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 solutions are finally compared using specific drivers related to brake mean effective pressure, brake specific fuel consumption and global energy conversion efficiency.
An original methodology to compare different classes of engines is described and utilized to make the global evaluation easier through a multidimensional point of view.
At the end, the leading philosophy beyond the apparently not well focused research strategy appears very clear. The Piaggio FAST (Fully Atomized Stratified Turbulence) solution, the first mass produced 2T SI engine for vehicle applications, is as the logical conclusion of the twenty years research.