Any discussion on possible alternatives to fuel ethers needs to take into account their potential air quality impact as well. The potential of a petrol component to impact ozone is highly related to both its boiling point temperature and its ability to generate ozone in the atmosphere once the VOC (volatile organic compound) is emitted into the atmosphere. The boiling temperature is also important since components that boil above 100°C do not fully vaporise during the engine warm-up period, which then leads to increased VOC out of the exhaust.
Of all the alternatives, fuel ethers generate the least amount of ozone in the atmosphere, as is illustrated in the chart below. Alkylates and isomerates branched hydrocarbons have low ozone yields compared to aromatics but, like ethanol, will still generate at least twice as much ozone as fuel ethers in the atmosphere. A good portion of the alkylates also boils above 100°C, which will contribute to increased exhaust VOC before the catalytic converter warms up.
Fuel ethers thus appear the most effective from both octane supply and air quality perspectives. They also have the added advantage of energy diversity, since alkylate production is dependent on refinery feedstocks derived from crude oil. Fuel ethers can also be made outside the refinery from a number of feedstocks, including those derived from natural gas, which can thereby reduce dependency on crude oil for the production of transportation fuels.
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