Abstract
Keywords- Computational Fluid Dynamics, Combustion, Downsizing, Tumble, Particle Image Velocimetry.
Abstract - Steady-state testing trials demonstrated the interest of Renaults 2.0l GDI turbocharged combustion concept relative to its PFI counterpart when considering low-end torque. The GDI superior knock resistance was correlated to its higher combustion speeds, especially under retarded spark advances. The potential of both combustion concepts was then analyzed with 3D combustion simulation. The two concepts respectively featured the following characteristics : a) PFI / No structured aerodynamics / Volumetric Ratio=9.5 /
Pentroof combustion chamber (referred to as PFI) and b) GDI central injection / significant tumble / Volumetric Ratio = 11/Dome combustion chamber (referred to as GDI). The compression and combustion-expansion strokes were first obtained with KIVA-2, without simulation of the intake stroke. Then, the IVC aerodynamic field was computed thanks to unsteady VECTIS simulations, with extensive inlet port and combustion chamber geometry. These calculations served as initial conditions for rest of the combustion calculations, and were validated by PIV measurements performed on a transparent single cylinder engine.
As a first step, we applied the computing methodology to both combustion systems. This provided a comparison between both engines, which demonstrated the GDI concepts higher potential and was confirmed by the aforementioned experimental results.
The second step was to perform a parametric study to investigate on the combustion differences and quantify the importance of each design parameter. The PFI concept features (combustion chamber shape, internal aerodynamics, volumetric ratio) were gradually modified until it was literally transformed into its GDI counterpart. Thus, the contribution of each parameter into the GDI potential was isolated. This parametric study showed that the most influent parameter was internal aerodynamics, which leads to high turbulence levels near TDC, and favors combustion initiation and propagation. Volumetric ratio also has a positive effect on combustion initiation, because it increases pressure and temperature at the end of the compression stroke, hence increasing laminar flame speed in the first steps of the combustion process. Eventually, the GDI chamber geometry is not beneficial to the combustion propagation, mostly because of the bumps on the piston sides (flame propagation is hindered in the last 50% of the combustion process).
Besides the important methodological aspects that were investigated in relation with turbocharged operation (validation of the internal aerodynamics computation, validation of the combustion code), this work also provided precious guidelines for the design of future downsized SI engines.