MODELING MIXTURE FORMATION IN A GDI ENGINE

Abstract

Mixture formation and combustion in a Gasoline Direct Injection (GDI) engine
were studied. A swirl-type nozzle, with an inwardly opening pintle, was used to inject the fuel
directly in a 4 stroke, 4 cylinder, 4 valves per cylinder engine. The atomization of the hollow
cone fuel spray was modeled by using an hybrid approach validated at first in a quiescent
chamber at ambient pressure and temperature, comparing numerical penetration and spray
shapes with the experimental ones. For both stoichiometric and stratified operation mode the
interaction of the liquid jet and the surrounding air was studied. The most important obstacle
in the development of GDI engines is that the control of the stratified-charge combustion over
the entire operating range is very difficult. Since the location of the ignition source is fixed in
SI engines the mixture cloud must be controlled both temporally and spatially for a wide
range of operating conditions. The development of a successful combustion system depends
on the design of the fuel injection system and the matching with the in-cylinder flow field.
Results show that the stratification at part load appears to be the most crucial and critical
step, and if the air motion is not well coupled with the fuel spray it would lead to an increase
of unburned hydrocarbon emission and consumption