Publications

This project will provide scientific understanding needed to design, optimize, and calibrate the next generations of off-road diesel engines that comply with increasingly stringent pollutant emission regulations while achieving thermal efficiencies exceeding 50%.

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.

The need to reduce the carbon footprint from medium- and heavy-duty diesel engines is clear; low-carbon biofuels are a powerful means to achieve this. Liquid fuels are rapidly deployed because existing infrastructure can be utilized for their production, transport, and distribution. Their impact is unique as they can decrease the greenhouse gas (GHG) emissions of existing vehicles and in applications resistant to electrification. However, introducing new diesel-like bio-blends into the market is very challenging. At a minimum, it requires a comprehensive understanding of the life-cycle GHG emissions of the fuels, the implications for refinery optimization and economics, the fuel’s impact on the infrastructure, the effect on the combustion performance of current and future vehicle fleets, and finally the implications for exhaust aftertreatment systems and compliance with emissions regulations. Such understanding is sought within the Co-Optima project.

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.

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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.

Most multidimensional engine simulations spend much time solving for non-equilibrium spray dynamics (atomization, collision, vaporization). However, their accuracy is limited by significant grid dependency, and the need for extensive calibration. This is critical for modeling cold-start diesel fuel post injections, which occur at low temperatures and pressures, far from typical model validation ranges. At the same time, resolving micron-scale spray phenomena would render full Eulerian multiphase calculations prohibitive. In this study, an improved phase equilibrium based approach was implemented and assessed for simulating diesel catalyst heating operation strategies. A phase equilibrium solver based on the model by Yue and Reitz [1] was implemented: a fully multiphase CFD solver is employed with an engineering-size engine grid, and fuel injection is modeled using the standard Lagrangian parcels approach. Mass and energy from the liquid parcels are released to the Eulerian multiphase mixture according to an equilibrium-based liquid jet model. An improved phase equilibrium solver was developed to handle large real-gas mixtures such as those from accurate chemical kinetics mechanisms. The liquid-jet model was improved such that momentum transfer to the Eulerian solver better reproduces the physical spray jet structure. Validation of liquid/vapor penetration predictions showed that the model yields accurate results with very limited tuning and low sensitivity to the few calibration constants. In-cylinder simulations of diesel catalyst heating operation strategies showed that capturing spray structure is paramount when short, transient injection pulses and low temperatures are present. Furthermore, the EP model provides improved predictions of post-injection spray structure and ignitability, while conventional spray modeling does not capture the increase of liquid penetration during the expansion stroke. Finally, the only important EP model calibration constant, Cliq, does not affect momentum transfer, but it changes the local charge cooling distribution through the local energy transfer, which makes it candidate to additional research. The results confirm that non-equilibrium spray processes do not need to be resolved in engineering simulations of high-pressure diesel sprays.

Lagrangian spray modeling represents a critical boundary condition for multidimensional simulations of in-cylinder flow structure, mixture formation and combustion in internal combustion engines. Segregated models for injection, breakup, collision and vaporization are usually employed to pass appropriate momentum, mass, and energy source terms to the gas-phase solver. Careful calibration of each sub-model generally produces appropriate results. Yet, the predictiveness of this modeling approach has been questioned by recent experimental observations, which showed that at trans- A nd super-critical conditions relevant to diesel injection, classical atomization and vaporization behavior is replaced by a mixing-controlled phase transition process of a dense fluid. In this work, we assessed the shortcomings of classical spray modeling with respect to real-gas and phase-change behavior, employing a multicomponent phase equilibrium solver and liquid-jet theory. A Peng-Robinson Equation of State (PR-EoS) model was implemented, and EoS-neutral thermodynamics derivatives were introduced in the FRESCO CFD platform turbulent NS solver. A phase equilibrium solver based on Gibbs free energy minimization was implemented to test phase stability and to compute phase equilibrium. Zero-dimensional flash calculations were employed to validate the solver with single- A nd multi-component fuels, at conditions relevant to diesel injection. The validation showed that 2-phase mixture temperature in the jet core can deviate up to 40K from the single-phase solution. Surface equilibrium with Raoult's law employed for drop vaporization calculation was observed to deviate up to 100% from the actual multiphase real-gas behavior. Liquid-jet spray structure in high pressure fuel injection CFD calculations was modeled using an equilibrium-phase (EP) Lagrangian injection model, where liquid fuel mass is released to the Eulerian liquid phase, assuming phase-equilibrium in every cell. Comparison to state-of-the-art modeling featuring KH-RT breakup and multicomponent fuel vaporization highlighted the superior predictive capabilities of the EP model in capturing liquid spray structure at several conditions with limited calibration efforts.

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Stepped-lip diesel pistons can enhance in-cylinder vortex formation and thereby improve the thermal efficiency and emissions behavior of a diesel engine. Further improvements to diesel combustion systems may be realized through improved understanding of the mechanisms by which fuel sprays interact with pistons to form vortices. Analysis of computational fluid dynamics simulations provides insight about vorticity formation in one particular region of a particular stepped-lip combustion chamber. Interactions at the boundary between the sprays and the piston surface are a source of new vorticity that is transported upward and outward. This process is believed to be the origin of an energetic vortex that has been experimentally observed in the outermost region of the combustion chamber during the mixing-controlled combustion process, and is associated with improved turbulent mixing.

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Diesel engines remain a cost-effective, efficient, powerful propulsion source for many light- and medium-duty vehicle applications. Modest efficiency improvements in these engines can eliminate millions of tons of CO₂ emissions per year, but these improvements will require improved understanding of how diesel combustion chamber geometry influences mixture preparation, combustion, and pollutant formation processes. The research focus for this performance period is to provide insight into spray-wall interactions in stepped-lip combustion chambers. These interactions are believed to promote the formation of recirculating flow structures that improve thermal efficiency and reduce soot emissions, but these benefits are only fully realized for late main injection timings. A detailed mechanistic understanding of these processes can lead to cleaner, more efficient combustion chamber designs. This project will provide scientific understanding needed to design, optimize, and calibrate the next generations of light- and medium-duty diesel engines that comply with increasingly stringent pollutant emission regulations while achieving thermal efficiencies approaching 50%.

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