WPS500X In-cylinder compression test (petrol, cranking)

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

This is a new Guided Test in BETA form. Please feel free to pass any feedback, suggestions, corrections or hints and tips to us via the comments function at the bottom of this page.

The purpose of this test is to evaluate the cylinder efficiency of a petrol engine through 720° of crankshaft rotation, during cranking. The Pico standard compression hose used during this test does not contain a Schrader valve, therefore the rise and fall in cylinder pressures will be recorded relative to crankshaft position.

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

During the various stages of crankshaft rotation, cylinder pressures may be either positive or negative. For the purpose of our test, Atmospheric pressure = 0 bar, therefore any value above 0 bar relates to positive pressure, and any value below 0 bar relates to negative pressure (vacuum).

All values obtained with the WPS500X during this test are referenced to gauge pressure.

All numerical readings quoted in this Guided Test are typical and not specific to all engine styles.

Ensure that the WPS500X is fully charged before starting this test.

How to perform the test

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 (exact adaptor depends upon spark plug thread and depth).

PicoScope settings

  1. Channel A Probe setting: WPS500X Range 1
  2. Channel A range: -2 .. 14 bar
  3. Timebase 500 ms/div

Note: Engine must be prohibited from starting and fuel injection is cut from all cylinders.

  1. Remove the relevant spark plug ensuring 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. Introduce the standard compression hose into the spark plug aperture and tighten into the cylinder head / combustion chamber. This is an identical procedure to a conventional compression test gauge application.
  5. Connect BNC to BNC lead to Channel A of your scope and the BNC socket of the WPS500X pressure transducer.
  6. 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.
  7. Connect the standard compression hose to the inlet port of the WPS500X.
  8. To prevent exhaust catalyst damage, engine oil contamination  and bore washing, ensure fuel injection is prohibited to all cylinders.
  9. Run your scope software by pressing either the space bar on your keyboard or the Go button in PicoScope.
  10. Ensure the throttle is held wide open and crank the engine for approximately 5 seconds.
  11. Cease cranking and stop your scope by pressing either the space bar on your keyboard or the Stop button in PicoScope.

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

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)

Peak positive pressures recorded during cranking (11.79 bar) are confirmed using the signal ruler where the value is recorded in the ruler legend  (See Diagnosis Figure 2).

Signal ruler indicating 0 bar or atmospheric pressure.

Compression peak (tower) indicating the symmetrical rise and fall in cylinder pressure during cranking  (see Diagnosis Figure 2 and More Information, item 2).

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 to display two consecutive Compression Towers.

Expansion Pocket (Negative pressure – 207 mbar) formed as the piston descends the cylinder during the expansion stroke (see Diagnosis Figure 3). The negative pressure value is indicated by the signal ruler and recorded in the Ruler legend .

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) . This will denote 0 – 720° of rotation of the crankshaft relative to TDC and peak compression (see More Information).

Rotation ruler partitions can be added here by clicking on the ruler button  and selecting four Rotation Partitions from the dialogue box (see Diagnosis Figure 3 and More Information). The distance and time between the rotation rulers 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).

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, in order to measure the cranking speed recorded in the Frequency and RPM Legend .

The frequency and RPM legend displays the engine RPM relative to the position of both time rulers (see Diagnosis Figure 3).

Arrows denoting the direction of piston travel  labelled with the relevant stroke of the four-stroke cycle.

Zoom tools are at your disposal in order to zoom into the two consecutive compression peaks and towers.

Diagnosis

Figure 2

Compression towers and peaks (See More Information, item 2).

Peak cylinder pressure is achieved as the piston ascends the cylinder during the compression stroke. (Intake and exhaust valves closed). Using the Signal Rulers 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 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 (see More Information)

Using PicoScope's Rotation rulers and partitions , 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 dropping below the zero bar indicating the cylinder pressure to momentarily drop 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 (as per ) and the value displayed in the Ruler Legend  -207 mbar.

Cranking speed (= Frequency x 60)

With the Time rulers 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 .

More information

PressureInVoltageOut-WPS500

The WPS500X pressure transducer provides a means to convert positive or negative pressures into voltage. When combined with our automotive PicoScopes, we have the distinct advantage of displaying pressure values against time, allowing technicians to view the dynamic change in cylinder pressures during the four-stroke cycle. Assessing the cylinder efficiency using a WPS500X will reveal more information about the condition of an engine than was ever thought possible, given the resolution and speed of the pressure transducer and PicoScope. For this reason we have to be aware that the variety of engine designs, intakes, exhaust systems, and elaborate variable valve timings will all have an effect on the waveform and results that differ from vehicle to vehicle.

Based on the cylinder efficiency test and analysis procedure above, we can confirm:

  1. Peak cylinder pressure to be correct for the engine under test (allow for standard compression hose volume of 5 ml).
  2. Confirmation of uniform and symmetrical compression towers confirming the cylinder efficiency, not only as the pressure builds but also as it naturally decays during the expansion stroke. For the compression towers to be symmetrical, valve timing, the valve train and the mechanical integrity of the piston and cylinder must be efficient.
  3. The presence of the expansion pocket confirms our cylinder can hold a vacuum and must therefore be air tight (valve seat and piston ring integrity OK).
  4. Sufficient cranking speed
  5. Adequate intake and exhaust flow so achieving the correct peak pressure (no restrictions).
  6. Repeatability of peak compression for every completion of the four-stroke cycle.
  7. Cylinder pressure remains at zero bar during exhaust and intake events confirming no airflow restrictions during either stroke.

Rotation Rulers and Partitions (Refer to Figure 3a)

The Rotation Rulers are used to denote 0 and 720° of rotation about the captured cylinder pressure waveform. Given peak cylinder pressure occurs at TDC of the compression stroke, placing the rotation rulers at two consecutive compression peaks will denote 0 – 720° of crankshaft rotation relative to TDC and peak compression.

Based on our knowledge of the four-stroke cycle, we know the events that should take place between the 0 and 720° rotation rulers (TDC compression to TDC compression). To assist with diagnosis, simply dividing the distance between 0 and 720° into four equal divisions reveals the position of the crankshaft at key stages throughout the four-stroke cycle (TDC and BDC). Once we know the position of the crankshaft, we can identify each of the four-stroke cycles between compression events, attributing any anomalies found to specific four-stroke events (see Troubleshooting).

AT157-AddingPartitions

Figure 3a

Troubleshooting

Based on the knowledge gained above we can now look at a number of mechanical issues revealed in waveforms using the pressure transducer.

AT157 - Exhaust Valve Fails to open

Figure 4 - Exhaust valve fails to open

Figure 4 reveals an additional pressure peak during the exhaust event that should not be present. As the piston rises from BDC (bottom dead centre) of the expansion stroke (approximately 180° after TDC compression) the exhaust valve will open and release the cylinder pressure out to the atmosphere (atmospheric pressure) via the exhaust system.

The waveform indicates a pressure increase to approximately 7.425 bar during the exhaust stroke, because the exhaust valve is not opening! In effect we have another compression stroke during the four-stroke cycle: Compression-Expansion-Compression-Intake.

Air can be drawn into the cylinder and so compressed, hence the peak compression value (8.9 bar) but air cannot escape during the exhaust stroke.

Looking a little deeper at the exhaust stroke we can see what looks to be another compression tower as a result of the exhaust valve remaining closed, only this time the tower is no longer symmetrical. At approximately 360° of crankshaft rotation the intake valve opens abruptly releasing 7.425 bar of cylinder pressure into the intake manifold, hence the rapid drop in pressure and asymmetric tower. Such an event would manifest itself as a popping sound via the intake manifold.

AT157 - Intake Valve Fails To Open

Figure 5 - Intake valve fails to open

Figure 5 demonstrates perfectly the effect on peak compression as result of the intake valve failing to open (447 mbar) peak compression. While we have found poor compression, we can also evaluate the cause looking at events during the four-stroke cycle between compression peaks.

Looking at the expansion pocket a near perfect vacuum is achieved at -992 mbar. At approximately 180° of crankshaft rotation we can see the exhaust valve has opened, given the cylinder pressure returns from a vacuum at -992 mbar to zero bar (Atmospheric pressure).

Following on from 360° of crankshaft rotation, a second and prolonged vacuum develops within the cylinder as the exhaust valve closes and the intake valve fails to open (this is called the intake pocket).

Once again an exceptional vacuum develops to form a deep and prolonged intake pocket as a direct result of the piston descending the cylinder while the intake valve remains closed. The result of no air entering the cylinder is low peak cylinder pressure (compression) given there is no air present in the cylinder in which to compress.

AT157-3

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)

    £1,695.00

  • WPS500X Maxi Kit (with case and foam tray)

    £1,765.00

  • WPS500X Maxi Kit (with foam tray)

    £1,695.00

  • WPS500X Pressure Transducer

    £819.00

  • WPS500X Pressure Transducer Kit (with carry case)

    £1,155.00

  • WPS500X Pressure Transducer kit (with case and foam tray)

    £1,245.00

  • WPS500X Pressure Transducer Kit (with foam tray)

    £1,165.00

  • Adaptor M10 deep reach

    £33.00

  • Adaptor M10 short reach

    £33.00

  • Adaptor M12 deep reach

    £33.00

  • Adaptor M14 deep reach

    £33.00

  • Adaptor M14 short reach

    £33.00

  • Adaptor M16 Ford triton

    £33.00

  • Adaptor M18

    £33.00

  • WPS500X Adaptor Kit A

    WPS500X Adaptor Kit A

    £129.00

  • WPS500X Adaptor Kit B

    WPS500X Adaptor kit B

    £129.00

  • Universal vacuum adaptor

    £20.00

  • Vacuum hose

    £20.00

Share your experience

If you have any suggestions to improve this guided test please do so using the 'add comment' button.

3 comments | Add comment

Steve Smith
October 21 2016

Hi Ric,

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.


Hi Ron,

Only the relevant spark plug for the cylinder under test is removed, no need to remove them all.

Ric Stacey
October 04 2016

Waveform for various variations of a mildly blown head gasket would be interesting

Ron Vinsant
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…

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Guided test: In-cylinder compression test (Petrol engine-Cranking)