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.

Engine downsizing is gaining popularity in the high performance engine market sector, where a new generation of highly downsized engines with specific power outputs around or above 150 HP/litre is emerging. High-boost and downsizing, adopted to increase power density and reduce fuel consumption, have to face the increased risks of pre-ignition, knock or mega-knock. To counterbalance autoignition of fuel/air mixture, such engines usually operate with high fuel enrichments and delayed (sometimes negative) spark advances. The former is responsible for high fuel consumption levels, while the latter reduces performance and induces an even lower A/F ratio (below 11), to limit the turbine inlet temperature, with huge negative effects on BSFC. A relatively simple yet effective solution to increase knock resistance is investigated by means of 3-D CFD analyses in the paper: water is port injected to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock safety margins. Full-load engine operations of a currently made turbocharged GDI engine are investigated comparing the adopted fuel-only rich mixture with stoichiometric mixtures, for which water is added in the intake port under constant charge cooling in the combustion chamber. In order to find the optimum fuel/water balance, preliminary analyses are carried out using a chemical reactor to evaluate the effects of charge dilution and mixture modification on both autoignition delays and laminar flame speeds. Thanks to the lower chemical reactivity of the diluted end gases, the water-injected engine allows the Spark Advance (SA) to be increased; as a consequence, engine power target is met, or even crossed, with a simultaneous relevant reduction of fuel consumption.

The large-scale adoption of fuel cells system for sustainable power generation will require the combined use of both multidimensional models and of dedicated testing techniques, in order to evolve the current technology beyond its present status. This requires an unprecedented understanding of concurrent and interacting fluid dynamics, material and electrochemical processes. In this review article, Polymer Electrolyte Membrane Fuel Cells (PEMFC) are analysed. In the first part, the most common approaches for multi-phase/multi-physics modelling are presented in their governing equations, inherent limitations and accurate materials characterisation for diffusion layers, membrane and catalyst layers. This provides a thorough overview of key aspects to be included in multidimensional CFD models. In the second part, advanced diagnostic techniques are surveyed, indicating testing practices to accurately characterise the cell operation. These can be used to validate models, complementing the conventional observation of the current–voltage curve with key operating parameters, thus defining a joint modelling/testing environment. The two sections complement each other in portraying a unified framework of interrelated physical/chemical processes, laying the foundation of a robust and complete understanding of PEMFC. This is needed to advance the current technology and to consciously use the ever-growing availability of computational resources in the next future.

The paper reports the application of a look-up table approach within a LES combustion modelling framework for the prediction of knock limit in a highly downsized turbocharged DISI engine.During experimental investigations at the engine test bed, high cycle-to-cycle variability was detected even for relatively stable peak power / full load operations of the engine, where knock onset severely limited the overall engine performance.In order to overcome the excessive computational cost of a direct chemical solution within a LES framework, the use of look-up tables for auto-ignition modelling perfectly fits with the strict mesh requirements of a LES simulation, with an acceptable approximation of the actual chemical kinetics.The model here presented is a totally stand-alone tool for autoignition analysis integrated with look-up table reading from detailed chemical kinetic schemes for gasoline. The look-up table access is provided by a multi-linear interpolating routine internally developed at the “Gruppo Motori (GruMo)” of the University of Modena and Reggio Emilia. As the experimental tests were conducted operating the engine at knock-limited spark advance, the tool is at first validated for three different LES cycles in terms of knock tolerance, i.e. the safety margin to knock occurrence.As a second stage, the validation of the methodology is performed for discrete spark advance increases in order to assess the sensitivity of the modelling strategy to variations in engine operations. A detailed analysis of the unburnt gas physical state is performed which confirms the knock-limited condition suggested by the experimental tests.