WPS500X In-cylinder pressure during idle (gasoline)

The purpose of this test is to evaluate the in-cylinder pressures of a gasoline engine running at idle speed using the WPS500X pressure transducer.

How to perform the test

1. Disable the fuel injection and ignition system on the cylinder under test.
2. Connect a fully charged WPS500X pressure transducer to PicoScope Channel A.
3. Switch on the WPS500X and wait for the self-test to complete (LED will scroll from range 1 to 3 and revert to 1)
4. Remove the spark plug.
5. Assemble the compression hose with the correct thread adapter and install into the spark plug hole.
6. Connect the WPS500X to the compression hose.
7. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
8. Start the engine and allow it to idle.
9. Start the scope to see live data.
10. With your waveform on screen stop the scope.
11. Turn off the engine.
12. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

• 0 bar is expressed as a relative pressure and indicates atmospheric pressure.
• Initially, the in-cylinder pressure is below atmospheric pressure at around -650 to -700 mbar (indicating a relative vacuum within the cylinder).
• Just prior to the first large pressure pulse, there is a small pressure rise of around 50 mbar and then a partial fall, after which the pressure rises rapidly to around 4 bar.
• After the peak, the pressure rapidly falls back to around -650 to -700 mbar, to give a pressure pulse waveform that is symmetric about its peak.
• The pressure then rises to around 0 bar (atmospheric pressure) for a sustained period until it drops suddenly back to around -650 to -700 mbar.
• The pattern then repeats.

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

A WPS500X pressure transducer allows you to measure in-cylinder pressure changes throughout the engine cycle to reveal crucial details about cylinder integrity and valve operation:

Waveform analysis

PicoScope’s Rotation rulers (represented as a draggable blue-green circle at the right limit of the View’s time axis), Rulers (represented as a draggable white square at the left limit of the View’s time axis) and Zoom features are essential aids to in-cylinder pressure waveform analysis:

• Use the Zoom tool to select two consecutive waveform pulses in their entirety.
• Set Rotation rulers to align exactly with the peak pressure on each pulse.

As the peak pressure occurs at TDC after the compression stroke, the adjacent peaks are apart by 720° of crankshaft rotation.

• Click on the second Rotation ruler value label to change it from default 360° to 720°.
• Click on the Rulers pop-up menu and enter 4 as the Rotation Partition value in the Ruler Settings.

Your waveform will be partitioned every 180° to provide a visual indication of the boundaries between the 4-stroke cycle phases.

Waveform features

With both time Rulers and Rotation rulers on the time axis, the Ruler legend shows both time and degrees. By aligning Rulers with specific waveform features, it is possible to measure valve timing events relative to TDC and BDC (in degrees) to check them against manufacturer data.

When captured with the engine running at idle, the relationship between the in-cylinder pressure waveform features and engine events can be described in turn, as follows:

• A 50 mbar increase in pressure, and subsequent partial fall, prior to a significant pressure pulse, indicating when the cylinder is sealed by the inlet valve closing.
• A significant pressure pulse symmetric about TDC (0°), indicating the valves and piston rings/cylinder walls are sealed and no cylinder charge is escaping during compression.
• A reduced peak pressure compared to those measured during cranking (generally compressions range from 6 to 13 bar). However, the engine is running at idle with the throttle closed so the intake air volume is low.
• A continuous fall in pressure during the expansion stroke, as there is no combustion.
• A depression pocket as the expansion stroke heads towards BDC, indicating the cylinder remains sealed.
• An increase in pressure from the negative peak, back towards 0 bar, just prior to BDC (180° after TDC) indicating the opening of the exhaust valve.
• Approximately constant atmospheric pressure as the piston cycles through its exhaust stroke phase (180° to 360° after TDC). An increase in pressure here indicates exhaust restriction.
• A decrease in pressure to the range from -650 mbar to -750 mbar at some time after TDC, indicating the inlet valve opening and that the piston is cycling through its induction stroke. The precise crank angle where this event occurs depends on the valve overlap period for your engine.
• A sustained period of relatively constant pressure, below atmospheric pressure, as the piston cycles through its induction stroke phase (360° to 540° after TDC). Deviations here indicate intake issues.
• Once again, a small increase in pressure, with a subsequent partial fall, etc.

When two Rulers are placed on the time axis, the Frequency legend indicates the equivalent cycle frequency calculated from the time period (delta) between the rulers. The frequency is displayed in units of hertz and RPM. Therefore, if rulers are placed at 0° and 360°, the RPM value indicates the engine’s idle speed (which, in this test, is likely reduced due to the inactive cylinder).

Note

Actual pressures vary with engine and test conditions. Only make pressure value decisions based on comparison with manufacturer data.

GT785-EN