Tackling Diesel Air Pollutants

On 26/06/2022 / By David Jenkins

As featured in Australian Diesel Mechanic Magazine (ADMM) June 2022

Road transport is a source of air pollutants which can have adverse effects at various scales.

The air pollutants which are currently causing greatest concern in terms of local air quality, primarily because of their impacts on human health, are airborne particulate matter (PM), nitrogen dioxide (NO2) and ground-level ozone. Road transport is an important contributor to all three.

Emissions of nitrogen oxides (NOx) from road vehicles are also implicated in regional phenomena such as acidification, eutrophication and loss of biodiversity, as well as the formation of secondary PM in the atmosphere. Moreover, road transport is a major source of the greenhouse gases carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O).

The significance of road transport as a source of air pollution can be illustrated by reference to sectoral emissions for the EU-27 countries based on submissions to the UNECE Convention on Long-Range Transboundary Air Pollution (CLRTAP). Road transport is a significant contributor to NOx emissions (41 per cent in Europe) and is also a major contributor to PM emissions. In urban areas, its impact is even greater due to the density of the road network, the volume of traffic and the close proximity of the population to the emission source.

Legislation and strategies to reduce exhaust emissions from road vehicles have been in place for some time. Calculations have established that emissions of regulated pollutants from road transport have been reducing as controls on vehicles and fuels have tightened. However, in many urban areas the concentrations of NO2 and PM10 (particulate matter with an aerodynamic diameter of less than 10μm) still frequently exceed health-based limits and are not decreasing.

The importance of NO2 and particulate matter is explained in more detail below.

Nitrogen dioxide

NO2 is an irritant and oxidant which can damage cell membranes and proteins. It has been linked to a range of adverse health effects, including asthma and cancer, but the most consistent association has been found with respiratory outcomes.

NO2 is predominantly a secondary pollutant, its major atmospheric source being the oxidation of NO emitted from combustion sources – notably road-vehicle exhaust. However, some NO2 is emitted directly from vehicles and this is commonly referred to as ‘primary NO2’. Emissions of NOx from vehicle exhaust are regulated at type approval but NO2 emissions per se are not.

Analyses have indicated that a significant proportion of ambient NO2 must be emitted directly from vehicle exhaust and that the direct road-traffic contribution to ambient NO2 has increased in recent years. Two contributing factors have been cited:

  1. Diesel vehicles emit more NOx than petrol vehicles and with a larger proportion of NO2 in NOx.
  2. The average value of NO2 in diesel exhaust is increasing. This appears to be linked to the growth in the use of specific after-treatment technologies in modern diesel vehicles which involve in-situ generation of NO2 such as catalytically regenerative particle filters.

Background concentrations of ozone are also increasing. As the ozone concentration increases, the amount of NO converted to NO2 increases.

Furthermore, it seems likely that real-world NOx emissions from road vehicles are not decreasing as rapidly as models are predicting. While this does not, in itself, affect actual NO2 concentrations, it does suggest that NOx controls have not been sufficiently stringent or that vehicles are not performing as expected.

The overall consequence is that there is now a great deal of interest in the tighter regulation of NOx and NO2 emissions from diesel vehicles and the effects of different after-treatment devices. Direct-injection petrol engines with after-treatment technologies will also have an important impact on NOx emissions in the future.

Particulate matter

Epidemiological studies have shown that concentrations of airborne PM are correlated with hospital admissions and death rates. Initially, the mass concentration of airborne particles with a diameter of less than 10μm (PM10) is identified as a key metric in relation to health outcomes.

However, more recent research has suggested that smaller particles are more important. Attention has focused on particles having a diameter of less than 2.5μm (PM2.5), although there is still a debate as to whether it is actually the mass of even smaller particles, or indeed a non-mass metric such as particle number (PN)7, that is primarily responsible for health effects. In addition to health, airborne particles are responsible for a range of other adverse effects, including nuisance and visibility reduction.

Particles in diesel exhaust have a range of sizes and the shape of the size distribution depends on whether the weighting is by number or by mass. There are three distinct size modes: the nucleation mode (also referred to as nuclei or nanoparticles), the accumulation mode and the coarse mode.

The nucleation mode has traditionally been defined as particles with a diameter of less than 50μm. Accumulation-mode particles range in size from around 50μm to around 1μm, with particles smaller than 0.1μm being referred to as ‘ultrafine’. The nucleation mode contains many more particles than the accumulation mode, although because each particle is so small the total mass is lower. The coarse mode consists of particles larger than around 1μm.

The main implication of the particle-size distribution in vehicle exhaust is that the instruments used in testing need to be sensitive enough to measure particles in the relevant size.

Regulation of exhaust emissions at type approval

The primary tool for combating air pollution from road transport is vehicle emissions legislation. There are currently two main levels of emissions legislation: type approval and periodic in-service/roadworthiness technical inspection.

Emissions tests are normally required for the type approval of all new passenger car (M1, M2) and light-duty vehicles (N1, N2), and for the engines used in heavy-duty vehicles.

Emission limits have been applied to vehicles and engines at the type approval stage since the early 1970s. The exhaust pollutants which are regulated are CO, unburnt hydrocarbons (HC), NOx and PM. The limits have been reduced in stages since they were first introduced through progressive ‘Euro’ standards and changes have been made to the test methods to make them more realistic and effective. Emissions-control technologies have developed accordingly.

For cars and light-duty vehicles, the test procedures and limit values have been consolidated in the Euro 5 and Euro 6 legislation (regulation [EC] no. 692/2008). In the exhaust-emissions test, a production vehicle is placed on a power-absorbing chassis dynamometer. The driver must follow a driving cycle and the vehicle’s emissions are collected and analysed.

Emissions are measured over the New European Driving Cycle (NEDC), which is composed of low-speed ‘urban’ segments and one high-speed ‘extra-urban’ segment. The vehicle exhaust gases are diluted with filtered air to prevent condensation or reaction between the exhaust-gas components.

Dedicated analysers are used for CO, NOx, HC and CO2. For diesel vehicles up to and including Euro 4, PM was collected separately from the other pollutants on a filter. For Euro 5 and Euro 6 vehicles, PM mass and PN are measured using the new particulate-measurement procedure. The PN limit is designed to prevent the possibility of the PM mass limit being met using technologies that would enable a high number of ultrafine particles (less than 0.1μm diameter) to pass.

Periodic technical inspection

The purpose of the periodic inspection emissions test is to allow authorities to check that in-service vehicles are well maintained and conform as far as possible to their design emissions levels.

However, while type-approval tests target the manufacturer, are relatively detailed and require specialist and expensive laboratory equipment, by necessity a lower level of sophistication applies to in-service emission tests. In-service tests target the vehicle owner, are based on shorter, simplified operations of the vehicle, involve the measurement of fewer pollutants (typically CO, HC and diesel smoke) and make use of equipment that is less precise and less expensive than that used in the laboratory.

Periodic inspections are conducted every one or two years, whereas in-service inspections are designed to identify large faults rather than a gradual deterioration in the control of emissions. All types of road vehicles (passenger cars, light-duty vehicles and heavy-duty vehicles) are usually handled using similar procedures. 

Diesel-vehicle test procedure

In the diesel-smoke opacity test, the vehicle is operated through a sequence of so-called ‘free accelerations’ with the engine under no external load, the gear lever in neutral and the clutch engaged. The inspection is done in the following steps:

  • Visual inspection – This is conducted in the same way as the petrol-vehicle inspection.
  • Vehicle preconditioning – Vehicles may be tested without preconditioning, although for safety reasons checks should be made that the engine is warm and in a satisfactory mechanical condition.
  • Test procedure.

– The engine must be at idle before the start of each free acceleration cycle.

– The throttle pedal is fully depressed quickly and continuously (in less than one second) but not violently, so as to obtain maximum delivery from the injection pump.

– During each free-acceleration cycle, the engine shall reach the cut-off speed or the speed specified by the manufacturer before the throttle is released.

On September 18, 2015, the Environmental Protection Agency (USEPA) issued a notice of violation of the Clean Air Act to Volkswagen AG, Audi AG and Volkswagen Group of America Inc. The notice of violation alleged that four-cylinder Volkswagen and Audi diesel cars from model years 2009-2015 included software that circumvented USEPA emission standards for certain air pollutants.

There is now a need for the procedures and instruments to properly measure exhaust emissions during periodic inspections to be updated and implemented in the light of today’s technology and capability of modern measurement devices. In Europe, there is consideration of updating their periodic inspections due to this problem.

In Australia, the reality is we are already consistently several years behind EU standards. The European Directive adopted the Emission Standard Euro 5 in 2011 but Australia is only up to Euro 4.

If Australia is to achieve the Australian Government’s 2030 greenhouse-gas emissions target of 26-28 per cent of 2005 levels and net zero by 2050, then the government needs to implement more stringent standards for noxious air pollutant emissions and a standards regime for fuel efficiency (CO2).

While Australia’s air quality is considered good by international standards, our increasingly urbanised and ageing population may be more susceptible to the health impacts of noxious emissions. This is particularly pertinent when considering that in 2012, 66 per cent of Australians lived in a capital city and that combined population of our four largest cities is projected to increase by around 45 per cent (5.8 million people) to 18.6 million by 2031.

It is imperative the Australian Government strengthens its vehicle-emissions standards and practices for all vehicles. This should encompass the following key points:

  • That regular periodic inspections be mandated for all road vehicles, with attention being paid to NOx, CO2 and PM.
  • That the impending prospect of a 100 per cent imported vehicle market be supported by local type testing of vehicles.
  • High vehicle emissions rates are often a result of component ageing, component failure or generally poor maintenance. PTI emissions tests allow authorities to check that in-service vehicles are well maintained and conform as far as possible to their design emissions levels
  • Improve Australia’s air quality and reduce growing health costs associated with noxious gases.
  • Provide a wide range of health outcomes, including all-cause and cardiovascular mortality, cardiovascular morbidity, asthma and respiratory symptoms, lung function and chronic obstructive pulmonary disease, allergy and birth outcomes.
  • The adoption of Euro 6 standards should be immediate. All of the available technologies are in current use and with the retreat of Australian manufacture, the readily available models of vehicles will be all to a higher technological standard.

Fuel standards are the obvious roadblock to implementation of Euro 6. The implementation of Euro 6 vehicles with substandard fuel would limit the available benefits of lower fuel consumption and less toxic emissions.

In Australia, we take the manufacturer’s and importer’s word that their cars meet the emission standards. Vehicles are tested by the manufacturer in accordance with Australian Design Rules (ADRs), whereas in Europe all manufactured and imported models are tested to ensure emissions standards are met and are tested during their lifetime (depending on model and age) to ensure the car continues to meet the emissions standard.

Euro 6 emissions standards for light vehicles became mandatory in Europe from September 2014 and equivalent standards are currently in force in the US and Japan. The Euro 6 emissions standards for heavy vehicles commenced in the US and Japan in 2010.

It is widely recognised that vehicle-emissions levels are affected by the vehicle condition, service history, fuel quality, vehicle usage and environmental conditions.   As demonstrated by some manufacturers, the emission levels in real-world conditions may differ from the manufacturer’s standardised.

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