The Diesel Debate

Added: 13 February 2018

Getting back to basics, vehicles emit three things: NOx (NO & NO2), PM2.5, and CO2. These emissions have very different impacts on human health and the environment.

  • CO2 impacts global warming
  • NOx gives concern to local air quality
  • PM2.5 causes the same local air quality concerns as NOx

The EU places restrictions on the emissions of these pollutants, and the UK is currently only in breach of its NO2 obligations. Limits of this pollutant are being exceeded around UK roads, which is why Government is focusing on road emissions.

CO2 emissions

Higher numbers of vehicles on UK roads and a change in consumer preference for larger cars have affected the UK's progress in reducing vehicle CO2 emissions. 

Previously, average CO2 emission levels from new cars were decreasing by about 3.5% a year. But due to the increasing preference for larger vehicles, new car average CO2 emission levels decreased by just 1.1% in 2016. Also, there has been an increase in miles driven, meaning CO2 emissions have been rising for the last two years.

Diesel cars are more efficient than petrol, using less fuel and producing an average less 20% CO2 than their equivalents. Because of their efficiency, larger vehicles are mostly diesel and this has lessened the CO2 impact of motorists moving to larger cars.

The decline of the diesel market share in 2017 meant that average new car CO2 emissions increased for the first time in two decades.

NOx emissions

NOx levels are at their highest at the roadside and this is where the UK is in legal breach.

While the nature of diesel engines means that is has been difficult to lower NOx emission levels, new technologcial advances are changing this.

Specifically, selective catalytic reduction (SCR) systems have been a big step forward. They use an external reducing agent, known in Europe as AdBlue. It turns NOx into nitrogen.

These converters can achieve NOx conversion rates of 80-95% making diesel vehicles very clean and still efficient.

National Government Response to Air Pollution Problem

The Government is focused on reducing roadside NOx levels to meet their legal obligations. They have announced that Clean Air Zones (CAZs) and diesel taxation will be the main tools to achieve compliance with emission limits.

Clean Air Zones

CAZs can either be 'charging' or 'non-charging' zones. Charging CAZs have different classes of severity. Only under the strictest charging CAZ category will cars be included. London's Ultra Low Emission Zone (ULEZ) follows the same format as the strictest charging CAZ.

If a local authority decides to implement a charging CAZ of the strictest class, then cars that, based on their EU standards, do not meet entry requirements, will face a charge to enter the zone. In London, this has been set at £12.50 per day.

Car entry standards for a charging CAZ:

  • Petrol cars must be Euro 4 or above - 2006 or newer
  • Diesel cars must be Euro 6 - 2015 or newer

Local Government Response

Whilst Government is leading national action against air pollution, some local authorities are taking additional measures, chief of which are:

  • Zero Emissions Zones - a ZEZ requires that cars which drive into them emit no exhaust emissions while they are in them or face a steep fine. This would effectively limit cars in a ZEZ to being electric vehicles, plug-in hybrids in electric mode, or hydrogen vehicles. ZEZs aren't yet a major trend but a possble sign of things to come. Oxford City Council is looking to put a ZEZ in place from 2020 in their city centre. In London, a ZEZ in the heart of the city has been suggested for 2025.
  • Diesel Parking Charges - a number of local authorities are in the process of implementing higher parking charges for deisel cars. For some authorities, these charges only apply to pre-Euro 6 (pre 2015) diesels but for others they apply for diesels of any age. Some authorities are considering implementing charges are Islington (£2-an-hour surcharge for all diesel drivers using parking bays from January 2018), and Westminster which is trialling a £2.45-an-hour levy for pre-2015 diesels.

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