Research On Knock Diagnosis And Control Strategy Of Gasoline Engine

Recently,with the increasing scarcity of oil resources and substantial increase in fuel prices, more and more engine manufacturers began to use a higher compression ratio to achieve the purpose of improving fuel economy. At the same time, the strict emissions regulations require the use of lead-free gasoline. These would increase the possibility of the gasoline engine knock. Knock is an important factor which has a strong impact on the power and fuel enconomy of spar-ignition engine. The engine can never be allowed to work under conditions of a strong knock. However, when slight knock occurs, the engine is close to isometric burning, so the economy and power of the engine are enhanced. Many of the modern gasoline engines are equipted with knock control system, but the control principles, methods and accuracy have been constantly improving. To research the methods of knock signal feature extraction, intensity determination and diagnosis, and to achieve the real-time and accurate closed-loop ignition control based on the knock signal by modern electronic technology are of great significance to improving the gasoline engine performance, saving energy, and reducing emission of the engine.The article separately studied the feasibility and effectiveness of gasoline engine knock feature extraction methods, such as the Short-time Fourier Transform(STFT), smoothed pseudo Wigner-Ville distribution(SPWD) and the discrete wavelet transform(DWT). The study found that: when adopting the STFT and SPWD method, only under the condition of filtering out the low-frequency components of the signal can we extract the knock feature from the engine cylinder pressure signal and the cylinder head vibration signal; STFT and SPWD method also need a large amount of calculation and time consuming; DWT method can extract the slight knock feature well from the cylinder pressure signal and the cylinder head vibration signal; And compared with the previous two methods, DWT method is faster and has obvious superiority; Moreover, by adopting DWT method the slight knock feature of four cylinders can be extracted out under the condition of installing only one acceleration senor on the first cylinder head.Twenty time-domain parameters are analyzed and the indexes suited to describe knock features of wavelet transform domain signal are found out. A relative energy index is proposed to overcome the defect that other indexes are sensitive to the changes of working conditions. The relative energy index associated wavelet sub-band signal with the original vibration signals, has better adaptability to work condition changes. It is suitable for knock judgement. The method of using multiple eigenvalues as knock judgement indexes is proposed to compensate for deficiency of the single index in knock judgement.In the study of knock diagnosis methods, fuzzy cluster analysis and neural method are applied to knock diagnosis. The knock diagnosis methods of wavelet with fuzzy C-means clustering and wavelet with fuzzy neural network are put forward. The principles of the two methods also be discussed. The two methods are verified effective in distinguishing engine combustion modes by some simulation tests, and slight knock combustion is diagnosed by the two methods. Comparing the two methods, wavelet with fuzzy C-means method is more suitable for implementing by hardware in knock diagnosis.In order to conquer the shortcomings in real-time performance of the existing gasoline engine knock control strategy, a new knock control strategy combined the open-loop control with closed-loop control is put forward. Using this control strategy can complete the works including signal detection and ignition advance angle adjustment in one engine work cycle. The mehod of using neural network instead of traditional experimental MAP to determine the basic ignition advance Angle is studied. The simulation model of knock operation which using wavelet combined with fuzzy C-means clustering algorithm is established. The validity of the model has been proved by the simulation computation.Finally, the implement method of knock control is studied. Knock computing unit and data communication unit, which are two key parts of the knock control are developed. The wavelet combined with fuzzy C-means algorithm is realized in the FPGA hardware. Controller area network bus technology is applied to the data communicaiton betweeen FPGA and ECU. The effectiveness of the two units are verified by the hardware test.