Research on vibration diagnosis of valve leakage f

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Research on vibration diagnosis of engine valve leakage fault

valve and valve seat are one of the friction pairs with very bad working conditions in the engine. During the working process of the engine, the valve keeps opening and closing, and bears high impact load and high-speed impact of hot gas; As it is not easy to dissipate heat, it is in a high temperature state of 600 ℃ ~ 800 ℃ for a long time. Due to hard combustion products, carbon deposition, high-temperature corrosion and parts deformation, the conical surface of the valve seal is worn and ablated, so that the valve seal is not tight, resulting in the occurrence of air leakage, which destroys the air tightness of the combustion chamber, increases the exhaust temperature and decreases the power. Therefore, it is necessary to realize early prediction of valve leakage fault. When the engine is running, even experienced operators can hardly find the early air leakage fault of the valve. At present, it is not an effective method to judge the air leakage fault according to the exhaust pipe temperature and cylinder pressure during engine operation. Using the vibration signal on the cylinder head surface to detect the working state of the valve has the characteristics of convenient installation of the sensor and accurate detection results. 1 the acoustic characteristics of valve leakage and the mechanism of vibration diagnosis engine valve leakage is the blocking injection of high temperature and high pressure gas through a small gap between the valve and the valve seat in a limited cavity, which leads to local vibration on the cylinder head surface; On the other hand, the high-frequency injection noise formed propagates along the exhaust pipe. According to the tensor form of general wave equation in fluid injection, there is a generalized Lighthill equation:

where: ρ—— Fluid density

t - time

-- Laplace operator, =

x1, X2, X3 -- rectangular coordinates

i, J, K -- unit vector representing three coordinate directions

q (x, t) - simple sound source intensity, which is equal to the fluid mass increased per unit volume per unit time at point x (x1, X2, x3) and time t

fi (x, t) - Xi component of applied force

tij - Lighthill stress tensor. The first term on the right of equation (1) represents monopole source, the second term represents dipole source, and the third term represents quadrupole source. The injection noise generated by engine valve leakage includes simple sound source, solid sound source and turbulent sound source, which respectively correspond to the above three items. Among them, the simple sound source is related to the change rate of exhaust flow, which should be described in a larger time scale than that in the engine working cycle, and its frequency spectrum is characterized by low frequency; The solid sound source has a close relationship with the shape and structure of the engine exhaust duct, and its frequency spectrum shows the characteristics of medium and high frequency; The turbulent sound source must be studied on micro time scale, and its frequency spectrum is characterized by high frequency. Therefore, when using the cylinder head surface vibration signal to diagnose valve leakage, we should focus on selecting the quadrupole turbulent sound source that reflects the degree of leakage, study its high-frequency characteristics, and select the surface vibration signal when the pressure difference between inside and outside the cylinder is the largest for analysis. Specifically, the cylinder head vibration signal near the maximum combustion and explosion pressure shall be selected, and its high-frequency components shall be analyzed as the judgment standard for the center line of the plastering surface formed by connecting the upper and lower center points of the test piece with the valve leakage respectively. Since the vibration caused by gas explosion belongs to low-frequency components, there is no conflict between the two. 2 test research the test was conducted on f3l912 engine, with the speed of 900 r/min, idle speed and sampling frequency of 40 kHz. The measurement and diagnosis system is shown in Figure 1. In order to test the simplicity of the system, the fault is simulated as uniform air leakage, that is, if the exhaust valve clearance is adjusted to a negative value, the valve cannot be seated. The degree of air leakage is measured by the adjusting screw pitch of valve clearance. 3 eigenvalue extraction and fault diagnosis Figure 2 shows the time domain waveforms of cylinder head vibration signals under different air leakage states. A, B and C are the three states of normal operation, slight air leakage of exhaust valve and serious air leakage of exhaust valve respectively. Figure 3 shows the power spectrum analysis results. It can be seen from Figure 3 that the main revisions with the exhaust valve include: 1. Two new monomers that can be used to manufacture food contact materials; 2. The specific migration limit of dicyandiamide monomer is 60mg/kg; 3. The severity of air leakage of the original material 1 is changed. The energy below 10 kHz is almost unchanged, but the energy above 10 kHz is significantly increased. Therefore, the ratio eh/e of high frequency (> 10 kHz) energy to total energy is extracted as the characteristic parameter. Eh/4 in three states and the screws e at the jaw of regular inspection are listed in Table 1. The results shown are consistent with the above theoretical analysis. Figure 1 Measurement and diagnosis system

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