Question

Which of the following is present in emissions from unleaded petrol?

• A. Carbon monoxide
• B. Carbon dioxide
• C. Ethylene
• D. Hydrocarbons

Hint:

Removing lead from petrol does not stop unburnt fuel escaping in the exhaust.

Answer 1

The Correct Option is D

Step 1: Unleaded petrol simply means tetraethyl lead, the older anti knock additive, has been removed from the fuel formulation because lead compounds in exhaust are highly toxic and also poison the catalytic converter. Removing lead does not change how the fuel actually burns inside the engine cylinder.
Step 2: Combustion of petrol inside an engine is never perfectly complete. A fraction of the fuel escapes the combustion process only partially burnt or completely unburnt, and this unburnt or partially burnt fuel exits through the exhaust as volatile hydrocarbon vapours. These unburnt hydrocarbons are one of the defining pollutants of petrol engine exhaust, and they remain in the exhaust of unleaded petrol vehicles exactly as before, because delead only removes the lead additive, not the combustion inefficiency that produces unburnt fuel.
Step 3: Carbon monoxide also forms from incomplete combustion, but among the listed choices the one that matches the standard classification of exhaust pollutants carried over even after delead is hydrocarbons. Carbon dioxide is simply the fully oxidised, complete combustion product and is not the pollutant marker being tested here. Ethylene is not a standard listed component of ordinary petrol engine exhaust in this context.
Step 4: Hence hydrocarbons, option D, is the answer, representing the unburnt fuel component still present in unleaded petrol emissions.