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:
- 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.
- Tag the cylinder number one engine phases (given that the secondary ignition event occurs during the compression stroke).
- Find the corresponding cylinder number one exhaust pulse.
- 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).
