Knock Detection Based on Recursive Variational Mode Decomposition and Multilevel Semi-Supervised Local Fisher Discriminant Analysis

Knock is an abnormal combustion phenomenon in gasoline engines. Strong knocks will reduce the efficiency and durability of engine, while with slight knocks engines can run on a high-efficiency state. It is necessary to detect knock and control the state of knock in order to improve the thermal efficiency of engine. This paper proposes a novel approach for detecting engine knocks in various intensities based on vibration signal of engine block using variational mode decomposition (VMD) and semi-supervised local fisher discriminant analysis (SELF). Since the quadratic penalty of recursive VMD has a great influence on decomposition results, the approach establishes the connection between the quadratic penalty and the stop condition by analyzing a large amount of data and quantifies the relationship by polynomial fitting, which reduces the complexity and subjectivity of recursive VMD. A multilevel SELF is developed for solving the problem that SELFs sometimes may not find ideal embedding space under large scale dimensionality reduction. This method adopts multi embedding spaces, with gradually decreasing dimension, to reduce the dimensionality of original data by a series of small steps. Verifications show the proposed approach can achieve high classification accuracy in knock detection and is able to identify the intensity of knock. This research contributes to the field of engine abnormality detection and can be implemented on vibration-based faults diagnosis area.