The modern spark ignition engines, due to the introduced strategies for limiting the consumption without reducing the power, are sensitive to both the detonation and the increase of the inlet turbine temperature. In order to reduce the risk of detonation, the use of dilution with the products of combustion (EGR) is an established practice that has recently improved with the use of water vapor obtained via direct or indirect injection.The application and optimization of these strategies cannot ignore the knowledge of physical quantities characterizing the combustion such as the laminar flame speed and the ignition delay, both are intrinsic property of the fuel and are function of the mixture composition (mixture fraction and dilution) and of its thermodynamic conditions.The experimental measurements of the laminar flame speed and the ignition delay available in literature, rarely report the effects of dilution by EGR or water vapor. To overcome the limitations of the experimentation is possible to predict the value of the ignition delay using numerical models based on chemical kinetics theory. The increased performance of computing systems…

The heat transfer phenomena in Internal Combustion Engines (ICEs) are one of the main research topics that need to be addressed to enhance the performance in terms of power, efficiency, emissions and reliability. The present study is focused on the evaluation of the in-cylinder heat fluxes through the use of Computational Fluid Dynamic (CFD) simulations, with a wall function approach. In particular, the aim of this work is to present a new fully non-isothermal wall function obtained from the one-dimensional (1-D) energy balance equation for turbulent flows in the boundary layers, specifying all the steps and assumptions which have carried to the final fully compressible formulation. The new proposed wall function has been validated against experimental data of the General Motors (GM) Pancake Engine, representative of low Brake Mean Effective Pressure (bmep) operating point, comparing the results with other existing wall functions. With the objective of a mesh independency analysis, the wall functions considered have been tested with three different grids, varying the height of the first layer. Globally, it has been found that the…

The vehicle WLTP and RDE homologation test cycles are pushing the engine technology toward the implementation of different solutions aimed to the exhaust gases emission reduction. The tightening of the policy on the Auxiliary Emission Strategy (A.E.S.), including those for the engine component protection, faces the Spark Ignited (S.I.) engines with the need to replace the fuel enrichment as a means to cool down both unburnt mixture and exhaust gases to accomplish with the inlet temperature turbine (TiT) limit. Among the whole technology solutions conceived to make SI engine operating at lambda 1.0 on the whole operation map, the water injection is one of the valuable candidates. Despite the fact that the water injection has been exploited in the past, the renewed interest in it requires a deep investigation in order to outcome its potential as well as its limits. Many experimental campaigns have been performed while only few researches have deeply investigated the effect of the water injection on the air-fuel mixture under engine operating conditions. Since the experiments perform like a black box…

Metallic open-cell foams have proven to be valuable for many engineering applications. Their success is mainly related to mechanical strength, low density, high specific surface, good thermal exchange, low flow resistance and sound absorption properties.The present work aims to investigate three principal aspects of real foams: the geometrical characterization, the flow regime characterization, the effects of the pore size and the porosity on the pressure drop. The first aspect is very important, since the geometrical properties depend on other parameters, such as porosity, cell/pore size and specific surface. A statistical evaluation of the cell size of a foam sample is necessary to define both its geometrical characteristics and the flow pattern at a given input velocity. To this purpose, a procedure which statistically computes the number of cells and pores with a given size has been implemented in order to obtain the diameter distribution. In particular, a morphological characterization of an actual foam sample was performed by applying an image processing method. The analyzed foam was a cubic open-cell foam made by Silicon Carbide (SiC).…

The laminar burning speed is an important intrinsic property of an air-fuel mixture determining key combustion characteristics such as turbulent flame propagation. It is a function of the mixture composition (mixture fraction and residual gas mass fraction) and of the thermodynamic conditions.Experimental measurements of Laminar Flame Speeds (LFS) are common in literature, but initial pressure and temperature are limited to low values due to the test conditions: typical pressure values for LFS detection are lower than 25 bar, and temperature rarely exceeds 550 K.Actual trends in spark ignition engines are to increase specific power output by downsizing and supercharging, thus the flame front involves even more higher pressure and temperature since the beginning of combustion. The most widespread models used to extrapolate the experimental data to the engine like conditions are derived from that of Metghalchi and Keck, but they often fail to correctly predict LFS values outside the experimental space.Thanks to the development of accurate chemical kinetic models together with the increase of computer performance, it is possible to numerically predict the laminar flame…

The performance optimization of modern Spark Ignition engines is limited by knock occurrence: heavily downsized engines often are forced to work in the Knock-Limited Spark Advance (KLSA) range. Knock control systems monitor the combustion process, allowing to achieve a proper compromise between performance and reliability. Combustion monitoring is usually carried out by means of accelerometers or ion sensing systems, but recently the use of cylinder pressure sensors is also becoming frequent in motorsport applications. On the other hand, cylinder pressure signals are often available in the calibration stage, where SA feedback-control based on the pressure signal can be used to avoid damages to the engine during automatic calibration.A predictive real-time combustion model could help optimizing engine performance, without exceeding the allowed knock severity. Several knock models are available in the literature: most of those proposed for real-time applications are single zone or two-zone models, grounded on more complex CFD simulations. However, since the knock phenomenon is influenced by several factors, the real-time determination of KLSA requires the model to be adapted to the engine actual…

The primary target of the internal combustion engines design is to lower the fuel consumption and to enhance the combustion process quality, in order to reduce the raw emission levels without performances penalty.In this scenario the direct injection system plays a key role for both diesel and gasoline engines. The spray dynamic behaviour is crucial in defining the global and the local air index of the mixture, which in turns affects the combustion process development. At the same time it is widely recognized that the spray formation is influenced by numerous parameters, among which also the cavitation process inside every single hole of the injector nozzle. The proper prediction of the cavitation development inside the injector nozzle holes is crucial in predicting the liquid jet emerging from them. In this mechanism the CFD simulation is of great importance because of the too small dimension of the nozzle holes, which are mostly non suitable for an accurate experimental investigation and, when they are, these analyses need to be limited to a few cases for cost reasons.…

Metallic foams or sponges are materials with a cell structure suitable for many industrial applications, such as reformers, heat catalytic converters, etc. The success of these materials is due to the combination of various characteristics such as mechanical strength, low density, high specific surface, good thermal exchange properties, low flow resistance and sound absorption. Different materials and manufacturing processes produce different type of structure and properties for various applications. In this work a genetic algorithm has been developed and applied to support the design of catalytic devices. In particular, two substrates were considered, namely the traditional honeycomb and an alternative open-cell foam type. CFD simulations of pressure losses and literature based correlations for the heat and mass transfer were used to support the genetic algorithm in finding the best compromise between flow resistance and pollutant abatement. The CFD analysis was conducted by means of numerical simulations carried out on a geometry sample obtained by the micro-tomography technique to investigate the flow regime type and to extract pressure drop information. The result of this analysis was…

Large Eddy Simulation (LES) represents nowadays one of the most promising techniques for the evaluation of the dynamics and evolution of turbulent structures characterizing internal combustion engines (ICE). In the present paper, subdivided into two parts, the capabilities of the open-source CFD code OpenFOAM® v2.3.0 are assessed in order to evaluate its suitability for engine cold flow LES analyses. Firstly, the code dissipative attitude is evaluated through an inviscid vortex convection test to ensure that the levels of numerical dissipation are compatible with LES needs. Quality and completeness estimators for LES simulations are then proposed. In particular the Pope M parameter is used as a LES completeness indicator while the LSR parameter provides useful insights far calibrating the grid density. Other parameters such as the two-grid LESIQk index are also discussed. Then advanced SGS models such as the dynamic WALE and the Sigma models are compared with models traditionally used for LES simulations of ICE wall-bounded flows, i.e. the dynamic Smagorinsky and the WALE models. The proposed SGS models have been implemented in the open-source…