Multiple Injection in a Mixed Mode GDI Boosted Engine

A numerical investigation is performed with the aim of understanding the potential benefits of multiple injections in the mixed mode boosting operation of a Gasoline Direct Injection (GDI) engine. The study is carried out by firstly characterizing a high pressure multi-hole injector from the experimental point of view in the split injection operation. Measurements of the fuel injection rate are made through an AVL Meter operating on the Bosch principle. The injector is tested using gasoline in a double pulse strategy. The injection pressure is varied between 5.0 and 25.0 MPa with the pulse durations calibrated for delivering a total mass up to 50 mg/str. The choice of the dwell time between two successive injection events is achieved by firstly defining the minimum time compatible with the mechanical characteristics of both the injector and the injector driver. While the GDI injector driver is able to generate a proper trigger current without limitations on the duration of the time between consecutive injections, a current command with a too low dwell time would make for the injected fuel to not be split in two separate events.

The multidimensional modeling of the in-cylinder processes is realized within the AVL FIRE™ code environment. Gasoline injection is simulated by resorting to a properly developed model that uses measured injection mass flow rates and accounts for the dependence of the initial droplets size distribution upon injection pressure. It is shown that high-speed high-load conditions do not benefit of splitting injection, whereas two injection events are useful for improving the quality of mixture stratification at moderate-speed moderate-load conditions. The choice of the dwell time between two successive injections is crucial.