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The emission standards compliance is really becoming effective generally for all the class of engines and particularly for two-stroke SI. While for larger capacity and more valuable applications, such as scooters, snowmobiles, marine engines, the state of the art appears consolidated with the use of direct fuel injection systems, the engines dedicated to very light and simple applications need a different approach in terms of simplicity and cost. Aiming in this direction a system including a self - sustained air injection system in the exhaust and oxidation catalysts was prepared. The influence of the main design parameters governing the system performances and their mathematical relationships were experimentally studied with the aid of a DOE (Design of Experiment) technique methodology.

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.

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.