The purpose of this test is to evaluate the in-cylinder pressures of a gasoline engine during a snap throttle test 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 pressure pulses have positive pressure peaks around 4 bar and negative pressure peaks around -750 mbar (below atmospheric pressure).
At around 1.5 s, the throttle is opened and the pressure immediately increases from about -750 mbar to 0 bar (atmospheric pressure).
The peak of the subsequent pressure pulse is around 13 bar.
The pulses become more frequent as the engine speed increases.
The pulse peak pressures increase up to a maximum around 21 bar (at around 2.4 s) and then decrease gradually as the engine speed continues to increase.
The engine speed reaches around 2,500 RPM at around 2.5 s.
As the engine speed approaches 2,500 RPM, intermediate peaks, reaching a maximum of around 2.5 bar, form between the main pressure pulses.
At around 2.65 s, the throttle is closed and the negative pressure peaks return to around -750 mbar.
The peak of the next pressure pulse drops to around 11 bar, considerably less than its predecessor.
The pressure pulse peaks continue to decrease rapidly, to around 1.5 bar, and the pulses become less frequent as the engine overruns and its speed drops.
Eventually, the pressure pulse peaks start to steadily increase and the pulses become less frequent as the speed returns towards steady-state idle conditions.
When used with a snap throttle test, a WPS500X allows you to observe how in-cylinder pressures change over a range of air flows in and out of the engine.
It is possible to use PicoScope’s Zoom, Rulers, and Rotation rulers to analyse and identify similar features in your waveforms to those described in the In-cylinder pressure during cranking (gasoline) and In-cylinder pressure during idle (gasoline) guided tests. However, the snap test waveform allows us to look at other broader trends that occur as the air flow and engine speed change.
When captured during a snap throttle test, the relationship between the in-cylinder pressure waveform features and engine events can be described in turn, as follows:
It is possible to use the maximum pulse pressure to estimate the engine load during any given engine cycle within the waveform. For example, if the maximum pressure was 21 bar and the peak pulse pressure at idle was around 4 bar the calculated value is around 4 / 21 x 100 = 19 %, which is a typical engine load during idle.
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 two consecutive pulse peaks, which are separated by 720° of crankshaft rotation, the RPM value indicates half the average engine speed over the period.
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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