Browse Publications Technical Papers 2019-24-0167

Piston Bowl Geometry Effects on Combustion Development in a High-Speed Light-Duty Diesel Engine 2019-24-0167

In this work, we studied the effects of piston bowl design on combustion in a light-duty direct-injection diesel engine. Two bowl designs were compared: a conventional, omega-shaped bowl and a stepped-lip piston bowl. Experiments were carried out in the SNL single-cylinder optical engine facility, with a medium-load, mild-boosted operating condition featuring a pilot+main injection strategy. CFD simulations carried out with the FRESCO platform featuring full-geometric modeling of the engine, were validated against measured in-cylinder performance as well as soot natural luminosity images. Differences in combustion development were studied using the simulation results, and sensitivities to in-cylinder flow field (swirl ratio) and injection rate parameters were also analyzed. In-cylinder mixture formation analysis showed that ignition of the pilot injection mixture develops nearly as a homogeneous adiabatic reactor, being mostly advected, not mixed, by the bowl’s swirling motion, while its timing is influenced by the local flow field. Local in-cylinder flow is also more crucial than injection parameters to igniting the main injection’s premixed fuel, as it determines the relative overlap with the high-temperature pilot ignited mixture. Bowl geometry effects drive diffusive and late-cycle combustion, as structural differences of the main injection spray flames appear since the different impact geometries at the piston bowl rim. Better air utilization with the stepped-lip geometry, thanks greater azimuthal spreading at the rim, a strong recirculating vortex in the squish, and better mixing/lower swirl in the bowl, is responsible for better late-cycle combustion efficiency and lower soot emissions. A comparison between full- and sector-mesh CFD approaches also showed that flame structure is geometry-driven and well-captured by the sector, but combustion predictions are deteriorated by the sector symmetry’s inaccurate local flow field representation.


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