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Q6. a) Describe, with the aid of a sketch, an external system for reducing engine NOx emission, explaining the chemistry of the process. (8)

b) Explain why Urea is used in the selective catalytic reduction process instead of ammonia. (4)

(c) Explain why the exhaust gas quality must be monitored before and after the selective catalytic reduction unit, stating how such monitoring influences operation of the SCR unit. (4)