Steady-State and Transient Operation Simulation of a “Downsized” Turbocharged SI Engine

The paper reports the research activity related to the development of a “downsized” turbocharged Spark-Ignition (SI) engine. Both experimental and theoretical analyses are carried out to characterize the performance of this engine architecture, and particularly to analyze the matching conditions with the turbocharger and the combustion process at wide-open-throttle conditions. To this aim, a quasi-dimensional model for the simulation of the burning process is included as an external user-defined routine in a commercial 1D simulation code (GT-Power^®). The rate of heat release is computed through a two-zone model, based on a “fractal” representation of the turbulent flame front. A turbulence sub-model is included and it is properly tuned with respect to turbulence results computed by a 3D CFD code. A CAD procedure evaluating, at each crank-angle and flame radius, the intersections between the flame surface and the actual combustion chamber walls, is also presented. This allows to take into account the influence of the combustion chamber geometry on flame development and on in-cylinder pressure cycle.

A number of comparisons with experimental results are presented with reference to instantaneous pressure data inside the cylinders. Comparisons with overall test-bench data are also carried out in correspondence to different control strategies of the waste-gate valve. Once validated, the model is finally applied to predict the engine behaviour in transient operation, for different turbocharger groups.