Exhaust pulsation (ignition triggered) during cranking

The purpose of this test is to assess engine condition during cranking by using the FirstLook™ sensor to observe exhaust pulsation.

 

WARNING

Uninsulated HT pickups are designed to clip around double-insulated HT leads only – they are not designed for direct connection to a hazardous live voltage.

To prevent injury or death, when connecting or disconnecting an HT pickup:

  1. switch off the ignition
  2. clean the HT leads
  3. inspect them for damage
  4. clip the HT pickup over the desired undamaged HT lead
  5. ensure all test leads are kept clear of hot or rotating parts before starting the engine

 

How to perform the test

View connection guidance notes.

  1. Disable the fuel injection system before carrying out this test.
  2. Connect the FirstLook™ Sensor to PicoScope Channel A and insert it approximately 10 cm into the vehicle exhaust tailpipe.
  3. Connect a secondary ignition pick up to PicoScope Channel B and around the HT lead for cylinder number 1. Ensure the secondary ignition pick up earth fly lead has a good earth.
  4. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
  5. Start the scope to see live data.
  6. Crank the engine until the waveform stabilises (3 to 5 seconds).
  7. With your waveforms on screen stop the scope.
  8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

These known good waveforms have the following characteristics:

  • A repeating series of alternate peaks and troughs, representing the pulsations (Channel A).
  • All pulsation peak to peak amplitudes are approximately the same.
  • There are no missing or anomalous peaks, troughs or inflexions in the pulsation waveform.
  • The secondary voltage (Channel B) shows cylinder number 1’s ignition event, as a reference to identify the individual cylinder pulses.
  • The number of pulsations per secondary ignition event equals the number of engine cylinders.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select Exhaust / tailpipe pressure waveform.

Further guidance

The FirstLook™ sensor contains a piezo crystal device which converts pressure pulsations to a voltage signal output. The output can be taken as an indication of the underlying physical actions causing the pressure pulsations.

When checking engine condition by examination of exhaust pulsations, several factors must be taken into account:

  • Cylinder head design and valve operating characteristics, affecting flow through the engine.
  • Any smoothing and damping effects of a turbocharger.
  • The internal design and length of the exhaust system, which not only carries gas away from the engine but also filters and slows the gas speed to reduce noise (and hence the pulsation amplitudes).

For these reasons you should ensure the pulsations have stabilized before examining for engine diagnosis.

With constant engine operating conditions, each cylinder should produce the same pulsation and you should expect uniformity across cylinders.

There are generic pressure pulsation features that may be observed with an internal combustion, piston, engine:

  • An initial pulse peak formed by the high pressure shot of gases escaping from the cylinder toward the end of the power stroke.
  • A slight drop in sensor output as the main slug of gas is pushed out of the cylinder during the exhaust stroke.
  • A significant drop in sensor output between pulsations as each piston decelerates towards the end of its exhaust stroke.

These features may be more prominent within a cranking engine, where the overall gas speed is slower and less smooth than with a running engine.

Cylinder identification

Should you suspect an anomaly within the pattern then you will need to identify the offending cylinder. However, both of our captured waveforms introduce a small uncertainty as to the reference points we should use within that process. These uncertainties are:

  • For most spark ignition engines, the secondary spark occurs between 20 and 35 degrees before the piston reaches top dead centre, at the end of its compression stroke.
  • The pulse propagation time down the exhaust pipe delays its arrival at our measurement point, the tailpipe. Therefore, the pulsation waveform is delayed (shifted) relative to the secondary ignition waveform.

At low engine speeds (e.g. at idle), the pulse propagation delay can be just under half a pulse width in a 12-cylinder engine but it is proportionally less in engines having fewer cylinders. In all cases, the peak pulse amplitude will not align as expected with the identified engine phases (which are determined relative to the secondary ignition events) but they do appear within the expected phase.

With the above factors in mind, the process for cylinder identification with an exhaust pressure pulsation waveform is:

  1. Retard the engine cycle reference points relative to the secondary ignition events: they should be between 20 and 35 degrees afterwards. As a rule of thumb, place Phase rulers at “30/720 x the time between two consecutive ignition events” after each.
  2. Tag the cylinder number one engine phases (given that the secondary ignition event occurs during the compression stroke).
  3. Find the corresponding cylinder number one exhaust pulse.
  4. Mark the remaining pulsations in engine firing order, relative to the cylinder number one pulse.

Causes of waveform anomalies

Exhaust system faults which may cause waveform anomalies are:

  • General exhaust system leaks, manifold cracks, gaskets etc.
  • Burnt valves/seats.
  • Valve not closing.
  • Valve not opening.
  • Incorrect timing/valve clearance issues.
  • Badly worn cam profiles.

Other engine faults also may affect the waveform:

  • Lack of cylinder compression.
  • Induction issues (e.g. blocked filter).

Intake valvetrain issues (e.g. those listed for the exhaust valvetrain above).

GT129-EN

Disclaimer
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.

Suitable accessories

  • FirstLook Engine Diagnostic Sensor

    £449.00

  • Secondary ignition pickup (capacitive with BNC)

    £58.00

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Guided test: Exhaust pulsation (ignition triggered) during cranking