Which phenomenon primarily causes knocking in spark-ignition (SI) engines and limits their compression ratio?
Auto-ignition of unburned mixture ahead of flame front
In a spark-ignition engine the spark lights a flame that should spread smoothly across the mixture.
As this flame advances it compresses and heats the unburned mixture (the end gas) still ahead of it.
If that end gas gets hot enough it auto-ignites on its own before the flame arrives, releasing energy suddenly.
This sharp pressure spike is heard as knocking, and the fear of it forces designers to keep the compression ratio limited.
Hence, the answer is auto-ignition of unburned mixture ahead of flame front.
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