5.1 Petrol and diesel emission performance

Within an internal combustion engine (ICE), chemical reactions take place between the hydrocarbons (HCs) of the fossil fuel, any fuel additives and the oxygen and nitrogen gases in the atmosphere. These processes include complete and partial oxidation of the fuel, producing carbon dioxide (CO₂), water (H₂O) and carbon monoxide (CO). Nitrogen from the air is also oxidised to nitrogen oxides (NO_x). This reaction only takes place if the combustion temperature exceeds 1500 °C.

Partially burned and unburned fuel in the exhaust gases can form methane and a complex cocktail of volatile organic compounds (VOCs) together with small particles of sooty matter (‘particulates’ or PMs).

In bright sunshine tropospheric (low-level) ozone (O₃) is produced by the chemical action of sunlight on NO_x and VOCs. This has been a particular problem in sunny cities such as Los Angeles in the USA and Athens in Greece.

Petrol and diesel engines differ in their relative emissions performances. Petrol engines run with an almost constant fuel/air mixture. This makes designing for complete combustion relatively simple. In diesel engines the fuel/air mixture varies with the power output. At low power there is plenty of air to ensure complete combustion. However, at high power, there may be a high fuel/air ratio and (without proper clean-up technology) incomplete combustion can result in a pall of NO_x, black smoke and particulates from the exhaust pipe.

Overall petrol vehicles produce lower NO_x and particulate emissions. Despite the fact that DERV fuel, when burned, emits more CO₂ per MJ than petrol fuel, diesel vehicles produce less CO₂ per kilometre. This is because of the increased engine efficiency. The relatively high combustion temperatures attained in a diesel engine explain both diesel’s higher NO_x emissions (i.e. a combustion temperature greater than 1500 °C) and its lower CO₂ emissions (due to the engine’s higher efficiency). Levels of particulates up to 10 micrometres in size (termed PM₁₀) are also generally higher for diesel.

The CO₂ advantages of diesel engines, coupled with available technologies to reduce NO_x and particulate emissions, have led to diesel vehicles being promoted over petrol ones since 1990.