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What are exhaust emissions?

Diesel engines, like other internal combustion engines, convert chemical energy contained in the fuel into mechanical power. Diesel fuel is a mixture of hydrocarbons which—during an ideal combustion process—would produce only carbon dioxide (CO₂) and water vapour (H2O).

The concentrations depend on the engine load, with the content of CO2 and H2O increasing and that of O2 decreasing with increasing engine load. None of these principal diesel emissions (with the exception of CO2 for its greenhouse gas properties) have adverse health or environmental effects.

Diesel emissions on older equipment however do include other pollutants that can have adverse health and/or environmental effects. Most of these pollutants originate from various non-ideal processes during combustion, such as incomplete combustion of fuel, reactions between mixture components under high temperature and pressure, combustion of engine lubricating oil and oil additives as well as combustion of non-hydrocarbon components of diesel fuel, such as sulphur compounds and fuel additives. Common pollutants include unburnt hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOₓ) or particulate matter (PM). The total concentration of pollutants in diesel exhaust gases typically amounts to some tenths of one percent—this is schematically illustrated in Figure 1. Much lower, “near-zero” levels of pollutants are emitted from modern diesel engines equipped with emission after treatment devices such as NOₓ and CO reduction catalysts and particulate filters.

Emissions can be categorized into a particulate and gas/vapour phase, where each contains both organic and inorganic components. The particulate phase of diesel exhaust includes clusters of respirable particles composed mainly of carbon and is termed “diesel particulate matter” (DPM). A variety of chemicals are contained within or absorbed onto the diesel particulate matter which has the potential to affect the toxicity of the particulate.

The major constituents of the gas/vapour phase include carbon monoxide (CO), oxides of nitrogen (NOₓ), sulphur dioxide (SO₂), aldehydes and many polycyclic aromatic hydrocarbons (PAHs).

Exhaust emissions does not only pollute the air we breathe, but also changes the characteristics of landscapes through soil contamination and water pollution. The effect of non-compliance with health standards and recommendations could also impact the health of humans far outside the demography of such pollutant producing mechanisms.

What is Carbon Monoxide Poisoning?

Carbon monoxide poisoning is the most common type of fatal air poisoning in many countries. Carbon monoxide is colorless, odorless, and tasteless, but highly toxic. It combines with hemoglobin to produce carboxyhemoglobin, which usurps the space in hemoglobin that normally carries oxygen, but is ineffective for delivering oxygen to bodily tissues.

Concentrations as low as 667ppm may cause up to 50% of the body’s hemoglobin to convert to carboxyhemoglobin. A level of 50% carboxyhemoglobin may result in a seizure, a coma or even fatality. In the United States, the OSHA limits long-term workplace exposure levels above 50ppm.

The most common symptoms of carbon monoxide poisoning may resemble other types of poisonings and infections including symptoms such as headachesnauseavomitingdizzinessfatigue, and a feeling of weakness. Affected families often believe they are victims of food poisoning. Neurological signs include confusion, disorientation, visual disturbance, syncope and seizures.

What are emission measurements?

Emissions can be characterized, regulated, or controlled only if they can be accurately measured. The increased health
and environmental concerns about diesel emissions resulted in the development of a wide range of measurement
techniques of different levels of sophistication, equipment cost and accuracy to suit a variety of applications.
Many techniques, especially those used for regulatory purposes, are highly standardized to produce comparable
results even if performed at different testing laboratories. The main types of diesel emission measurements can be
grouped as follows:

  • Laboratory testing:

    • Regulatory testing

    • Emission research

    • Engine and emission control system development

  • Field testing:

    • Mobile emission laboratories

    • On-vehicle measurements

    • Inspection and maintenance (I&M) programs

    • Remote emission measurement

    • Emission-assisted equipment maintenance

    • Occupational health measurements

Laboratory emission testing methods generally use very complex and sophisticated equipment to deliver the highest
possible accuracy and repeatability. Test methods used for regulatory purposes, such as engine/vehicle emission
certification or compliance testing, are highly standardized. Detailed description of the measurement setup, type of
equipment, and test procedures are an important part of every emission regulation. These standard methods cover the
measurement of regulated pollutants, which traditionally include CO, HC, NOₓ, and PM, all of which are measured in
units of mass. Some regulations—for instance Euro 5—also include a solid particle number (PN) limit.

Measurement equipment and methods used for measuring engine emissions in the field vary from
“mobile laboratories”, with capabilities comparable to those of stationary emission laboratories, to simple, low-budget tools,
which can offer only very approximate results. The EM200E has been recommended by the CSIR as the most accurate
mobile device for measuring exhaust gas emissions.

DPM (diesel particulate matter) measurements can be taken at the exhaust tailpipe. The CSIR has purchased
a FLIR AIRTEC mobile DPM measurement device, and modified it to duplicate laboratory results.
John Ratcliffe is in the process of acquiring such a device to conduct DPM measurements at the exhaust tailpipe.

Another area of emission measurement and control is vehicle maintenance. Several US states and a number of countries
worldwide operate mandatory vehicle inspection and maintenance (I&M) programs, where vehicles must pass a periodic
emission check. The methods vary greatly, from relatively sophisticated chassis dynamometer tests (IM240 in the USA)
to a simple emission measurement at engine idle condition. In the case of heavy-duty diesel engines the most common
procedure had been to measure smoke opacity during engine acceleration. New technologies—such as remote exhaust
emission sensing [Baum 2000]—may also be used in future I&M programs.

What is Diesel Particulate Matter (DPM)?

Despite the considerable amount of basic research, neither the formation of DPM in the engine cylinder, nor its physical and
chemical properties or human health effects are fully understood. Nevertheless, the existing medical research suggests that
DPM is one of the major harmful emissions produced by diesel engines. This black smoke is not only carcinogenic, affecting
the operators’ lungs, but deposits will stain paintwork and/or contaminate products manufactured or stored where diesel
vehicles frequently work.

Diesel Particulate Matter (DPM) has been classified as a carcinogen to humans (Group 1) by the International Agency
for Research on Cancer (IARC) and the World Health Organisation (WHO) in June 2012. Diesel particulates are subject
to diesel emission regulations worldwide, and have become the focus in diesel emission control technology.

Diesel particulate matter, perhaps the most characteristic of diesel emissions, is responsible for the black smoke/soot
traditionally associated with diesel powered vehicles. The diesel particulate matter emission is usually abbreviated as
PM or DPM, the latter acronym being more common in occupational health applications. Diesel particulates form a very
complex aerosol system.

Diesel particulate matter also has many different types of particles that can be classified by size or composition. The size of
diesel particulates that are of greatest concern are in the categories of fine, and ultra-fine particles which can get deep into
the lung when inhaled.

The composition of these fine and ultra-fine particles may be composed of elemental carbon with absorbed compounds
such as organic compounds, sulphate, nitrate, metals and other trace elements.
The most common exposures to diesel particulate matter occur underground in mining, or indoors within factories and
storage facilities with the use of diesel-powered equipment.

What are diesel particulate filters (DPF)?


Diesel particulate filters (DPF) are devices that physically capture diesel particulates to prevent their release into the
atmosphere. Diesel particulate filter materials have been developed that show impressive filtration efficiencies, in excess
of 90%, as well as good mechanical and thermal durability. Diesel particulate filters have become the most effective
technology for the control of diesel particulate emissions—including particle mass and numbers—with high efficiencies.

Due to the particle deposition mechanisms in these devices, filters are most effective in controlling the solid fraction
of diesel particulates, including elemental carbon (soot) and the related black smoke emission. Filters may have limited
effectiveness or be totally ineffective in controlling non-solid fractions of PM emissions. To control total PM emissions
DPF systems are likely to incorporate additional functional components targeting gas emissions, typically oxidation
catalysts, while ultra-low sulphur fuels may be required to control sulphate particulates.

The term “diesel particulate trap” is sometimes used as a synonym for “diesel particulate filter” especially in older literature.
The term “trap” covers a wider class of particle separation devices. Several particle deposition mechanisms other than
filtration are commonly employed in industrial dust separation equipment. Examples include gravity settling,
centrifugal separation, or electrostatic trapping. None of these techniques could be adopted to control diesel PM emissions
due to the small particle size and low density of diesel soot.

It may be noted that diesel oxidation catalysts (DOC) can also capture diesel particulates, but provide a much lower overall
efficiency than diesel particulate filters.

What are Diesel Oxidation Catalysts (Purifiers/Catalytic Converters)?

A diesel purifier starts to work at an exhaust temperature of around 120°C, thereafter its efficiency rises very quickly.
At 230 °C it removes over 80% of carbons; at 300°C, over 90% of carbon monoxide and over 80% of hydrocarbons,
until at 350° C efficiency levels out, with the purifier eliminating over 90% of both pollutants.

As a forklift shifts heavier loads it produces more and more pollution. With a purifier fitted, the greater the load,
the more pollution is reduced. For example, at 1400 rev/min and with a full load, a typical diesel-powered fork-lift truck
produces nearly 3000ppm of carbon monoxide; the purifier reduces this to around 270ppm – a reduction of over 90%.

The benefits of such purifiers are therefore:

High reduction of carbon monoxide to reduce problems of dizziness and headaches which affects the concentration of
the operator.
Effective conversion of hydrocarbons and aldehydes.  This translates to less eye and throat irritation.  The diesel odours
are virtually eliminated.

It should be noted that a Purifier has a limited lifespan of between 5000 and 8000 hours. This lifespan will be influenced by:

  • The performance of the engine

  • The quality of the diesel used

  • Airflow rate

  • Sulphur content of engine oil used

The Purifiers convert CO to CO₂ through a chemical reaction with the precious metal coating on the honeycomb monolith.
This coating will, with time. be worn off hence the need for a replacement unit.

The Design

Whatever shapes the purifier take, the design principles are essentially the same.  Each purifier consists substantially of a
high-quality grade stainless steel case containing a ceramic honeycomb.

Purifiers are available in more than 400 different configurations to suit virtually every fork-lift, truck and bus on the market.

This honeycomb supports a platinum-based catalyst that reacts with pollutants to form carbon dioxide and water as follows:

  • (Carbon Monoxide) 2CO + O₂ → 2CO₂ converted to harmless

  • (Aldehydes) HCHO + O₂ → CO 2 + H₂O converted to harmless

  • (Hydrocarbons) 4HC +SO₂ → 4CO₂ + 2 H₂O converted to harmless

High-technology research and development goes into each purifier model.  For the optimum conversion of gases to occur,
the maximum surface area of the catalyst has to come into contact with the maximum volume of exhaust gas.
Yet the gas must not be blocked by the catalyst support in any significant way, otherwise engine efficiency will be impaired,
hence the honeycomb design, which creates a turbulent gas-flow to force the maximum amount of gas into contact
with the catalyst.

The cross-section of each cell in the honeycomb is made to particularly fine tolerances. The time path and
optimum cell size (to allow the emissions to flow freely) and catalyst surface area (to allow the gas to react easily) are all
crucial aspects of the purifier design.

Each model is made to particularly heavy-duty specifications and is resistant to vibration.  The purifiers are also compact,
to fit into small engine compartments, and should not interfere with normal engine maintenance.

What is a Sintered Metal Filter with Automatic Regeneration (SMF-AR)?

The SMF-AR system accommodates the engine type that does not reach the required exhaust temperatures when
regeneration is necessary. This process is accomplished by adding an additive and heating coils to the filter that
will reduce the flashpoint and increase the regeneration cycles.

The additive was originally manufactured to improve the engine and injector lifetime, and later proved to lower the flash
point of the soot. Refer to Annexure “D” for guarantees and OEM recommendations. 1L of additive should, as a rule, last
up to 2000L of diesel fuel. The display unit of the SMF-AR system will indicate both the diesel and additive tank levels.

Note: an electric level sensor should be available on the diesel tank.

This system will regenerate the filter before the back pressure reaches OEM maximum allowable levels.
These maximum OE back pressure and owner safeguard levels are programmed into the system at installation stage
as a safeguard.

​The SMF-AR system consists of:

​1. A sintered metal filter inside a stainless steel housing,
2. Engine control unit (ECU) to control the regenerating process, run diagnostics, log back pressure levels, over
(abuse), exhaust temperatures and fuel and additive tank levels.
3. Display unit
4. Warning buzzer,
5. Additive tank,
6. Mass Airflow Sensor (MAF),
7. Back pressure and temperature probes.

The SMF-AR system can accommodate engines up to 150kW depending on the maximum allowable back pressure ranges.
The filter size, in m², is determined by using these factors together with the available space for fitment and ease of access
for servicing of the unit. The diagram provides a quick reference of the filter size to be fitted.

What is an SMF filter system?

SMF’s trap the soot at the exhaust outlet resulting in zero black smoke being emitted into the environment that the forklift
operates in.

Sintered Metal Filters with manual regeneration:

These filters saturated with soot will ignite at +-400⁰C automatically forming ash that will be collected in the filter housing.
This ash, under normal conditions, needs only be removed every 1200 operating hours or with each service interval by
releasing two clamps. The cleaning process comprises of spraying the filter with a high pressure water hose and then
allowing the filter to dry before inserting it back inside the filter housing.

What is Collection and Regeneration?

Due to the low bulk density of diesel particulates, which is typically below 0.1 g/cm3 (the density depends on the degree of compactness, as an example, a number of 0.056 g/cm3 was reported by Wade [Wade 1981]), diesel particulate filters can quickly accumulate considerable volumes of soot. Several litres of soot per day may be collected from an older generation heavy-duty truck or bus engine. The collected particulates would eventually cause an excessively high exhaust gas pressure drop in the filter, which would negatively affect the engine operation. Therefore, diesel particulate filter systems have to provide a way of removing particulates from the filter to restore its soot collection capacity. This removal of particulates, known as the filter regeneration, can be performed either continuously, during regular operation of the filter, or periodically, after a pre-determined quantity of soot has been accumulated. In either case, the regeneration, which clean the blocked-up filter, should be “invisible” to the vehicle driver/operator and should be performed without his intervention.

How can we manage CO2 and Sulphur Dioxide emissions?

The way forward:

1. Ensure Latest Tier/Stage Engines are used

  • Older engines should be changed to at least TIER II technology. Advances from non-certified engines to
    TIER II engines could reduce DPM values to as much as 52%

  • Current engine technology also reduces oil consumption, due to better sealing of the combustion process this leads
    to lower DPM

  • Fuel savings from current available engine technology reduces DPM

  • A fuel and oil balance set up in the company operating procedure would be a good indicator to follow and prove to
    authorities that strategies are working

​2. Initiate Engine Maintenance Programmes

  • Diesel with a sulphur content of 50ppm or less is recommended – this will have an immediate impact on the DPM levels

  • Ensure that the correct fuel is used when newer TIER engines are being used

  • Ensure that the diesel is properly cared for up to the point of decanting​:

    • Install Breathers

    • Pr-filtering of particles to 10µm or less will prevent “sticky” injectors – “controlled combustion equals controlled DPM

    • Use Fuel/Water separators

    • Proper storage

    • Dirty air filters cause diesel engines to run dirtier and waste fuel – higher DPM

    • Loss of clean air in the combustion process can lead to higher internal engine operating temperatures and reduce
      engine life – higher DPM

    • Monitor higher fuel and oil consumption

    • Monitor an increase in exhaust emissions

    • Blue smoke – higher oil consumption

    • Black smoke – incomplete combustion

What are the allowable workplace exposure limits for Sulphur Dioxide?

The legal airborne permissible exposure limit (PEL) is 5ppm average over an 8-hour work shift.

Are there health problems that can be caused by sulphur dioxide?

Short term exposure to high enough levels of SO2 can be life threatening.  Generally, exposures to SO2 cause a burning sensation in the nose and throat.  Also, SO2 exposure can cause difficulty breathing, including changes in the body’s ability to take a breath or breathe deeply, or to take in as much air per breath.  Long term exposure to sulphur dioxide can cause changes in lung function and aggravate existing heart disease. Asthmatics may be sensitive to changes in respiratory effects due to SO2 exposure at even low concentrations. Sulphur Dioxide is not classified as a human carcinogen i.e. it has not been shown to cause cancer in humans.

How can I be exposed to sulphur dioxide?

You can be exposed to SO2 by breathing it in the air or getting it on your skin. People who live near industrial sources of sulphur dioxide may be exposed to it in the air. You are most likely to be exposed if you work in industries where SO2 is produced, such as copper smelting or power plants, or where SO2 is used like in the production of sulphuric acid, paper, food preservatives or fertilizers. People with malfunctioning appliances or chimneys in their homes may also be exposed to sulphur dioxide.

What is Sulphur Dioxide?

Sulphur dioxide (SO) is a colourless gas or liquid with a strong, choking odour.  It is produced from the burning of
fossil fuels (coal and oil) and the smelting of mineral ores (aluminium, copper, zinc, lead and iron) that contain sulphur.

Most of the sulphur dioxide released into the environment comes from electric utilities, especially those that burn coal. 
Some other sources of sulphur dioxide include petroleum refineries, cement manufacturing, paper pulp manufacturing
and metal smelting and processing facilities.  Locomotives, large ships, and some non-road diesel equipment currently
burn high sulphur fuel and release sulphur dioxide into the air.

Sulphur dioxide dissolves easily in water to form sulphuric acid.  Sulphuric acid is a major component of acid rain. 
Acid rain can damage forests and crops, change the acidity of soils, and make lakes and streams acidic and unsuitable
for fish. Sulphur dioxide also contributes to the decay of building materials and paints, including monuments and statues.

What are the allowable workplace exposure limits for CO2?

The legal airborne permissible exposure limit (PEL) is 25ppm, not to be exceeded at any time.

What are the Health Effects of Nitrogen Oxides?

Small levels of nitrogen oxides can irritate the eyes and/or nose, cause nausea, may cause fluid to form in the lungs, and cause shortness of breath. Exposure to high levels of nitrogen oxides can lead to rapid, burning spasms, cause the throat to swell, reduce oxygen intake, cause a larger build-up of fluids in the lungs, and sometimes even death. Many scientific studies have found a relationship between high levels of nitrogen oxides and sulphur dioxide and increased sickness and premature death from cardiac and respiratory disorders.

What are the Sources of Nitrogen Oxide?

Nitrogen is all around us and is normally relatively un-reactive in the atmosphere. Nitrogen will only react with oxygen when put under specific conditions to form nitrogen oxides such as nitrogen monoxide, and nitrogen dioxide. The most common method that causes nitrogen and oxygen to react and form nitrogen oxides is by combusting fuel, a process where fuel is burned at high temperatures. Everyday activities such as burning fuel for transportation, or using electricity in our homes produced by burning coal or oil, or in power plants, are all activities that directly, or indirectly, cause the production of nitrogen oxide emissions.

What is Oxides of Nitrogen?

The term “nitrogen oxide” refers to a combination of gases made up of oxygen and nitrogen. The two most common nitrogen oxide (NOₓ) gases are known as nitrogen monoxide (NO) and nitrogen dioxide (NO2). Nitrogen monoxide is also commonly known as nitric oxide.

These two gases are both harmful to human health, and the environment. Nitrogen oxides also play an important role in the production of other harmful pollutants when they react with other gases in the atmosphere such as ground-level ozone, smog, and acidic rain.

The main environmental concern regarding nitrogen oxides is when they combine with sulphur dioxide (SO₂) to form acid rain. Acid rain forms when nitric and sulphuric acid reacts with water droplets to form a rain cloud. The precipitation that comes out of this rain cloud will be high in acidity. Acid rain also affects infrastructures by corroding cars, buildings, and historical monuments. This costs cities millions in damages every year!

What are the OSHA standards for CO exposure?
  • The OSHA PEL is 50 parts per million (ppm). OSHA standards prohibit worker exposure to more than 50 parts of the
    gas per million parts of air averaged during an 8-hour time period.

  • The 8-hour PEL for CO in maritime operations is also 50 ppm. Maritime workers, however, must be removed from
    exposure if the CO concentration in the atmosphere exceeds 100ppm. The peak CO level for employees engaged in
    Ro-Ro operations (roll-on roll-off operations during cargo loading and unloading) is 200ppm.

What are the exposure limits for Diesel Particulate Matter?

The Australian Institute of Occupational Hygienists (AIOH) have issued an updated guidance on diesel exhaust particulate and health risks. The document retains the original recommendation that levels of DPM should be controlled to below 100µg/m³, as an 8 hour average value, measured as submicron elemental carbon (EC).

In South Africa, the regulatory authority (Department of Minerals and Energy) for Mine Health and Safety would normally promulgate regulations controlling the exposure of the workforce to below a specified Occupational Exposure Limit (OEL), which has a similar definition to a TLV.  Under the Mine Health and Safety Act (Act 29 of 1996), a guideline for a mandatory code of practice on the use of diesel engines should also be considered from a health and explosion prevention perspective. The South African Bureau of Standards (SABS) also publishes engine performance standards that can be made legally binding to OEM’s and industry when referred to in legislation.

There are currently no personal occupational exposure limits or legally binding tailpipe emissions standards in South Africa for DPM. Mining companies are however obliged to conduct risk assessments in terms of Section 11 of the Mine Health and Safety Act (Act 29 of 1996) on all factors that could adversely affect the health and safety of the workforce and institute appropriate mitigation measures. Where no local regulations exist, international best practice should be utilised. OEMs are also required to provide a full disclosure, in terms of Section 21 of the Mine Health and Safety Act (Act 29 of 1996) of the health and safety impact of the equipment being sold to a mining company and advice on appropriate measures that can be taken to eliminate or reduce the risk.

The Group Environmental Engineers “GEE” committee recognise that several international agencies have imposed limits for DPM, but also that these limits have been developed in countries where:

  • Higher quality diesel fuel with low sulphur content is used
  • Latest generation diesel engines are used
  • Maintenance staff is adequately trained and available for employment to work on these units
  • Exhaust purification systems are used extensively

To this extent the GEE’s recommendation for action is to introduce an interim DPM exposure control value and gradually lower this exposure control value by means of a “phased-in” approach as follows:

  1. A DPM exposure control value of 350µg/m3 (TC) up to 31 December 2013
  2. A DPM exposure control value of 250µg/m3 (TC) for 01 January 2014 to 31 December 2014
  3. A DPM exposure control value of 200µg/m3 (TC) for 01 January 2015
  4. As from 01 January 2016 a DPM exposure control value of 160ug/m3 (TC) will be adopted
  5. This level is however subjected to review should new knowledge on the risks associated with excessive exposure to DPM become available.
  6. Currently, exposure limits are legislated in Australia, Canada, United Kingdom and United States of America

These limits are based on the measurement of particulate constituents as indicated in the table below:

Regulatory/Agency Measured

Exposure Guidelines/Limits


Canada (Ontario province


Total Carbon (TC)



Total Carbon (TC)



Total Carbon (TC)



Elemental Carbon (EC)

Internationally, DPM is regulated via two mechanisms:

  1. Occupational Health and Safety Standards
  2. Tailpipe Emission Standards. Where diesel engines are used in confined spaces, their operation is regulated by occupational health standards in addition to tailpipe emissions.
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