Solutions to climate change and air pollution from vehicles: diesel cars and NOx emissions

Key facts

  • G-20 account for 90 percent of global vehicle sales, and 17 out of 20 member countries follow the European regulatory pathway for vehicle emissions control [1];
  • In 2016 registrations of new diesel cars in the UK made up 49% market share while overall car sales were at a record high. This market share is largely reflected across Europe [2];
  • Elsewhere in the world, diesel cars are much less prevalent. For example, less than one in 20 cars in the United States of America run on diesel [3];
  • Many diesel car models breach emission standards for NOx by massive amounts, despite passing emission tests conducted in a laboratory on a rolling road. It has been estimated that the tougher Real-driving Emission testing (RDE) required for cars sold in Europe from September 2017 will reduce NOx emissions; but only from an average of seven times the legal limit to four times the limit [4];
  • NOx emissions from diesel cars are more than double those of diesel lorries (heavy goods vehicles) per kilometre [5].

Introduction

Fossil-fuel powered road transport is a major cause of pollution; see Brake’s fact page on climate change and air pollution [6]. One way governments are tackling the problem is through increasingly stringent exhaust emission standards for new vehicles. In recent years, many diesel cars have been revealed as having NOx emissions that exceed, or massively exceed, their current exhaust emission standard. This fact sheet explains why, and what is being done about it.

What are the European NOx emission standards and testing procedures for cars?

Vehicles sold in the UK must currently meet emission standards set in Europe. European emission standards affect other countries too. Countries that are members of G-20 account for 90 percent of global vehicle sales, and 17 out of 20 member countries follow the European regulatory pathway for vehicle emissions control [7].

New models of cars and vans sold in the UK and Europe must have met the EU’s “Euro 6” emission standard, which includes limits on NOx emissions (a greenhouse gas that is also a major cause of respiratory disease). The single biggest contributor to health-impairing NOx pollution is road transport [8].

To comply with Euro 6, a diesel car’s allowable NOx emissions must be six times less (and its particulate emissions ten times less) than the standard required in the year 2000 (when the Euro 3 standard was in place) [9].

The NOx emissions standard for diesel cars is almost the same as that required for petrol cars [10].

Currently, new car models must demonstrate they meet Euro 6 by passing a test on a “rolling road” (a fake road set up in a laboratory).

What are the real levels of NOx released by diesel cars ‘on the road’?

The hoped-for reductions in NOx emissions due to increasingly stringent Euro standards has not been achieved. It has emerged, through independent emission testing, that many diesel car models that have passed the Euro 6 rolling road test are emitting wildly and illegally higher amounts of NOx when actually used on the road [11].

In one set of tests, diesel car models that had passed Euro 6 were found, on average, to have NOx emissions seven times higher than the Euro 6 limit, and up to 25 times higher than the limit [12]. Volkswagen has particularly repeatedly hit the headlines as a poor performer, but other manufacturers have performed badly too.

It has emerged that manufacturers have built diesel cars with technology that enables them to pass the test, but then emit more NOx when on the road. It is only possible to guess at reasons why manufacturers have done this, but academics have speculated that it is probably for reasons of fuel efficiency, or to increase performance, or to reduce maintenance required on a car (for example, how frequently an engine’s ‘diesel exhaust fluid’ needs topping up). [13]

Testing by the International Council on Clean Transportation (ICCT) has concluded that this means that while NOx emission limits for diesel cars in the EU were lowered by 85% between 2000 (Euro 3) and 2014 (Euro 6), on-road emission levels, on average per diesel car, have decreased only about 40% [14].

Are there lots of diesel cars in the UK and Europe?

The NOx diesel problem in Europe and the UK has been massively exacerbated by rapid growth in numbers, and market share, of diesel cars, which has risen steadily since the early 1990s. Between 1990 and 2006, the European market share of diesel cars rose from one in seven new cars to one in two new cars. Since 2006, the market share has remained about half [15]. 

In the UK, about half new cars registered have been diesel since 2010 [16], with 2016 registrations of new diesel cars at 49% market share, and overall car sales at a record high [17].

This rise is for a number of reasons but partly due to government taxation regimes that have taxed petrol more heavily than diesel because diesel can be more fuel efficient (be used slower) and produce less CO2. In the UK the same amount of tax currently applies, per litre, to both diesel and petrol (just under 58p tax per litre) [18]; but because diesel vehicles are generally more fuel efficient compared with petrol, it is generally still cheaper to use diesel.

The high market share of diesel cars in Europe is not reflected worldwide. For example, less than one in 20 cars in the USA run on diesel [19].

What’s happening to improve EU exhaust emission testing of diesel cars?

Because the Euro rolling road testing is not working in practice, a ‘real driving emissions test procedure’ (known as RDE), that more closely represents how a vehicle will perform, is being introduced by the EU. It tests vehicle emission performance on the road using a portable device. The RDE was proposed by the EU in 2012, but will be implemented from September 2017.  

Research by ICCT concludes the new test will help, but not fix, the diesel NOx problem. ICCT experts estimate RDE will initially reduce NOx emissions from new diesel vehicles significantly but nowhere near the limit. It estimates the RDE will reduce NOx emissions per vehicle from seven times the Euro 6 limit on average, to four times the limit. ICCT says further improvements to the RDE test, enabling better testing of vehicles’ NOx emissions in a wider range of driving conditions, should be implemented as fast as possible to require manufacturers to produce vehicles that can really meet the Euro 6 limit. [20] 

Is it possible for manufacturers to meet the Euro 6 standard through use of existing technology?

It is a ‘challenging but not insurmountable problem’ for manufacturers to make diesel cars that meet the Euro 6 limits [21]. It is harder to limit NOx emissions from a diesel engine compared with a petrol engine because of the higher temperatures in diesel engines, causing release of NOx in higher amounts. NOx levels have to be combatted through:

  1. Engine modifications to try to reduce the engine temperature. Exhaust gas recirculation recirculating a portion of an engine's exhaust gas back to the engine cylinders and, through a chemical process, reduce temperatures in the engine’s cylinders, consequently reducing NOx emissions;
  2. After-market systems known as NOx “control technologies”, already in existence and fitted by manufacturers, most commonly [22]:
    1. Lean NOx traps (LNTs): NOx is adsorbed onto a catalyst during lean engine operation (when there is a higher ratio of air to fuel). NOX is then catalytically reduced in short periods of fuel-rich operation of the vehicle;
    2. Selectic Catalytic Reduction (SCR): A catalyst reduces NOx to different chemicals; gaseous nitrogen and water in the presence of ammonia. Usually, this is achieved through a urea solution (sometimes known as diesel exhaust fluid) that has to be kept topped up.

LNTs and SCR technology is not overly expensive for manufacturers to fit (a few hundred pounds) [23]. The ICCT says NOx control technologies can work, and “implementation of NOx control technologies by a few manufacturers is delivering acceptable results”. Some diesel cars fitted with NOx control technologies performed fairly well in tests by ICCT, but others, also fitted with the technologies, performed very poorly [24].

ICCT says: “This casts a shadow of doubt over the real-world performance of all current (pre-RDE) NOX control approaches, especially those relying on LNTs, and underscores the importance of engine and after-treatment calibration to realise the full potential of available technologies and achieve satisfactory real-world performance.” [25]

Will this problem go away because manufacturers switch to using other fuels?  

The large market share of diesel cars in Europe and the UK means diesel cars will not disappear overnight. However, one study claims the market share will have shrunk back from half to under one in ten new vehicles by 2030 [26].

Electric and hybrid cars (normally a combination of a petrol or diesel engine and an electric motor) are beginning to increase their (very small) market share thanks to lower manufacturing costs, greater availability of models, building of charging stations and government incentives.

Governments, including the UK government, are also likely to start introducing policies to dissuade people from owning diesel cars, particularly older diesel cars with the highest NOx emissions. Policies could include higher parking charges, charges to enter a ‘low emission zone’, higher taxation of diesel for cars, and a financial incentive to scrap older diesel cars.

What about diesel-fuelled heavy goods vehicles (lorries) and buses?

Most diesel vehicles are cars. Worldwide, there are about five diesel cars sold for every diesel goods vehicle [27]. However, nearly all new lorries are fuelled by diesel, rather than petrol or other fuels.

The equivalent test to Euro 6, for lorries, is, similarly, called Euro VI. However, ICCT says that, overall, lorries are meeting Euro VI standards for NOx emissions on the road. ICCT also says that despite lower fuel consumption compared with lorries, NOx emissions from diesel cars are “more than double those of diesel lorries on a per kilometre basis”.  However, lorries are higher emitters of CO2 (on average five times the amount emitted by a car) due to their lower fuel efficiency. [28]


End notes

[1] Briefing paper: A technical summary of Euro 6/VI vehicle emission standards, ICCT, 2016
[2] Car Registrations: UK new car market achieves record growth of 2.69 million registrations in 2016 with fifth year of growth, SMMT, 2017
[3] ] BTS Fact Sheets: Diesel-powered passenger cars and light trucks, Bureau of Transportation Statistics, 2015 
[4] Fact sheet Europe: Impact of improved regulation of real-world NOx emissions from diesel passenger cars in the EU, 2015-2030, ICCT, 2016
[5] NOx emissions from heavy-duty and light-duty diesel vehicles in the EU: Comparison of real-world performance and current type approval requirements, ICCT, 2016  
[6] Climate change, air pollution and the contribution of fossil-fuelled motorised transport, Brake, 2017
[7] Briefing paper: A technical summary of Euro 6/VI vehicle emission standards, ICCT, 2016
[8] Transport Emissions: Air pollutants from road transport, European Commission, 2016
[9] Briefing paper: A technical summary of Euro 6/VI vehicle emission standards, ICCT, 2016
[10] Transport Emissions: Air pollutants from road transport, European Commission, 2016
[11] Franco, V et al., Real World Exhaust Emissions from Diesel Cars: A meta-analysis of PEMS emissions data from EU (Euro 6) and US (Tier 2 BIN 5/ULEV II) diesel passenger cars, ICCT, 2014
[12] Ibid
[13] Mackenzie, D. Stated in a webinar for the ‘MIT alumni association’, featuring Anup Bandivadekar the Passenger Vehicles Program Director at the International Council on Clean Transportation (ICCT), 20 November 2015
[14] Fact sheet Europe: Real world emissions from diesel cars, ICCT, 2014
[15] Share of Diesel in New Passenger Cars, European Automobile Manufacturers Organisation, 2017
[16] Vehicle Licencing Statistics: 2013, Department for Transport, 2014
[17] Car Registrations: UK new car market achieves record growth of 2.69 million registrations in 2016 with fifth year of growth, SMMT, 2017
[18] Tax on Shopping and Services: Fuel duty, Gov.uk, 2017
[19] BTS Fact Sheets: Diesel-powered passenger cars and light trucks, Bureau of Transportation Statistics, 2015 
[20] Fact sheet Europe: Impact of improved regulation of real-world NOx emissions from diesel passenger cars in the EU, 2015-2030, ICCT, 2016
[21] Mackenzie, D. Stated in a webinar for the ‘MIT alumni association’, featuring Anup Bandivadekar the Passenger Vehicles Program Director at the International Council on Clean Transportation (ICCT), 20 November 2015
[22] Franco, V et al., Real World Exhaust Emissions from Diesel Cars: A meta-analysis of PEMS emissions data from EU (Euro 6) and US (Tier 2 BIN 5/ULEV II) diesel passenger cars, ICCT, 2014
[23] Estimated cost of emission reduction technologies for light-duty vehicles, ICCT, 2012
[24] Franco, V et al., NOx Control Technologies for Euro 6 Diesel Passenger Cars: Market penetration and experimental performance assessment, ICCT, 2015
[25] Ibid
[26] A Watershed Moment for the Automotive Industry: The AlixPartners Global Automotive Outlook 2016, AlixPartners, 2016
[27] NOx emissions from heavy-duty and light-duty diesel vehicles in the EU: Comparison of real-world performance and current type approval requirements, ICCT, 2016  
[28] Ibid 

Date posted: March 2017