WPS500X In-cylinder pressure during snap test (gasoline)

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.

How to perform the test

View connection guidance notes.

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. Fully and rapidly depress the accelerator pedal to snap the throttle open, hold the position for 1 to 2 s, then quickly release the pedal so the throttle snaps closed.
11. With your waveform on screen stop the scope.
12. Turn off the engine.
13. 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 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.

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

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.

Waveform analysis

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.

Waveform features

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:

• Each pressure pulse peak occurs at peak compression during the compression stroke. Therefore, the peaks are separated by 720° of crankshaft rotation.
• Initially, the pressure pulses have a form equivalent to those described in the In-cylinder pressure during idle (gasoline) guided test.
• With the intake throttle and inlet valve open, air can fill the cylinder causing the in-cylinder pressure to rise rapidly to 0 bar (atmospheric pressure).
• The engine speed and air intake increase as more fuel is injected to meet the torque demand signalled by the depression of the accelerator pedal.
• Increased intake air significantly increases the peak compression pressures. Any fault causing a loss of volumetric efficiency (e.g. blocked intake or exhaust) will limit the peak pressures.
• The maximum compression pressure indicates the point of maximum engine load during the test. Thus, it also indicates the engine speed delivering the maximum torque (around 2,000 RPM in the above example).
• Beyond a certain engine speed (dependent on the engine design), the cylinder is less able to receive air. This drop in volumetric efficiency is signalled by a decrease in the peak compression pressures.
• The intermediate peaks that form between the compression pulses occur when the exhaust valve is open and reflect the back pressure within the exhaust system. Any exhaust restriction, whether by fault or design, will increase these peak pressures.
• The compression pressures drop significantly on engine overrun, indicating the significantly decreased air intake when the throttle is closed.

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.

Note

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

GT892-EN