Discussion
Assertion (A): A diesel needs separate devices for oxidation and for nitrogen oxide reduction.
Reason (R): Its exhaust is oxygen-rich, and reduction cannot proceed in the presence of excess oxygen.
Select the correct answer from the options below.
Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the second is the reason. A diesel runs lean, so its exhaust carries substantial oxygen - which suits oxidation perfectly and prevents reduction entirely, since a reducing reaction cannot compete with the oxygen present.
Separate devices are therefore required: an oxidation catalyst, and either exhaust gas recirculation to prevent nitrogen oxides forming or a urea system supplying ammonia to reduce them selectively. A petrol engine at stoichiometric needs only one device because both chemistries can proceed together there.
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