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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.

Following general remarks about high pressure direct injection systems impact on two stroke S.I. engine applications, further advantages, both on emissions as well as fuel consumption, obtainable with a variable timing injection system are presented. An exhaustive evaluation, carried out on a single cylinder air cooled engine, clearly shows that sensible reductions, basically in HC emissions as well as in fuel consumptions can be obtained delaying the injection beginning depending on RPM and engine load. Further advantages appear available at idle, injecting every other cycle, if a suitably conceived system is adopted. Finally an electronically controlled high Pressure injection system, derived from a low cost and dependable solution for small engines is proposed.

The nowadays two-stroke engine technique is directed toward new concepts to reduce fuel consumption and pollution. Typical features are direct injection of fuel and no lost oil lubrication: a separate scavenging system is becoming so a must. Following general consideration about different scavenging pumps, a regenerative blower solution is proposed. Regenerative blowers present in fact several advantages in comparison with other solutions like simplicity in construction and on engine fitting, no maintenance and low cost. An exhaustive experimental evaluation was carried out on a direct injection two-stroke S.I. engine with a separate scavenge, already described in a previous work by one of the authors. The results show the practical feasibility of the proposed solution. In fact, the regenerative blower is suitable to match the engine air breath demand all over its utilization range.

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.

The future urban mobility strategies, especially in cities with high population density, include the individual transportation means as one of the main components to contribute to the solution of the two fundamental problems, namely the traffic concentration and the individual satisfaction. Following the consolidated trend, the 2000's vehicles shall be characterized by extremely reduced environmental impact in terms of pollutant emissions, noise, fuel consumption (low carbon dioxide production). In this sense the two-wheelers appear the most favorable candidate provided that a suitable propulsive unit is available. The present work describes the development of a propulsive unit characterized by a direct injected two-stroke engine with FAST technology, an electronic engine management system and a very efficient CVT transmission.

The advantages of air assisted direct fuel injection systems to achieve high atomization degree into the combustion chamber of SI engines are well-known. The solutions up to now proposed appear anyway poorly tailored to be suitable for small engine applications. In fact scaling down such existing systems for automotive applications, they present mainly two drawbacks: the costs and a difficult tuning of the very low quantity of fuel required per cycle. Moreover the amount of electric energy required makes the engines not self-sufficient. To overcome the above mentioned problems, Piaggio has developed a completely mechanical low cost fuel injection system, named FAST (Fully Atomized Stratified Turbulence), which does consent a very atomized and stratified mixture lean combustion process, i.e. a dramatic improvement of emissions and fuel consumption. After general considerations, the application of such system to a small capacity 2T engine is analyzed.