Numerical and experimental investigation on combustion characteristics of a spark ignition engine with an early intake valve closing load control

In this paper a new intake port configuration has been designed, analyzed by means of 3D CFD simulation and experimentally tested on a turbocharged Spark Ignition (SI) engine, with the aim of addressing the issue of the poor in-cylinder turbulence levels which are typical of the Early-Intake-Valve-Closing (EIVC) strategies adopted in Variable Valve Actuation (VVA) systems at part load to reduce pumping losses. The proposed intake port layout promotes turbulence by increasing the tumble motion at low valve lifts in order to achieve a proper flame propagation speed at part load. The new layout was proved to have a significant and positive effect in improving the EGR tolerance and in shortening the combustion process, especially at the lower loads, which are the more critical for VVA systems using an EIVC strategy. However, under full load operating conditions the new design (which enhances the tumble motion at the low valve lifts used at part load, but decreases the tumble intensity under the full lift operation used at full load) did not reach the performance targets, since the knock mitigation was not sufficient to compensate for the loss in combustion efficiency due to the slower combustion. The proposed solution could therefore be exploited only if a reduction of the engine full load performance is allowed in view of the significant benefits during part load operation. Finally, the calculated in cylinder flow characteristics were related to the experimental combustion durations, identifying, on a quantitative basis, the relationship between the turbulent kinetic energy and the burning process durations, and thus providing guidelines for further possible modifications of the engine geometry aimed to achieve a suitable combustion speed over the whole engine operating map