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How to check the air tightness of automobile

In engine manufacturing,Air tightness detectorIt plays an important role in ensuring the quality of products. Chery Automotive Engine Division adoptsHarrisThrough years of testing and application, the online air tight leak testing technology has accumulated valuable experience in the setting of leak testing parameters and the use of leak testing equipment. To manage early Vehicles & amp; ldquo; Air leakage, water leakage, oil leakage & amp; rdquo; In the process of engine manufacturing, in the past, the manufacturers used & amp; ldquo; Immersion leak detection & amp; rdquo; To detect the leakage of cylinder block, cylinder head, exhaust pipe and intake pipe to find the unqualified parts. This method can not accurately get the leakage of the measured workpiece, and the liquid tension occurs in the inner side of the leakage hole, which increases the flow resistance. This method often fails to detect, and at the same time, it also has the problems of drying and automation. With the advent of air tight leak detection equipment and the maturity of technology, this problem has been greatly improved.

With the improvement of casting quality, processing quality and assembly quality, as well as the application of on-line air tight leak detection technology since 1980s, the three leakage problems of engine have been thoroughly controlled, and the quality of engine has been reliably guaranteed. In the cylinder block, cylinder head and other production lines, using the leak detector of Shenzhen Harris, the on-line air tight leak test can effectively avoid the problems found in the leak test after assembly, so as to reduce unnecessary losses. It can effectively reduce the quality problems and safety risks caused by leakage due to unsealing.

In the production line of engine cylinder block and cylinder head, according to the requirements of sealing leakage, we have developed professional accessories leakage testing equipment and final air tight leakage testing equipment. Some leakage testing processes are equipped with water testing equipment according to the needs, which is mainly used to check and determine the leakage points of unqualified workpieces in gas test. Of course, not all leakage points can be found through water test. This paper mainly introduces the application of our company's on-line leak test technology in cylinder block and cylinder head production line.

Principle of air tightness test

Air has compressibility and low viscosity, which can be ignored, which means that the speed of air passing through the leakage point is 100-400 times faster than that of liquid. Air essentially has no surface tension, which makes it easier to pass through small leaks than liquid. However, leakage through which air can pass is not necessarily leakage for liquids. This is why it is necessary to define a maximum allowable air leakage when using air as the medium for leakage test. People usually say that the detection of leakage is to ensure the tightness and waterproof of the product. As long as the air leaks out within the allowable range, it is OK. Because air leakage may cause water leakage. Oil leakage. In this case, we want to achieve the sealing, waterproof, leakage effect.

The result of physics research is & amp; ldquo; Ideal gas & amp; rdquo; Research on the law of state change & amp; ldquo; Equation of state of ideal gas & amp; rdquo;, Namely:


Where p is the gas pressure, V is the gas volume, t is the gas temperature, and R is the gas universal constant, which is generally applicable to all gases (Note: air is infinitely close to the ideal gas).

This equation shows that, under the condition of constant temperature, after a certain period of time (generally more than 20s), the volume of the inner cavity does not change, and if the pressure in the cavity drops by a certain value △ P, then the workpiece & amp; amp; nbsp; can be determined; ldquo; There is a leak & amp; rdquo;。 This is the basic principle of using gas to test the sealing performance of the workpiece. The electronic air leak tester based on this principle has become the fastest, most effective and most practical automatic leak tester so far.

Of course, whether the workpiece leaks or not is a relative concept. Every workpiece has a certain amount of air leakage, absolute & amp; ldquo; Don't miss & amp; rdquo; It doesn't exist. When we say that the workpiece has no liquid leakage, we can define an air leakage rate based on it. We use air as the medium to test most parts to ensure that there is no liquid leakage, so we cannot define that these parts do not leak air or other gases.

Main parameters of air tightness test

In order to test the volume of the workpiece, the manufacturer of the leak detector should reasonably determine the following parameters according to the requirements of the production rhythm of the user's production line and the actual working conditions of the workpiece

1. Test pressure

The test pressure is generally equivalent to the actual pressure of various media during the operation of the engine.

2. Test beat

The leakage test of workpiece generally includes four stages, namely inflation, pressure stabilization, test and exhaust. The time setting of each stage should first ensure that the fixture does not leak, and then set a long enough inflation time for testing. If the inflation time is not enough, it is likely that it will not reach the required detection pressure.

Theoretically, the longer the test time is, the more accurate the test accuracy will be obtained. However, this is not suitable for the actual needs of the production line. The beat of the leak tester should match the overall beat of the production line while ensuring the test accuracy. In addition, the use of filler with reduced volume in the fixture helps to speed up the test beat. When the test pressure of the workpiece is high, the test volume of the workpiece is large, the leakage rate of the workpiece is small, and the pressure deformation of the workpiece and the sealing part of the fixture occurs, it is necessary to extend the test time to ensure the test accuracy of the leakage rate of the workpiece.

3. Leakage rate

The leakage rate standard of engine parts and assemblies should meet the requirements of the actual working conditions when the engine is running, and it is closely related to the material, structure of the workpiece and the technical state of the leak detector. The parameters of the leak detector of Shenzhen Harris meet the requirements. Too high leakage rate will cause the quality problems of oil leakage, water leakage and air leakage, If it is too low, the material rate of the blank will increase and the manufacturing cost of the engine will increase.

We can use the leak detector to counter detect the leakage rate of the sample, and calculate the critical point of leakage as the leakage detection standard. It is a parameter usually used in leakage rate test of engine parts and assemblies.

In addition, in order to ensure the test accuracy, the air tightness test has certain requirements on the material selection and structural design, sealing design, workpiece positioning tightening device and sealing fixture of the test sealing element, such as: the sealing element should have no elastic creep, oil resistance and pressure resistance; Seal design helps to reduce the uncertainty of unnecessary seal test; The sealing fixture must provide appropriate pressure to block the workpiece under test, and the clamping force should be about 3 times of the test pressure. At the same time, the strength of the fixture support frame should be enough to support the pressure, and each sealing cylinder should have reliable guidance to ensure the accurate sealing position. There is no leakage in the fixture.

Problems in air tightness test

1. Quality of compressed air

The compressed air must be clean and dry. If there is water and oil in the leakage tester, it will cause the failure of the precision pressure sensor. In order to prevent water and oil from mixing, it is necessary to install oil mist separator and air dryer as prefilter. Especially in the negative pressure operation mode, impurities are absolutely not allowed to enter the leakage tester.

The stability of compressed air will also affect the test accuracy. The compressed air with large flow, adjusted and higher than the test pressure (at least 105Pa higher than the test pressure) should be supplied to the pressure regulating valve to improve the stability of the test.

2. Cleanliness of workpiece

For the workpieces that need to be leak tested, they should be cleaned and dried first to avoid the influence of foreign matters on the test accuracy or the wear of sealing elements. When gas test is carried out on the leakage hole which has had liquid leakage, the leakage will become smaller or disappear, so it is difficult to accurately measure. Therefore, in the comparative test of leakage, the gas leakage test must be carried out first.

3. Workpiece and ambient temperature

According to the law of Boltzmann Marx (PV = NRT), when the volume is constant, the change of temperature will cause the change of pressure, which will eventually affect the test accuracy. In order to ensure the test accuracy of leakage rate, the temperature of workpiece and environment should be consistent. In the cylinder block and cylinder head production line, the workpiece should be heated and cleaned before leakage test. In order to ensure that the temperature of the workpiece and the environment is consistent, it is necessary to set a cooling device at the outlet of the cleaning machine. In order to ensure the test accuracy, temperature compensation device can also be used. The leakage detection instrument of Shenzhen Harris has built-in temperature compensation system, which can effectively reduce the leakage detection error

4. Requirements for test gas pipe and accessories

It is necessary to use rigid nylon pipe and jacket embedded joint with less pressure expansion to reduce the measurement. In order not to move in a short time, the piping should be fixed as much as possible. Copper pipe and steel pipe are not suitable for piping, because these materials are easily affected by ambient temperature, resulting in test error; It is also not allowed to use the quick plug connector, because the deformation of the internal O-ring will affect the test accuracy.

5. Spot inspection and maintenance of leak tester

In order to ensure the accuracy of the sealing test, in addition to the above, it is also important to do a good job in the routine point inspection and maintenance of the leakage tester to ensure that the leak detector is in a good technical state.

In engine manufacturing, air tightness test plays an important role in ensuring the quality of products, and it is an important index to control the quality of engine. Therefore, air tightness leak detector of Shenzhen Harris Automation Technology Co., Ltd. can be used to do & amp; ldquo; 100% tested, 100% qualified & amp; rdquo;。


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