Discussion
Aftertreatment devices on a modern heavy diesel.
| Device | Duty | Consumable |
|---|---|---|
| Oxidation catalyst | Carbon monoxide and hydrocarbons | None |
| Particulate filter | Soot | Fuel for regeneration |
| Urea reduction catalyst | Oxides of nitrogen | Urea solution |
| Exhaust gas recirculation | Oxides of nitrogen in-cylinder | None |
Why does a diesel need several devices where a petrol engine needs one?
Answer: C
Correct answer: Lean exhaust prevents reduction
Read the table's duty column. Oxidation and filtration both work in oxygen-rich exhaust, but nitrogen oxide reduction does not - reduction requires the removal of oxygen, and a diesel's exhaust is full of it.
A three-way catalyst avoids the problem by working at stoichiometric, where oxidation and reduction can proceed together. A diesel cannot run there, so it needs separate devices: one to oxidise, one to filter, and either recirculation or urea to deal with nitrogen oxides.
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