Correct answer
When the air is compressed and very hot.
The air is compressed and very hot. Compression raises its temperature; injected fuel mixes with it and self-ignites under suitable operating conditions, without an ignition spark plug. This describes the stated diesel engine, not all combustion engines. Glow plugs can assist cold starting and are not spark plugs producing a spark on each cycle.
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Before the cylinder fills with air.
When the air is compressed and very hot.
At the same time as the spark plug fires.
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