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The problems of Two-Stroke SI engines regarding high fuel consumption and unburned hydrocarbon emissions, both caused by the short circuiting of fresh homogeneous mixture during the scavenge process, are well-known. The progress of Piaggio since 1977 in state-of-art direct fuel injection systems, oriented to development of Hi-Tech solutions for 2T SI engines to overcome the above drawbacks, is analyzed. The analysis includes several streams of research ranging from conventional crankcase scavenged engines with direct solid mechanically controlled fuel injection to solutions with separate scavenging pump with electronically controlled injection units, and from low pressure injectors to air-assisted fuel injection with stratified charge. Each solution is examined with presentation of typical engine parameters and cost data.

The Hi-Tech Two-Stroke SI engine is finding good appreciation in various fields of vehicle application and the final solution appears to include air assisted direct fuel injection and several solutions are now evaluated by the major world engine manufacturers. Those solutions anyway are poorly tailored to be fitted on very small engines such as for moped or light scooter applications both for the complications they require, consequentely the costs, and the difficult tuning of low load and idle caused by the very low quantity of fuel per cycle required. The proposed completely mechanical solution instead does consent with small modifications to the engine, only a new cylinder head is in fact required, to manage very well the engine all over its utilization range. After general considerations of the philosophy of the system and on the main parameters evaluation, a practical application on a 50cc scooter engine is described.

Following general considerations about the advantages of two stroke S.I. engines equipped with direct fuel injection systems, especially for light vehicle applications, two solutions, developed for mass production, are presented. The first one deals with a mechanical high pressure D.I. system characterized by an essential fuel delivery control. The second one is concerned with an electronic low pressure semi - D.I. in view of a reduction of the involved costs. While the mechanical solution achieves a dramatic reduction in HC emission and fuel consumption but with high costs, the electronic one still appears a good compromise between improvements and costs when very fast injectors are industrially available.

Following the state - of - the - art analysis of typical applications of two-stroke SI engines in road vehicles, new solutions for this type of engine are examined. The two-stroke engine appears as an extremely attractive one during part-throttle opera tions especially from the point of view of fuel consumption, when the problems caused by fresh mixture short-circuiting are overcome. This fact is well confirmed by the renewed interest in the two-stroke cycle SI Engine. A new solution, with direct fuel injection and separate scavenging pump, that shows very low fuel consumption data down to very low B.M.E.P. levels while mantaining a high specific power output and acceptable HC emissions, is then presented.