To replace pollutant fuels (petrol and diesel), cleaner alternative fuels are currently being developed. They include: compressed natural gas (CNG); liquefied petroleum gas (LPG); city diesel; hydrogen; alcohol fuels; and battery operated vehicles.
On a cycle representing congested urban traffic, both LPG and CNG have lower emissions of carbon monoxide (CO) than for petrol-powered vehicles. Indeed, emissions of carbon monoxide from CNG-powered vehicles are of the same order as those emitted by diesel vehicles. However, emissions of hydrocarbons from CNG vehicles are relatively high because of methane, the major component of natural gas. Although methane is a small contributor to the formation of low level ozone it is a major factor in global warming. Emissions of nitrogen oxides and particulates from both LPG- and CNG-powered vehicles are significantly lower than those from diesel vehicles. Moreover, emissions of nitrogen oxides from CNG vehicles are half those from equivalent petrol-engined vehicles.
City diesel is a petroleum-based lower emission diesel developed in Sweden but now available in many European Countries including the UK. Exhaust emissions from vehicles fuelled with city diesel compare favourably with exhaust emissions from equivalent vehicles fuelled with conventional diesel. The main benefit of city diesel is that its combustion reduces particulate emissions by 34 - 84% depending on engine type and type of particulate measured. An additional benefit of city diesel is that it is a low sulphur fuel, which is necessary for the optimum running of oxidation catalytic converters.
Whilst pollution from road transport may be reduced by using alternative fuels to petrol and diesel, ultimately it is cheaper to improve conventional fuels than to use many of the alternatives, and no investment is needed for new storage tanks and service stations.