During the last years the deep re-examination of the engine
design for lowering engine emissions involved two-wheel vehicles
too. The IC engine overall efficiency plays a fundamental role in
determining final raw emissions. From this point of view, the
optimization of the in-cylinder flow organization is mandatory. In
detail, in SI engines the generation of a coherent tumble vortex
having dimensions comparable to the engine stroke could be of
primary importance to extend the engines' ignition limits
toward the field of the dilute/lean mixtures.
For motorbike and motor scooter applications, the optimization
of the tumble generation is considered an effective way to improve
the combustion system efficiency and to lower emissions,
considering also that the two-wheels layout represents an obstacle
in adopting the advanced post-treatment concepts designed for
automotive applications.
The aim of the paper is to use the 3D-CFD simulation tool to
assess the intake duct geometry influence on the tumble motion
generation during both the intake and the compression strokes. All
the CFD simulations presented in the paper were performed on a SI
4-valve engine characterized by a unit displacement of 250 cm₃. The
tumble structure was changed during the analysis by changing the
angle set defining the intake port shape. The stroke-to-bore engine
ratio was kept constant to 0.7. The effects of the tumble
variations were evaluated on the in-cylinder vortex characteristic
length and vorticity, and on the cylinder mean flow structure at
IVC.
3D-CFD simulations were performed by AVL-FIRE v.2010 CFD
code.