The purpose of this test is to evaluate the in-cylinder compression of a petrol engine through 720° of crankshaft rotation, during cranking.
General connection advice:
Ensure that the WPS500X is fully charged before starting this test.
The engine must be prohibited from starting and the fuel injection system disabled, NO fuel delivery.
We advise you to recharge your WPS500X after use to ensure it is ready for future measurements.
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.
We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.
December 02 2017
Could you not test diesel working pressure by using an adaptor in a glowplug hole? Is the transducer unable to handle a)the working pressure, or b) the heat of a running diesel engine?
I just noticed this is a page for testing petrol engines - the question is still relevant to a diesel page if there is one.
If the transducer can handle the pressure / heat of a petrol engine, I’d consider investigating drilling and tapping a hole in a cylinder head just to see the results. I understand the complexities and dangers of doing so, but the results from working cylinders would be much more valuable than from a non-working cylinder.
October 21 2016
Waveforms for various variations of a mildly blown head gasket would be superb to see, however we don’t currently have access to PicoScope data of that sort.
Only the relevant spark plug for the cylinder under test is removed, no need to remove them all.
October 04 2016
Waveform for various variations of a mildly blown head gasket would be interesting
September 30 2016
It is unclear to me that the test is done with only one spark plug removed or all plugs removed.
I would think it would be one at a time…