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Motor Vehicle Emission Controls: Technologies

Introduction

The number of vehicles on Britain's roads is continually increasing - between 1970 and 1995 the number of vehicles almost doubled. It now stands at 29 million. With a large rise in traffic numbers, it becomes increasingly important to keep pollutant emissions to a minimum. There are presently a number of ways in which road traffic pollution can be reduced, including the use of technological solutions.

Catalytic Converters

Since January 1993, all new cars sold in the European Union (EU) have been fitted with a catalytic converter (CAT). This is made up of a very thin layer of platinum group metals on a honeycomb structure. The surface area of a typical 3-way CAT covers the equivalent of two football pitches. As exhaust gasses pass through the catalyst a chemical reaction occurs which converts carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx) to less harmful compounds (water, nitrogen and carbon dioxide).

Schematic of a Catalytic Converter


To work most effectively, a catalytic converter needs to reach an optimum temperature. It may not reach this in a short journey. Devises to pre-warm the catalyst are being developed which improve the overall performance of catalytic converters.

The use of catalytic converters leads to a dramatic reduction in the emissions of CO, HC and NOx. However, they also result in an increase in CO2 emissions, which do not cause a problem for urban air quality, but may contribute to global warming. The efficiency of a CAT can be as high as 90%.

Oxidation Catalysts

Oxidation catalysts may be fitted to either petrol or diesel cars. The catalyst oxidises the pollutants formed by incomplete combustion to carbon dioxide and water, and is effective for hydrocarbons, aldehydes and carbon monoxide. However, they do not reduce NOx emissions.

Carbon Canisters

Carbon canisters are being fitted to petrol tanks to reduce evaporative emissions of petrol (petroleum). Vapour rises into the carbon canister, which then returns the petrol to the tank, hence avoiding a loss due to evaporation.

Lean Burn Engines

Lean burn engines are more fuel-efficient than other engines and produce less NOx and CO, but they do emit higher levels of hydrocarbons. At the present time they are still under development and have not yet been able to meet EU emission standards, especially at high speeds.

Exhaust Gas Recirculation

This involves returning exhaust air to the fuel inlet, which results in a reduction in peak engine temperatures and emissions of NOx gasses from petrol vehicles. Levels of CO and HC are also reduced.

Cleaner Cars

Smaller, lighter cars use less fuel and hence produce less pollution. Technological development using lighter materials for construction may therefore reduce emissions.

Electric Transport

The above technologies all provide a reduction in emissions from vehicles. Electric transport is an alternative development that could lead to a large reduction in pollution at ground level, if it became more wide spread. Electric transport produces no emissions at the point of use, although pollution is emitted during the production of electricity. The amount of pollution created is lower than that from individual vehicles, and the emissions occur away from city centres where the public live and work. Electric vehicles also produce less noise pollution than diesel and petrol vehicles.

The main drawback for electric vehicles is the need to recharge batteries. The maximum range of these vehicles is currently around 50 - 100 miles. However, they can reach maximum speeds of 60 - 90 miles per hour and have good acceleration. They have lower fuel and maintenance costs than petrol and diesel but involve a higher capital investment.

Conclusion

Whilst technical fixes, such as those outlined above, may provide cleaner air for the next 10 - 15 years, they do not represent a long term solution to transport related urban air pollution. They need to be combined with management schemes to reduce traffic in city centres, education to encourage the public to use their cars less, and the further development of alternative fuels that are not harmful to the environment.