Measurement and Analysis of Primary Ignition Voltage Signal Using Automotive Oscilloscope

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Measurement and Analysis of Primary Ignition Voltage Signal Using Automotive Oscilloscope [Copy link]

The gasoline engine ignition system has the following three main functions:

1. Produce a spark. The ignition system must be able to generate a high enough voltage to ignite the mixture and maintain the spark long enough to achieve complete combustion.
2. Control ignition timing. The ignition system must be able to change the ignition timing as the engine speed and load change, as well as the requirements of special operating conditions.
3. Distributing the spark. The ignition system must deliver the spark to the correct cylinder at the appropriate moment in the compression stroke to begin the combustion process.

The primary ignition waveform is essentially the trigger induction waveform of the secondary ignition. Its waveform can reflect the quality of the ignition coil and the igniter. By observing the primary voltage change waveform, you can observe the conduction time of the primary current of the ignition coil, the voltage drop during conduction, and find out the damage of the ignition coil and igniter, as well as the short circuit, open circuit, poor contact and other faults in the ignition circuit.

According to the composition of the ignition system, it can be divided into two categories: traditional ignition system (i.e. distributor ignition system) and electronic ignition system. According to the display method of the waveform, it can be divided into single-cylinder ignition primary waveform and multi-cylinder parallel and parallel waveform. At present, most cars have electronic ignition systems, and traditional ignition systems are rare. Here we will take the electronic ignition system as an example to explain to you.

Oscilloscope connection and settings

Connect a BNC to banana cable to the oscilloscope channel, connect a black alligator clip to the black connector (negative) of the BNC to banana cable, and connect a red piercing needle to the BNC to banana cable (positive). Connect the black alligator clip to the negative terminal of the battery for grounding, and connect the red piercing needle to the negative terminal of the ignition coil.

Adjust the channel attenuation ratio of the channel connected to the oscilloscope to be consistent with the probe, adjust the time base to 1ms, and then adjust the vertical gear so that the voltage measurement range is greater than the maximum value of the measured signal. Of course, you can also adjust it after the signal comes out. In order to eliminate interference, you can turn on the low pass of the oscilloscope and set it to L 30K, and the channel coupling mode is DC coupling.

Oscilloscopes with built-in automotive software packages can complete the relevant setting parameters of the oscilloscope with one click. It is more efficient and convenient.

The figure above is the measured primary ignition waveform. In the figure, 1 represents the grounding of the engine electronic control unit, 2 represents the induced voltage generated by the engine electronic control unit disconnecting the electric shock, 3 represents the combustion line, and 4 represents the coil oscillation.

Primary ignition signal waveform analysis

The following figure shows a single-bar primary waveform of an electronic ignition system. From the waveform, it can be seen that the primary induced voltage of this primary voltage signal is a maximum value of 296V. Seeing that the time base is 1ms, the closing time is about 1.9ms and the burning time is about 1.2ms.

It can be seen that the horizontal voltage line on the left side of the waveform is a constant voltage of approximately 40 volts.

No current flows through the primary circuit of the coil until the closing phase begins, when the coil is grounded and the voltage drops to zero. The closing phase is controlled by the ignition amplifier and is essentially the duration of the ignition primary coil being energized. The length of the closing phase is determined by the time required to build up a current of approximately 8 amperes. When this preset current is reached, the amplifier stops increasing the primary current and maintains this current until the coil ground is disconnected, at which point the ignition occurs.

The vertical line in the middle of the waveform is called the primary induced voltage, which is generated by the magnetic induction process. At the moment of ignition, the ground loop of the coil is disconnected, and the magnetic field through the coil quickly collapses, which in turn induces an average voltage of 150 to 350 volts. The high voltage output of the coil is proportional to this induced voltage. The height of this induced voltage is sometimes also called the primary peak voltage.