Basic Concepts on GDI Systems

Abstract

Gasoline Direct Engines offer many advantages as compared to PFI engines, as regard efficiency and specific power. To fully exploit this potential a particular attention must be paid to the in-cylinder formation process of air/fuel mixture. More demanding performance is required to the combustion system, since injectors must provide a fine fuel atomization in considerably short time, achieving a spray pattern able to interact with in-cylinder air motion and piston top surface. This is made possible through the Common Rail technology allowing an injection pressure one order of magnitude higher as compared with that of conventional PFI engines. Fuel economy can be obtained regulating load by mixture leaning, minimizing throttle usage at low loads where pumping losses are more significant, and requiring charge stratification for a stable ignition and combustion. Charge stratification can be pursued based mainly on the sole action of the fuel spray or on its interaction with a specially shaped surface on piston top or with the air bulk motion. Depending on the modality of stratification attainment, different combustion systems can be considered. The injector design has in turn a key role being the final element of fuel metering required to the desired spray pattern, injected fuel mass per injection event, resistance to thermal stress and deposits. Injector housing and orientation with respect to the combustion chamber has to be carefully chosen, exploiting in this regard the indications of computational fluid dynamics (CFD), provided by 3D simulations. Some fundamental scheme is provided for the whole high pressure fuel delivery plant, as employed in current vehicles equipped with GDI spark ignition engines.