Riccardo Scarcelli

High-Pressure Gaseous Injection: A Comprehensive Analysis of Gas Dynamics and Mixing Effects

While the transportation field is mostly characterized by the use of liquid fuels, gaseous fuels like hydrogen and natural gas have shown high thermal efficiency and low exhaust emissions when used in internal combustion engines (ICEs). In particular, high-pressure direct injection of a gaseous fuel within the cylinder overcomes the loss of volumetric efficiency and allows stratifying the mixture around the spark plug at the ignition time. Direct injection and mixture stratification can extend the lean flammability limit and improve efficiency and emissions of ICEs.Compared to liquid sprays, the phenomena involved in the evolution of gaseous jets are less complex to understand and model. Nevertheless, the numerical simulation of a high-pressure gas jet is not a simple task. At high injection pressure, immediately downstream of the nozzle exit the flow is supersonic, the gas is under-expanded, and a large series of shocks occurs due to the effect of compressibility. To simulate and capture these phenomena, grid resolution, computational time-step, discretization scheme, and turbulence model need to be properly set.The research group on hydrogen ICEs at Argonne National Laboratory has been extensively working on validating numerical results on gaseous direct injection and mixture formation against PIV and PLIF data from an optically accessible engine. While a good general agreement was observed, simulations still could not perfectly predict the mixing of fuel with the surrounding air, which sometimes led to significant under-prediction of fuel dispersion. The challenge is to correctly describe the gas dynamic phenomena of under-expanded gas jets. To this aim, x-ray radiography was performed at the Advanced Photon Source (APS) at Argonne to provide high-detail data of the mass distribution within a high-pressure gas jet, with the main focus on the under-expanded region.In this paper, the numerical simulation of high-pressure (100 bar) injection of argon in a cylindrical chamber is performed using the computational fluid dynamic (CFD) solver Fluent. Numerical results of jet penetration and mass distribution are compared with x-ray data. The simplest nozzle geometry, consisting of one hole with a diameter of 1 mm directed along the injector axis, is chosen as a canonical case for modeling validation. A sector (90°) mesh, with high resolution in the under-expanded region, is used and the assumption of symmetry is made. Results show good agreement between CFD and x-ray data. Gas dynamics and mass distribution within the jet are well predicted by numerical simulations.Copyright

Update on the Progress of Hydrogen-Fueled Internal Combustion Engines

This chapter provides an overview on the use of hydrogen as a fuel for internal combustion engines (ICEs). First, pros and cons are discussed for using hydrogen to fuel ICEs versus fuel cells. Then, the properties of hydrogen pertinent to engine operation are briefly reviewed, after which the present state of the art of hydrogen engines is discussed. Ongoing research efforts are highlighted next, which primarily aim at maximizing engine efficiency throughout the load range, while keeping emissions at ultralow levels. Finally, the challenges for reaching these goals and translating laboratory results to production are discussed.

Evaluation of the efficiency and the drive cycle emissions for a hydrogen direct-injection engine

Hydrogen is seen as a sustainable energy carrier for transportation because it can be generated using renewable energy sources and it is a favorable fuel for clean vehicle powertrains. Hydrogen internal-combustion engines have been identified as a cost-effective consumer of hydrogen in the near term to aid in the development of a large-scale hydrogen infrastructure. Current research on hydrogen internal-combustion engines is directed by a series of efficiency and emissions targets defined by the US Department of Energy including a peak brake thermal efficiency of 45% and nitrogen oxide emissions of less than 0.07 g/mile. A high-efficiency hydrogen direct-injection engine was developed at Argonne National Laboratory to take advantage of the combustion characteristics of hydrogen. The engine employs a lean control strategy with turbocharging for power density comparable with that of gasoline engines. The injection strategy was optimized through collaborative three-dimensional computational fluid dynamics and experimental efforts to achieve mixture stratification that is beneficial for both a high efficiency and low nitrogen oxide emissions. The efficiency maps of the hydrogen engine demonstrate a peak brake thermal efficiency of 45.5% together with nitrogen oxide maps showing emissions of less than 0.10 g/kW h in much of the operating regime. In order to evaluate the driving-cycle nitrogen oxide emissions, the engine maps were fed into a vehicle simulation assuming a midsize sedan with a conventional (non-hybrid) powertrain. With a 3.0 l hydrogen engine, nitrogen oxide emissions from a Urban Dynamometer Driving Schedule cycle are 0.017 g/mile which fulfills the project goal and are even sufficiently low to meet the Super-Ultra-Low-Emissions Vehicle II emissions specification. The city or highway fuel economy, normalized to gallons of gasoline, is 32.4/51.5 mile/gal(US) for a combined average of 38.9 mile/gal(US), exceeding the 2016 Corporate Average Fuel Economy standard. Further vehicle simulations were performed to show the effect of engine downsizing. With a smaller 2.0 l engine, nitrogen oxide emissions increase to 0.028 g/mile, which still exceeds the US Department of Energy target together with the benefit of a fuel economy improvement to 45.4 mile/gal(US) (combined).

CFD and Optical Investigations of Fluid Dynamics and Mixture Formation in a DI-H2ICE

This paper reports the validation of a three-dimensional numerical simulation of the in-cylinder processes during gas-exchange, injection, and compression in a direct-injection, hydrogen-fueled internal combustion engine. Computational results from the commercial code Fluent are compared to experimental data acquired by laser-based measurements in a corresponding optically accessible engine. The simulation includes the intake-port geometry as well as the injection event with its supersonic hydrogen jet. The cylinder geometry is typical of passenger-car sized spark-ignited engines. Gaseous hydrogen is injected from a high-pressure injector with a single-hole nozzle. Numerically and experimentally determined flow fields in the vertical, central symmetry plane are compared for a series of crank angles during the compression stroke, with and without fuel injection. With hydrogen injection, the fuel mole-fraction in the same data plane is included in the comparison as well. The results show that the simulation predicts the flow field without injection reasonably well, with increasing numerical-experimental disagreement towards the end of the compression stroke. The injection event completely disrupts the intake-induced flow, and the simulation predicts the post-injection velocity fields much better than the flow without injection at the same crank-angles. The two-dimensional tumble ratio is evaluated to quantify the coherent barrel motion of the charge. Without fuel injection, the simulation significantly over-predicts tumble during most of the compression stroke, but with injection, the numerical and experimental tumble ratio track each other closely. The evolution of hydrogen mole-fraction during the compression stroke shows conflicting trends. Jet penetration and jet-wall interaction are well captured, while fuel dispersion appears under-predicted. Possible causes of this latter discrepancy are discussed.Copyright

Extending Lean and EGR-Dilute Operating Limits of a Modern GDI Engine Using a Low-Energy Transient Plasma Ignition System

The efficiency improvement and emissions reduction potential of lean and EGR dilute operation of spark-ignition gasoline engines is well understood and documented. However, dilute operation is generally limited by deteriorating combustion stability with increasing inert gas levels. The combustion stability decreases due to reduced mixture flame speeds resulting in significantly increased combustion initiation periods and burn durations.A study was designed and executed to evaluate the potential to extend lean and EGR-dilute limits using a low-energy transient plasma ignition system. The low-energy transient plasma was generated by nano-second pulses and its performance compared to a conventional transistorized coil ignition system operated on an automotive, gasoline direct injection (GDI) single-cylinder research engine. The experimental assessment was focused on steady-state experiments at the part load condition of 1500 rpm 5.6 bar IMEP, where dilution tolerance is particularly critical to improving efficiency and emissions performance.Experimental results suggest that the energy delivery process of the low-energy transient plasma ignition system significantly improves part load dilution tolerance by reducing the early flame development period. Statistical analysis of relevant combustion metrics was performed in order to further investigate the effects of the advanced ignition system on combustion stability. Results confirm that at select operating conditions EGR tolerance and lean limit could be improved by as much as 20% (from 22.7 to 27.1% EGR) and nearly 10% (from λ=1.55 to 1.7) with the low-energy transient plasma ignition system.Copyright

Effects of Ignition and Injection Perturbation under Lean and Dilute GDI Engine Operation

Turbocharged gasoline direct injection (GDI) engines are quickly becoming more prominent in light-duty automotive applications because of their potential improvements in efficiency and fuel economy. While EGR dilute and lean operation serve as potential pathways to further improve efficiencies and emissions in GDI engines, they also pose challenges for stable engine operation. Tests were performed on a single-cylinder research engine that is representative of current automotive-style GDI engines. Baseline cases were performed under steady-state operating conditions where combustion phasing and dilution levels were varied to determine the effects on indicated efficiency and combustion stability. Sensitivity studies were then carried out by introducing binary low-high perturbation of spark timing and injection duration on a cycle-by-cycle basis under EGR dilute and lean operation to determine dominant feedback mechanisms. Ignition perturbation was phased early/late of MBT timing, and injection perturbation was set fuel rich/lean of the given air-to-fuel ratio. COVIMEP was used to define acceptable operation limits when comparing different perturbation cases. Overall sensitivity data shows COVIMEP is more sensitive to injection perturbation over ignition perturbation. This is because of the greater effect injection perturbation has on combustion phasing, ignition delay, and combustion duration.

The Observation of Cyclic Variation in Engine Simulations When Using RANS Turbulence Modeling

State-of-the art engine technologies are susceptible to high cycle-to-cycle variability. Researchers have successfully used Large Eddy Simulations (LES) to capture this cyclic variation with CFD. However, LES is computationally expensive. The current work demonstrates that using RANS turbulence models can also exhibit cyclic variation if the simulation approach minimizes numerical viscosity. This is accomplished by using fine mesh resolution, non-morphing mesh motion, higher-order accurate numerical schemes, and small timesteps.RANS turbulence models act to destroy time-varying smaller eddies and replace the mixing effects of these eddies with enhanced viscosity. In an IC engine, larger-scale eddies can change from cycle to cycle, and may not be small enough to be dampened out by the RANS turbulence viscosity. By minimizing the numerical viscosity, the length scale at which eddies are destroyed is reduced and more structure is seen in the simulated flowfield. If the injection and combustion strategy in an engine is susceptible to cyclic changes in these large-scale eddies, then cyclic variation will be apparent in the simulation when using a RANS model.This work will also demonstrate that perturbations in initial conditions, boundary conditions, or numerical settings can give run-to-run variability in simulation consistent with cycle-to-cycle variability in an actual engine.For the current work, three studies are performed to show that the use of a RANS turbulence model does not always yield an ensemble average result. One of the studies is a basic cylinder-in-cross-flow case. The other two studies are for engines. One of the engine studies focuses on global mixing parameters and compares to TCC (Transparent Combustion Chamber) experimental data. The other engine study looks at cycle-to-cycle variation in combustion predictions.Copyright

Capturing Cyclic Variability in EGR Dilute SI Combustion Using Multi-Cycle RANS

Dilute combustion is an effective approach to increase the thermal efficiency of spark-ignition (SI) internal combustion engines (ICEs). However, high dilution levels typically result in large cycle-to-cycle variations (CCV) and poor combustion stability, therefore limiting the efficiency improvement. In order to extend the dilution tolerance of SI engines, advanced ignition systems are the subject of extensive research.When simulating the effect of the ignition characteristics on CCV, providing a numerical result matching the measured average in-cylinder pressure trace does not deliver useful information regarding combustion stability. Typically Large Eddy Simulations (LES) are performed to simulate cyclic engine variations, since Reynold-Averaged Navier-Stokes (RANS) modeling is expected to deliver an ensemble-averaged result.In this paper it is shown that, when using RANS, the cyclic perturbations coming from different initial conditions at each cycle are not damped out even after many simulated cycles. As a result, multi-cycle RANS results feature cyclic variability. This allows evaluating the effect of advanced ignition sources on combustion stability but requires validation against the entire cycle-resolved experimental dataset.A single-cylinder GDI research engine is simulated using RANS and the numerical results for 20 consecutive engine cycles are evaluated for several operating conditions, including stoichiometric as well as EGR dilute operation. The effect of the ignition characteristics on CCV is also evaluated. Results show not only that multi-cycle RANS simulations can capture cyclic variability and deliver similar trends as the experimental data, but more importantly that RANS might be an effective, lower-cost alternative to LES for the evaluation of ignition strategies for combustion systems that operate close to the stability limit.© 2015 ASME

Numerical and Experimental Analysis of Ignition and Combustion Stability in EGR Dilute GDI Operation

This paper discusses the characteristics of EGR dilute GDI engines in terms of combustion stability. A combined approach consisting of RANS numerical simulations integrated with experimental engine testing is used to analyze the effect of the ignition source on flame propagation under dilute operating conditions.A programmable spark-based ignition system is compared to a production spark system in terms of cyclic variability and ultimately indicated efficiency. 3D-CFD simulations are carried out for multiple cycles with the goal of establishing correlations between the characteristics of the ignition system and flame propagation as well as cycle-to-cycle variations. Numerical results are compared to engine data in terms of in-cylinder pressure traces.The results show that an improved control over the energy released to the fluid surrounding the spark domain during the ignition process has beneficial effects on combustion stability. This allows extending the dilution tolerance for fuel/air mixtures. Although affected by cyclic variability, numerical results show good qualitative agreement with experimental data. The result is a simple but promising approach for relatively quick assessment of stability improvements from advanced and alternative ignition strategies.Copyright

Numerical Study of the Combustion Characteristics of a Diesel Micro Pilot Ignited DI Gasoline Engine With Turbocharging and Cooled EGR

Advanced technologies combining turbocharging, downsizing, direct injection, and cooled EGR are being intensively investigated in order to significantly improve the fuel economy of spark-ignition (SI) gasoline engines. To avoid the occurrence of knock and to improve the thermal efficiency, a significant fraction of EGR is often used. Due to the significant fraction of EGR, the ignition source needs to be enhanced to ensure high combustion stability. In addition to advanced spark-based solutions, diesel micro-pilot (DMP) technology has been proposed in recent years where the diesel fuel replaces the spark-plug as the ignition source.This paper studies the combustion characteristics of a diesel micro pilot ignited gasoline engine, employing direct injection of gasoline and diesel as well as turbocharging and cooled EGR. A multi-dimensional CFD code with a chemical kinetic calculation capability was extensively validated across the engine speed and load range in a previous study [1]. This paper explores the influence of a number of parameters on DMP combustion behavior, including: diesel pilot mass fraction, start of injection (SOI), DMP injection strategy, as well as EGR rate, air/fuel ratio, and DI gasoline/air mixture inhomogeneity.Besides, the comparison of DMP ignited combustion with traditional spark ignited combustion is also made in terms of EGR tolerance, lean burn limit, and DI gasoline air mixture inhomogeneity. Finally, numerical simulations aimed at optimizing both gasoline and diesel injection parameters, as well as EGR rate in order to enhance the engine performance in the DMP combustion mode, are discussed.Copyright