During which stroke of the engine cycle are nitrogen oxides produced in the highest amount?
Power stroke
Nitrogen oxides form when nitrogen and oxygen in the cylinder combine, and this reaction needs very high temperature.
The highest in-cylinder temperature and pressure occur just after combustion, during the power (expansion) stroke when the burnt gases push the piston down.
At this peak heat the nitrogen and oxygen react most strongly, so the largest quantity of nitrogen oxides is produced then.
The intake, compression and exhaust strokes do not reach the peak flame temperature needed for heavy NOx formation.
Hence, the answer is Power stroke.
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