mm. Detailed flow field analysis showed lower magnitude flow vectors were observed for 4.5 mm depth as the jet impinged more on the floor of the piston bowl than the wall. The decreased wall bounce-off flow for deeper injector tip protrusion also led to lower flow turbulence measured in the r -θ plane. This outperformed higher fuel vaporisation expected, and thus the injector tip protrusion depth of 3 mm showed the most advanced combustion phasing and lowest cyclic variations.