Cut-price supermarket petrol causes higher emissions

Emissions of all regulated pollutants from cars are considerably higher when they are run on petrol without detergent additives, Shell claimed in October in the latest stage of its price war with the supermarkets. The increase in emissions appears to be large enough for regulators to look at the need to mandate the use of detergents.

The major oil companies have been engaging in an increasingly bitter price war with the supermarkets as their petrol sales have declined. Selling at 15 pence per gallon less than the average price at other filling stations, the supermarkets now have a 16% and rising market share.

In April, Shell became the first oil company to play a "green" card in the dispute, warning that cut-throat competition from the supermarket chains was reducing the oil companies' ability to pay for land and groundwater contamination beneath their petrol stations. One-third of its own sites have been found to be contaminated (ENDS Report 219, pp 4-5 ).

Shell has now introduced another environmental issue into the debate. Almost all the petrol sold by supermarkets contains no detergents. They claim that there is no proof that these are effective. But Shell says its own studies have proved them wrong.

Detergents are used to keep components such as carburettors, fuel injectors and inlet valves free of carbon deposits. The advent of the fuel-injected car, which needs precise amounts of air and fuel metered to the engine to operate smoothly, has made the prevention of these deposits more crucial. More than 20 motor manufacturers now recommend the use of petrol with detergent after finding themselves having to pay for engine decarbonisation during the warranty period.

As part of the development work on its detergent package, Shell carried out trials with a cross-section of cars currently on the market. Pairs of each model were fuelled with petrol with and without detergent and run for 10,000 miles. Their emissions during the normal European test cycle were measured at the end of this programme.

The company's message for motorists was that after 10,000 miles the models run with the detergent additive had an average 2-3% better fuel economy and quicker acceleration. But their environmental impact was also considerably better.

According to Dr Cathryn Hickey, Technical Fuels Manager at Shell UK, as well as a 2-3% average reduction in carbon dioxide emissions, the models run with detergent showed on average 12.7% lower emissions of carbon monoxide and 7% lower emissions of hydrocarbons. No difference was found in emissions of nitrogen oxides.

Dr Hickey argues that motorists have a financial incentive to buy petrol with detergent despite its higher price. As well as losing fuel efficiency, cars run on detergent-free petrol will need to have their engines decarbonised after 30,000 miles - or as little as 10,000 miles for models susceptible to carbon formation. She estimates the cost of the operation at £400 plus VAT.

The use of detergents in petrol is not mandatory anywhere in the world at present, but the Californian authorities intend to make it a legal requirement in 1995.

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