The main drawback of an in-cylinder Low Pressure Direct Injection (LPDI) in a two-stroke engine is the difficulty of achieving a satisfactory vaporization level in low load conditions. The liquid droplets are characterized by large diameters and, when the temperature level and the velocity of the scavenging flow field are low, the time needed for the droplet vaporization and the homogenization with fresh air becomes too long to guarantee a suitable mixture formation. A transfer port injection allows a higher flexibility, due to the possibility of performing a mixed injection either directly in the cylinder or indirectly in the crank case, depending on the load request or engine speed. Also, an even lower injection pressure can be adopted with respect to an in-cylinder LPDI injection, which is relevant in case of lightweight and low power applications. On the other hand, the time available for the direct in-cylinder injection is limited to the scavenge phase.
In the present work, a detailed numerical analysis has been performed on a 15kW 300 cm3 two stroke engine for evaluating the potential of a transfer duct installation of a low pressure injector. The influence of the injection system positioning has been investigated by means of high fidelity three dimensional CFD simulations of both the scavenge and the spray processes. The engine model has been validated with experimental data acquired at the test bench on the engine operated in a homogenously scavenged configuration, equipped with a standard carburettor. The spray model has been validated with experimental data acquired during both an imaging test campaign for the analysis of the global spray evolution over time and a Phase Doppler Anemometry (PDA) analysis for a detailed sizing characterization of the spray. The paper shows the results of a sensitivity analysis on the fuel vaporization and short circuit, as well as air fuel mixing and homogenization, by varying the following parameters: choice of the transfer port (from first to fifth), injection direction (co-current or counter-flow) and injection timing.