Automobile Inspection Technology

Chapter 5 Automotive Engine Detection (1)
Knowledge points
1. Main evaluation parameters of engine technical performance;
2. The principle and method of engine no-load dynamometer;
3. The main evaluation parameters and test methods of cylinder tightness;

4. Main parameters and detection methods of gasoline engine ignition system;
5. The main parameters and test methods of diesel engine oil supply;

6. Detection of engine lubricating oil pressure and oil quality;
7. Use the engine comprehensive detector for detection.

Learn
1. Master the main evaluation parameters and related standards of engine technical performance;

2. Understand the structure and principle of each testing equipment of the engine;

3. Master the detection methods of the main parameters of the engine;
4. Master the use of engine comprehensive detector for detection.

(Section [-]) Main parameters for evaluating engine technical performance

During the use of the engine, the technical condition is constantly changing.The main symptoms are decreased power, increased fuel and lubricating oil consumption, increased harmful gas content in exhaust gas, water leakage, air leakage, difficulty in starting, and abnormal noise during operation.

The main diagnostic parameters for evaluating the technical condition of the engine are as follows:

1. Engine power

Engine power is an overall technical indicator of the engine.The wear and tear of various parts of the engine and the poor operation of ignition, fuel supply, lubrication, cooling and other systems will cause the decline of engine power.Therefore, it can comprehensively reflect the technical status of the engine.

2. Engine fuel consumption
Engine fuel consumption is a comprehensive technical indicator for evaluating engine economy.It is not only related to the technical status of the engine's fuel supply system, but also affected by the technical status of the engine's ignition system and cooling system.

3. Cylinder tightness

Cylinder tightness is reflected by engine cylinder compression pressure.The pressure at the end of cylinder compression is related to the cylinder compression ratio, crankshaft speed, oil viscosity and the technical condition of the cylinder piston group.After the cylinder piston assembly is worn, the gap increases, the piston ring is insufficiently elastic and stuck, and the valve and valve seat are not well matched, all of which will cause the cylinder compression pressure to drop.

4. Exhaust purification

The purification performance of engine exhaust means that the combustion quality in the engine combustion chamber can be determined by detecting the content of CO, HC, NOx, etc. in the exhaust gas from the engine.The quality of combustion mainly depends on the technical status of the supply system, but also related to the technical status of the ignition system.

5. Crankcase blow-by
When the cylinder, piston, and piston rings are worn, the amount of gas that escapes into the crankcase will increase.Therefore, the amount of blow-by gas in the crankcase mainly reflects the technical status of the cylinder piston group.

6. Cylinder air leakage rate

When the engine is not working, the compressed air is filled into the cylinder, and the wear condition of the cylinder piston and valve assembly can be diagnosed by measuring the leakage rate of the compressed air, so as to judge the technical condition of the engine.

7. Intake manifold vacuum

The vacuum degree of the engine changes with the wear of the cylinder piston group, and is also related to the condition of the parts of the valve train and the adjustment of the ignition system and supply system.However, the vacuum degree of the intake manifold can only be used to judge the general technical condition of the engine, and cannot determine the exact location of the fault. Therefore, the inspection of the vacuum degree of the intake manifold can only be used as an auxiliary means for the diagnosis of the engine without disassembly.

8. Working condition parameters of the ignition system

The parameters that reflect the technical status of the ignition system include battery (commonly known as battery) voltage, ignition voltage, dwell angle, overlap angle, ignition advance angle, ignition waveform, etc.Through the analysis, the condition of the parts and circuits of the ignition system can be judged.

9. Main parameters of diesel engine oil supply system
The technical status of the fuel supply system of the diesel engine has a great influence on the working performance of the diesel engine, and it is the main parameter to detect the performance of the diesel engine. Device technical status, etc.

10. Lubricating oil pressure

Usually, the lubricating oil pressure of the engine at idle speed should not be lower than 0.1MPa (the specific value of the lubricating oil pressure should refer to the original factory regulations).The level of lubricating oil pressure reflects the technical status of the lubrication system and the wear of the bearings through which the lubricating oil flows.

11. Content of heavy metal impurities in lubricating oil
When the engine is working, the lubricating oil not only lubricates the surface of the parts, but also takes away the wear products (such as iron, copper, aluminum and other metal abrasive particles). The abrasive particles exist in the lubricating oil in a suspended state and cause damage to the lubricating parts.During detection, by measuring the content of abrasive particles in the lubricating oil, the degree of wear of the machine parts can be determined, and at the same time, a reasonable oil change period can also be determined.Therefore, it can be used as an auxiliary means of engine diagnosis.

12. Engine operating temperature

The engine operating temperature can be used as an auxiliary measurement parameter when the engine is not disassembled for diagnosis. In addition to indicating the technical status of the cooling system, it can also reflect whether the cylinder-piston group clearance is appropriate, whether the ignition timing is correct, and whether the valve timing is out of alignment.

13. Abnormal sound and vibration of the engine

With the increase of the wear of various parts of the engine, the matching clearance of the parts becomes larger, and vibration and vibration will occur when the parts are working.
sound.Therefore, the abnormal sound and vibration when the engine is working is a strong proof that the engine is not working well.

During engine testing, engine testing equipment can be used to focus on detecting diagnostic parameters related to engine power, fuel consumption, exhaust purification and wear, and compare them with diagnostic parameter standards for analysis, judgment and evaluation.These diagnostic parameters not only indicate the working performance and wear condition of the engine, but also an important basis for deciding whether the car should continue to run or be repaired in the factory.

Other diagnostic parameters of the engine are shown in Table 1-1.

In engine comprehensive performance testing, many testing equipments are used.Such as engine no-load dynamometer, cylinder pressure gauge, cylinder air leakage (rate) detector, crankcase air leakage detector, vacuum gauge, ignition timing detector, fuel supply timing detector, gasoline pump tester , multimeter, industrial fiberscope, decoder, oscilloscope and engine comprehensive performance analyzer, etc., have become indispensable testing equipment.

(Section [-]) Detection and Analysis of Engine Power
[-]. Steady state dynamometer and dynamic dynamometer

1. Steady state dynamometer

Steady-state dynamometer refers to a method of measuring engine power on a dynamometer when the engine is in a stable state with a constant throttle opening (or the position of the oil volume adjustment mechanism), a constant speed, and other parameters.Common dynamometers include hydraulic dynamometers, electric dynamometers and eddy current dynamometers.The dynamometer can measure the speed and torque of the engine, and then use the formula to calculate the rated power of the engine in the steady state when the throttle valve is fully open (or the position of the oil quantity adjustment mechanism is limited to the position of the circulating oil supply of the rated power) In the case of dynamometer, the rated load is applied to the crankshaft of the engine to make it run stably at the rated speed, and the corresponding torque is measured. Regardless of the stroke and type of the engine, the effective power can be calculated using the above formula.

The results of steady-state dynamometer are relatively accurate and reliable, and are mostly used in engine design, manufacturing, performance tests in colleges and scientific research units.The disadvantage is that the dynamometer is time-consuming, labor-intensive, expensive, and requires a large, fixed-installation dynamometer.Therefore, it is not widely used in general automobile transportation enterprises, automobile maintenance enterprises and automobile inspection stations.

Since the steady-state dynamometer needs to apply an external load to the engine, it is also called a load dynamometer or an external load dynamometer.

2. Dynamic dynamometer

Dynamic dynamometer is a method of measuring engine power when the throttle opening and speed of the engine are changing.Since the dynamic dynamometer does not need to apply an external load to the engine, it is also called no-load dynamometer or no external load dynamometer.

The basic method of this kind of dynamometer is: when the engine is idling or at a certain speed without load, suddenly open the throttle valve fully, so that the engine can overcome its inertia and various internal motion resistances and accelerate. The acceleration performance can be directly measured. Reflects the size of the engine power.Therefore, as long as a certain parameter of the engine is measured during the acceleration process, the corresponding maximum power can be obtained.

Since there is no need to apply load to the engine during the dynamic dynamometer, there is no need for large-scale equipment like a dynamometer, and a small no-load dynamometer can be used for on-vehicle testing.Although its measurement accuracy is worse than that of steady-state dynamometer, this method is especially suitable for the detection of in-use vehicle engines. It is time-saving, labor-saving and convenient to measure, so it is widely used by general transportation companies, maintenance companies and testing stations.

[-]. The principle of no-load dynamometer
The no-load dynamometer is based on the principle of dynamics.When the engine is running at idling speed or at a certain no-load low speed, the throttle valve is suddenly fully opened to accelerate the operation. At this time, the power generated by the engine will accelerate the crankshaft in addition to overcoming various internal motion resistance torques, that is, the engine moves by itself. The parts are accelerated for the load.If the effective power of the engine under test is greater and the instantaneous angular acceleration of the crankshaft is also greater, the acceleration time will be shorter.Therefore, as long as the angular acceleration and acceleration time are measured, the engine power can be obtained indirectly.

1. The relationship between angular acceleration torque and angular acceleration is K in the formula——correction coefficient (because the engine acceleration process is an unsteady working condition, the actual measured power value is smaller than the steady-state dynamometer value at the same speed, so make corrections).

The above formula shows that during the acceleration process of the engine, the effective power at a certain speed is proportional to the instantaneous acceleration at that speed.Therefore, as long as the rotational speed and the corresponding instantaneous acceleration are measured during the acceleration process, the effective power at this rotational speed can be obtained.For a certain type of engine, its moment of inertia is a constant.The value of the correction factor K can be obtained through bench comparison tests.

2. Measure acceleration time

It can be seen from the above formula that the average effective acceleration power of the engine in the range of starting and ending speeds is inversely proportional to its acceleration time.That is, when the throttle valve of the engine is suddenly fully opened, the longer the time for the engine to accelerate from the initial speed to the final speed, the smaller the effective acceleration power; on the contrary, the greater it is.Therefore, as long as the acceleration time of the engine within the set speed range is measured, the average effective acceleration power can be obtained.

In addition, it is necessary to find out the relationship between the average power of steady-state characteristics and the maximum power Penax of external characteristics through bench tests.Among them, the relationship between the acceleration time AT and the maximum power Penax can be calibrated to the no-load dynamometer tester, and input into the microcomputer, so that the power number can be read directly by measuring the acceleration time, and some can draw the relationship between them Form a graph or a table so that the corresponding power value can be found in the graph or table after the acceleration time is measured.

[-]. No-load dynamometer method
The no-load dynamometer is divided into two categories: one is to obtain instantaneous power by measuring the acceleration of a certain speed during the acceleration process, such as the YT-416 engine tester; the other is to test the acceleration time within a certain speed range during the acceleration process Obtain the average effective acceleration power, such as WFJ-1 engine tester.

1. Portable no-load dynamometer and test method
(1) Preparation before the test
① Adjust the engine's gas distribution mechanism, oil supply system and ignition system to make them in a technically sound state; preheat the engine to normal operating temperature (0-90°C); adjust the engine idle speed to make it run stably within the specified range.

② Turn on the power, preheat the instrument and adjust to zero, connect the sensor to the specified position as required, and pull out the antenna if there is no connection requirement.

③ For the instrument for measuring acceleration time-average power, the state 1 and state 2 should be adjusted as required.

④ It is necessary to set the moment of inertia J of the instrument, and the moment of inertia J of the engine under test should be placed in the instrument.If the moment of inertia of the engine under test is unknown, its moment of inertia should be measured first.Its method is:
First select a known maximum power Penax engine of the same type, and set its moment of inertia as J1, use the no-load dynamometer to measure the power of the engine several times, if the measured maximum power is , then the measured The moment of inertia J of the engine can be approximated by the following formula.

⑤Press other necessary keys, such as model (gasoline engine, diesel engine) selection key, cylinder number selection key and "test" key, etc.

(2) Power test method
① Idle speed acceleration method.The engine runs stably at idle speed, and then the accelerator pedal is stepped on to the bottom suddenly, and the engine speed rises rapidly. When the speed exceeds the stop speed, the meter displays the measured power value.After that, the accelerator pedal should be released immediately to avoid the engine running at high speed for a long time.After writing down or printing out the reading, press the "Reset" key to reset the indicating device to zero.In order to ensure the reliability of the test results, the average value is usually repeated three times.This test method is applicable to both gasoline and diesel engines.

② Start the acceleration method.First, depress the accelerator pedal to the bottom, then start the engine to allow it to run freely. When the speed exceeds the stop speed, the instrument will display the test value.The start-up acceleration method can avoid the error caused by the rapid acceleration operation of the engine, and can also eliminate the influence of the additional fuel supply effect of the accelerator pump of the carburetor gasoline engine, but this method is not suitable for EFI engines.

(3) Precautions
① The initial speed of detection <1> is generally slightly higher than the idle speed.The end speed <2- generally selects 80% of the maximum speed.

② The moment of inertia of the engine under test should be placed in the instrument for the instrument that needs to put the moment of inertia.

③ Some instruments cannot be turned off when the engine speed rises to W2.During operation, when the rotational speed exceeds W2, the accelerator pedal should be released immediately, and the engine should not be operated at high speed for a long time.

2. Engine comprehensive tester detection

The engine comprehensive performance analyzer is a comprehensive instrument with many test items.Generally, it is the part with or without load dynamometer.The method of using the engine comprehensive performance analyzer is introduced in the section "Engine Comprehensive Performance Test".

According to the national standard, the power of the engine in use shall not be lower than 75% of the original rated power, and the maximum power of the engine after overhaul shall not be lower than 90% of the original design rated value.The detected engine power is compared with the standard value to determine whether the car engine needs overhaul or whether the repair is qualified.

[-]. Single cylinder power detection

The no-load dynamometer can not only detect the overall power of the engine, but also the power of a single cylinder.

(1) Single cylinder flameout power change
The no-load dynamometer can detect both the overall power of the engine and the single power of a certain cylinder.The method of detecting the power of a single cylinder is: first measure the power of the engine as a whole, and then measure the power of the engine when a certain cylinder is out of fire, and the difference between the two powers is the power of the cylinder under fire.Check each cylinder in turn. For an engine in good technical condition, the power of each cylinder should be consistent, that is, the power difference of each cylinder should be roughly equal, otherwise the engine will not run smoothly.By comparing the power of each single cylinder, the working condition of each cylinder can be judged.

(2) Variation of single-cylinder flameout speed

Using the measured engine speed drop value in the case of a single cylinder fire failure, the working conditions of each cylinder of the engine can also be evaluated.

When the engine is running at a certain speed, if a certain cylinder suddenly loses fire, the output power of the engine will decrease, so the speed will also decrease.If the power of each cylinder is balanced, then when each cylinder turns off the fire, the magnitude of the speed drop should be basically the same.Conversely, if the magnitude of the speed drop is very different, it means that some cylinders are not working properly.The magnitude of the speed drop is related to the number of cylinders.Obviously, the more cylinders there are, the smaller the speed drop after a single cylinder fires off.Table 2-1 shows the value of the normal decrease in speed caused by disconnecting one cylinder when the engine is working stably at 800r/min. It is required that the difference between the highest and the lowest decrease value is not greater than 30% of the average decrease value.If the speed drop value is low, it means that the fire cut-off cylinder is not working well.

For example, using the Yuanzheng EA-2000 engine comprehensive performance tester, by extracting the ignition signal of one cylinder and the primary signal of the ignition system, in the main menu of the system test, click "power balance" to start, and the computer will automatically cut off the ignition of each cylinder in turn. In this way, the rotation speed of each cylinder before the fire is cut off, the speed after the fire is cut off, and the percentage of the speed drop are obtained.

[-]. Cylinder efficiency test

According to the characteristics of the microscopic fluctuation of the speed caused by the intermittent operation of each cylinder of the automobile engine, the ignition of each cylinder is collected at a high speed

By calculating the interval time of the ignition of each cylinder, the difference between the instantaneous speed and the average speed of each single cylinder is obtained as an index for judging the working capacity of each cylinder and comparing the work uniformity of each cylinder.

Using the EA-2000 comprehensive engine tester, click the icon of "Cylinder Efficiency Analysis" in the "Gasoline Engine Test Menu" with the left mouse button, and the system will enter the test state, as shown in Figure 2-1.

Compared with the power balance, the cylinder efficiency test does not need to carry out the cylinder disconnection test, so the exhaust temperature will not be too high and the poisoning of the catalytic conversion enzyme will not occur, and it is more suitable for the electronic fuel injection vehicle.

(Section [-]) Cylinder tightness inspection

Cylinder sealing is mainly used to reflect the technical status of the engine cylinder piston group.The cylinder piston group includes cylinders, pistons, piston rings, valves, cylinder heads and cylinder gaskets and other parts that surround the working medium of the engine. During use, due to wear, ablation, gelation, carbon deposits, etc. The condition deteriorates, so that the cylinder seal is poor and the engine starts
Power and economy decline.

The main parameters to evaluate cylinder tightness are cylinder compression pressure, cylinder air leakage rate, crankcase blow-by gas volume, intake pipe vacuum degree, etc.These parameters have different emphases and have different usage characteristics, and you should pay attention to their respective applicability when using them.

[-]. Measurement of cylinder compression pressure
According to the relevant conclusions of thermodynamics, the cylinder compression pressure is directly related to the thermal efficiency of the engine and the average indicated pressure. The torque and power of the engine depend on the average pressure in each cylinder. Cylinder sealing is the basic condition to ensure the normal compression pressure of the engine cylinder.Cylinder compression pressure is the most direct index to evaluate cylinder sealing, and because the instrument used is simple and easy to measure, it is widely used.

1. Using cylinder pressure gauge detection method
(End of this chapter)