WPS500X In-cylinder compression – cranking (petrol)

The purpose of this test is to evaluate the in-cylinder compression of a petrol engine during cranking using the WPS500X pressure transducer.

Connection guidance

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.

  • Ensure that the WPS500X is fully charged before use.
  • To avoid possible damage to both engine and equipment ensure all connections are clean and dry.

Note:

The engine must be prohibited from starting. Disable the ignition and fuel systems before testing.

How to perform the test

  1. Ensure fuel delivery and ignition systems are disabled before testing.
  2. Remove the spark plug.
  3. Assemble the compression hose with the correct thread adapter and install into the spark plug hole.
  4. Connect the fully charged WPX500X pressure transducer to PicoScope Channel A.
  5. Switch on the WPS500X and wait for the self-test to complete (LED will scroll from range 1 to 3 and revert to 1). Do not connect the compression hose until the self-test is complete.
  6. Connect the WPS500X to the compression hose.
  7. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  8. Start the scope.
  9. With the throttle fully open crank the engine to capture the waveform.
  10. Stop the scope.
  11. Use the Waveform Buffer and Zoom tools to examine your waveform.

Waveform notes

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.

Typical values
Engine cranking:

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.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, Cylinder pressure waveform

Further guidance

Figure 2

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).

Figure 3

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.

Expansion Pocket

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.

GT157-4

Disclaimer
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.

Suitable accessories

  • WPS500X Maxi Kit thumbnail

    WPS500X Maxi Kit (with carry case)

    £1,585.00

  • WPS500X Pressure Transducer Kit (with carry case)

    £1,055.00

  • WPS500X Pressure Transducer

    £749.00

  • WPS500X Adaptor Kit A

    WPS500X Adaptor Kit A

    £119.00

  • WPS500X Adaptor Kit B

    WPS500X Adaptor kit B

    £119.00

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Guided test: In-cylinder compression - cranking (petrol)