Analysis and application of ignition test technology in engine cold test

The engine ignition test is an important part of the engine factory inspection. In order to ensure the quality of the engine, a rapid detection cold test technology is proposed to meet the needs of online detection during the engine assembly process.

Figure 1 Engine ignition principle

The engine ignition test is an important part of the engine factory inspection. In order to ensure the quality of the engine, a rapid detection cold test technology is proposed to meet the needs of online detection during the engine assembly process.

The engine ignition test is an important part of the engine factory inspection. Traditional engine tests are based on engine hot tests, so it is impossible to perform rapid online inspection on the assembly line, and it will generate a lot of exhaust gas and noise, causing environmental pollution.

In this test method, the servo motor drives the engine to run at a steady speed. During the test, the engine itself does not work and still runs according to the four strokes. During these four processes, there is no combustion due to the lack of fuel injection, but the ignition process always exists. The magnetic field sensor close to the four ignition coils on the test bench is used to sense the changes in the magnetic field during the ignition process and record the induced waveform, so that the function of rapid online detection of engine ignition can be realized. Compared with hot tests, engine cold test technology has obvious advantages and is currently widely used by major automobile manufacturers such as GM and Volkswagen.

Figure 2 Ignition coil structure

Ignition test principle

At present, most automobile manufacturers have adopted pencil-type ignition coils, and its principle is shown in Figure 1. Figure 1 is an ignition principle diagram of an eight-cylinder engine using a pencil-type ignition coil. Its ignition timing signal is given by the crankshaft transmitter and the camshaft transmitter. The PCM uses these signals to give a certain ignition sequence, and distributes the boost pressure to the spark plugs of each cylinder in turn through the ignition coil to make them break down, thereby achieving the purpose of igniting the oil-gas mixture. Since there is no fuel injection during the cold test process, there is no combustion, but the spark plug has been broken down. The ignition coil of the spark plug plays a vital role in the entire ignition process. Its structure is shown in Figure 2. It is a structure similar to a transformer, with two windings, primary and secondary. By turning the primary winding on and off, the secondary winding can sense instantaneous high voltage and break down the spark plug. [page]

The ignition test utilizes the mutual induction characteristics of the primary and secondary windings of the ignition coil. For an inline four-cylinder engine, there are four independent magnetic induction coils in the test bench, which are used to sense the voltage changes of the primary coils in the ignition process. By observing the sensed waveforms, the quality of the engine ignition system can be judged, and potential problems can be quickly, accurately and simply determined.

Figure 3 Engine ignition timing signal (EST)

The whole process of engine ignition cold test is very simple. The engine ECU generates EST (electron spark timing) signal according to the signal provided by the crankshaft position sensor. This is a signal similar to a pulse sequence, as shown in Figure 3. The time difference between the rising edge and the falling edge of the signal is the charging time (dwell time) supplied to the primary winding of the ignition coil. The charging time is usually fixed in the whole vehicle, but this time is adjustable during the engine cold ignition test. The EST signal is generally a voltage signal with a fixed amplitude. In the cold test bench, there will be a voltage-adjustable power supply specifically used to charge the primary winding of the ignition coil. The charging time can be controlled by the EST signal. In the whole vehicle, the charging time determined by the EST signal and the charging voltage supplied to the primary winding of the ignition coil are usually fixed, but in the engine cold test bench, these two parameters are adjustable. In the actual test process, the purpose of ignition coil testing is achieved by adjusting these two parameters.

Cold test fire test waveform analysis

In the actual production process, in order to discover more defects in the engine ignition system as early as possible, two strategies, high-voltage ignition and low-voltage ignition, are often used. The essence is to adjust the charging time of the EST signal and the primary coil supply voltage. In high-voltage ignition, a relatively high EST voltage amplitude and a longer charging time are often used, while in low-voltage ignition, a lower voltage amplitude and a shorter charging time are often used. These two ignition methods can better detect potential problems in the engine ignition system.

Figure 4 Low voltage ignition test waveform

1. Low voltage ignition test waveform analysis

The low-voltage ignition test is performed using a lower than normal charging voltage value. When the voltage value is too low, the spark plug cannot break down. The method of low-voltage testing is to find a suitable primary coil charging voltage to make the spark plug in a critical breakdown state. At this time, the spark plug does not break down, but when the charging voltage is slightly higher, it will cause breakdown. Figure 4 is a typical low-voltage ignition test waveform. During this test, the amplitude of the EST voltage signal is low, as shown in the low-voltage waveform in Figure 5. Since the spark plug has not broken down, its waveform enters the decay oscillation link after the first peak until the energy in the ignition coil is exhausted. Low-voltage ignition can be used to detect the spark plug gap. When the spark plug gap is small, it is easier to break down than the normal gap. The energy accumulated by low-voltage ignition can break down the spark plug with a relatively small gap, so its ignition waveform will also be significantly different from the normal spark plug gap.

2. High voltage ignition test waveform analysis

When the amplitude of the primary coil charging voltage signal voltage is relatively high, as shown in the high voltage part of Figure 5, the amplitude and duration of its breakdown voltage will become higher accordingly. When the charging voltage is adjusted to a higher value, its ignition waveform is shown in Figure 6. Figure 6 is a typical high-voltage ignition test waveform. During this test, the primary coil charging voltage is relatively high. Since the voltage amplitude of 5V in the whole vehicle can guarantee the breakdown of the spark plug, the spark plug in the ignition circuit has been completely broken down as long as the voltage is greater than 5V. Due to its high voltage amplitude, its first peak value is also relatively high from its waveform. There is a long duration thereafter and a smaller peak pulse will be generated at the end until the energy in the coil is exhausted.

Figure 5 Low voltage ignition and high voltage ignition primary coil charging voltage waveform

When the charging time is the same, the voltage amplitude will directly determine the amount of energy stored in the ignition coil. Therefore, the energy stored in the high-voltage ignition is too high, and the waveform sensed by the ignition is shown in Figure 6. The energy stored in the low-voltage ignition coil is relatively low and cannot break through the normal spark plug, so its ignition waveform is shown in Figure 4.

During the actual test, the initial peak value, secondary peak value, duration and other obvious characteristic values ​​of the waveform displayed by the test will be recorded and compared with the test records of a normal engine. If there is an obvious difference, it can be determined that this is an engine with a problem in the ignition system. Usually, high-voltage ignition can detect many problems that cannot be found in engine hot tests. This test method can detect one or more faults in the ignition system, such as the connection of the line, the wide spark plug gap, and potential problems in the ignition coil.

Figure 6 High voltage ignition test waveform

Conclusion

The ignition test during the engine cold test can help the production line discover problems with the ignition circuit as early as possible, such as a spark plug gap that is too wide or too narrow, problems with the number of turns of the primary and secondary coils in the ignition coil, and a broken circuit in the ignition coil. It has higher sensitivity than an engine hot test.

At present, the engine ignition test is part of the cold test bench. Its low-voltage non-breakdown ignition is often set to test at high speed, and the high-voltage breakdown ignition is set to test at low speed. Both are a certain stage of the cold test process. Now many manufacturers set up a separate station for ignition test in addition to the independent cold test station, so that the ignition test can be carried out without the engine dragging, and the accuracy and reliability of the test are further improved. In addition to the method of using magnetic field sensors to sense the ignition waveform mentioned at the beginning of the article, many manufacturers also use direct measurement of the induced voltage of the primary circuit of the ignition coil to obtain the ignition waveform. This method has higher sensitivity, but the basic principles are the same and are currently widely used.