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 sub-model for the gasoline spray spatial-temporal dynamics is first assessed on the ground of experimental data collected by injecting fuel in a constant volume vessel containing nitrogen at controlled pressure and temperature. The measured instantaneous injected mass flow rate and the mean value of the measured cone angles are used as input variables for the model. Initial droplets size at the nozzle exit section is assumed accordingly to a probabilistic log-normal distribution of given variance and expected value. Choosing the variance as linearly dependent on injection pressure, after a simple tuning of the constants entering the breakup model, furnishes well predictive results as regards spray penetration length, overall spray structure and local particle size at a given distance from the holes exit section. The simulation of the complete engine cycle is realised by using boundary conditions set on the ground of a one-dimensional (1D) simulation of the entire propulsion system, performed by means of the 1Dime code. This last exploits information derived by a preliminary evaluation of the intake and exhaust valves discharge coefficients. Representative engine operating conditions are discussed, relevant to stoichiometric full load, and lean and stoichiometric low load operations. The effects on the in-cylinder mixture formation process of parameters as the Start Of Injection (SOI) and its duration (namely injection pressure at constant injected mass) are analysed.
The work is finalised to derive preliminary guidelines towards an optimised design of the engine in the mixed mode stratified charge direct injection mode.