CFD analysis of combustion and knock in an optically accessible GDI engine

The occurrence of knock is the most limiting hindrance for modern Spark-Ignition (SI) engines. In order to understand its origin and move the operating condition as close as possible to onset of this potentially harmful phenomenon, a joint experimental and numerical investigation is the most recommended approach. A preliminary experimental activity was carried out at IM-CNR on a 0.4 liter GDI unit, equipped with a flat transparent piston. The analysis of flame front morphology allowed to correlate high levels of flame front wrinkling and negative curvature to knock prone operating conditions, such as increased spark timings or high levels of exhaust back-pressure. In this study a detailed CFD analysis is carried out for the same engine and operating point as the experiments. The aim of this activity is to deeper investigate the reasons behind the main outcomes of the experimental campaign. A tabulated knock model is presented, based on detailed chemical mechanism for the surrogate gasoline. Combustion and knock simulations are carried out in a RANS framework through the use of validated models and the results are compared with cycle-resolved acquisition from the test-bed. The results of the CFD analysis explain the experimentally observed flame behavior and allow to proficiently understand the reasons of the sensitivity to knock of the analyzed unit.