Petrol is a hydrocarbon fuel obtained from crude oil by fractional distillation. When petrol undergoes incomplete combustion in a car engine, pollutants are released into the atmosphere. Explain why incomplete combustion occurs in a car engine and describe the environmental and health problems caused by the pollutants produced during incomplete combustion.

Incomplete combustion occurs when the amount of oxygen supplied to the engine is insufficient for the complete oxidation of the hydrocarbon fuel. In a car engine the air–fuel mixture is often rich, especially during cold starts or acceleration, so the fuel cannot react fully with O₂.

The main pollutant from incomplete combustion is carbon monoxide (CO). CO is produced when carbon in the fuel is oxidised only to CO rather than CO₂.

CO is toxic to humans because it binds to haemoglobin with a much higher affinity than oxygen, forming carboxyhaemoglobin and preventing oxygen delivery to tissues.

Another pollutant is carbon particulates, or soot, which are unburnt carbon particles formed when the fuel is not fully oxidised.

Soot contributes to environmental problems such as global dimming by scattering sunlight, and it also causes respiratory health problems in humans, including bronchitis and asthma.