Diesel is a mixture of hydrocarbons distilled from petroleum and is used as fuel in a particular type of engine invented by German engineer Rudolf Diesel. When compressed to a very high pressure, diesel automatically ignites and burns, releasing energy to power the vehicle. This is in contrast to a petrol engine, which requires a spark plug to ignite the fuel. The diesel engine is more efficient than the normal petrol engine, and when combined with the high energy content of diesel fuel, this means that a diesel vehicle can achieve up to 40% better mileage per gallon in comparison to a petrol car. This increased fuel economy also results in reduced CO2 emissions. However, a diesel engine does produce more toxic emissions, in particular, oxides of nitrogen (NOx) and particulate matter (PM10).

Diesel has a Cetane Number (CN) rather than the Research Octane Number (RON) that petrol has. It can normally be found in two CN ranges: 40-46 for regular diesel, and 45-50 for premium. Diesel engines tend to run best with a CN between 45 to 50; however, there is no performance or emission advantage when the CN is raised past 50.

Diesel is high in sulphur; this acts as a lubricant for the engine, but is harmful to the environment and can prevent filters from controlling diesel particulate emissions. There are number of different types of diesel sold at the pumps; these include standard diesel, ultra low sulphur diesel and zero sulphur diesel.

Standard Diesel

Standard diesel contains up to 350 ppm sulphur, which leads to significantly higher exhaust emissions than those seen with Ultra Low Sulphur Diesel (ULSD). In January 2005, the maximum legal permitted level of sulphur allowed in diesel in the UK was lowered to just 50 ppm, which had the effect of prohibiting the sale of standard diesel from that date.

Ultra Low Sulphur Diesel (ULSD)

ULSD is now the normal diesel fuel sold in the UK, and contains less than 50 parts per million of sulphur. ULSD gives good rate of miles per gallon, low CO2 emissions, but fairly high emissions of particulate matter and oxides of nitrogen.

Zero Sulphur Diesel

In 2005, zero sulphur diesel was introduced, containing less than 10 ppm of sulphur. This low level of sulphur allows the use of more effective exhaust after-treatment, such as exhaust catalysts and particulate traps. Not only does this help to reduce the emissions of pollutants such as oxides of nitrogen and particles, but it also improves fuel efficiency.

Diesel cars:

  • release less carbon dioxide than petrol powered cars.
  • have a 30-40% improved fuel efficiency than petrol powered vehicles.
  • last longer, which reduces the need to repair or purchase a new car.
  • are fuelled from non-renewable energy sources, which uses up the worlds natural resources.
  • release more volatile organic compounds and oxides of nitrogen contributing to ozone smog.