Oscilloscope measurement of automobile engine cylinder pressure test signal

Insufficient engine power is a common fault in automobile maintenance. The cause of this fault may be ignition system failure, fuel injection system failure, intake failure or mechanical failure. It is difficult for us to directly determine mechanical failures. Generally, we use instruments to test indirectly, and cylinder pressure detection is one of them. We can use a cylinder pressure tester (the disadvantage is that it can only see the final cylinder pressure, and cannot see the process changes), or use an oscilloscope with a pressure probe to perform cylinder pressure testing.

Before starting the test, make sure the battery is fully charged and the starter is running strongly. Warm up the engine so that the water temperature gauge needle indicates the middle position. Make sure the injection system and ignition system of the engine cylinder are turned off. Connect a pressure probe to channel 1 of the oscilloscope (the probe is a pressure sensor that can convert the pressure signal into an electrical signal and input it into the oscilloscope). Clean the area around the spark plug and remove the spark plug on the engine cylinder to reduce the resistance of the engine. Start the car engine and enter the idle state to clear all the fuel remaining in the cylinder, and install the pressure probe to connect the removed gap. With the throttle fully open, press the accelerator pedal to ensure sufficient intake air and start the engine.

Open the channel setting of the oscilloscope, turn on the oscilloscope low-pass filter of about 30K, change the probe type to pressure, set the pressure range and the corresponding voltage range. Open the oscilloscope measurement item, select the maximum value, minimum value and frequency. Set the time base to about 20ms. Adjust the vertical scale of the channel to make the waveform fill the entire screen as much as possible. Some oscilloscopes have built-in car package software, which can complete one-click settings.

The figure below is the waveform of Mazda 6-cylinder pressure test compression (in operation).

It can be seen that the waveform is initially below 0 (atmospheric pressure), then there is a small pressure rise and then a drop, followed by a large pulse rise and rapid drop, and so on and so forth. It can be seen that the frequency of this repetition is about 7.3 Hz.

When the test result is lower than the standard value, it means that there is a leak in the cylinder, but it is not certain whether it is the upper valve and cylinder gasket that is leaking, or the lower piston and cylinder that are leaking. Add 10-20ml of engine oil into the cylinder, run a few circles to temporarily seal the gap between the piston and the cylinder, and measure the cylinder pressure again. If the pressure value does not change, it means that there is a leak on the top, and if the pressure value recovers, it means that there is a leak on the bottom. Use the same method to test other cylinders. The minimum value of the waveform cannot be lower than the manufacturer's standard, and the maximum and minimum values ​​should not exceed a certain ratio. Regarding the standards, different engines are slightly different, please refer to the relevant manual for details. If the pressure values ​​of two adjacent cylinders are low, there may be a cylinder cross-flow phenomenon.