How modern diesel engines are helping reduce motoring’s impact on local air quality

A combination of breakthrough emission control technologies have lowered NOx and particulates, making modern diesel a responsible choice for an urban driver.


What do you need to know about urban air quality?

According to the European Environment Agency (EEA), air pollution is the number one environmental cause of premature death in the European Union. Sources of urban air pollution include industrial emissions, heating and road transport such as cars and trucks.

Air quality is assessed by measuring a number of pollutants, such as ozone, nitrogen dioxide (NO2) and particulate matter (PM), whose values should not exceed certain benchmarks. While carbon dioxide (CO2) emissions play a role in climate change, they are not responsible for damaging local air quality.

In Europe, Euro 6 standards define the maximum amount of those pollutants that vehicles may emit. In 2017, so-called Real Driving Emissions (RDE) tests were introduced to ensure that cars deliver low emissions when driven in the real world, rather than just in a laboratory setting.


What impact do cars have on local air quality?

Air quality is a serious issue and the transport sector has made significant improvements to reduce harmful emissions. Efforts are underway across the European Union to improve the efficiency of engines to move to zero-emission vehicles in the long term.

If all urban motorists today drove new RDE-compliant Euro 6 diesel cars, then our cities would see a substantial reduction in NOx air pollution.

In the near future, combustion-engine cars are unlikely to disappear from European roads. To minimise their impact on air quality, improvements are constantly being made to engines in modern vehicles.


Diesel pump - air quality

Modern diesel engines combine great fuel economy with near-zero pollutant emissions

Despite negative public perception, diesel engine technology has evolved dramatically in recent years to limit air pollutants and reduce CO2 emissions.

In fact, modern and cleaner diesel cars combine great fuel economy with near-zero emissions of ultrafine particles and nitrogen oxides, making them one of the most cost-effective options for reducing transport emissions in Europe.

Here are some of the innovations in diesel engine technology that actively combat air quality problems:

  • Diesel Particulate Filters (DPFs) remove 99.9% of particles coming from the engine, including ultrafine particles. Ceramic wall flow filters remove nearly all carbon particulates, including fine particles less than 100 nanometres (nm) in diameter. Since the Euro 5b exhaust emissions legislation was introduced in 2011, DPFs are effectively mandatory.
  • DeNOx exhaust aftertreatment systems such as Selective Catalytic Reduction (SCR) and NOx traps further reduce and control tailpipe NOx emissions of diesel cars. In the SCR system, ammonia is used to convert over 70% (up to 95%) of NO and NO2 into nitrogen over a special catalyst system. AdBlue®, for example is a urea solution which is carefully injected from a separate tank into a diesel car’s exhaust system where it hydrolyses into ammonia ahead of the SCR catalyst. A growing number of diesel cars registered after September 2015 (predominantly Euro 6-compliant vehicles) are equipped with this technology.
  • Oxidation catalysts remain a key technology for diesel engines and convert carbon monoxide (CO) and hydrocarbons (HC) into CO2 and water.


How do we measure the real impact of motoring?

Since 1992, successive Euro emission standards have set stricter limits on the amount of pollutant emissions a vehicle can emit, such as PM, NOx, unburnt hydrocarbons and CO.

A European diesel passenger vehicle was permitted to emit 140 milligrams of PM per kilometre in 1992, but this number dropped to just 4.5 milligrams in 2014. In addition, NOx and unburnt hydrocarbon emission limits have seen reductions of more than 90% over the same period.


City motorway - air quality

Since 1992, diesel passenger vehicles have seen a 96% drop in particulate mass output per kilometre

Measuring the real impact of motoring is dependent on the study’s location. For years, car emissions were measured only in a laboratory setting that aimed to simulate road conditions. These tests came under fire for exhibiting loopholes that were exploited in the now infamous ‘Dieselgate’ scandal.

In 2017, the European Union introduced two new complementary tests to measure air pollutants emitted by cars in a variety of real-world driving conditions to avoid loopholes and measurement discrepancies.


What emissions innovations can we expect next?

European vehicle manufacturers and their suppliers lead the world in the development of state-of-the-art diesel engine technology. New diesel engines not only emit fewer pollutants that damage air quality, but also produce lower levels of CO2.

Modern, cleaner diesel cars combine great fuel economy with near-zero emissions of ultrafine particles and nitrogen oxides, making them one of the most cost-effective options for reducing transport emissions in Europe.

In the coming years, partial-to-full hybridisation of the diesel powertrain (e.g. with 48V technology or plug-in hybrid) in combination with advanced emissions control technologies will allow further reduction of both harmful pollutants and CO2 emissions.

However, European drivers can already make a difference to local air quality in towns and cities. If all urban motorists today drove existing new RDE-compliant Euro 6 diesel cars (registered from September 2017), then our cities would see a substantial reduction in NOx air pollution.

NOx and Particle Number (PN) emissions of RDE-compliant cars tested on the road are publicly available. NOx and PN emissions from the latest diesel cars are well within the Not-To-Exceed emissions area as shown below.


Diesel emissions in not-to-exceed area

The latest diesel cars have NOx and PN emissions that are well within the Not-To-Exceed emissions area

The ADAC in Germany provides a list of RDE-compliant cars already available on the market.

Armed with this data and with further technological updates in the pipeline, drivers are well placed to make choices that will impact positively on the quality of air in their towns and cities.