In the field of engines for light motorcycles, two-stroke cycle survival is submitted to the application of direct fuel injection and charge stratification, even in the case of low-cost small engines. However, charge stratification is a difficult target in two-stroke engines, chiefly because timings of late injection (necessary for charge stratification) and of early injection (necessary for homogeneous charge) are much closer than in four-stroke engines. The compatibility between stratified and homogeneous charge operations needs a thorough CFD study of injection and mixing processes, with the support of techniques of spray visualization. Results strongly depend on the possibility of optimising the interaction between in-cylinder gas-dynamic field and spray; experimental activity is necessary as data source and verification of computational prediction. This paper shows the latest CFD investigation, experimental tests and results concerning a 50 cm3 engine for light motorcycles. The injection is of the liquid type with wall-and-air guided spray produced by a swirl injector. The research has been focused on the attainment of charge stratification at every engine speed. Spray actual characteristics have been investigated, attesting suitable repeatability and proper variation versus backpressure. Engine satisfactory behaviour even at light loads in unthrottled condition is proved by good fuel economy and engine stability in dynamometric bench tests. Exhaust gas analysis and indicated pressure behaviour confirm stratification and combustion correctness.

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