On the Effect of the Injector Position on Fuel-Air Mixture Preparation in a Two-Stroke GDI Engine

Modern injection systems are characterized by low cost, light weight and diversified components based on a mature technology. In addition, the constant growth of computational resources allows an in-depth understanding and control of the injection process. In this scenario, increasing interest is presently being paid to understand if an application of such technologies to small two-stroke engines could lead to a return to popularity in place of the more widespread use of the four-stroke engine. Indeed, the possibility of achieving a drastic reduction of both specific fuel consumption and pollutant emissions would completely reverse the future prospect of the two-stroke engine. The authors in previous studies developed a low pressure direct injection (LPDI) system for a 300 cm3 two-stroke engine that was ensuring a performance consistent with a standard four-stroke engine of similar size. The main drawbacks of the system were the large time required for delivering the fuel and the incomplete vaporization in some working conditions, due to the large size of the injected droplets. In this study, the use of a single high pressure injector with an operating pressure of 100 bar was analyzed. An optimization study was carried out in order to identify the best injector configuration for the GDI system. The results of the preliminary 3-D CFD study are here reported. The effect of the injector positioning and injection timing on the spray vaporization, mixture homogenization and fuel short-circuit was evaluated at different engine operating points. The results will show that also in case of a high pressure injection the best performance can be obtained when a suitable interaction between the liquid jet of fuel and the flow of scavenging air is ensured, as well as with the appropriate choice of the injection timing.