Discussion
Consider the following statements about emissions and mixture strength:
1. Rich running raises carbon monoxide and hydrocarbons.
2. Nitrogen oxides peak slightly lean of stoichiometric.
3. Deeply lean running raises nitrogen oxides further still.
Which of the statements given above is or are correct?
Answer: C
Correct answer: 1 and 2 only
The first two are correct: rich running gives incomplete combustion and so raises carbon monoxide and hydrocarbons, and nitrogen oxides peak slightly lean of stoichiometric where flame temperature and available oxygen coincide.
The third reverses the trend beyond the peak. Deeply lean running lowers nitrogen oxides, because the excess air cools the flame - which is the principle a lean-burn engine exploits, at the cost of losing catalytic reduction.
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