Page 45 - Energize July 2021

TECHNICAL

Diesel exhaust exposure is widespread in the modern world Soot
and, consequently, diesel engines are considered one of the largest Perhaps the most controversial of all regulated emissions is the soot
contributors to environmental pollution caused by exhaust emissions, portion of diesel exhaust, which is made up of particles such as carbon,
which have been classified as a potential carcinogen by various organic materials like PAHs, and traces of metallic compounds.
international environmental protection and health agencies like the World PM or soot is created during the incomplete combustion of diesel
Health Organisation (WHO). fuel. Its composition often comprises hundreds of chemical elements,
Diesel fuel is a mixture of hydrocarbons which, during an ideal including sulphates, ammonium, nitrates, elemental carbon, condensed
combustion process, would produce only carbon dioxide (CO₂) and organic compounds and heavy metals such as arsenic, selenium,
water vapor. Unfortunately, exhaust from diesel engines brings a cadmium and zinc. Though just a fraction of the width of a human hair,
complex mixture of soot and gases to our roadways and cities. Health particulate matter varies in size from coarse particulates (less than 10
concerns about diesel exhaust relate not only to cancer, but also to other microns in diameter) to fine particulates (less than 2,5 µm) to ultrafine
health problems such as lung and heart diseases. particulates (less than 0,1 µm).
The four main pollutant emissions generated by diesel engines are Ultrafine particulates, which are small enough to penetrate the cells
carbon monoxide (CO), hydrocarbons (HC), particulate matter (PM) and of the lungs, make up 80 to 95% of diesel soot pollution. When one
nitrogen oxides (NOx). NOx is a general term referring mainly to nitric inhales these microscopic particulates, they can become embedded in
oxide (NO) and nitrogen dioxide (NO₂) gas - a family of poisonous and your lungs and impair their function. As a result of this, diesel PM was
highly reactive compounds. officially classified as carcinogenic by the WHO in 2012.
When broken down, diesel exhaust is made up of two main parts:
gases and PM (otherwise known as soot). Each of these, in turn, is made The emission standards
up of many more different compounds. Emission standards are legal requirements governing air pollutants released
The gas portion of diesel exhaust is mostly CO₂, CO, NOx, into the atmosphere. Emission standards set quantitative limits on the
sulphur dioxides (SO₂), and HCs, including polycyclic aromatic permissible amount of specific air pollutants that may be released from
hydrocarbons (PAHs). specific sources over specific timeframes. They are generally designed to
CO and HCs are generated in the exhaust as the result of achieve air quality standards and to protect human life. Different regions
incomplete combustion of fuel, but exhaust hydrocarbons can also and countries have different standards for engine emissions.
come from the lubricant. In order to conform to these emission standards, engines need to
SO₂ is generated from the sulphur present in diesel fuel so the produce cleaner exhaust emissions by producing less harmful by-products.
concentration of SO₂ in the exhaust gas depends on the sulphur content There are largely four main sets of standards: United States (TIER),
of the fuel. Oxidation of SO₂ produces sulphur trioxide (SO₃) which is the Japanese (CEC - Central Environment Council), India (BHARAT) and
precursor of sulphuric acid which, in turn, is responsible for the sulphate European (EURO) with various markets outside of these regions mostly
particulate emissions and acid rain. using these as their base. The European standards are the most widely-
Out of the various compounds produced, NOx gas and PM are followed vehicle emission guidelines in the world, and as such South
typically portrayed as the two bad boys of diesel exhaust and have Africa has elected to follow this standard - although in a somewhat
proven to be the most challenging of regulated pollutants when it comes laggardly fashion.
to diesel engine design that is compliant with emission standards. Although emissions regulations date back to 1970, the first EU-wide
NOx gases are generated from nitrogen and oxygen under the standard – known as Euro I – wasn’t introduced until 1992. Since then,
high pressures and temperature conditions in the engine cylinders. there has been a series of Euro emissions standards, leading to the
Diesel engines run both hotter and at higher pressures than their petrol current Euro VI, introduced in September 2015.
counterparts and subsequently produce more NOx gases. The aim of Euro emissions standards is to reduce the levels of
Diesel emissions of NOx contribute to the formation of ground level harmful exhaust emissions, primarily NOx, CO, HC, PM and additionally
ozone, which irritates the respiratory system, causing coughing, choking ammonia (NH₃) emissions in the case of Euro VI-compliant engines.
and reduced lung capacity. Ground level ozone pollution, formed when Every evolution of emissions standards leads to more advanced and
nitrogen oxides and hydrocarbon emissions combine in the presence efficient emission control technologies. Table 1 below summarises the
of sunlight, presents a hazard for both healthy adults and individuals progressions of technologies that meet increasingly stringent emissions
suffering from respiratory problems. standards for diesel engines in heavy duty vehicles.

Table 1: Summary of emission standards and technologies
Emission standards Technology
Euro II and Euro III • Emissions are controlled via air and fuel mixing strategies
• No aftertreatment system
Euro IV • NOx control through vanadium-based, open-loop selective catalytic reduction (SCR) system or exhaust gas recirculation (EGR)
• PM control through the use of a diesel oxidation catalyst (DOC) in some engines.
• Aftertreatment system comprising DOC and SCR

Euro V • Same technology as Euro IV with combinations of EGR and SCR
• Small changes to NOx limits which are met with minimum hardware changes – mainly engine calibrations
• Aftertreatment system comprising DOC and SCR

Euro VI • NOx control through zeolite-based, closed-loop SCR systems or a combination of SCR and EGR.
• Ammonia slip catalyst (ASC) for ammonia control as this is a by-product of SCR reactions.
• PM control through DOC and diesel particulate filters (DPF)
• Aftertreatment system is composed of DOC, DPF, SCR and ASC.

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