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The work relates to the use of multidimensional modelling as a tool for improving the robustness of combustion of a Gasoline Direct Injection (GDI) Spark Ignition (SI) engine. A procedure is assessed for the prediction of the thermo-fluid-dynamic processes occurring in a single-cylinder, four-stroke engine, characterised by a bore-to-stroke ratio close to the unity, and a pent-roof head with four valves. The engine is at a design stage, under development for application on two wheels vehicles. A new generation six-holes Bosch injector is considered as realising a jet guided combustion mode. This last is preferred for its potential in realising effective charge stratification and great combustion stability under various operating conditions. The three-dimensional (3D) numerical model is developed within the AVL FIRE™ software environment.

A fuel matrix consisting of twelve gasolines was tested in presence of Exhaust Gas Recirculation (EGR). The fuels have different percentages of aromatics (20÷35% vol.), olefins (5÷15% vol.) and oxygen (0÷2% wgt). Four different oxygenated compounds (MTBE, ETBE, TAME, DIPE) were chosen as additives. Tests were carried out on a MPI premixed spark ignition engine at steady operating conditions (2000 rpm, 2 bar BMEP, 13.5% EGR) and stoichiometric air/fuel ratio. Regulated and unregulated pollutants were measured upstream the catalytic converter. Cyclic variation of Indicated Mean Effective Pressure (IMEP) in presence of EGR was also evaluated. The adoption of EGR increases PAH and aldehydes emissions, and decreases benzene emissions of unoxygenated fuels. Conversion efficiencies of CO and of total HC are lowered by EGR. An increase of aromatics content in an unoxygenated fuel leads to higher engine out NOx emission. This effect is reduced if MTBE is added.