The purpose of this test is to evaluate the in-cylinder compression of a petrol engine during cranking using the WPS500X pressure transducer.
Connection for diagnostic work will vary dependent on application.
Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.
The engine must be prohibited from starting. Disable the ignition and fuel systems before testing.
All values included in the Example waveforms are typical and not specific to all vehicle types.
Channel A: Indicates the rise and fall in-cylinder pressure over 5 seconds of cranking.
Refer to vehicle technical data for specific test conditions and results.
Overview and Zoomed (Figures 2 and 3)
1. Peak positive pressures recorded during cranking (11.79 bar) are confirmed using the signal ruler where the value is recorded in the ruler legend (marker 4).
2. Signal ruler indicating 0 bar or atmospheric pressure.
3. Compression peak (tower) indicating the symmetrical rise and fall in cylinder pressure during cranking.
4. The ruler legend records the numerical pressure, time and degree values relative to the position of the signal, time and rotation rulers.
Zoomed (Figure 3)
Use PicoScope zoom functions (marker 11) to display two consecutive compression towers.
5. Expansion pocket (Negative pressure – 207 mbar) formed as the piston descends the cylinder during the expansion stroke. The negative pressure value is indicated by the signal ruler and recorded in the ruler legend (marker 4).
6. Rotation ruler handle position. Click on the rotation ruler handle and drag to positions on the waveform that align with two consecutive TDC (Top Dead Centre) compression peaks (towers) (marker 1). This will denote 0 – 720° of rotation of the crankshaft relative to TDC and peak compression.
7. Rotation ruler partitions can be added here by clicking on the ruler button (marker 7) and selecting four rotation partitions from the dialogue box. The distance and time between the rotation rulers (marker 6) will now be partitioned into four equal areas, each representing 180° of rotation of the crankshaft, relative to the positions of the rotation rulers (placed at TDC of compression stroke).
8. The time ruler handle is located at the bottom left-hand corner of the waveform. Drag both time rulers to align with the 0 and 360° rotation rulers to measure the cranking speed recorded in the frequency and RPM legend (marker 9).
9. The frequency and RPM legend displays the engine RPM relative to the position of both time rulers.
10. Arrows denoting the direction of piston travel, labelled with the relevant stroke of the four-stroke cycle.
11. Zoom tools are at your disposal in order to zoom into the two consecutive compression peaks and towers.
Compression towers and peaks
cylinder pressure is achieved as the piston ascends the cylinder during the compression stroke. (Intake and exhaust valves closed). By using the signal rulers (marker 1) we reveal the cylinder compression peaks at 11.79 bar as would a typical compression tester. However, we can now see repeated, even and symmetrical compression peaks as the crankshaft rotates and, more importantly, events taking place between compressions that could not be seen with our conventional compression tester. The signal ruler (marker 2) denotes zero bar (atmospheric pressure) where the cylinder pressure should remain throughout 360° of crankshaft rotation during the exhaust and intake strokes.
Note: Peak cylinder pressure of the compression stroke can be considered as TDC (top dead centre).
Rotation rulers and partitions
By using PicoScope's rotation rulers (marker 6) and partitions (marker 7), we can equally divide the distance between compression events into four equal divisions to reveal the position of the crankshaft (degrees of rotation). If we know the position of the crankshaft we can identify each of the four stroke cycles between compression events.
At the base of each compression tower during the expansion stroke, you can see the expansion pocket (marker 5) dropping below the zero bar. The cylinder pressure momentarily drops to negative (vacuum). This indicates adequate sealing of both intake and exhaust valves that should remain closed as the piston descends down the cylinder towards the end of the power stroke (referred to here as the expansion stroke as there is no combustion). Valve timing, the integrity of the piston compression rings and cylinder face can also be confirmed via the expansion pocket. The depth of the expansion pocket (and so the vacuum level) can be measured using the signal ruler (marker 5) and the value displayed in the ruler legend (marker 4) -207 mbar.
Cranking speed (= Frequency x 60)
With the Time rulers (marker 8) placed at the 0° and 360° rotation rulers, we can also measure and display the cranking speed. The time taken (frequency in Hz) for the crankshaft to rotate 360° (measured by the time rulers) is multiplied by 60 to reveal the cranking speed, where the value (278 RPM) will be displayed in the frequency and RPM legend (marker 9).
You can read more about Compression testing and find our troubleshooter for compression testing on our training page.
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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