On line air leakage test technology of engine

In the manufacturing of the engine, the air tightness test plays a vital role in ensuring the quality of the product. The Chery motor industry department uses the on-line air tight leak test technology. Through many years of test application, it has tired the precious experience in the setting of the leakage test parameters and the use of the leak test equipment.

In order to control the three leakage problem of "leakage, leakage and oil leakage" of the early vehicles, especially the three leakage of the engine, in the process of engine manufacturing, the previous manufacturers used "immersion leak" to leak out the cylinder body, cylinder head, exhaust pipe and intake pipe of the engine, so as to present the unqualified workpiece. This method can not accurately get the leakage of the measured workpiece, and the liquid tension inside the leakage hole, increase the flow resistance, this method often leak detection, and there is still need to dry and can not achieve automation and other problems.

With the improvement of casting quality, processing quality and assembly quality, and the application of on-line gas leakage testing technology since 1980s, the three leakage problem of engine has finally been completely controlled, and the quality of the engine has also been assured. The on-line air tightness test on the production line of cylinder block and cylinder head can effectively avoid the problem when the assembly is done, so that the unnecessary loss can be reduced.

In the engine cylinder block and cylinder head production line of our company, according to the arrangement of the process, we have intermediate air tight leak test (see Figure 1) and the final leak test (see Figure 2). Some of the leakage procedures are equipped with water test equipment according to the needs. It is mainly used to check the leakage point of the gas test unqualified workpiece, of course, not All the leaks can be found by water test. This article mainly introduces the application of online leak testing technology in cylinder block and cylinder head production line.

Principle of gas tightness test

The air is compressible and has low viscosity, which means that the air passing through the leak point is 100~400 times faster than the liquid. Air has no surface tension in nature, which makes it easier to pass small leakage points than liquids. However, the leakage of air can not be leaked to the liquid. This is why a maximum allowable air leakage is defined when air is used as a medium to test leakage.

The "state equation of ideal gas" is derived from the study of the "changing state of ideal gas" in physics.

PV/T=R

The P is the gas pressure, the V is the volume of the gas, the T is the temperature of the gas, and the R is the universal constant of the gas, that is, the universal constant for all gases (the air is close to the ideal gas).

This equation shows that when the temperature is constant, after a certain period of time (generally greater than 20s), the volume of the inner cavity does not change, and if the pressure of the cavity is reduced by a definite value Delta P, the "leakage" of the workpiece can be determined. This is the basic principle of using gas to test the sealing performance of workpiece. The electronic air leak tester based on this principle for leak test has become the fastest, most effective and practical leakage automatic testing instrument up to now.

Of course, leakage of workpiece or leakage is a relative concept. Every work piece has a certain air leakage. Absolute "no leakage" does not exist. When we say that there is no liquid leakage, we can define an air leakage rate accordingly. We test most of the spare parts with air as a medium to ensure no liquid leakage, so we can not define these parts without leakage of air or other gases.

Main parameters of gas tightness test

In order to test the volume of the workpiece, the manufacturer should combine the requirements of the production line with the production line of the user and the actual working conditions of the work piece, and determine the following parameters.

1. test pressure

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

2. test rhythm

The leakage test of workpieces usually consists of 4 stages, that is, inflation, balance, testing and exhaust. The setting of each stage is first to ensure that the measured object or fixture is not leaked. Secondly, it is necessary to set up sufficient air time for testing, and then slowly shorten the inflating time to obtain the shortest inflation and balance time when the leakage value is close to zero.

The longer the test time is, the more accurate the test accuracy is obtained, but it is not suitable for the actual needs of the production line. The beat of the leak test machine should match the overall beat of the production line in the case of ensuring the accuracy of the test. In addition, reducing the volume of filling in the fixture will help speed up the test. 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 workpiece and the fixture seal has pressure deformation, it is necessary to extend the test time to ensure the testing accuracy of the leakage rate of the workpiece.

3. leakage rate

The leakage rate standard of each part and assembly of the engine should meet the requirements of the actual operating conditions of the engine, and closely related to the material, structure and technical state of the workpiece. The high leakage rate will cause the quality problems of oil leakage, leakage and leakage of the engine. The low rate will lead to the increase of the material rate of the blank. Add the cost of the engine.

We can obtain the leakage rate standard of a workpiece from the supplier of the leak test machine manufacturer and the leakage tester, and also refer to the leakage rate of similar products. It is used to test the leakage rate of components and assemblies of engine.

In addition, in order to ensure the accuracy of the test, the air tightness test is used to test the material selection and structural design, sealing design, workpiece positioning and tightening device and sealing of the sealing elements.