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Sensitivity Analysis of Knock Indices and Combustion Noise Evaluation for a S.I. Engine

Present work focuses on the development of two techniques for knock phenomena and combustion noise evaluation on the acquired in-cylinder pressure data. Experimental investigations are performed on a S.I. four-cylinder naturally aspirated engine at different engine speeds. Data are contemporarily acquired by four pressure transducers located in each cylinder in order to test the proposed methodology in each cylinder. Specifically, 300 consecutive experimental pressure cycles are acquired at fixed load and for each investigated engine speed. Firstly, a deep study on the determination of different knock indexes to evaluate their sensitivity and consequently their reliability and precision in establishing when knock occurs, is presented. The identified index also allows to define a proper threshold level and then to distinguish soft and heavy knock presence, independently from the particular operating condition in terms of reached engine speed. This peculiarity makes the index more suitable among all the others for calibration during the experimental measurements on the engine installed at the bench-test. Secondly, a methodology based on the decomposition of the in-cylinder pressure data for combustion noise analysis, is presented. The proposed techniques have demonstrated their capability to evaluate knock phenomena as well as identify combustion noise.