Katerina Swanson
Diesel engines emit a complex mixture of gaseous and solid pollutants, known as diesel particulate matter (DPM). DPM includes hundreds of chemical elements, including elemental carbon, sulfates, nitrates, ammonium, organic compounds, carcinogenic compounds, and even heavy metals such as arsenic, selenium, cadmium, and zinc. The formation of DPM is influenced by factors such as combustion processes, fuel quality, lubrication oil quality, and consumption rates. Incomplete combustion of diesel fuel results in the emission of particulates, with soot being the most prevalent component, comprising over 50% of total DPM emissions. While modern diesel engines emit lower levels of pollutants, the solid particles emitted by diesel engines contribute significantly to air pollution and pose risks to public health and the environment.
| Characteristics | Values |
|---|---|
| Composition | Diesel exhaust is a mixture of gases and solid particles. |
| Gases | CO2, carbon monoxide, volatile organic compounds, organic carbon, and black carbon particles. |
| Solid Particles | Elemental carbon, condensed organic compounds, carcinogenic compounds, heavy metals (arsenic, selenium, cadmium, zinc), soot, sulfates, ammonium, nitrates, and more. |
| Size | Particulates are smaller than 10 microns in diameter. Fine particulates are smaller than 2.5 microns, and ultrafine particulates are smaller than 0.1 microns. |
| Ultrafine Particulates | Make up 80-95% of diesel soot pollution. |
| Pollutants | Nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbons (HC), and particulate matter (PM). |
| Pollutant Levels | Modern diesel engines emit much lower levels of pollutants, almost "near-zero", when equipped with emission aftertreatment devices. |
| Emission Reduction | Mixing diesel with biodiesel or bioethers (e.g., dimethyl ether) can reduce pollutants. |
| Health Effects | Diesel exhaust causes lung cancer, asthma, and premature deaths. It poses substantial risks to public health and the environment. |
| Environmental Effects | Black carbon emissions have a warming impact on the climate, 460-1500 times stronger than CO2 per unit of mass. |
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What You'll Learn
Diesel particulate matter (DPM)
Diesel engines emit a complex mixture of air pollutants, including gaseous and solid material. The solid material in diesel exhaust is known as diesel particulate matter (DPM). DPM is a subset of particulate matter less than 2.5 microns in diameter (PM2.5). PM2.5 is the size of ambient particulate matter air pollution most associated with adverse health effects among air pollutants that have ambient air quality standards.
DPM is composed of an elemental carbon core with several organic compounds, sulfates, nitrogen oxides, metals, and irritants (such as acrolein, ammonia, acids, fuel vapors, and unburnt lubricating oils) adsorbed to its surface. The formation process of DPM emissions depends on several factors, including the combustion and expansion process, fuel quality (sulfur and ash content), lubrication oil quality, consumption, combustion temperature, and exhaust gas cooling.
DPM emissions result from the combustion process and may originate from the agglomeration of very small particles of partly burned fuel, partly burned lube oil, ash content of fuel oil, cylinder lube oil, or sulfates and water. Most particulate matter results from the incomplete combustion of hydrocarbons in the fuel and lube oil. In a typical particle composition of a heavy-duty diesel engine, Kittelson (1998) found that particulate matter consisted of 41% carbon, 7% unburned fuel, 25% unburned oil, 14% sulfate and water, 13% ash, and other components.
DPM emissions can be reduced through various methods, including the use of cleaner-burning diesel fuel, retrofitting engines with particle-trapping filters, introducing new advanced technologies that reduce particle emissions, and using alternative fuels such as biodiesel or dimethyl ether. Biodiesel combustion results in lower mass emissions and smoke opacity compared to mineral diesel, although it may increase the number concentration of nuclei-mode particles due to their smaller size. Dimethyl ether (DME), for instance, results in near-nonexistent particulate matter emissions, and the use of diesel particulate filters can be omitted.
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Nitrogen oxides (NOx)
NOx emissions from diesel engines have been a significant environmental concern. The release of these gases contributes to air pollution, smog formation, and acid rain. Additionally, NOx can affect agricultural crops, ecosystems, and human health. Exposure to NOx has been linked to adverse respiratory effects, including exacerbating asthma symptoms and causing irritation to the eyes and respiratory tract.
To address the issue of NOx emissions, several methods have been employed:
- Lowering combustion temperature: One approach to minimize NOx creation is by lowering the combustion temperature. This can be achieved through techniques like Exhaust Gas Recirculation (EGR), where cooled exhaust gas is injected back into the combustion chamber, reducing the oxygen levels available for combustion.
- Aftertreatment devices: Aftertreatment devices, such as NOx reduction catalysts and particulate filters, are used to remove NOx from the exhaust. Selective Catalytic Reduction (SCR) is a common method where ammonia or urea is injected into the exhaust flow, reacting with NOx gases over a catalyst to convert them into harmless nitrogen and water.
- Alternative fuels: Transitioning to alternative fuels, such as dimethyl ether and other bioethers, can effectively reduce NOx emissions. These fuels can be carbon-neutral and significantly decrease particulate matter emissions.
- Regulatory efforts: Emission standards and regulations have been tightened globally, extending beyond on-road vehicles to include farm vehicles, marine vessels, and stationary generators. These regulations aim to reduce NOx emissions and encourage the development of more efficient engine technologies.
While progress has been made, challenges remain. Evidence suggests that diesel manufacturers have employed strategies to circumvent vehicle emissions tests, highlighting the need for stricter enforcement and continued innovation in emission reduction technologies.
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Heavy metals
Diesel engines emit a complex mixture of air pollutants, including both gaseous and solid material. The solid material in diesel exhaust is known as diesel particulate matter (DPM). The composition of diesel exhaust may vary with the fuel type, rate of consumption, or speed of engine operation.
DPM is of particular concern as it presents different health concerns and is rarely produced in significant quantities by spark-ignition engines. These especially harmful particulate contaminants are at their peak when such engines are run without sufficient oxygen to fully combust the fuel. When a diesel engine runs at idle, enough oxygen is usually present to burn the fuel completely.
The metals emitted from diesel engines can be divided into two categories: crust elements and anthropogenic elements. Crust elements, including Al, Ca, Fe, Mg, and Si, are emitted in higher concentrations than anthropogenic elements, which include Ag, Ba, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Sb, Sr, Ti, V, and Zn. The increase in engine speeds results in a higher fraction of metal contents in particulate matters.
Studies have shown that the emission of heavy metals from diesel engines is a significant issue. For example, the annual emission rates of crust and anthropogenic elements from all diesel engine vehicles were significantly higher than those from coal power plants and coke ovens. The relatively high amount of metal contents emitted from diesel engines suggests that the control of metal contents in diesel fuel should be addressed.
The use of alternative fuels or biofuel blends has been proposed as a way to reduce the environmental impact of increasing traffic. Studies have shown that the total contents of Ba, Ce, Cd, Cr, Cu, Fe, Mn, Ni, Pb, V, and Zn were higher in diesel engines than in spark-ignition engines. Additionally, the bioaccessibility of metals in emissions was assessed by extraction in water and simulated lung fluid, with diesel engines emitting higher levels of metals.
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Soot
Diesel engines produce soot more readily than gasoline engines due to differences in how fuel is injected and ignited. In gasoline engines, fuel is injected during the intake stroke and ignited with a spark, whereas in diesel engines, fuel is injected during the compression stroke and ignited spontaneously from the pressure. This late fuel injection in diesel engines produces fuel-dense pockets in the combustion chamber that produce soot when ignited. Additionally, the lean-burning nature of diesel engines and the high temperatures and pressures of the combustion process result in significant production of nitrogen oxides (NOx), another air pollutant.
The formation of soot in diesel engines can be influenced by various factors, such as driving conditions, low-quality fuel, and driving habits. Worn-out rings or injectors, excessive idling, poor fuel spray patterns, and incorrect air-fuel ratios also contribute to excessive soot formation. Soot buildup can lead to decreased fuel efficiency, reduced acceleration, rough idling, and increased engine noise. It can also cause engine damage by leading to the loss of oil dispersancy, where the oil's ability to suspend and carry away pollutants like soot is diminished.
To address the issue of soot in diesel engines, several technologies and strategies have been developed. Diesel particulate filters (DPFs) are devices designed to assist in reducing sooty particulate matter entering the atmosphere and have proven effective in specific operating conditions. Emission system devices are designed to collect and burn excess particulate matter, but they may not always function optimally due to various variables. Proactive maintenance is crucial for managing soot accumulation, and certain additives, such as BG Diesel engine oil conditioner, can help prevent soot chain formation and lower operating temperatures. Changing to alternative fuels, such as dimethyl ether or bioethers, can also effectively reduce soot and other pollutants.
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Solid particles
DPM is composed of hundreds of chemical elements, including volatile organic compounds, organic carbon, elemental carbon, and black carbon particles. Black carbon, in particular, has a warming impact on the climate that is 460 to 1,500 times stronger than CO2 per unit of mass. Other components of DPM include sulfates, ammonium, nitrates, condensed organic compounds, carcinogenic compounds, and heavy metals such as arsenic, selenium, cadmium, and zinc.
The formation of DPM is influenced by various factors, including the combustion and expansion process, fuel quality (such as sulfur and ash content), lubrication oil quality, combustion temperature, and exhaust gas cooling. The use of biodiesel, for instance, can reduce the mass and size of particulate matter emissions compared to mineral diesel. This is due to the suppression of soot formation and increased in-cylinder pressure during biodiesel combustion.
DPM emissions pose significant risks to public health and the environment. They have been linked to lung cancer, asthma, and other diseases, contributing to premature deaths. Additionally, DPM emissions impact the environment, with black carbon having a substantial warming impact on the climate.
To mitigate the harmful effects of DPM, several strategies have been employed, including the use of diesel particulate filters, which can reduce DPM emissions by over 90%. Other approaches include transitioning away from fossil fuels and internal combustion engines (decarbonization) and adopting new technologies that reduce particle emissions.
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Frequently asked questions
Diesel particulates, or diesel particulate matter (DPM), are the solid particles found in diesel exhaust. They are a mixture of gases and solid particles, including soot, sulphates, ammonium, nitrates, elemental carbon, condensed organic compounds, carcinogenic compounds, and heavy metals.
Diesel particulates are harmful to human health and have been linked to lung cancer, asthma, and other diseases. They are also a major contributor to climate change, with black carbon emissions having a warming impact on the climate that is 460-1,500 times stronger than CO2 per unit of mass.
Diesel particulates are formed during the combustion process, which can be incomplete in diesel engines. This results in a mixture of gases and solid particles, with particulate matter consisting of partly burned fuel, lubricating oil, ash content of fuel oil, cylinder lube oil, sulphates, and water.
Diesel particulate emissions can be reduced by using alternative fuels such as biodiesel, dimethyl ether, or other bioethers, as well as through the use of diesel particulate filters, new technologies that reduce particle emissions, and cleaner-burning diesel fuel.
