In-cylinder pressure during cranking (gasoline)

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

How to perform the test

View connection guidance notes.

1. Disable the vehicle’s fuel injection and ignition systems.
2. Connect the fully charged WPX500X 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 scope to see live data.
9. With the throttle fully open crank the engine until the waveform stabilises, usually around 5 s.
10. With your waveforms on screen stop the scope.
11. 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.
• A series of symmetrical pressure pulses with peaks around 11 to 12 bar.
• A small depression, reaching around -0.25 bar (-250 mbar below atmospheric pressure), immediately after each pulse.
• Following the small depression, the pressure remains around atmospheric pressure until the start of the next pulse.

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 the time 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 during cranking, the relationship between the in-cylinder pressure waveform features and engine events can be described in turn, as follows:

• A 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 continuous fall in pressure during the expansion stroke, as there is no combustion.
• A depression pocket as the expansion stroke heads towards BDC (180° after TDC), indicating the cylinder remains sealed.
• An increase in pressure from the negative peak, back towards 0 bar, 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.
• Approximately constant atmospheric pressure as the piston cycles through its induction stroke phase (360° to 540° after TDC). Deviations here indicate intake issues.
• An increase in pressure, as the piston cycles through its compression stroke (540° to 720° after TDC), indicating intake valve closure.
• Once more, a symmetrical pressure pulse.

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 cranking speed.

Note

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

GT157-EN