Experimental and numerical approaches for the quantification of tumble intensity in high-performance SI engines

The activity presented in the paper is focused on the development of a high-performance engine specifically dedicated to CNG fueling. The engine features a variable valve actuation system as well as different possible compression ratios ranging between 12 and 14. More specifically, the present work carries out an experimental and numerical characterization of the steady-state tumble flow from the engine head and deals with the development and assessment of a numerical model for the engine-cycle transient simulation. Experimental tests were carried out at Fiat Research Center (CRF) on a flow test rig equipped with two different devices for the tumble measurement, based on the Ricardo method and on the two-dimensional HWA acquisition of the axial velocity, respectively. It is worth highlighting that the HWA technique was purposely developed by CRF for the tumble characterization. The experimental results were also used for the calibration of a ‘virtual flow box’ numerical model, which was in turn exploited to gain complimentary information about the characteristics of the tumbling flow and to define the requirements for the transient CFD model of the real engine, both in terms of discretization mesh and turbulence modeling. The results showed that the HWA technique represents a factual alternative to the integral techniques for tumble characterization. It also provides additional information about the unevenness of the flow distribution generated by the intake system. The ‘virtual flow box’ simulations pointed out that good results can be obtained by using the Realizable k- model, whereas both the Realizable and the RNG k- models proved to be appropriate for the engine transient simulations. Finally, the CFD models showed to be fairly accurate and reliable, especially when the relative changes from one engine geometry and/or valve actuation to another need to be accounted for.