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.
A one-dimensional, non-equilibrium, compressible law of the wall model is proposed to increase the accuracy of heat transfer predictions from computational fluid dynamics (CFD) simulations of internal combustion engine flows on engineering grids. Our 1D model solves the transient turbulent Navier-Stokes equations for mass, momentum, energy and turbulence under the thin-layer assumption, using a finite-difference spatial scheme and a high-order implicit time integration method. A new algebraic eddy-viscosity closure, derived from the Han-Reitz equilibrium law of the wall, with enhanced Prandtl number sensitivity and compressibility effects, was developed for optimal performance. Several eddy viscosity sub-models were tested for turbulence closure, including the two-equation k-epsilon and k-omega, which gave insufficient performance. Validation against pulsating channel flow experiments highlighted the superior capability of the 1D model to capture transient near-wall velocity and temperature profiles, and the need to appropriately model the eddy viscosity using a low-Reynolds method, which could not be achieved with the standard two-equation models. The results indicate that the non-equilibrium model can capture the near-wall velocity profile dynamics (including velocity profile inversion) while equilibrium models cannot, and simultaneously reduce heat flux prediction errors by up to one order of magnitude. The proposed optimal configuration reduced heat flux error for the pulsating channel flow case from 18.4#x00025; (Launder-Spalding law of the wall) down to 1.67#x00025;.
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.
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.
Appropriate spray modeling in multidimensional simulations of diesel engines is well known to affect the overall accuracy of the results. More and more accurate models are being developed to deal with drop dynamics, breakup, collisions, and vaporization/multiphase processes; the latter ones being the most computationally demanding. In fact, in parallel calculations, the droplets occupy a physical region of the in-cylinder domain, which is generally very different than the topology-driven finite-volume mesh decomposition. This makes the CPU decomposition of the spray cloud severely uneven when many CPUs are employed, yielding poor parallel performance of the spray computation. Furthermore, mesh-independent models such as collision calculations require checking of each possible droplet pair, which leads to a practically intractable O(np2/2) computational cost, np being the total number of droplets in the spray cloud, and additional overhead for parallel communications. This problem is usually overcome by employing O°Rourke°s same-cell collision condition, which, however, introduces severe mesh dependency. In this work, we introduced two strategies to achieve optimal load balancing for fast spray calculations with mesh-independent models. Both methods were implemented in the FRESCO CFD code. For drop collisions, a mesh-independent collision detection algorithm with high parallel efficiency was developed. This method pre-sorts eligible collision pairs using a high-performance three-dimensional clustering algorithm similar to what is used for on-the-fly chemistry model reduction; these are then filtered again based on deterministic impact parameters and assembled in parallel into a global sparse adjacency structure. For the particle-in-cell vaporization/multiphase solver, we developed a solution-preserving load balancing algorithm. At each timestep, the parallel cell-ownership-based spray cloud structure is re-sorted into cell-owner bins, which are used to distribute the spray parcels across all CPUs along with their cell thermodynamic states; the distributed solution results are then sent back to the cell owners. The combination of both methods achieved more than one order of magnitude speed-up in spray solution for diesel engine simulations with a full and sector cylinder geometry.