Development of a low pressure direct injection system for a small 2S engine. Part II - Experimental analysis of the engine performance and pollutant emissions

High specific fuel consumption and pollutant emissions are the main drawbacks of the small crankcase-scavenged two-stroke engine. The symmetrical port timing combined with a carburetor or an indirect injection system leads to a lower scavenging efficiency than a four-stroke engine and to the short-circuit of fresh air-fuel mixture. The use of fuel supply systems as the indirect injection and the carburetor is the standard solution for small two-stroke engine equipment, due to the necessity of reducing the complexity, weight, overall dimensions and costs. This paper presents the results of a detailed study on the application of an innovative Low Pressure Direct Injection system (LPDI) on an existing 300 cm3 cylinder formerly equipped with a carburetor. The proposed solution is characterized by two injectors working at 5 bar of injection pressure. The injection nozzles are located in the liner wall at the opposite side of the exhaust port, with the axes oriented towards the piston at the BDC (Bottom Dead Center). The injectors are positioned above all of the cylinder ports in order to have an injection timing independent from the ports timing. The only constraint to the injection timing is related to the piston motion, with great benefits in terms of short-circuit reduction in all of the engine operating conditions. Numerical and experimental activity was carried out in order to identify the best injector configuration and to assess the system performance. The results of the experimental study are reported here. Standard low cost components from the automotive market were used, without a significant increase of weight. The benefits of the innovative two-stroke LPDI engine were evaluated at the test bench and compared with the performance of the configurations equipped with the carburetor and the indirect injection system. Once the position of the injectors inside the cylinder was defined, the development of the system at the test bench was focused primarily on the definition of the best injection timing, also taking advantage of the numerical results of the CFD analysis. The experimental setup and the fine-tuning methodology are discussed in detail. The goal was to reach the best thermodynamic efficiency and the lowest pollutant emissions of the LPDI system while ensuring the same engine power with respect to the other two configurations, without increasing the manufacturing costs.