Reduction of fuel consumption of a GDI engine by split injections

The paper reports the main results of single objective optimization analyses aimed at increasing the useful work of a high performance GDI engine under split injection strategies. Both single and double injection events are preliminary experimentally characterized on a Bosch tube to gain the instantaneous mass flow rates. A 3D model of the in-cylinder processes is then presented, used to evaluate the pressure cycle under two main operating conditions, namely a high-speed high-load operation employing a stoichiometric mixture, and a moderate-speed moderate-load operation with an overall lean mixture stratified around the spark plug. Possible benefits deriving from the use of split injections are investigated through a parametric analysis at high speed, and through an optimization procedure under the stratified charge mode. The discussion, in this last case, starts with the synchronization of a single injection event within the working cycle: choice of both start of injection and time of spark advance is realized by means of the Simplex algorithm, in order to minimize fuel consumption. The same criterion is adopted in the numerical study of the mixture formation and combustion processes consequent a double injection. The start of the first pulse and the dwell time between the two pulses, both characterized by the same gasoline masses, are assumed as input variables. The optimal solution is found to improve the engine performance with respect to the case injection is realized in one shot, thus confirming the possibility to use split injections to enhance the charge stratification under lean operation.