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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.

Cycle-to-cycle variation is one of the main factors for high fuel consumption and emissions of a two-stroke engine during the low-load and low-speed running. The increase of residual gas ratio due to the lower delivered amount of fresh scavenging air leads to a lower flame front speed and, therefore, to a slow combustion or even misfiring. The consequence is a very high level of unburnt hydrocarbons, since a large amount of fuel does not take part in the combustion process. The use of a direct injection system allows a more flexible management of the injection of fuel over subsequent engine cycles. Under a low-load condition, the low request in terms of brake mean effective pressure (BMEP) can be achieved by performing a load control based on an intermittent injection, thus reducing the need for intake throttling and avoiding the loss of fresh fuel resulting from cycles without combustion.