Discussion
Three of the following are diesel aftertreatment devices. Which belongs to a petrol engine?
Three-way catalyst
Diesel oxidation catalyst
Particulate filter
Urea reduction catalyst
Answer: A
Correct answer: Three-way catalyst
A three-way catalyst works only in exhaust near stoichiometric, which is a petrol engine's condition. A diesel runs lean, so the oxygen-rich exhaust prevents the reduction half of the chemistry entirely.
The other three are diesel devices: an oxidation catalyst for carbon monoxide and hydrocarbons, a filter for soot, and a urea reduction catalyst for nitrogen oxides. The diesel needs three because no single device can do in lean exhaust what the three-way catalyst does at stoichiometric.
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