October 2021

OCTOBER 2021 AFTERMARKET 27 essence of integrated ignition, the delivery of electrical energy across the spark plug initiates a progressive flame front throughout the air fuel mixture within the cylinder. Depending on physical load, temperature, AFR, timing, available energy, advance angle and induction turbulence, this flame front can be erratic, even extinguish and re- ignite momentarily, shunt or completely collapse. All these events will result in a reduction of spark burn time. This is unavoidable and is calculated in the reserve energy capacity of the ignition system. It our job to ensure there are no additional loads reducing the energy delivered still further. Please also note that if the ignition energy has no physical load within the cylinder, it will appear longer than normal. Observation How else may we observe ignition performance? First, visual inspection of spark plugs and components. I know intrusion is a dirty word these days, but this is gold dust especially as plug condition tells a great story about the combustion properties.Second, serial data. There is an impressive amount of data available relating to ignition, which is not possible to observe when using an oscilloscope, and it is easy to access. Remember not to treat the ignition system in isolation. I recommend observing load request and actual load, a problem here will cause the PCM to pull back the actual value. However, this can vary according to atmospheric conditions, temperature and humidity, fuel quality, turbo boost value, and air intake temperatures, for example insufficient intercooling. Exhaust gas exit temperature bank 1, AFR, and ignition set back; This is the retardation of the ignition point from the calculated value, primarily caused by ignition knock. Do not forget to check fuel quality. There should not be more than 3 degrees of retardation at full load. The third way to observe is via the scope, more on which in a moment, and the fourth is vibration analysis, which we will cover in greater detail at the end of this topic. So, let’s look at using the scope. Although ignition is a low frequency event, I still recommend a high sample rate of 10ms or higher. This is because you may wish to capture several events in free run, i.e., no trigger, then zoom in for close inspection, as many ignition anomalies are intermittent in nature. If you do choose to use a trigger to stabilise the image, consider triggering on the current ramp its more stable and does not have voltage spikes from other induction events which often de-stabilise triggering. Do not use filtering when sampling to smooth the image. Once the data is in the buffer you can remove unwanted noise during the display options. A quick word of advice here, always monitor current in the primary circuit, this can often be done from a common voltage supply. If you observe an unusual reduction in burn time with no reduction in current profile, it must be a secondary problem. Combustion So how does NVH or vibration monitoring lend itself to ignition diagnostics? First you need to take a step back and ask, what is combustion? In short, heat energy created by There is an impressive amount of data available relating to ignition ” Fig.1 Fig.2 Fig.3 Fig.4