Adopting HLA also brings some problems. The compressibility of HLA high pressure cavity oils has made HLA stiffness lower. When the engine is working, the air bubbles generated by the crankshaft's agitation of the oil in the oil pan are random oil and enter the HLA oil storage chamber and high pressure chamber through the oil passage, so that the volumetric elastic modulus of the oil in the high pressure chamber is reduced due to the mixed gas, further reducing the The stiffness of the HLA may deteriorate the dynamic performance of the valve mechanism, causing noise and damage. There have been experiments and simulations to study the effect of engine oil mixing on valve train operation, and proposed measures to reduce the amount of mixed gas and reduce the negative impact of mixed gas on valve train work.
In the dynamic simulation of valve trains with HLA, in order to obtain the accurate dynamic characteristics of each part, it is obviously necessary to consider the reduction of the bulk modulus of elasticity caused by the mixture of HLA high pressure chamber oil. In principle, the air that has been dissolved in the oil can be ignored, and only the volume fraction of undissolved air in the HLA high pressure oil chamber needs to be given in advance.
However, due to a variety of complicated factors affecting the HLA mixture of air at work, it is difficult for HLA high-pressure chambers to accurately calculate the amount of gas mixture, and the high-speed movement of HLA and the small size of the high-pressure chamber also make the mixture here. Gas volume cannot be measured. Therefore, there has been no report on the determination of the amount of HLA in the high-pressure chamber.
In the study of a car engine valve mechanism and its direct-acting HLA, the author developed a method to estimate the HLA high-pressure chamber mixed gas volume during the valve mechanism work process based on experimental data. The estimated mixed gas is used in the valve mechanism dynamics calculation. The calculated results are basically consistent with the measured valve mechanism dynamic characteristics, thus verifying the feasibility of the method.
1 Based on the calculation of the air-fuel mixture in the high-pressure chamber In the process of valve opening and closing, the height of HLA is changed. The height change is divided into two parts, one is the elastic deformation of the high pressure oil column, and the other is caused by the leakage or compensation of the oil in the high pressure chamber. When the HLA check valve is closed (without compensation), if the change caused by the leak is subtracted from the measured HLA height variation, the elasticity of the oil column of the high pressure chamber mixer relative to its initial height can be obtained at each moment. Deformation. On the other hand, based on the valve lift and acceleration, the HLA force of the valve stem at each moment can be calculated. From the corresponding force and elastic deformation, the stiffness of the oil column of the air-mixer and its bulk modulus can be calculated. Finally, according to the relationship between the volumetric elastic modulus of the oil column of the air-blower and the volumetric elastic modulus and air-blending volume of the pure oil and air, the volume fraction of air mixed in the oil can be calculated back.
Valve mechanism dynamic measurement Valve acceleration and HLA height variation at five camshaft speeds were measured on a valve mechanism test bench. The valve acceleration is directly measured by an acceleration sensor fixed to the valve head, and the HLA height variation is measured by an indirect method. As shown in b, the two valve springs are moved under the cylinder head and directly against the valve head; two high-precision non-contact eddy current displacement sensors are fixed on the upper part of the valve rod, and the gap between the sensor and the HLA is measured. Changes can be made to HLA working height changes.
To perform the above calculations, it is also necessary to measure the leakage rate of HLA under different loads. The test was conducted on a specially designed device.
Based on the measured relationship between the position of the HLA plunger and the time under the 7 fixed loads, the sinking speed under different loads can be obtained.
Calculate the process of mixing the air in the high pressure cavity The calculation of the force between the HLA and the valve stem When the valve mechanism is working, the valve stem force on the HLA is the sum of the valve spring force and the valve assembly inertia force (ignoring the external damping force). When the valve mechanism is modified according to b, the valve assembly includes a valve, a valve spring, a spring seat, an acceleration sensor, a displacement sensor and a mounting bracket thereof, and a moving part of the wire. The inertial force of the valve assembly is calculated from the measured valve acceleration and the mass of the valve assembly. The valve spring force depends on the spring stiffness and valve lift. Since the actual lift of the valve differs from the theoretical lift very little, the spring force is calculated according to the theoretical lift.
2) Calculate the HLA drop rate According to the relationship between the HLA drop velocity v and the force F obtained from the leak test and the calculated HLA force size, the HLA leak rate under each cam angle is calculated. Here, the static leakage performance in the leakage test is considered as the dynamic leakage performance in the HLA work, and the influence of different gas mixing amounts on the leakage in the two cases is ignored, which will bring about a certain calculation deviation.
It should be noted that the leak test and valve train test were conducted at two different oil temperatures of 16°C and 64°C, respectively.
Since the oil viscosity decreases with increasing oil temperature, the HLA leak rate at 64°C oil temperature is greater than at 16°C. Therefore, in the calculation of this step, the HLA leakage characteristics obtained at 16°C were not used directly. Instead, the leak rate at the 16°C leak test was first inversely proportional to the viscosity of the oil. The relation between speed and load is converted into the relationship between equivalent leakage velocity and load at 64°C.
3) Calculate the HLA height change caused by the leak. Integrate the HLA leakage rate obtained in step 2 to obtain the HLA height change due to leakage at each cam angle.
4) Elastic deformation of computer oil column 4 If there is no oil flow through the check valve, subtract the part caused by the leakage from the measured HLA height change to obtain the elastic deformation of the oil column. It has been pointed out that it takes a certain time for the cam to lift the HLA to close the check valve, and the check valve may open when the valve approaches the seat. In other words, the HLA check valve may not be closed within a range of cam angles corresponding to the valve opening and before the seating, and the amount of elastic deformation of the computer oil column cannot be used in the above method. Therefore, this step calculation is only performed within a range of 50° before and after the cam lobe. Within this cam angle range, the check valve is considered to be closed.
5) Stiffness of the computer oil column The stiffness of the oil column can be obtained by dividing the HLA force of the valve stem obtained in the first step by the elastic deformation of the oil column.
6) Calculate the volumetric elastic modulus of the gas mixture in the high pressure chamber The k 0 oil column stiffness in the high pressure chamber is proportional to the volumetric elastic modulus K of the mixture oil and the cross sectional area A 0 of the oil column, and the height of the oil column h Inversely. Accordingly, K = k 0 h A 0 = k 0 (h 0 - h) A 0, (1) where h varies with the relative displacement of the plunger and the plunger sleeve, and h 0 corresponds to the cam base circle. The initial height of the HLA high pressure chamber, h is the height variation of the high pressure chamber (shortened to a positive value), which is the measured HLA height variation.
7) Calculate the amount of air in the high pressure chamber Ignore the air that has dissolved in the oil. Let the volume fraction of undissolved air in the gas mixture oil be air and the volumetric elastic modulus of the mixture oil be K, and the bulk modulus of pure oil and air be K oil and K air, respectively. The results of the HLA high pressure chamber mixing on the valve train test bench.
In the valve opening and closing process, the instantaneous volume component of the air fluctuates with the change of the camshaft rotation angle ,, and tends to decrease. This may be because the solubility of air in the engine oil increases with the pressure. The change in pressure in the high pressure chamber will cause the amount of air dissolved in the engine oil to change, and the proportion of air that is not dissolved in the engine oil will also change. The oil is more likely to leak than the oil, and as the cumulative leakage increases, the amount of air remaining in the high pressure chamber tends to decrease.
At higher camshaft speeds, the check valve may open, leaking oil through the check valve. In addition, as the rotational speed increases, the eccentric force added to the HLA increases, causing it to be skewed, which may cause uneven radial clearance between the plunger and the plunger sleeve, and increase leakage flow. This means that HLA has a greater amount of leakage at high speeds. Although there is air and organic oil leaked out, the air has a low viscosity and easily leaks, and relatively large amounts of air leak out. This may be the reason why the average volume fraction of air in the HLA high pressure chamber in b decreases as the camshaft speed n increases.
It should be noted that on the valve mechanism test bench, the oil is not agitated by the crankshaft because the camshaft is directly pulled by the motor. The mixture of oil in the oil is mainly due to the oil pump return pipe from the oil surface a certain distance, the impact of the oil surface when the return oil flows into the tank, so the oil in the lubricating oil road at different speeds is basically the same. In the actual engine, with the increase of the rotational speed, the agitation of the oil on the crankshaft is strengthened, and the amount of gas mixture in the oil passage is also increased correspondingly. Therefore, the tendency of the mixture of HLA high pressure chamber engine oil to change with the speed may be different from the results in the drag test. This article just introduced a method to estimate the amount of gas mixture.
3 The dynamics calculation of valve mechanism The valve mechanism dynamic model including HLA hydraulic model was used to calculate the original valve mechanism without modification, and the calculated results were compared with the measurement results on the valve mechanism test bench. It should be noted that the measured valve acceleration given in the comparison is measured on the original valve mechanism, while the HLA height reduction is measured on the modified valve mechanism. The amount of gas mixture used in the calculation is derived from the experimental results of the valve mechanism that was changed, which is taken from Figure 4b.
The comparison shows that the text shows that the mixture of HLA high-pressure cavity oil has a great impact on the valve mechanism dynamics calculation, and can not be ignored, especially in terms of the height variation h of HLA. Since the mixed gas calculation results are basically consistent with the actual measurement results, it shows that the method proposed in this paper is feasible to estimate the amount of gas mixture.
4 Conclusions 1) When analyzing the dynamics of valve trains with HLA, consider the effect of the decrease in the bulk modulus of oil on the HLA high pressure chamber.
2) It is feasible to calculate the mixture of HLA high pressure chamber engine oil based on HLA leakage test and valve mechanism dynamic measurement results. This method is applicable to various types of HLA and can be used to analyze valve mechanism test results and accumulate mixed gas statistics.
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