Università degli studi di Modena e Reggio Emilia

In recent years, internal combustion engine (ICE) downsizing coupled with turbocharging was considered the most effective path to improve engine efficiency at low load, without penalizing rated power/torque performance at full load. On the other side, issues related to knocking combustion and excessive exhaust gas temperatures obliged adopting countermeasures that highly affect the efficiency, such as fuel enrichment and delayed combustion. Powertrain electrification allows operating the ICE mostly at medium/high loads, shifting design needs and constraints towards targeting high efficiency under those operating conditions. Conversely, engine efficiency at low loads becomes a less important issue. In this track, the aim of this work is the investigation of the potential of the oversizing of a small Variable Valve ActuationSpark Ignition gasoline engine towards efficiency increase and tailpipe emission reduction. To enhance the potential improvements of such an approach, a lean combustion concept is adopted, where the flame speed propagation is supported by doping gasoline with the addition of a percentage of hydrogen (10% by mass). The analysis is carried out by a 1D simulation tool, widely validated for the base engine supplied with pure gasoline and under stoichiometric/rich combustion. The combustion and knock models are here extended to handle the flame speed and auto-ignition characteristics of gasoline/H2 blends. The comparison between the base gasoline engine and the oversized gasoline/H2 variant highlights significant efficiency advantages at full load operations, which are due to the possibility to remove fuel enrichment and combustion delays. Exceptfor unburned hydrocarbons, pollutants and CO2 emissionsare reducedthanks to the synergic effects of H2addition and ultra-lean mixtures. A certain penalization of efficiency arises at very low loads, where engine oversizingdegrades the combustion process.

The statistical tendency of a GDI spark-ignition engine to undergo knocking combustion as a consequence of spark timing variation is numerically investigated. In particular, attention is focused on the importance to match combustion-relevant and knock-relevant fuel properties to ensure consistency with the experimental evidence. An inhouse surrogate formulation methodology is used to emulate real gasoline properties, comparing fuel models of increasing complexity. Knock is investigated using a proprietary statistical knock model (GruMo Knock Model, GK-PDF). The model can infer a log-normal distribution of knock intensity within a RANS formalism, by means of transport equations for variances and turbulence-derived probability density functions (PDFs) for physical quantities. The calculated distributions are compared to measured statistical distributions. The proposed numerical/experimental comparison constitutes an advancement in synthetic chemistry integration into 3D-CFD combustion simulations.

The evaluation of Internal Combustion Engine (ICE) flows by 3D-CFD strongly depends on a combination of mutually interacting factors, among which grid resolution, closure model, numerics. A careful choice should be made in order to limit the extremely high computational cost and numerical problems arising from the combination of refined grids, high-order numeric schemes and complex geometries typical of ICEs. The paper focuses on the comparison between different grid strategies: in particular, attention is focused firstly on near-wall grid through the comparison between multi-layer and single-layer grids, and secondly on core grid density. The performance of each grid strategy is assessed in terms of accuracy and computational efficiency. A detailed comparison is presented against PIV flow measurements of the Spray Guided Darmstadt Engine available at the Darmstadt University of Technology. As many research groups are simultaneously working on the Darmstadt engine using different CFD codes and meshing approaches, it constitutes a perfect environment for both method validation and scientific cooperation. A motored engine condition is chosen and the flow evolution throughout the engine cycle is evaluated on two different section planes. Pros and cons of each grid strategy are highlighted and motivated.

The present engine development pathway for increased specific power and efficiency is moving Spark-Ignition engines towards unprecedented levels of mean thermo-mechanical loading. This in turn promotes undesired abnormal combustion events in the unburnt mixture (also called “engine knock”), leading to solid parts failure and constituting a severe upper constraint to engine efficiency. In this context, CFD simulations are regularly used to investigate the fluid-dynamic reasons for engine knock and to address knock suppression strategies, using dedicated models to simulate the chemical reaction rate of the fuel/air/residual mixture at the same thermodynamics states as those encountered in engines. In this paper three different approaches are coherently compared to simulate knock occurrence on a turbocharged GDI engine, representing some of the most popular choices for modelers in the RANS framework. The first one considers the on-the-fly solution of chemical reactions, which represents the state-of-the-art knock modelling approach albeit its problematic computational cost for industrial turnaround times. The other two methods consider pre-calculated libraries of ignition delay times (calculated at constant pressure and volume, respectively) for the same fuel model, and knock timing is predicted using a classical Livengood-Wu approach coupled to the same main combustion model. All the analyzed models for the end-gas reaction rate are coupled with a dedicated combustion model for propagating flame (G-equation). A comprehensive analysis of computational cost and of knock prediction accuracy is carried out for library-based methods against the detailed chemistry model. Finally, results are critically discussed and explained using combined ignition delay time maps and traces for thermodynamic in-cylinder states, and guidelines for the a priori choice for constant pressure- or volume-generated libraries are provided. In this context, the use of a synthetic knock model combined with libraries of ignition delays calculated at constant volume emerges as an accurate and efficient modelling strategy. The study outlines a method for the well-supported use of simplified CPU-efficient models, with a promoted confidence in simulation results from the comparison with detailed chemistry.