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In this paper, an experimental and numerical analysis of combustion process and knock occurrence in a small displacement spark-ignition engine is presented. A wide experimental campaign is preliminarily carried out in order to fully characterize the engine behavior in different operating conditions. In particular, the acquisition of a large number of consecutive pressure cycle is realized to analyze the Cyclic Variability (CV) effects in terms of Indicated Mean Effective Pressure (IMEP) Coefficient of Variation (CoV). The spark advance is also changed up to incipient knocking conditions, basing on a proper definition of a knock index. The latter is estimated through the decomposition and the FFT analysis of the instantaneous pressure cycles. Contemporary, a quasi-dimensional combustion and knock model, included within a whole engine one-dimensional (1D) modeling framework, are developed. Combustion and knock models are extended to include the CV effects, too.

Water injection (WI) could be a viable tool for the reduction of CO2 emissions of spark-ignition engines. At high loads, the performances of this kind of engines are constrained by knock phenomena, thermal limits of engine components and maximum tolerable in-cylinder pressure. Water injection, mainly due to its cooling effect, helps mitigating knock and reducing the exhaust gas temperature. Furthermore, it allows to obtain greater spark advances, better combustion phasing and leaner mixtures with a consequent improvement in terms of engine efficiency. In this work, the authors investigated the effects of a particular direct water injection (DWI) strategy on the performance of a turbocharged PFI spark-ignition engine at high load operation. The analysis has been carried out using a validated 1D model that reproduces the entire engine layout. A knock model allows to identify the knock-limited parameters in the various operating points analyzed.