A Modelling Study to Analyse the Compression Ratio Effects on Combustion and Knock Phenomena in a High-Performance Spark-Ignition GDI Engine

Modern internal combustion engines show complex architectures in order to improve their performance in terms of brake torque and fuel consumption. Among the different solutions, the compression ratio (CR) increase represents a well assessed path to achieve the above result. However, CR has to be limited in order to comply with the mechanical and thermal engine safety and to avoid knocking combustion. In the present work, a 10-cylinder naturally aspirated spark ignition engine is investigated to evaluate the effects of an increased CR on performance. In a preliminary stage, the engine is experimentally tested under full load operation for a base CR of 12.6. Main performance parameters and in-cylinder pressure cycles are measured. The engine is schematized in a one-dimensional model (GT-Power™), where “user routines” are implemented to simulate turbulence, combustion and knock phenomena. 1D model is validated against the experimental data, denoting a good accuracy. The model is then used to estimate the engine performance variations passing from the base CR up to a CR value of 13.3. The results underline a reduced improvement of the engine performance for the higher CR, mainly deriving from a higher thermodynamic efficiency. The proposed methodology shows the capability to predict the effects of a partial engine re-design on completely theoretical basis and presents the potential to be very helpful in reducing the related experimental costs and time-to-market.