Air Quality & Health Effects
Engine and vehicle emissions
If we could burn petrol or diesel perfectly in pure oxygen it would produce only carbon dioxide (CO2), water vapour, and energy. However in reality there are always some emissions of unburned and partially burned fuel, giving carbon monoxide (CO), hydrocarbon (HC) and - especially for diesel engines - particulate matter (PM), plus nitrogen oxides (NOx) formed from nitrogen present in the air. There is more detail on the engine and vehicle emissions page.
Motor vehicles have played a major role in urban air quality problems and consequent health effects due to these emissions. Although kerbside, 'street canyon' (streets surrounded by high buildings) or local emissions are of particular concern because the concentration of pollutants is likely to be highest in these situations, effects can also occur away from city or town centres as the pollutants react with each other and are distributed by air movement.
Health risks from vehicle pollution
Nitrogen oxides (NOx) react with hydrocarbons in sunlight to form harmful ozone and photochemical smog. NOx can increase respiratory illnesses and is a contributor to acid rain. Ozone causes breathing difficulties and damages plants.
Particulate matter (PM) is mainly soot particles with volatile hydrocarbons and some sulfate and metallic residues from the fuel and engine lubricant. Particles are found in the air in a range of sizes. Diesel engines are responsible for the majority of ultra-fine particulates (less than one micron in diameter or PM1). Particulates emissions from modern Direct Injection petrol engines are also characterized by a higher number of smaller particles than traditional port fuel injection systems. These small particles (mostly below 100 nanometers diameter) are present in large numbers in untreated exhaust, but amount to only a tiny fraction of the weight of particulate matter. There is evidence that fine and ultra-fine particles are linked to increased rates of premature death for causes such as cardiovascular and lung disease. Also, WHO IARC has classified untreated Diesel exhaust as carcinogenic to humans. In addition, Diesel PM is primarily made of Black Carbon which is a short-lived climate forcer with a high Global Warming potential.
Carbon dioxide (CO2) is the final product of all combustion processes and the major contributor to the 'greenhouse' effect. Catalysts do not increase overall CO2 emissions from cars because all the carbon burnt in the engine eventually ends up as CO2, so CO2 emissions can only be limited by reducing the amount of fuel used. Use of particulate filters or NOx traps gives a small (typically 1 to 2%) increase in CO2 because a small amount of extra fuel is used to regenerate them from time to time, but Selective Catalytic Reduction (SCR) can reduce fuel consumption and hence CO2 by up to 5% by allowing engine developers to use more fuel-efficient conditions instead of trading fuel consumption for a reduction in combustion NOx emissions.
Lead was, in the past, added to petrol to boost the octane number. Health concerns focussed on the effect that low levels of ambient lead can have on the educational and behavioural development of children. Lead poisons catalytic converters and since 2000, sales of leaded petrol have been banned in the European Union. For the remaining non-catalyst engines that rely on lead to prevent valve recession, other additives have been introduced.
Controlling pollution from internal combustion engines
Autocatalysts, traps and filters are used on new vehicles as part of an integrated approach to emissions control which includes the combustion system, fuel and reductant quality and electronic control systems. Autocatalysts, traps and filters are extremely durable and effective means of controlling emissions from engines and vehicles to ensure that modern vehicles contribute to improving air quality and are able to meet legislative limits on emissions. They form the most effective technologies in an engine or vehicle manufacturer's armoury of tools to control emissions, with efficiencies of over 95%. The technologies are used on all types of engine and vehicle - cars, trucks, buses, motorcycles, construction equipment, boats and railway engines.
Some of these technologies such as particulate filters can also be 'retrofitted' to older vehicles to improve their emissions performance. Many European Member States have incentive scheme for retrofitting, especially for trucks and buses. More information on retrofit can be found here.