Publications

Diesel piston-bowl shape is a key design parameter that affects spray-wall interactions and turbulent flow development, and in turn affects the engine’s thermal efficiency and emissions. It is hypothesized that thermal efficiency can be improved by enhancing squish-region vortices as they are hypothesized to promote fuel-air mixing, leading to faster heat-release rates. However, the strength and longevity of these vortices decrease with advanced injection timings for typical stepped-lip (SL) piston geometries. Dimple stepped-lip (DSL) pistons enhance vortex formation at early injection timings. Previous engine experiments with such a bowl show 1.4% thermal efficiency gains over an SL piston. However, soot was increased dramatically [SAE 2022-01-0400]. In a previous study, a new DSL bowl was designed using non-combusting computational fluid dynamic simulations. This improved DSL bowl is predicted to promote stronger, more rotationally energetic vortices than the baseline DSL piston: it employs shallower, narrower, and steeper-curved dimples that are placed further out into the squish region. In the current experimental study, this improved bowl is tested in a medium-duty diesel engine and compared against the SL piston over an injection timing sweep at low-load and part-load operating conditions. No substantial thermal efficiency gains are achieved at the early injection timing with the improved DSL design, but soot emissions are lowered by 45% relative to the production SL piston, likely due to improved air utilization and soot oxidation. However, these benefits are lost at late injection timings, where the DSL piston renders a lower thermal efficiency than that of the SL piston. Energy balance analyses show higher wall heat transfer with the DSL piston than with the SL piston despite a 1.3% reduction in the piston surface area. Vortex enhancement may not necessarily lead to improved efficiency as more energetic squish-region vortices can lead to higher convective heat transfer losses.

To comply with increasingly stringent pollutant emissions regulations, diesel engine operation in a catalyst-heating mode is critical to achieve rapid light-off of exhaust aftertreatment catalysts during the first minutes of cold starting. Current approaches to catalyst-heating operation typically involve one or more late post injections to retard combustion phasing and increase exhaust temperatures. The ability to retard post injection timing(s) while maintaining acceptable pollutant emissions levels is pivotal for improved catalyst-heating calibrations. Higher fuel cetane number has been reported to enable later post injections with increased exhaust heat and decreased pollutant emissions, but the mechanism is not well understood. The purpose of this experimental and numerical simulation study is to provide further insight into the ways in which fuel cetane number affects combustion and pollutant formation in a medium-duty diesel engine. Three full boiling-range diesel fuels with cetane numbers of approximately 45, 50, and 55 are employed in this study with a well-controlled set of calibrations employing a five-injection strategy. The two post injections are block-shifted to increasingly retarded timings, and the effects on exhaust heat and pollutant emissions are quantified for each fuel. For a given injection strategy calibration, increasing cetane number enables increased exhaust temperature and decreased hydrocarbon and carbon monoxide emissions for a fixed load. The increase in exhaust temperature is attributed to an increased fueling requirement to compensate for additional wall heat losses caused by earlier, more robust pilot combustion with the more reactive fuels. Formaldehyde is predicted to form in the fuel-lean periphery of the first pilot injection spray and can persist until exhaust valve opening in the absence of direct interactions with subsequent injections. Unreacted fuel-air mixture in the fuel-rich interior of the first-pilot spray is likely too cool for any significant reactions, and can persist until exhaust valve opening in the absence of turbulence/chemistry interactions and/or direct heating through interactions with subsequent injections.

Spray-wall interactions in diesel engines have a strong influence on turbulent flow evolution and mixing, which influences the engine's thermal efficiency and pollutant-emissions behavior. Previous optical experiments and numerical investigations of a stepped-lip diesel piston bowl focused on how spray-wall interactions influence the formation of squish-region vortices and their sensitivity to injection timing. Such vortices are stronger and longer-lived at retarded injection timings and are correlated with faster late-cycle heat release and soot reductions, but are weaker and shorter-lived as injection timing is advanced. Computational fluid dynamics (CFD) simulations predict that piston bowls with more space in the squish region can enhance the strength of these vortices at near-TDC injection timings, which is hypothesized to further improve peak thermal efficiency and reduce emissions. The dimpled stepped-lip (DSL) piston is such a design. In this study, the in-cylinder flow is simulated with a DSL piston to investigate the effects of dimple geometry parameters on squish-region vortex formation via a design sensitivity study. The rotational energy and size of the squish-region vortices are quantified. The results suggest that the DSL piston is capable of enhancing vortex formation compared to the stepped-lip piston at near-TDC injection timings. The sensitivity study led to the design of an improved DSL bowl with shallower, narrower, and steeper-curved dimples that are further out into the squish region, which enhances predicted vortex formation with 27#x00025; larger and 44#x00025; more rotationally energetic vortices compared to the baseline DSL bowl. Engine experiments with the baseline DSL piston demonstrate that it can reduce combustion duration and improve thermal efficiency by as much as 1.4#x00025; with main injection timings near TDC, due to improved rotational energy, but with 69#x00025; increased soot emissions and no penalty in NOx emissions.

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Compliance with future ultra-low nitrogen oxide regulations with diesel engines requires the fastest possible heating of the exhaust aftertreatment system to its proper operating temperature upon cold starting. Late post injections are commonly integrated into catalyst-heating operating strategies. This experimental study provides insight into the complex interactions between the injection-strategy calibration and the tradeoffs between exhaust heat and pollutant emissions. Experiments are performed with certification diesel fuel and blends of diesel fuel with butylal and hexyl hexanoate. Further analyses of experimental data provide insight into fuel reactivity and oxygen content as potential enablers for improved catalyst-heating operation. A statistical design-of-experiments approach is developed to investigate a wide range of injection strategy calibrations at three different intake dilution levels. Thermodynamic and exhaust emissions measurements are taken using a new medium-duty, single-cylinder research engine. Analysis of the results provides insight into the effects of exhaust gas recirculation, oxygenated fuel blends, and fuel reactivity on exhaust heat and pollutant emissions. Late-cycle heat release is an important factor in determining exhaust temperatures. Intake dilution and fuel properties certainly affect late-cycle heat release, but the methods applied in this work are not sufficient to reproduce or explain the mechanisms by which improved fuel cetane rating promotes operation with hotter exhaust and lower pollutant emissions.