Final answer:
In a diesel engine with a rich air-fuel ratio, hydrocarbons (HC) emissions would likely increase due to incomplete combustion.
Therefore, the correct answer is: option d) HC.
Step-by-step explanation:
Any vehicle that uses energy as its source of propulsion but produces no exhaust gas or other pollutants is considered a zero-emissions vehicle. If a diesel engine is run with a rich air-fuel ratio, the tailpipe emissions that would likely increase is hydrocarbons (HC).
Running an engine with a rich mixture means there is more fuel in the mixture than what is needed for complete combustion. As a result, not all fuel is burned and excess hydrocarbons are emitted in the exhaust.
While nitrogen dioxide (NO2) and other nitrogen oxides (NOx) are associated with high combustion temperatures which occur in lean burn conditions when there is more air than fuel, HC emissions increase when the fuel ratio is high relative to air, leading to incomplete combustion.
The level of unburned hydrocarbons (HC) in the exhaust gases of gasoline fueled spark-ignition engines is typically 1 to 2% of the fuel: in diesels it is much less.