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Flame Front vs. Knock “Explosion”

EFI Tuning Fundamentals

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Andre,

I don’t understand why a normal spark plug initiated flame front is relatively slow and controlled and a post-ignition, knock event is fast and explosive. The two events are taking place in the same volume of space in the same A/F environment—what factor(s) make the nature of these two “ignition events” so drastically different?

Does it have something to do with the knock event occurring post-spark event, in a much higher pressure and temperature environment and since an increase in these conditions speeds up flame front propagation to the point of it being more of an instantaneous “explosion?”

Thanks,

Rich Laws

Akron, OH

As I understand it (I may stand to be corrected), you're close, with there being two similar issues.

With a controlled ignition the fuel burns the fuel progressively - the flame front - but this also creates a pressure front that is compressing the fuel-air mixture ahead of the flame and increasing it's temperature. If the localised pressure generated temperature is high enough (poor quench/deck clearances), and/or there is a localised 'hot spot', it can ignite the unburned fuel-air mixture, starting a second flame/pressure front, and it is the collision of the pressure fronts, and the resulting pressure spike, that causes the audible 'knock'. These pressure spikes can be several times the normal combustion pressure and are what causes the engine damage - especially as the colliding pressure fronts can spontaneously ignite the mixture between from the temperature build up.

I understand this as "detonation"

The similar problem is where the pressure and temperature increase, usually aided by 'hot spots', is such that the fuel is ignited BEFORE the spark ignites the mixture, still causing the pressure front "knock" but for a slightly different reason. This problem, of a 'hot spot' initiating ignition, is why some old carb' engines would run on when the ignition was turned off, some of the older folks may remember it being referred to as 'dieseling', which is basically what it was doing, and the anti-dieseling techniques of the early emissions engines.

I understand this as "pre-ignition".

From what I learnt about knock - Gord is absolutely correct. It has also something to do with octane rating. Petrol fuel hydrocarbon molecules are not that physically stable (comparing to othe types of fuel - spirits for example, etc) and tend to part in high temperature and pressure environment going through oxidation process. The lower the octane rating of petrol the quicker oxidation process is resulting in quick forming of hydrogen peroxide. Hydrogen peroxide is highly explosive and causes small local blasts that start knocking process as Gord described...

Thanks for the feedback and for shedding some light on this question for me…the increase of pressure/heat, post ignition seems to be the differentiator between the fuel/air igniting at time of spark vs. immediately after.

Thanks again!

~Rich

You can download it for free

Knock: A Century of Research

https://saemobilus.sae.org/content/03-15-01-0004/

Thanks sir