Large-Eddy Simulations of Spray Variability Effects on Flow Variability in a Direct-Injection Spark-Ignition Engine Under Non-Combusting Operating Conditions

Large-eddy Simulations (LES) have been carried out to investigate spray variability and its effect on cycle-to-cycle flow variability in a direct-injection spark-ignition (DISI) engine under non-reacting conditions. Initial simulations were performed of an injector in a constant volume spray chamber to validate the simulation spray set-up. Comparisons showed good agreement in global spray measures such as the penetration. Local mixing data and shot-to-shot variability were also compared using Rayleigh-scattering images and probability contours. The simulations were found to reasonably match the local mixing data and shot-to-shot variability using a random-seed perturbation methodology. After validation, the same spray set-up with only minor changes was used to simulate the same injector in an optically accessible DISI engine. Particle Image Velocimetry (PIV) measurements were used to quantify the flow velocity in a horizontal plane intersecting the spark plug gap. The engine was operated in a skip-fired operating mode and comparisons focused on cycles that included fuel injection, but no spark event and therefore no combustion. 105 total LES engine cycles were simulated using a parallel cycle simulation approach and 3 different perturbation methods in an attempt to isolate the effects of shot-to-shot spray variability and the initial turbulent flow field as well as their interaction effects on overall engine CCVs. The experimental mean and standard deviations were reasonably well matched by the simulations, though quantitative comparisons near the injection event during the intake stroke were difficult due to the high uncertainty in the PIV measurements at these crank angles. The 3 simulation perturbation methods resulted in very similar results, though further analysis found the current parallel cycle approach may be limiting the ability of the simulations to isolate the spray and flow effects.This research was conducted as part of the Co-Optimization of Fuels & Engines (Co-Optima) project sponsored by the U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE), Bioenergy Technologies and Vehicle Technologies Offices. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (Argonne). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DEAC02- 06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable wsorldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. The research at Argonne was funded by DOE’s Office of Vehicle Technologies, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC02-06CH11357.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143834/1/2018-01-0196.pd

Numerical Methodology for Optimization of Compression-Ignited Engines Considering Combustion Noise Control

[EN] It is challenging to develop highly efficient and clean engines while meeting user expectations in terms of performance, comfort and drivability. One of the critical aspects in this regard is combustion noise control. Combustion noise accounts for about 40 percent of the overall engine noise in typical turbocharged diesel engines. The experimental investigation of noise generation is difficult due to its inherent complexity and measurement limitations. Therefore, it is important to develop efficient numerical strategies in order to gain a better understanding of the combustion noise mechanisms. In this work, a novel methodology was developed, combining computational fluid dynamics (CFD) modeling and genetic algorithm (GA) technique to optimize the combustion system hardware design of a high-speed direct injection (HSDI) diesel engine, with respect to various emissions and performance targets including combustion noise. The CFD model was specifically set up to reproduce the unsteady pressure field inside the combustion chamber, thereby allowing an accurate prediction of the acoustic response of the combustion phenomena. The model was validated by simulating several steady operating conditions and comparing the numerical results against experimental data, in both temporal and frequency domains. Thereafter, a GA optimization was performed with the goal of minimizing indicated specific fuel consumption (ISFC) and combustion noise, while restricting pollutant (soot and NOx) emissions to their respective baseline values. Eight design variables were selected pertaining to piston bowl geometry, nozzle inclusion angle, number of injector nozzle holes and in-cylinder swirl. An objective merit function based on the emissions, ISFC and combustion noise, was constructed to quantify the strength of the engine designs, and was determined using the CFD model as the function evaluator. The in-cylinder noise level was characterized by the total resonance energy of local pressure oscillations. The optimum engine configuration thus obtained, showed a significant improvement in terms of efficiency and combustion noise compared to the baseline system, along with both soot and NOx emissions within their respective constraints. This optimum configuration included a deeper and tighter bowl geometry with higher swirl and larger number of nozzle holes. Subsequently, a more detailed acoustics analysis based on proper orthogonal decomposition (POD) technique was carried out to further explore the combustion noise benefits achieved by the GA optimum. This computational study is a first of its kind (to the best of the authors¿ knowledge), which demonstrates a comprehensive framework to incorporate combustion noise into a numerical optimization strategy for engine design.The equipment used in this work was partially supported by FEDER and the Spanish Government through grant no. DPI2015-70464-R and by Fondo Europeo de Desarrollo Regional (FEDER) project funds "Dotacion de infraestructuras cientifico tecnicas para el Centro Integral de Mejora Energetica y Medioambiental de Sistemas de Transporte (CiMeT), (FEDER-ICTS-2012-06)," framed in the operational program of unique scientific and technical infrastructure of the Spanish Ministerio de Economia y Competitividad. J. Gomez-Soriano was partially supported through contract FPI-S2-2016-1353 of the "Programa de Apoyo para la Investigacion y Desarrollo (PAID-01-16)" of Universitat Politecnica de Valencia. The submitted manuscript was created partly by UChicago Argonne, LLC, Operator of Argonne National Laboratory. Argonne, a US Department of Energy (DOE) Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. This research was partly funded by the US DOE Office of Vehicle Technologies, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC02-06CH11357. The authors wish to thank Gurpreet Singh and Leo Breton, program managers at DOE, for their support.Broatch, A.; Novella Rosa, R.; Gómez-Soriano, J.; Pinaki, P.; Som, S. (2018). Numerical Methodology for Optimization of Compression-Ignited Engines Considering Combustion Noise Control. SAE International Journal of Engines. 11(6):625-642. https://doi.org/10.4271/2018-01-0193S62564211

Development of a Virtual CFR Engine Model for Knocking Combustion Analysis

[EN] Knock is a major bottleneck to achieving higher thermal efficiency in spark-ignited (SI) engines. The overall tendency to knock is highly dependent on fuel anti-knock quality as well as engine operating conditions. It is, therefore, critical to gain a better understanding of fuel-engine interactions in order to develop robust knock mitigation strategies. In the present work, a numerical model based on three-dimensional (3-D) computational fluid dynamics (CFD) was developed to capture knock in a Cooperative Fuel Research (CFR) engine. For combustion modeling, a hybrid approach incorporating the G-equation model to track turbulent flame propagation, and a homogeneous reactor multi-zone model to predict end-gas auto-ignition ahead of the flame front and post-flame oxidation in the burned zone, was employed. In addition, a novel methodology was implemented wherein a laminar flame speed lookup table generated a priori from a chemical kinetic mechanism could be used to provide flame speed as an input to the G-equation model, instead of using conventional empirical correlations. Multi-cycle Reynolds-Averaged Navier Stokes (RANS) simulations were performed for two different spark timings (STs) corresponding to non-knocking and knocking conditions, with other operating conditions kept the same as those of a standard Research Octane Number (RON) test. Iso-octane was considered as the fuel for the numerical study. Two different reduced kinetic mechanisms were employed to describe end-gas auto-ignition chemistry and to generate the flame speed lookup table. Experimental data, including intake/exhaust boundary conditions, was provided by a spark timing sweep study conducted in an in-house CFR engine. Moreover, cylinder wall/valve/port surface temperatures and residual gas fraction (RGF) were estimated using a well-calibrated one-dimensional (1-D) model. On the other hand, a novel methodology was also developed to analyze experimental data for the knocking case and identify the most representative cycle. For the non-knocking case, a good agreement was found between experiment and CFD simulation, with respect to cycle-averaged values of 10% burn point (CA10), 50% burn point (CA50) and peak pressure magnitude/location. The virtual CFR engine model was also demonstrated to be capable of predicting average knock characteristics for the knocking case, such as knock point, knock intensity and energy of resonance, with good accuracy.The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (Argonne). Argonne, a U.S. Department of Energy (DOE) Office of Science laboratory, is operated under Contract No. DEAC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in the said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. This research was partially funded by DOE's Office of Vehicle Technologies and Office of Energy Efficiency and Renewable Energy (EERE) under Contract No. DE-AC02-06CH11357. The authors wish to thank Gurpreet Singh, Kevin Stork, and Leo Breton, program managers at DOE, for their support. This research was conducted as part of the Co-Optimization of Fuels and Engines (Co-Optima) project sponsored by the U.S. DOE Office of EERE, Bioenergy Technologies and Vehicle Technologies OfficesPal, P.; Kolodziej, C.; Choi, S.; Som, S.; Broatch, A.; Gómez-Soriano, J.; Wu, Y.... (2018). Development of a Virtual CFR Engine Model for Knocking Combustion Analysis. SAE International Journal of Engines. 11(6):1069-1082. https://doi.org/10.4271/2018-01-0187S1069108211

Experimental and numerical study of lift-off length and ignition delay of a two-component diesel surrogate

Understanding and controlling mixing and combustion processes is fundamental for the ever more demanding pollutant regulations and fuel consumption standards of direct injection diesel engines. The fundamentals of these processes have been long studied from both experimental and numerical perspectives. As numerical models become more advanced, the need for adequate experimental data increases. Hence, experimental methodologies and scientific databases need to be enhanced with more quantitative, accurate, consistent, and reliable information in order to evaluate the models in a robust fashion. The present study seeks to enhance the current state-of-the-art by further evaluating the combustion performance of a two-component diesel surrogate for multi-dimensional compression ignition engine simulations, composed of n-dodecane and m-xylene. This surrogate is expected to better represent diesel fuel combustion than the standard Engine Combustion Network (ECN) fuel (n-dodecane), since it contains an alkyl-benzene which represents an important chemical class present in diesel fuels. Experiments and numerical simulations have been performed on lift-off length and ignition delay in a wide range of conditions for a single-hole injector from ECN. The experiments were carried out in a constant-pressure flow facility able of reproducing engine-like thermodynamic conditions. The experiments focused in characterizing the ignition delay through the Schlieren technique, and the lift-off length through OH* chemiluminescence visualization, at ECN suggested test conditions. On the other hand, computational fluid dynamics (CFD) simulations were performed using a multi-flamelet representative interactive flamelet (mRIF) model by taking consideration of turbulence-chemistry-interaction (TCI) with a beta-function on the form of scalar probability density functions (PDFs). This model is evaluated extensively over a wider range of parametric variations in this study. Encouraging results were obtained compared to the experiments with regards to the predictions of ignition delay and lift-off length at different ambient temperatures, ambient densities and fuel-injection pressures. Under predictions were found at less reactive conditions, which leave room for improvement in the future. (C) 2014 Elsevier Ltd. All rights reserved.The research was funded by DOE's Office of Vehicle Technologies, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC02-06CH11357. The authors wish to thank Gurpreet Singh and Leo Breton, program managers at DOE, for his support.Payri, R.; Viera Sotillo, JP.; Pei, Y.; Som, S. (2015). Experimental and numerical study of lift-off length and ignition delay of a two-component diesel surrogate. Fuel. 158:957-967. https://doi.org/10.1016/j.fuel.2014.11.072S95796715

Development and validation of spray models for investigating diesel engine combustion and emissions.

Development and validation of spray models for investigating diesel engine combustion and emissions

Large-Eddy Simulation (LES) of Spray Transients: Start and End of Injection Phenomena

This work reports investigations on Diesel spray transients, accounting for internal nozzle flow and needle motion, and demonstrates how seamless calculations of internal flow and external jet can be accomplished in a Large-Eddy Simulation (LES) framework using an Eulerian mixture model. Sub-grid stresses are modeled with the Dynamic Structure (DS) model, a non-viscosity based one-equation LES model. Two problems are studied with high level of spatial and temporal resolution. The first one concerns an End-Of-Injection (EOI) case where gas ingestion, cavitation, and dribble formation are resolved. The second case is a Start-Of-Injection (SOI) simulation that aims at analyzing the effect of residual gas trapped inside the injector sac on spray penetration and rate of fuel injection. Simulation results are compared against experiments carried out at Argonne National Laboratory (ANL) using synchrotron X-ray. A mesh sensitivity analysis is conducted to assess the quality of the LES approach by evaluating the resolved turbulent kinetic energy budget and comparing the outcomes with a length-scale resolution index. LES of both EOI and SOI processes have been carried out on a single hole Diesel injector, providing insights in to the physics of the processes, with internal and external flow details, and linking the phenomena at the end of an injection event to those at the start of a new injection. Concerning the EOI, the model predicts ligament formation and gas ingestion, as observed experimentally, and the amount of residual gas in the nozzle sac matches with the available data. The fast dynamics of the process is described in detail. The simulation provides unique insights into the physics at the EOI. Similarly, the SOI simulation shows how gas is ejected first, and liquid fuel starts being injected with a delay. The simulation starts from a very low needle lift and is able to predict the actual Rate-Of-Injection (ROI) and jet penetration, based only on the prescribed needle motion. Finally, guidelines and future improvements of the model are discussed concerning the simulation of the transient injection phases

Effect of Nozzle Orifice Geometry on Spray, Combustion, and Emission Characteristics under Diesel Engine Conditions

Diesel engine performance and emissions are strongly coupled with fuel atomization and spray processes, which in turn are strongly influenced by injector flow dynamics. Modern engines employ micro-orifices with different orifice designs. It is critical to characterize the effects of various designs on engine performance and emissions. In this study, a recently developed primary breakup model (KH-ACT), which accounts for the effects of cavitation and turbulence generated inside the injector nozzle is incorporated into a CFD software CONVERGE for comprehensive engine simulations. The effects of orifice geometry on inner nozzle flow, spray, and combustion processes are examined by coupling the injector flow and spray simulations. Results indicate that conicity and hydrogrinding reduce cavitation and turbulence inside the nozzle orifice, which slows down primary breakup, increasing spray penetration, and reducing dispersion. Consequently, with conical and hydroground nozzles, the vaporization rate and fuel air mixing are reduced, and ignition occurs further downstream. The flame lift-off lengths are the highest and lowest for the hydroground and conical nozzles, respectively. This can be related to the rate of fuel injection, which is higher for the hydroground nozzle, leading to richer mixtures and lower flame base speeds. A modified flame index is employed to resolve the flame structure, which indicates a dual combustion mode. For the conical nozzle, the relative role of rich premixed combustion is enhanced and that of diffusion combustion reduced compared to the other two nozzles. In contrast, for the hydroground nozzle, the role of rich premixed combustion is reduced and that of non-premixed combustion is enhanced. Consequently, the amount of soot produced is the highest for the conical nozzle, while the amount of NOx produced is the highest for the hydroground nozzle, indicating the classical tradeoff between them