WPS500X In-cylinder compression – idle (petrol)

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.
 

The purpose of this test is to evaluate the cylinder efficiency of a petrol engine throughout 720 degrees of the crankshaft rotation at idle speed.

Please note that the Pico standard compression hose used for this test does not contain a Schrader valve. Therefore, the rise and fall in cylinder pressures will be recorded relative to the crankshaft position.

Note: Cylinder efficiency is dependent upon engine speed, adequate intake and exhaust flow, the correct valve timing and the mechanical condition of the engine.

All numerical readings quoted in this help topic are typical and not specific to all engine styles

Note: Engines with Variable Valve Lift will produce waveforms with characteristics varying from the examples below.

How to perform the test

  • Make sure that the WPS500X is fully charged.
  • Make sure that the fuel injection is cut from the cylinder under test.
  • Amend crankshaft rotation.

 

Accessories
  1. BNC to BNC Test lead
  2. WPS500X Pressure Transducer
  3. TA212 Standard compression hose
  4. TA213 to TA220 Standard compression hose spark plug adaptors (select the appropriate adaptor for the vehicle under test).
PicoScope settings
  • WPS500X set to Range 1
  • Channel A - 2 to 7 bar
  • Time base 50 ms/div
  1. Remove the relevant spark plug.
    Make sure that the surrounding area is clean, dry and free of debris
  2. Measure the spark plug thread pitch and length and select the appropriate compression hose adaptor.
  3. Securely attach the selected compression hose adaptor to the standard compression hose.
  4. Place the standard compression hose into the spark plug aperture and tighten into the cylinder head/combustion chamber.
    This procedure is identical to a conventional compression test gauge application.
  5. Connect the BNC to BNC test lead to Channel A on your scope and to the BNC socket of the WPS500X pressure transducer.
  6. Switch on the WPS500X and wait for the self-test to complete (LEDs will scroll from range 1 to 3 and revert to 1).
  7. Connect the standard compression hose to the inlet port on the WPS500X.
  8. To prevent exhaust catalyst damage, engine oil contamination and bore washing, make sure that the fuel injection to the cylinder is prohibited during the test.
  9. Run your scope by pressing the space bar on your keyboard or the Go button in PicoScope
  10. Crank and start the engine. Give it sufficient time to near stabilization at idle speed.
  11. With the engine idling, stop capturing by pressing either the space bar on your keyboard or the Stop button in PicoScope.
  12. Switch off the engine.

All values obtained below with the WPS500X are referenced to Gauge pressure.

We advise you to recharge your WPS500X after use to ensure it is ready for future measurements.

Waveform notes

During the various stages of crankshaft rotation, cylinder pressures may be either positive or negative.
For the purpose of this test:

Atmospheric pressure = 0 bar

Any value above 0 bar = Positive pressure

Any value below 0 bar = Negative pressure (vacuum)

All values are typical and not specific to all vehicle types.

Channel A indicates the rise and fall in-cylinder pressure with the engine at idle speed.

Refer to vehicle technical data for specific test conditions and results.

  1. Peak positive pressure recorded during idle (3.967 bar) confirmed using the signal ruler. The value is recorded in the ruler legend (marker 4).
  2. Marker indicating 0 bar or atmospheric pressure
  3. Compression tower indicating the symmetrical rise and fall in cylinder pressure during idling.
  4. The ruler legend recording the numerical pressure, time and degree values relative to the position of the signal, time and rotation rulers.
  5. Expansion pocket (negative pressure -698 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 handles’ rest position. Click on the handle and drag to positions on the waveform that align with two consecutive TDC compression towers (marker 1) This will denote 0–720 degrees of rotation of the crankshaft relative to TDC/peak compression.
  7. Rotation ruler partitions can be added here by clicking on the Rulers button (marker 7) and selecting four rotation partitions from the popup box. The distance/time between the rotation rulers (marker 6) will now be split into four equal partitions to indicate 180-degree rotations of the crankshaft relative to the positions of the rotation rulers (placed at TDC compression stroke).
  8. The time ruler handles are located at the bottom left-hand corner of the waveform. Drag both time rulers to align with the 0 and 360-degree rotation ruler partitions in order to measure the engine speed recorded in the frequency / RPM legend (marker 9).
  9. The frequency/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 at your disposal to zoom to two consecutive compression towers
  12. Intake pocket (negative pressure -698 mbar) formed as the piston descends the cylinder during the intake stroke. The negative pressure value is indicated by the signal ruler and recorded in the ruler legend (marker 4).

Diagnosis

Peak cylinder pressure is achieved as the piston ascends the cylinder during the compression stroke. (Intake and exhaust valves closed).

Using the Signal Rulers (marker 1) we reveal the cylinder compression peaks at 3.967 bar which appears low but quite normal given the throttle is closed and starving the engine of airflow (low air intake = low compression). However, we can now see repeated, even and symmetrical compression towers as the crankshaft rotates and more importantly, events taking place between compressions that could never be visible with a compression tester. Marker 2 denotes 0 bar (atmospheric pressure) where the cylinder pressure should remain from approximately 180 to 360 degrees of crankshaft rotation during the exhaust stroke

NB! Peak cylinder pressure of the compression stroke can be considered as Top Dead Centre (TDC)

At the base of each compression tower during the expansion stroke, you can see the expansion pocket (marker 5) dropping below 0 bar, indicating the cylinder pressure to momentarily drop 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 by looking at the expansion pocket. You can measure the depth of the expansion pocket (and so the vacuum level) with the signal ruler (marker 5) and the value displayed in the ruler legend (marker 4) at -698.6 mbar.

At approximately 360 degrees of crankshaft rotation, you can see the commencement of the intake pocket dropping below 0 bar indicating the cylinder pressure to now fall into a vacuum. This indicates adequate sealing of the exhaust valve that will now close as the piston descends the cylinder during the intake stroke. Valve timing, the integrity of the air intake, piston compression rings and cylinder face, can also be confirmed by looking at intake pocket. The depth of the intake pocket (the vacuum level) can be measured with the signal ruler (marker 5) and the value displayed in the ruler legend (marker 4) at -698.6 mbar.

In most petrol engines at idle speed (throttle closed) the depth (vacuum level) of the expansion pocket is equal to the depth of the intake pocket. Identified here by the signal ruler (marker 5)

Engine speed (= Frequency x 60)

You can use the time rulers (marker 8) placed at the 0-degree rotation ruler and 360-degree rotation partition, to measure and display the engine speed. The time it took (frequency in Hz) for the crankshaft to rotate 360 degrees (measured by the time rulers) is multiplied by 60 to reveal the engine speed where the value (719 RPM) will be displayed in the Frequency / RPM legend (marker 9).

Technical information

The rotation rulers are used to denote 0 to 720 degrees of rotation about the captured cylinder pressure waveform. Given that peak cylinder pressure occurs at TDC of the compression stroke, positioning the rotation rulers at two consecutive compression peaks/towers will denote 0–720 degrees of crankshaft rotation relative to TDC / peak compression.

Based on our knowledge of the four stroke cycle, we know the events that should take place between the rotation rulers at 0 and 720 degrees (TDC Compression to TDC Compression). To make the diagnosis easier, you can divide the distance between 0 and 720 degrees into four equal divisions. This will reveal the position of the crankshaft at key stages throughout the four-stroke cycle (TDC and BDC). When you know the position of the crankshaft, you can identify each of the four-stroke cycles between compression events and attributing anomalies found to specific four-stroke events/activities.

You can read more about Compression testing and find our troubleshooter for compression testing on our training page.

AT785-1

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 Pressure Transducer Kit (with carry case)

    £1,055.00

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Guided test: In-cylinder compression – idle (petrol)