Automobile Inspection Technology

Chapter 7 Automotive Engine Detection (3)
Obviously, the amount of blow-by gas in the crankcase is related to the operating conditions.However, under certain working conditions, the amount of blow-by gas in the crankcase can reflect the technical condition or wear degree of the cylinder-piston group.Figure 2-10 shows the relationship between crankcase blow-by and power and fuel consumption.with the crankcase

As the amount of blow-by gas increases, the output power of the engine decreases gradually, while the fuel consumption increases linearly.

Therefore, detecting the amount of gas that escapes into the crankcase per unit time under the working state of the engine can evaluate the tightness of the cylinder-piston matching pair.

2. Detection method

Since the gas escaping from the crankcase has the characteristics of high temperature, small volume, pulsation, and turbidity, it is difficult to detect.

Crankcase blow-by can be detected by crankcase blow-by detector.The detectors produced in the early stage consisted of a gas flow meter, a hose connected to it, and a gas collecting head.The exhaust gas from the crankcase is transported to the gas flow meter through the gas collecting head and the hose, and the flow rate of the exhaust gas flowing through the gas flow meter per unit time is measured.At present, the crankcase gas blow-by detector uses a micro-pressure sensor. When the exhaust gas flows through the hole of the sampling probe, a negative pressure is generated at the small measuring hole, which is measured by the micro-pressure sensor and converted into an electrical signal.The greater the amount of exhaust gas flowing through the hole of the gas collecting head, the greater the negative pressure generated by the measuring hole, and the stronger the electrical signal output by the micro-pressure sensor.The signal is sent to the instrument box, and the instrument indicates the size to reflect the amount of blow-by gas in the crankcase.

The test steps are as follows:

①Turn on the power switch, and pre-adjust the detector according to the requirements of the instrument manual.

②Seal the crankcase, that is, block the oil gauge opening, crankcase ventilation inlet and outlet, etc., and insert the sampling probe into the oil filling port.

③Start the engine, and after it is preheated to normal working temperature and runs smoothly, the indicated value of the meter in the instrument box is the blow-by gas volume of the engine crankcase at this speed.Crankcase blow-by is not only related to the technical condition of the engine cylinder piston pair, but also related to the engine speed and load.

Therefore, when testing, the engine should be loaded, the throttle valve should be fully opened (or the maximum fuel supply of the diesel engine), and the test should be performed at the maximum torque speed (the amount of blow-by gas is the largest at this time).Engine loading can be achieved on a chassis dynamometer.The loading device of the dynamometer can conveniently load the engine through the drum, driving wheels and transmission system, which can make the engine run at any speed from the maximum torque speed to the rated speed under full load conditions, so the crankcase can be used The blow-by gas detector detects the blow-by gas of the crankcase under any working condition.

The large amount of blow-by gas in the crankcase is generally caused by the large wear of the cylinder, piston, and piston ring, the increased fit clearance, or the alignment of the piston ring, glue, carbon deposits, loss of elasticity, fracture, and cylinder wall strain. Use, maintenance and accessories quality analysis and judgment.

(Section [-]) Detection of gasoline engine ignition system

[-]. Ignition waveform detection and analysis
Whether it is a mechanical contact ignition system, or a non-contact electronic ignition or computer-controlled ignition system, the ignition coil converts low-voltage electricity into high-voltage electricity through mutual inductance, and ignites the mixture through spark plugs to do work.The change process of the low pressure part and high pressure part of the ignition system is regular.Therefore, the actual measured ignition system ignition voltage waveform
By comparing and analyzing the ignition voltage waveform under normal working conditions, the technical status of the ignition system and the location of the fault can be detected.

1. Oscilloscope detection principle

The oscilloscope is a test instrument that can visually express the relationship between the ignition system voltage and the crankshaft angle or camshaft angle with waveforms for easy observation and analysis.All voltage, current and other non-electrical quantities that can be converted into voltage and current through sensors, such as pressure, vibration, temperature, flow, etc., can be observed with an oscilloscope.The ignition waveform can be observed with a special oscilloscope (such as a domestic QDS-1A oscilloscope), or it can be detected with a comprehensive engine tester.Most engine comprehensive detectors (such as domestic QFC-3/QFC-4 and WFJ-1 engine comprehensive detectors) are equipped with oscilloscopes for observing ignition waveforms, cylinder pressure waveforms, oil pressure waveforms, injector needle valves, etc. Lift waveform and abnormal noise vibration waveform, etc.

Figure 2-12 is a working principle diagram of the oscilloscope.An oscilloscope is composed of picture tubes, sensors and electronic circuits.In an oscilloscope's picture tube, an electron gun shoots a beam of electrons at a fluorescent screen, creating a bright spot.A horizontal deflection plate and a vertical deflection plate are arranged in the picture tube.The horizontal deflection plate is set vertically, so that the electron beam is bent in the horizontal direction, and the bright spots sweep across the fluorescent screen from left to right.

A bright line is formed; the vertical deflection plate is set horizontally, and the charge is received from the engine ignition circuit through the oscilloscope circuit, and the amount of this charge is proportional to the instantaneous change of the ignition system voltage, and changes as the electron beam scans from left to right The charge causes it to bend in the vertical direction because:

The voltage of the fire system changes with time (camshaft angle). 1-electron gun;-horizontal deflection plate;-the coordinates of the vertical deflection plate to the curve graph, the horizontal direction represents time (camshaft rotates 4 electron beams; fluorescent screen; bright spot angle); the vertical direction represents voltage, and the baseline is the standard, upwards is Positive voltage, down to negative voltage.

2. Waveform detection

(1) Sensor connection

The primary ignition waveform signal of the mechanical ignition system is collected from both ends of the breaker contacts, so it is also called platinum waveform; and the secondary ignition waveform is collected from the high voltage line of the ignition ring.When using the WFJ-1 engine comprehensive detector, the connection of the sensor is shown in Figure 2-13. The red fish clip of the low-voltage ignition sensor (platinum signal) is clamped on the low-voltage terminal of the distributor, and the black fish clip is clamped on the metal of the vacuum regulator. On the ground, the high-voltage ignition sensor is set on the high-voltage line of the ignition coil; the speed sensor is plugged into the spark plug of the I cylinder to collect the speed, ignition time and ignition sequence signals.

In the non-contact electronic ignition device, after the primary current flows through the primary winding of the ignition coil, it does not flow through the distributor, but is grounded through the ignition controller.Therefore, the redfish clip of the low-voltage ignition sensor should be clipped on the negative terminal of the ignition coil.

(2) Detection steps

① According to the requirements of the instruction manual of the engine ignition oscilloscope or the engine comprehensive tester, power on the instrument to preheat, check and calibrate.

②Start the engine and warm it up to normal operating temperature.

③ Connect correctly as required, that is, connect various sensors to relevant parts of the engine.

④ The overlapping wave, parallel wave, parallel wave and single-cylinder selection wave of the engine can be measured respectively by pressing the button or inputting the operation code.Adjust the "brightness", "contrast", "horizontal position", "horizontal amplitude", "vertical position", "vertical amplitude" and "oscilloscope synchronization" knobs on the detector to make the brightness, contrast, waveform position, and waveform amplitude on the fluorescent screen meet the observation requirements.At the same time, when observing the waveform, the engine should be operated at the specified speed.

Parallel waves, parallel waves, overlapping waves and single-cylinder selective waves can be selected according to the detection purpose.

Parallel waves: Arranged end-to-end from left to right according to the ignition sequence, it is easy to compare the height of the firing line of each cylinder.

Parallel waves: According to the ignition sequence from bottom to top, the length of the spark line and the length of the closed section of the primary circuit can be compared.Overlapping wave: Align and overlap the heads of the waveforms of each cylinder to compare the difference between the ignition cycle, closed interval and open interval of each cylinder.

Single-cylinder selection wave: Select the waveform of one cylinder one by one according to the ignition sequence to display, and elongate the abscissa to see the changes of each stage of the ignition waveform, as well as the length and height of the spark line.The display of the single-cylinder selective wave is very beneficial to the display and analysis of the low-frequency oscillation stage of the spark wire box.

3. Waveform analysis
Waveform analysis refers to the process of comparing the actual ignition waveform of the ignition system of the automobile engine with the standard waveform to judge the failure of the ignition system.

(1) Standard waveform
The primary current, primary voltage, and secondary voltage waveforms of the traditional contact type are shown in Figure 2-17; the main difference between the secondary ignition waveform of the electronic ignition system and the ignition waveform of the mechanical ignition system is that the voltage at the rear of the closed section rises slightly.some

(a) Primary current
(b) Primary voltage
Figure 2-17 Waveform diagram of traditional ignition process

The waveform also has a slight voltage fluctuation in the middle of the closed segment, which reflects the effect of the current limiting circuit in the ignition controller (electronic module).In addition, the length of the closed section of the electronic ignition waveform changes with the change of the rotational speed.The electronic ignition waveform is shown in Figure 2-18.

([-]) Fault reflection area on the waveform
If the actual secondary ignition voltage waveform of the engine measured by the oscilloscope is different from the standard waveform, it means that the ignition system is faulty.There are four main reflection areas on the waveform (take the secondary waveform as an example) of the failure of the mechanical contact ignition system, as shown in Figure 2-19.

① Area C is the ignition area: when the primary circuit is cut off, the current in the primary winding of the ignition coil decreases rapidly, the generated magnetic field rapidly decays, and the high voltage (15000-20000V) generated in the secondary winding causes the spark plug gap to be broken down.The breakdown voltage is generally 4000-8000V.After the spark plug electrode is broken down to generate electricity, the secondary ignition voltage drops accordingly.Abnormality in this area indicates a bad capacitor or breaker contact.

② Area D is the combustion area: when the spark plug electrode gap is broken down, an arc is formed between the electrodes to ignite the mixture.The spark discharge process generally lasts 0.6-1.5ms, forming a spark line on the secondary ignition voltage waveform.A difference in this area indicates a bad distributor or spark plug.

③B area is the oscillation area: when the discharge of the spark plug ends and the energy in the ignition coil cannot maintain the spark discharge, the residual energy is exhausted in the form of damped oscillation.At this point, low frequency oscillations with visible pulses appear on the ignition voltage waveform.An abnormality in this area indicates that the ignition coil or capacitor is not working properly.

④ Area A is the closed area: After the primary circuit is closed again, the secondary circuit induces an induced voltage of 1500-2000V opposite to the battery voltage.A rapidly falling vertical line appears on the ignition waveform, and then rises and transitions to a horizontal line.Abnormalities in this area are usually caused by improper operation of the distributor.

(3) Typical fault waveform analysis
①Ignition wire analysis: When the speed is stable, choose to display parallel waves of each cylinder. If the ignition voltage is higher than the standard value, it means that the high-voltage circuit has high resistance.

a. If all cylinders are high, it means that the high resistance occurs between the ignition coil socket and the distributor, such as high-voltage disconnection, poor contact, and dirt on the distributor.

b. The voltage of individual cylinders is high, indicating that the spark plug gap of this cylinder is too large, the high-voltage line is not in good contact, or the spark plug is in poor contact with the high-voltage line of the cylinder.

c. If the voltage of all cylinders or individual cylinders is too low, the reason is that the spark plug is dirty, the gap is too small or the high voltage is short-circuited.

d. There are redundant waveforms at the lower end of the ignition wire, which is usually caused by ablation or poor contact of platinum contacts.

When the parallel waves of each cylinder are displayed, and the high-voltage line of any cylinder other than the first cylinder is unplugged (the high-voltage line of the first cylinder includes the sensor of the oscilloscope), the gap between the high-voltage line end and the grounding part gradually increases. At this time The ignition voltage value is called the single-cylinder open-circuit high-voltage value. From the waveform, the ignition wire of the cylinder should rise obviously, and its voltage value should be the highest output voltage of the ignition coil.For traditional ignition systems, this voltage should be higher than 20kV; for electronic ignition systems, this voltage should be higher than 30kV, otherwise it means that the insulation of high-voltage wires and distributor covers is poor or the performance of ignition coils and capacitors is poor.If the unplugged high-voltage wire is grounded, the ignition wire should be significantly shortened, and its value should be lower than 5kV, otherwise it means that the gap between the electrodes of the distributor or the distributor cover jack is large, or the high-voltage wire of the sub-cylinder is in poor contact with the jack.

When the secondary ignition parallel wave is displayed on the fluorescent screen, if the engine speed suddenly increases, the corresponding speed performance of the ignition lines of all cylinders is good.If the ignition wire of one cylinder or several cylinders cannot
Rising, indicating that the spark plug has carbon deposits or the electrode gap is too small.If the high-voltage line of a certain cylinder is unplugged and the voltage of the ignition wire increases, the peak value of the high-voltage of a certain cylinder rises very high, which means that the spark plug electrode gap of this cylinder is too large or the electrode is ablated.

②Spark line analysis: It is easier to observe the spark line of the cylinder by using the single-cylinder selective wave. On the oscilloscope with millisecond scanning device, the spark line duration and ignition voltage value can be read from the scale (such as the American BEAR-200 engine The detector can display the spark line duration in milliseconds).For most cars equipped with electronic ignition systems, the spark duration is about 1000ms at a speed of 1.5r/min.When the spark duration is less than 0.8ms, the complete combustion of the mixture cannot be guaranteed, and at the same time, the exhaust pollution increases and the power performance decreases; if the spark duration exceeds 2ms, the life of the spark plug electrode will be significantly shortened.The length of the spark line of the mechanical ignition system is generally 0.6-0.8ms, and the voltage of the combustion zone is generally 1-2kV.

If the spark line is too short, the reasons are generally:

a. Spark plug gap is too large.

b. The electrodes of the distributor and the distributor cover are ablated or the gap between the two is too large.

c. High voltage line resistance is too high.

d. The mixture is too lean.

If the spark line is too long, the reasons are generally:

a. The spark plug is dirty.

b. Spark plug gap is too small.

High voltage wire or spark plug short circuit.

When observing the ignition waveform with some engine comprehensive detectors, although the specific length of the spark line cannot be determined, by comparing the ignition waveforms of each cylinder, cylinders with shorter spark duration and lower voltage can also be found.

③ Analysis of low-frequency oscillation area: When the engine ignition system is in good technical condition, there should be more than 5 visible pulses in the low-frequency oscillation area; the pulses generated by high-power coils will be more than 8.The reasons for the small number of oscillation pulses and small amplitude are:
a. The ignition coil is short-circuited.

b. Capacitor leakage.

The ignition coil primary circuit connector or line connection is poor, and the resistance is too large.

If the number of oscillation pulses is too large, it indicates that the capacity of the capacitor is too large.

If it rises evenly, it means that the spark plugs of each cylinder are working normally. For the electronic ignition system, when the low frequency oscillation area is abnormal, it only means that the technical condition of the ignition coil is abnormal, and has nothing to do with the capacitor. This is because the electronic ignition system has no capacitor.

④Closed area analysis: For the traditional ignition system, when the contacts are closed, a vertical downward line is generated on the ignition waveform, where clutter indicates platinum contact ablation, poor contact or insufficient spring force of the contact spring, as shown in the figure 2-21.In the same way, if there is clutter before the firing line at the end of the closed area, it also indicates that the platinum contact technology is in poor condition.

For the electronic ignition system, although the waveform of the closed area is very similar to that of the traditional ignition system, the reverse voltage and breakdown voltage are generated due to the conduction and cut-off of the primary current by the transistor.Therefore, the two waveform anomalies are due to the poor technical condition of the transistors.The length and shape of the closed zone waveform of the electronic ignition system are different from those of the traditional ignition system. The main manifestations are: the closed zone is elongated at high speed, and there are ripples or bulges in the closed zone; some electronic ignition systems, before the closed zone ends, first Produces a jagged rising ramp followed by an ignition line.All of the above are normal conditions.

⑤Waveform inversion: When the positive and negative poles of the ignition coil are reversed, the engine can also start, but the ignition consumption capacity increases.This is because when the spark plug is working, the temperature of the central electrode is higher than that of the side electrodes, and it is easier for electrons to move from the center electrode to the side electrodes; vice versa.When the positive and negative poles of the ignition coil are connected correctly, the ignition wire is upward; when the polarity is reversed, the ignition wire is downward, as shown in Figure 2-22.

⑥ Dwelling angle detection: During the ignition process of the gasoline engine, the camshaft rotation angle corresponding to the conduction stage of the primary circuit is called the dwelling angle.For a traditional ignition system, the dwell angle is the camshaft rotation angle occupied by the platinum contact closure period; for an electronic ignition system, it is the camshaft rotation angle occupied by the transistor conduction.

The closing angle of each cylinder can be easily observed by using the primary parallel wave (Fig. 2-15), and the closing angle should be within the following range:

For the mechanical contact ignition system, the measured closing angle is small, indicating that the contact gap is too large, the contact closing time is short, and the primary current does not increase to the required value, which will cause insufficient ignition energy; if the closing angle is too large , indicating that the contact gap is small, which will cause arc discharge between the contacts, which will weaken the ignition energy and is not conducive to normal ignition.

When the dwell angle is the same, the closing time is short when the engine speed is high, and the closing time is long when the engine speed is low.Therefore, in order to ensure reliable ignition, the dwell angle should vary with the engine speed.The ignition controller in the electronic ignition system can control and adjust the dwell angle: at low speed, reduce the dwell angle; at high speed, increase the dwell angle.

⑦ Overlap angle detection: The ignition waveforms of each cylinder are aligned, and the camshaft rotation angle occupied by the difference between the longest waveform and the shortest waveform length is called the overlap angle (Figure 2-16).

重叠角应不大于点火间隔的5%，即卩：4缸发动机<4.5°；6缸发动机<3°；8缸发动机<2.25。

The size of the overlap angle reflects the consistency of the ignition interval of the multi-cylinder engine. The larger the overlap angle, the more uneven the ignition interval.This will not only affect the power and economy of the engine, but also affect the stability of the engine operation.If the overlap angle is too large, it is caused by uneven wear of the distributor cam or loose wear and bending deformation of the distributor shaft.

[-]. Test of ignition advance angle

The ignition timing of the engine can be roughly checked and corrected based on experience, but the precise detection of the ignition advance angle must rely on instruments.Commonly used detection methods are stroboscopic method and cylinder pressure method.

1. Stroboscopic detection

(End of this chapter)