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.
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.
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:
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:
As the peak pressure occurs at TDC after the compression stroke, the adjacent peaks are apart by 720° of crankshaft rotation.
Your waveform will be partitioned every 180° to provide a visual indication of the boundaries between the 4-stroke cycle phases.
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:
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).
Actual pressures vary with engine and test conditions. Only make pressure value decisions based on comparison with manufacturer data.
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.
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