Diesel cars not as green as perceived

Diesel cars from 2010 and later only emit 1.5 per cent less CO₂ than petrol cars from the same year. Photo: © donfiore - Fotolia.com

In Europe, diesel cars have been promoted based on the assumption that they emit less greenhouse gases (GHG) than petrol-fuelled cars and many EU member states have reduced taxes on diesel fuel and diesel car sales. This led to a boom in the diesel car market between 1995 and 2009, resulting in an additional 47 million more diesel vehicles on the road since the mid-1990s. These cars have the potential to have long-term impacts on emissions, as each car is likely to last approximately 16 years.

A recent study has reviewed the evidence for the claim that diesel cars are better for the environment than petrol cars.

Laboratory studies suggest that diesel cars are 35 per cent more efficient than petrol cars. However, diesel fuel contains about 14 per cent more carbon per litre. When improvements are measured in grams of CO₂ emitted per kilometre (gCO₂/km), i.e. considering the emission intensity, diesel is only 15 per cent more CO₂ efficient.

Furthermore, when the actual car fleet is examined, the advantages of diesel cars reduce even more. Until 2005, diesel cars emitted 5–10 per cent less CO₂ than petrol cars. However, designs of petrol cars have improved and by 2010 diesel cars emitted only 1.5 per cent less CO₂. The researchers note that a trend for greater size and power of diesel cars may be partly responsible for the comparative reduced efficiency of the diesel fleet. However, they argue that this increase in size and power is itself partly due to excessive diesel fuel subsidies.

In Europe, CO₂ emissions from newly registered cars dropped from over 180 gCO₂/km in 1995 to 140–150 gCO₂/km in 2009. In contrast, emissions from the Japanese car fleet reduced from very similar levels in 1995 to 120–130 gCO₂/km in 2009. In Japan, diesel cars have been phased out and efforts have been made to promote hybrid petrol-electric cars.

The researchers also point to the fact that diesel cars emit more black carbon, which also contributes to global warming, and estimate that for diesel cars produced between 1995 and 2003, the negative effects on the climate of black carbon outweigh CO₂ savings. That said, post-2003 cars are often fitted with particle filters and, for these vehicles, the diesel car retains a slight advantage of 4 gCO₂/km, based on laboratory tests.

In addition to climate change effects, diesel cars emit high levels of nitrogen oxides (NOx), causing damage to human health and contributing to ecosystem damage through eutrophication, acidification and ground-level ozone. It is concluded that NOx emissions from current on-road diesel cars are 10 to 100 times higher than those from petrol cars, if petrol-fuelled hybrid cars are considered. This fact was ignored when setting up economic policy on diesel fuel and diesel car sales.

The researchers conclude that, while the move away from petrol cars is essential to tackle global warming, replacing them with diesel-fuelled cars is not the solution. They suggest that the Japanese approach of producing relatively affordable hybrids has been much more successful in reducing GHG emissions from the transport sector.

Source: Science for Environment Policy, 5 December 2013
Study: Critical evaluation of the European diesel car boom – global comparison, environmental effects and various national strategies (2013). By M. Cames & E. Helmers. Published in Environmental Sciences Europe. www.enveurope.com/content/25/1/15