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Development of a Low Pressure Direct Injection System for a Small 2S Engine. Part I - CFD Analysis of the Injection Process
Abstract:
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 preliminary 3-D CFD study are here reported. The best positioning of the injectors was determined at the operating condition of maximum power by means of a sensitivity analysis of the circumferential position and spray orientation. The performance comparison was evaluated in terms of fuel short-circuit and spray vaporization. The optimized configuration was also verified at lower revolution speeds and partial loads showing a great potential for all of the working conditions. The experiments on a real prototype on the test bench confirmed the predictions showing significant improvements in comparison with the original configuration.