Discussion
Arrange the events of a cold start in order:
1. The engine starts on a rich mixture for stability
2. Untreated exhaust passes through a cold catalyst
3. The catalyst reaches light-off temperature
4. Conversion efficiency rises above ninety per cent
Select the correct sequence.
Answer: C
Correct answer: 1, 2, 3, 4
The sequence explains why cold start emissions are so high. The engine starts rich, because a cold intake condenses fuel and a lean mixture will not run smoothly.
That rich exhaust passes through a catalyst still at ambient temperature, which converts almost nothing. Only when the exhaust has warmed the substrate to light-off, around 250 to 300 degrees, does conversion begin, and efficiency then rises above 90 per cent. The worst exhaust meets the least effective catalyst.
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