Stefano Fontanesi

Large-Eddy simulation analysis of spark configuration effect on cycle-to-cycle variability of combustion and knock

Cycle-to-cycle variability is numerically simulated for high-speed, full-load operation of a turbocharged gasoline direct injection engine. Large-Eddy simulation is adopted to replicate the fluctuations of the flow field affecting the turbulent combustion. Experimental data were provided at knock onset, and large-Eddy simulation was validated for the same condition. In the original engine configuration, the spark plug is displaced toward the exhaust side, while the electrodes orientation is arbitrary. A 90° rotation is imposed to evaluate the effects of the aerodynamic obstruction caused by the electrode with respect to the flow field and the flame kernel growth. A second speculative analysis is performed modifying the position of the spark plug. The electrodes are shifted 2mm toward the intake side since this variation is compatible with the cylinder head layout. For both variations in orientation and position, the effects on the flow field around the spark plug are investigated. Statistical analysis is carried out on early flame kernel formation and knock tendency. The results highlight that the orientation of the electrodes affects the flow field for each cycle but plays a negligible role on the statistical cyclic variability, indirectly justifying the lack of an imposed orientation. As for the spark plug position, the numerical analysis indicate that the shifting of the electrodes toward the intake side slightly improves the knock limit mainly because of a reduction in in-cylinder peak pressure. In general, it is inferred that improvements may be achieved only through a simultaneous modification of the fuel jet orientation and phasing.

CFD Investigation of Fuel Film Formation within a GDI Engine under Cold Start Cranking Operation

A numerical study on the investigation of spray evolution and liquid film formation within the combustion chamber of a current production automotive Gasoline Direct Injected (GDI) engine characterised by a swirl-type side mounted injector is presented. Particularly, the paper focuses on low-temperature cranking operation of the engine, when, in view of the high injected fuel amount and the strongly reduced fuel vaporisation, wall wetting becomes a critical issue and plays a fundamental role on the early combustion stages. In fact, under such conditions, fuel deposits around the spark plug region can affect the ignition process, and even prevent engine start-up. In order to properly investigate and understand the many involved phenomena, experimental visualisation of the full injection process by means of an optically accessible engine would be a very useful tool. Nevertheless, the application of such technique, far from being feasible from an industrial point of view, appears to be very difficult even in research laboratories, due to the relevant wall wetting at cranking conditions. A numerical program was therefore carried out in order to analyze in depth and investigate the wall/spray interaction and the subsequent fuel deposit distribution on the combustion chamber walls. The CFD model describing the spray conditions at the injector nozzle was previously implemented and validated against experimental evidence. Many different injection strategies were tested and results compared in terms of both fuel film characteristics and fuel/air mixture distribution within the combustion chamber. Low-temperature cranking conditions proved to be an open challenge for the in-cylinder numerical simulations, due to the simultaneous presence of many physical sub-models (spray evolution, droplet-droplet interaction, droplet-wall interaction, liquid-film) and the very low motored engine speed. Nevertheless, the use of a properly customized and validated numerical setup led to a good understanding of the overall injection process as well as of the effects of both injection strategy and spray orientation modifications on both the air/fuel and fuel/wall interaction.Copyright

Numerical Analysis of Swirl Control Strategies in a Four Valve HSDI Diesel Engine

Recent four value HSDI Diesel engines are able to control the swirl intensity, in order to enhance the in-cylinder flow field at partial load without decreasing breathing capabilities at full load. Making reference to a current production engine, the purpose of this paper is to envestiage the influence of port design and flow-control strategies on both engine permeability and in-cylinder flow field. Using previously validated models, 3-D CFD simulations of the intake and compression strokes are performed in order to predict the in-cylinder flow patterns originated by the different configurations. The comparison between the two configurations in terms of airflow at full load indicates that Geometry 2 can trap 3.03% more air than Geometry 1, while the swirl intensity at IVC is reduced (−30%). The closure of one intake valve (the left one) is very effective to enhance the swirl intensity at partial load: the Swirl Ratio at IVC passes from 0.7 to 2.6 for Geometry 1, while for Geometry 2 it varies from 0.4 to 2.9.Copyright

Comparison Between Steady and Unsteady CFD Simulations of Two Different Port Designs in a Four Valve HSDI Diesel Engine: Swirl Intensity and Engine Permeability

Swirl control strategies are useful methods for controlling mixture formation in HSDI Diesel engines. Test rigs allows only steady state measurements of the Swirl number, and give only a rough estimation of the charge motion during the actual compression stroke within the engine. On the contrary, CFD simulations are powerful tools to characterize the air flow drawn into the cylinder, since they allow not only steady state operations, but also full dynamic modeling of the intake and compression strokes. This paper studies an application of computational fluid dynamics for predicting intake swirl intensity in an automotive 4 valve per cylinder C.I. Diesel engine. Two different intake ports are compared and the best trade off between engine permeability and swirl intensity is assessed. Both steady state and dynamic simulations of the induction process are carried out, and results demonstrate that steady state analysis is a reliable tool for predicting the port permeability, while the same capability is not proved in investigating the organized charge motion within the chamber.Copyright

CFD Methodology Assessment for the Investigation of Convective Heat Transfer Properties of Engine Coolants Under Boiling Conditions

The paper presents a combined experimental and numerical program directed at defining a cost/effective methodology for conjugate heat transfer CFD simulations of engine water cooling jackets. As a first step in the process, deficiencies in current numerical strategies for the analysis of conjugate heat transfer problems under typical engine operating conditions are exposed and commented. Results are shown form a wide validation program based on the comparison between experimental measurements from a test facility at Villanova University and CFD predictions at the University of Modena. On the experimental side, the test apparatus consists of a test section, pump, accumulator tank, rejection heat exchanger and required pumping. The test section is provided with a constant volumetric flow rate, and consists of a cylindrical aluminum body with a drilled horizontal flow channel. The section is heated by ten cartridge heaters located at a constant radial distance from the cylinder axis. The test section is connected to the flow loop by means of two calming sections, respectively at the cylinder inlet and exit. Twenty thermocouples are used to measure the test section local temperature along a radial plane cutting the cylinder. Water / ethylene-glycol binary mixture and pure water are tested and compared during the experimental program, in order to reproduce a set of thermal situations as close as possible to actual engine cooling system operation. On the CFD side, an extensive program reproducing the experiments is carried out in order to assess the predictive capabilities of some of the most commonly used eddy viscosity models available in literature. Both non-evaporating and evaporating conditions are tested, showing severe limitations to the use of simplified boiling models to correctly capture the complex interaction between turbulent boundary layer and vapor bubble dynamics. In order to overcome the above stated deficiencies under boiling conditions, a methodology is then proposed to both improve the accuracy of the CFD forecasts and reduce the computational costs of the simulations. A few preliminary results from the validation process are shown and briefly discussed at the end of the paper.Copyright

Combustion optimization of a marine di diesel engine

Enhanced calibration strategies and innovative engine combustion technologies are required to meet the new limits on exhaust gas emissions enforced in the field of marine propulsion and on-board energy production. The goal of the paper is to optimize the control parameters of a 4.2 dm3 unit displacement marine DI Diesel engine, in order to enhance the efficiency of the combustion system and reduce engine out emissions. The investigation is carried out by means of experimental tests and CFD simulations. For a better control of the testing conditions, the experimental activity is performed on a single cylinder prototype, while the engine test bench is specifically designed to simulate different levels of boosting. The numerical investigations are carried out using a set of different CFD tools: GT-Power for the engine cycle analysis, STAR-CD for the study of the in-cylinder flow, and a customized version of the KIVA-3V code for combustion. All the models are calibrated through the above mentioned experimental campaign. Then, CFD simulations are applied to optimize the injection parameters and to explore the potential of the Miller combustion concept. It is found that the reduction of the charge temperature, ensuing the adoption of an early intake valve closing strategy, strongly affects combustion. With a proper valve actuation strategy, an increase of boost pressure and an optimized injection advance, a 40% reduction of NOx emissions can be obtained, along with a significant reduction of in-cylinder peak pressure, without penalizing fuel efficiency.