BDN Automotive ® CA-6 On-board angle based real-time combustion analyser
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During the combustion phase the spark plug fires the air-fuel mixture, which starts to burn radially. As the volume of combustion products are higher than the unburned mixture, a pressure wave arises between the burned and unburned zone of the combustion chamber. This wave then reflects from the cylinder walls, pistons, and other mechanical components. Detonation occurs when an abnormal, auto-ignited pressure wave collides with a normal one creating a local pressure peak.
The effect of knock could be monitored on the pressure trace, as it being overlaid by a high frequency oscillation. The frequency of it depends mainly on the bore, as it is a superposition of the different armonics of the pressure wave reflections inside the cylinder.
The detonation that makes the pressure peaks of “knock” could be created in the two following ways. The difference between those two phenomena is the generating cause.
Auto-ignition and Pre-ignition
Auto-ignition is the phenomenon usually assigned with knock. The spark generated combustion leads towards increased cylinder pressure and temperature. This phenomenon occurs when the local peak temperature in the unburned zone reaches the auto-ignition temperature of air-fuel mixture and the secondary flame starts to propagate from that spontaneous ignition point. The generated pressure wave collides with the normal one. Auto ignition is triggered by the combustion coming from the spark event, and therefore could be controlled with altering the spark advance.
Pre-ignition is created in the compression stroke before spark would ignite the mixture. In that case a point of the combustion chamber (hot spot) gets too hot and exceeds the mixture’s auto-ignition temperature. Therefore, the combustion starts even without the presence of the spark. As the volume is still decreasing (the engine is before TDC) that type combustion could cause enormously high cylinder pressure. Furthermore, the combustion generated by a pre-ignition process could cause auto-ignition as well. The pre-ignition is not triggered by a spark event, therefore it could not be controlled with altering the spark event’s position. Auto ignition could be distinguished from pre ignition by the location of the generated pressure peaks. The amplitude of pre-ignition is usually magnitudes higher then a standard knock, thus it is commonly named as super-knock.