Automobile Inspection Technology

Chapter 6 Automotive Engine Detection (2)
(1) cylinder pressure gauge

Cylinder pressure gauge (Figure 2-2) is a special pressure gauge, generally composed of gauge head, conduit, check valve and joint.There are two types of cylinder pressure gauge joints: threaded pipe joints and tapered or stepped rubber joints.The threaded pipe joint can be screwed into the threaded hole of the spark plug or the fuel injector; the rubber joint can be pressed into the spark plug or the fuel injector hole.When the one-way valve is in the closed position, the measured cylinder compression pressure reading can be maintained (maintain the pressure gauge pointer position); when the one-way valve is opened, the pressure gauge pointer can be returned to zero for the next measurement.

Detection method

①The engine should run to normal operating temperature, the coolant temperature of water-cooled engine is 75-95V, and the oil temperature of air-cooled engine is 80-90V.

② Remove all spark plugs or fuel injectors (diesel engine).

③ Put the throttle valve and choke valve at the fully open position.

④ Press the tapered rubber joint of the cylinder pressure gauge into the spark plug hole of the measured cylinder, or screw the threaded pipe joint on the spark plug hole.

⑤ Use the starter to drive the crankshaft to rotate for 3-5s, read the reading after the pointer is stable, and then press the one-way valve to make the pointer return to zero.

The number of measurements for each cylinder should not be less than twice, and the average value of the measurement results should be taken.

⑥Measure each cylinder at one time according to the above method.

(3) Influencing factors of test results The cylinder compression pressure measured by the cylinder pressure gauge is not only related to the sealing of the cylinder, but also affected by the engine speed, that is, it is closely related to the duration of the piston's compression stroke in the cylinder.Figure 2-3 is the relationship curve between cylinder compression pressure and engine crankshaft speed.It can be seen from the figure that when the starter drives the engine to run in a lower speed range, even a small speed difference Aw can cause a large change Ap in the detection result of the cylinder compression pressure.Only when the crankshaft speed of the engine exceeds a certain value (generally 150r/min), the detection result will be less affected by the speed.Therefore, the rotational speed during testing should comply with the manufacturer's regulations.

Table 2-2 Standard values ​​of engine cylinder compression pressure for several models

During detection, the engine speed depends on the technical condition of the battery and the starter, as well as the frictional torque when the engine rotates.Therefore, the battery and starter are required to be in good technical condition; at the same time, the engine is required to have good lubrication conditions and run to a normal thermal condition to reduce frictional resistance during operation.

The main reason for the large error in the test results obtained by the cylinder pressure gauge is that the starting speed does not meet the speed requirements when testing the cylinder compression pressure.Therefore, when detecting the cylinder pressure, if the crankshaft speed can be monitored, it is very important to reduce the measurement error and obtain correct detection and analysis results.

(4) Analysis of test results

When the detected value of the cylinder compression pressure is lower than the standard value, the reason for the poor sealing of the cylinder is often determined by the following method according to the characteristics of the lubricating oil having a sealing effect.

After injecting an appropriate amount (generally 20-30mL) of lubricating oil from the spark plug or injector hole, check the cylinder compression pressure again, and compare the two test results.like:

① The second test result is higher than the first time and close to the standard value, indicating that the poor sealing of the cylinder is caused by excessive wear of the cylinder, piston ring, and piston, or the alignment of the piston ring, jamming, fracture, and cylinder wall strain. cause.

②The result of the second test is similar to the first one, indicating that the reason for the poor sealing of the cylinder is that the intake and exhaust valves or the cylinder liner are not sealed (it is difficult for dripping lubricating oil to reach these parts).

③The two test results show that the compression pressure of two adjacent cylinders is low, which may be caused by the burning of the cylinder liner adjacent to the two cylinders.

If the cylinder compression pressure is higher than the standard value, it does not necessarily mean that the cylinder is well sealed; the specific reason should be analyzed in combination with the use and maintenance conditions.Too much carbon deposits in the combustion chamber, too thin cylinder liner, or excessive processing of the joint plane between the cylinder block and the cylinder head after repeated repairs will cause the cylinder compression pressure to be too high.At the same time, if the cylinder compression pressure is higher than the standard value, it will often lead to the occurrence of abnormal combustion conditions such as deflagration and early combustion.

The standard value of cylinder compression pressure is generally provided by the manufacturer through the automobile instruction manual.Commonly used automotive engine compression pressure standard values ​​are shown in Table 2-2.

The cylinder compression pressure is directly related to the compression ratio of the engine, so it can also be approximated according to the following formula, but for new cars, the calculation value of this formula is relatively low.

根据GB18565-2001《营运车辆综合性能要求和检验方法》，发动机各汽缸压缩压力应不小于原设计的85%；各缸压力与平均压力的差：汽油发动机应不大于8%，柴油发动机应不大于10%。

2. Use the electronic cylinder compression pressure measuring instrument to detect

(1) Detection principle

The electronic cylinder compression pressure gauge can measure the compression pressure of each cylinder of the engine without removing the spark plug or fuel injector.The typical detection principle is to use the current sensor to measure the change waveform of the starting current during the starting process of the starter to measure the compression pressure of each cylinder of the engine.

When the starter drives the engine, the starting resistance torque has a linear relationship with the starting current, that is, the greater the starting resistance torque, the greater the starting current.The starting resistance torque of the engine is composed of two parts: the mechanical resistance torque and the reaction torque of the compressed gas in the cylinder. Under normal circumstances, the mechanical resistance torque can be considered as a constant, while the reaction torque of the compressed gas in the cylinder is a variable that fluctuates with the cylinder compression process. .Therefore, when the engine is started, there is a corresponding relationship between the change of the starting current and the change of the cylinder compression pressure, so the cylinder compression pressure can be determined by measuring the relationship between the starter current and the crank angle that reflects the fluctuation of the resistance torque.

During detection, if the amplitude of the displayed current waveform of each cylinder is consistent and the peak value is within the specified range, it means that the compression pressure of each cylinder meets the requirements; Insufficient cylinder compression pressure.

Domestically produced QFC-5 and WFJ-1 engine comprehensive detectors detect the compression pressure of engine cylinders based on the above principles.There are also many engine testers that turn the waveform of the starting current into a histogram to display the cylinder compression pressure of each cylinder, which is very intuitive.

Detection method

①Run the engine to normal operating temperature (coolant temperature reaches 80-90°C) and then stop.

②Connect the power supply of the measuring instrument and the wiring of the sensor, and preheat and adjust the measuring instrument to a normal state.

③ Operate according to the detection regulations of the measuring instrument, so that the starter drives the engine to run at the specified speed but does not catch fire.

④ The screen of the measuring instrument will display the starting current curve or the histogram of the relative cylinder compression pressure, and the compression pressure of each cylinder.

⑤Print out the test results as needed.

(3) Detection characteristics

The detection speed is fast and the efficiency is high, and it is suitable for the qualitative inspection of the general technical condition of the engine.

[-]. Detection of cylinder air leakage volume and air leakage rate

The air leakage of the cylinder can also be used to detect the tightness of the cylinder.

1. Structural principle of cylinder air leakage detector

The external air source pressure is generally 600-800kPa, and an oil-water separator is equipped.Compressed air enters the cylinder air leakage detector in the direction of the arrow, and its pressure is displayed by the intake pressure gauge.Then, through the pressure reducing valve, calibration orifice plate, rubber hose, quick-change joint, and inflation nozzle, it enters the cylinder at the top dead center of compression, and the pressure change in the cylinder is displayed by the measuring table.This pressure change indicates the tightness of the cylinder bank.

2. Detection method of cylinder air leakage

①Preheat the engine to normal operating temperature, and blow off the dirt around the cylinder head and spark plug with compressed air.

②Set the piston of the first cylinder at the top dead center of the compression stroke.

③ Install the piston position indicator on the distributor cover, and the top dead center scale of the first cylinder of the indicator is aligned with the center of the distributor (the distributor with a needle point can be used to replace the original distributor), as shown in Figure 2-5.In order to ensure that the compressed air does not push the piston down after entering the cylinder, the transmission can be put into the first gear and the hand brake can be tightened.

④ Connect the instrument to the power supply. When the air outlet of the instrument is completely sealed, adjust the pressure reducing valve so that the pointer of the measuring gauge points to 400kPa.

⑤ Connect the quick-change pipe joint to the charging nozzle of the first cylinder, inflate the first cylinder, read the reading after the needle is stable, and record it. and the spark plug hole, and listen to whether there is an air leakage sound.The same as the cylinder pressure test, judge the location of the specific air leakage by listening to whether there is air leakage sound at the intake pipe, exhaust pipe and crankcase vent.Air leakage from the intake pipe indicates that the intake valve leaks; air leakage from the exhaust pipe indicates that the exhaust valve leaks; air leakage from the crankcase vent indicates that the piston, piston ring and cylinder are not tightly sealed; Bubbles indicate that the cylinder head gasket is leaking or the cylinder head has cracks.

If the air leakage of two adjacent cylinders is large, it means that the cylinder head gasket is leaking.The piston can be moved to the bottom dead center at the beginning of compression. The difference between the measured air leakage and the air leakage at the compression top dead center indicates the air leakage of the piston, piston ring and cylinder. .Because the cylinder wears the most at the top dead center, and there is basically no cylinder wear at the bottom dead center, so the difference in air leakage at the top and bottom dead centers of the compression stroke represents the size of the cylinder wear.Such a measurement method excludes the influence of intake and exhaust valve leakage.

⑥Turn the crankshaft of the engine, and align the pointer of the piston position indicator with the scale line of the next cylinder.In the same way, detect the air leakage of the next cylinder.Check all cylinders sequentially according to the ignition sequence of the engine.

⑦After the measurement of each cylinder, repeat the detection of all cylinders, and take the arithmetic mean value of the two measurement values.

⑧ Analyze the measurement results of each cylinder.When the reading of the gauge is greater than 250kPa, it indicates that the sealing condition of the cylinder piston group meets the requirements, and the engine can continue to be used; if the reading of the gauge is less than 250kPa, it indicates that the sealing condition of the cylinder piston group does not meet the requirements, and the engine needs to replace the ring or bore the cylinder.

The same test method can also be used to check the cylinder air leakage rate.In the air leakage rate test, the calibration unit of the measuring meter is not kPa or MPa, but is calibrated as a percentage, that is, when the external air source is connected and the outlet of the instrument is sealed, the pointer of the measuring meter is , indicating that there is no air leakage; when the outlet When it is fully open and connected to the atmosphere, the pointer of the gauge is "00%", indicating that 2% of the compressed air in the cylinder has been leaked; while the pointer between the two intuitively indicates that a few percent of the compressed air has been leaked.During measurement, as shown in Figure 5-[-], crank the crankshaft from the position I at the beginning of the compression stroke pointed by the pointer of the piston position indicator to the top dead center position n at the end of the compression stroke.Detect the air leakage rate and air leakage position of each cylinder at different stages in the whole compression process.

It is generally believed that the air leakage rate of 0-10% means that the cylinder seal is in good condition, 10%-20% is average, 20%-30% means that the cylinder seal is poor, and when the reading of the gauge reaches 30%-40%. , If it can be confirmed that the seals of the intake and exhaust valves, cylinder gaskets, cylinder head and cylinder are no problem, it means that the wear of the cylinder piston matching pair has reached the limit, and it is necessary to replace the piston ring or grind the cylinder.

[-]. Detection of intake manifold vacuum degree

The vacuum of the intake pipe refers to the difference between the intake pressure in the intake pipe and the external atmospheric pressure.By detecting the vacuum of the engine intake manifold to evaluate the cylinder sealing performance of the engine, it is mainly for gasoline engines.

1. Detection principle

The gasoline engine load is adjusted by "quantity", that is, the amount of mixed gas entering the cylinder is controlled by the change of the throttle opening to change the engine output power.At idle speed, the throttle opening is small, the air intake throttling effect is large, and the vacuum degree in the intake pipe is relatively high; when the throttle valve is fully opened, the vacuum degree in the intake pipe is relatively small.It can be seen that the vacuum degree of the intake pipe depends first on the working state of the engine.Most of the detection of the vacuum degree of the intake pipe is carried out under the condition of idling speed, because when the gasoline engine in good technical condition is idling, the vacuum degree of the intake pipe has a relatively stable value (about 57-70kPa for the carburetor engine), The trachea has a high degree of vacuum and is sensitive to the drop in vacuum caused by poor sealing of the intake pipe and cylinder.

The vacuum degree of the intake pipe can also reflect the tightness of the cylinder piston group and the intake pipe.If the seal of the intake pipe gasket, vacuum ignition advance mechanism, etc. is poor, the gap between the cylinder piston group and the valve mechanism increases due to wear or failure, and the adjustment of the ignition system and fuel supply system will affect the vacuum degree of the engine intake pipe.

2. Check with a vacuum gauge

Intake Manifold Vacuum Detection Procedures

The vacuum gauge is the most commonly used tool to detect the vacuum degree of the gasoline engine intake manifold.It is mainly composed of a gauge head and a hose. One end of the hose is fixed on the vacuum gauge, and the other end can be conveniently connected to the detection hole of the intake manifold.

The detection of the vacuum degree is usually carried out under the condition of idling speed, because the vacuum degree of the intake pipe is relatively high, and when the gasoline engine in good technical condition is idling, the vacuum degree of the intake pipe has a relatively stable value. Adverse conditions are the most sensitive.Intake manifold vacuum detection steps are as follows:
①The engine warms up to normal working temperature.

②Use a vacuum tube about 30cm long to connect the vacuum gauge to the intake manifold. This length is chosen to prevent excessive swinging of the gauge needle.

③The transmission is in the "N" position, and the engine is running at idle speed.

④ Read the reading of the vacuum gauge.

Considering the influence of atmospheric pressure, the parameter standard of vacuum degree should be corrected according to the altitude of the measurement site.Generally, every time the altitude increases by 1000m, the vacuum degree will decrease by 10kPa.

⑵Analysis of test results

① Under the conditions equivalent to the altitude, when the engine is running at idle speed, the pointer of the vacuum gauge is stable in the range of 57-71kPa, and the fluctuation value is less than or equal to 5kPa.When the throttle valve is quickly opened and immediately closed, the gauge needle can swing between 6.8-84kPa, which indicates that the cylinder is well sealed.

②When idling, the pointer of the vacuum gauge swings between 50.6-67.6kPa, indicating that the valve is sticky or there is a problem with the ignition system.

③When idling, if the pointer of the vacuum gauge is lower than the normal value, it is mainly caused by the leakage of the piston ring, intake pipe or carburetor gasket, and it may also be related to the late ignition or late gas distribution.In this case, if the throttle valve is suddenly opened and closed, the pointer will fall back to but not jump back to 84kPa.

④ At idle speed, the pointer of the vacuum gauge swings slowly between 33.8-74.3kPa, and the swing intensifies as the engine speed increases, indicating that the valve spring is insufficiently elastic, the valve guide is worn or the cylinder gasket is leaking.

⑤ When idling, the pointer of the vacuum gauge drops regularly, indicating that a certain valve is burned.When the burnt valve works, the pointer drops.

⑥Gelling, carbon deposition and fragmentation of the three-way catalytic converter cause partial blockage of the exhaust system, increase the negative pressure of the exhaust, and the vacuum is too low, resulting in insufficient intake and incomplete exhaust.At this time, the idle vacuum is only 57kPa, and even quickly drops to zero. If the blockage is serious, the engine can only maintain low speed operation.

⑦ At idle speed, the pointer of the vacuum gauge swings rapidly between 27-67.6kPa, but the pointer is stable when the engine speeds up, indicating that the intake valve stem and its guide tube are worn and loose.

When carrying out the flameout test, the vacuum degree must drop obviously; when the ignition advance angle is increased or decreased, the vacuum degree will decrease.When the degree of vacuum is lower than the normal value, you can rotate the outer shell of the distributor until the value indicated by the degree of vacuum is the maximum, which is the optimum ignition advance angle.

电控燃油喷射发动机冷车时进气压力在40-46kPa之间，达正常温度后会在36.5-40kPa之间。一缸火花塞不跳火，进气压力会升高6.7kPa。一缸进气门漏气，进气压力会上升13.4kPa。点火正时比标准值提前进气压力会下降3.3kPa。

Intake manifold vacuum is a comprehensive diagnostic parameter.If the vacuum degree of the intake manifold meets the requirements, it not only indicates that the sealing of the cylinder meets the requirements, but also indicates that the ignition timing, valve timing and air-fuel ratio also meet the requirements.

3. Check with an oscilloscope

The engine will inevitably cause intake pressure pulsation during the working process, resulting in the fluctuation of the vacuum degree of the intake manifold, and the cylinder sealing condition will affect the waveform of the vacuum degree fluctuation of the intake manifold. Therefore, the vacuum degree waveform of the engine intake manifold is detected by an oscilloscope , can analyze and judge the cylinder tightness and diagnose the faults of related parts.

(1) Intake manifold vacuum waveform detection

The voltage signal of the vacuum degree of the intake manifold collected by the sensor is sent to the oscilloscope after being processed by the instrument, so the waveform of the vacuum degree of the intake manifold is displayed on the instrument screen.

(2) Waveform analysis of intake manifold vacuum degree
When the engine is in good technical condition, the waveforms of the vacuum degree of the intake manifold of each cylinder are basically similar, but the shape and section of the intake manifold are not the same, resulting in a slight difference in the waveform of the intake vacuum degree.However, if the structural parameters or technical conditions of the cylinder change, the vacuum waveform of the intake manifold will change significantly, such as the wear of the matching pair of the cylinder and the piston to make the sealing performance worse, cylinder liner or valve leakage, and insufficient valve spring , too rich or too lean mixture will cause changes in the vacuum waveform of the intake manifold, so it is very convenient and effective to judge engine failure.

As shown in Figure 2-6 and Figure 2-7, the standard waveforms of the vacuum degree of the intake manifold of the four-cylinder and six-cylinder engines are respectively. It can be seen from the figure that the negative pressure of the intake manifold caused by the intake process of each cylinder Basically the same, indicating that the engine intake system and piston group are in normal technical condition.

[-]. Detection of crankcase air leakage

1. Detection principle

Cylinder-piston group matching pair wear, piston ring elasticity reduction or bonding will reduce the sealing performance, and the working medium and gas will enter the crankcase from the unsealed place.The more gas that escapes into the crankcase, the higher the leakiness between the cylinder and the piston and piston ring.The channels through which the exhaust gas that escapes into the crankcase can overflow include: the oil filling port, the oil dipstick opening and the forced ventilation valve of the crankcase, see Figure 2-9.

(End of this chapter)