The small two-stroke engine represents a strategic typology of propulsion system for applications in which lightweight and high power density are required. However, the conventional two-stroke engine will not be compliant with forthcoming legislations about pollutant emissions and new solutions, such as electrification, are seriously taken into account by industry to overcome the two-stroke engine drawbacks. In this scenario, a promising way to allow the two-stroke engine to be competitive is represented by the use of direct injection systems, in order to overcome the long-standing issue of short circuiting fuel. The authors in previous studies developed a low-pressure direct injection (LPDI) system for a 300 cm3 two-stroke engine that was ensuring the same power output of the engine in carbureted configuration and raw pollutant emissions consistent with a four-stroke engine of similar performance. The main drawbacks of the system were the large time required for delivering the fuel and the incomplete vaporization in some working conditions; as a result, the engine operation was limited at high revolution speed, as well as the cycle to cycle variation was amplified at very low loads.
In this study, the LPDI system was replaced by a GDI system, with a single high pressure injector installed in the engine head capable of working up to an operating pressure of 150 bar. After a preliminary numerical activity to identify the best injector configuration, the system performance was evaluated at the test bench. The experiments show that the GDI system allows reaching higher revolution speeds, thanks to the shorter injection duration, with the same benefits in terms of fuel consumption reduction obtained with the LPDI technology. In-depth investigations on the injection timing and the injection pressure were carried out in order to minimize both hydrocarbon emissions and brake specific fuel consumption.