- Distinguish from Diesel exhaust fluid (DEF), which is an aqueous urea solution made of 32.5% high purity urea and 67.5% deionized water, used in selective catalytic reduction (SCR) to reduce NOx concentration in exhaust emissions of diesel engines.
Diesel exhaust is produced inside diesel engines, where conditions differ considerably from spark-ignition engines. Diesel engine power is directly controlled by the fuel supply, not by controlling the air supply as in conventional gasoline engines. When the engine runs at idle, enough oxygen is present to burn the fuel completely. Diesel engines only make significant amounts of smoke when running without enough oxygen. This is usually mitigated in a turbocharged diesel engine.
Diesel exhaust is known for its characteristic smell, but this has largely disappeared in recent years following reductions in sulfur content.
Diesel exhaust is believed to contain toxic air contaminants and is listed as a possible carcinogen for humans by the IARC in group 1. Diesel fuel also contains fine particles associated with negative health effects. Diesel exhaust pollution was thought to account for around one quarter of the pollution in the air in previous decades, and a high share of sickness caused by automotive pollution.
The lean-burning nature of diesel engines and the high temperatures and pressures of the combustion process result in significant production of nitrogen oxides, and provides a unique challenge in reducing these compounds. Modern on-road diesel engines typically use selective catalytic reduction to meet emissions laws, as other methods such as exhaust gas recirculation cannot adequately reduce NOx to meet newer standards in many jurisdictions. However, the fine particulate matter (sometimes visible as opaque dark-colored smoke) has traditionally been of greater concern in the realm of diesel exhaust, as it presents different health concerns and is rarely produced in significant quantities by spark-ignition engines.
Diesel engines produce very little carbon monoxide as they burn the fuel in excess air even at full load, at which point the quantity of fuel injected per cycle is still about 50 percent lean of stoichiometric.
Occupational health effects
Exposure to diesel exhaust and diesel particulate matter (DPM) is an occupational hazard to truckers, railroad workers, and miners using diesel-powered equipment in underground mines. Adverse health effects have also been observed in the general population at ambient atmospheric particle concentrations well below the concentrations in occupational settings.
In March 2012, U.S. government scientists showed that underground miners exposed to high levels of diesel fumes have a threefold increased risk for contracting lung cancer compared with those exposed to low levels. The $11.5 million Diesel Exhaust in Miners Study (DEMS) followed 12,315 miners, controlling for key carcinogens such as cigarette smoke, radon, and asbestos. This allowed scientists to isolate the effects of diesel fumes.
For over 10 years, concerns have been raised in the USA regarding children's exposure to DPM as they ride diesel-powered school buses to and from school. The Environmental Protection Agency (EPA) has established the Clean School Bus USA initiative in an effort to unite private and public organizations in curbing student exposures.
Particulate health effects
Diesel combustion exhaust is a source of atmospheric soot and fine particles, which is a fraction of air pollution implicated in human cancer, heart and lung damage, and mental functioning. Diesel exhaust also contains nanoparticles, which have additional health impacts, and are as yet poorly understood.
Diesel particulate matter (DPM), sometimes also called diesel exhaust particles (DEP), is the particulate component of diesel exhaust, which includes diesel soot and aerosols such as ash particulates, metallic abrasion particles, sulfates, and silicates. When released into the atmosphere, DPM can take the form of individual particles or chain aggregates, with most in the invisible sub-micrometre range of 100 nanometers, also known as ultrafine particles (UFP) or PM0.1.
The main particulate fraction of diesel exhaust consists of fine particles. Because of their small size, inhaled particles may easily penetrate deep into the lungs. The rough surfaces of these particles makes it easy for them to bind with other toxins in the environment, thus increasing the hazards of particle inhalation.
Exposures have been linked with acute short-term symptoms such as headache, dizziness, light-headedness, nausea, coughing, difficult or labored breathing, tightness of chest, and irritation of the eyes and nose and throat. Long-term exposures could lead to chronic, more serious health problems such as cardiovascular disease, cardiopulmonary disease, and lung cancer. The NERC-HPA funded 'Traffic Pollution and Health in London' project at King's College London is currently seeking to refine our understanding of the health effects of traffic pollution. Ambient traffic-related air pollution was associated with decreased cognitive function in older men.
Mortality from diesel soot exposure in 2001 was at least 14,400 out of the German population of 82 million, according to the official report 2352 of the Umweltbundesamt Berlin (Federal Environmental Agency of Germany).
The study of nanoparticles and nanotoxicology is still in its infancy, but the full health effects from nanoparticles produced by all types of diesel is still being uncovered. It is already clear enough, however, that the health detriments of fine particle emissions are severe and pervasive. Although one study found no significant evidence that short term exposure to diesel exhaust results in adverse extra-pulmonary effects, effects that are often correlated with an increase in cardiovascular disease, a 2011 study in The Lancet concluded that traffic exposure is the single most serious preventable trigger of heart attack in the general public, the cause of 7.4% of all attacks. It is impossible to tell how much of this effect is due to the stress of being in traffic and how much is due to exposure to exhaust.
Since the study of the detrimental health effects of nanoparticles (nanotoxicology) is still in its infancy, and the nature and extent of negative health impacts from diesel exhaust continues to be discovered. There is little controversy however that the public health impact of diesels is higher than that of petrol fueled vehicles despite the wide uncertainties.
Variation with engine conditions
The types and quantities of nanoparticles can vary according to operating temperatures and pressures, presence of an open flame, fundamental fuel type and fuel mixture, and even atmospheric mixtures. As such, the resulting types of nanoparticles from different engine technologies and even different fuels are not necessarily comparable. One study has shown that the volatile component of 95% of diesel nanoparticles is unburned lubricating oil. Long term effects still need to be further clarified, as well as the effects on susceptible groups of people with cardiopulmonary diseases.
Diesel engines can produce black soot (or more specifically diesel particulate matter) from their exhaust. The black smoke consists of carbon compounds that were not combusted, because of local low temperatures where the fuel is not fully atomized. These local low temperatures occur at the cylinder walls, and at the outside of large droplets of fuel. At these areas where it is relatively cold, the mixture is rich (contrary to the overall mixture which is lean). The rich mixture has less air to burn and some of the fuel turns into a carbon deposit. Modern car engines use a diesel particulate filter (DPF) to capture carbon particles and then intermittently burn them using extra fuel injected directly into the filter. This prevents carbon buildup at the expense of wasting a small quantity of fuel.
The full load limit of a diesel engine in normal service is defined by the "black smoke limit", beyond which point the fuel cannot be completely combusted. As the "black smoke limit" is still considerably lean of stoichiometric, it is possible to obtain more power by exceeding it, but the resultant inefficient combustion means that the extra power comes at the price of reduced combustion efficiency, high fuel consumption and dense clouds of smoke. This is only done in specialized applications (such as tractor pulling competitions) where these disadvantages are of little concern.
When starting from cold, the engine's combustion efficiency is reduced because the cold engine block draws heat out of the cylinder in the compression stroke. The result is that fuel is not combusted fully, resulting in blue and white smoke and lower power outputs until the engine has warmed. This is especially the case with indirect injection engines, which are less thermally efficient. With electronic injection, the timing and length of the injection sequence can be altered to compensate for this. Older engines with mechanical injection can have mechanical and hydraulic governor control to alter the timing, and multi-phase electrically controlled glow plugs, that stay on for a period after start-up to ensure clean combustion—the plugs are automatically switched to a lower power to prevent their burning out.
This is a list of chemical components that have been found in diesel exhaust.
Although the US Mine Safety and Health Administration (MSHA) issued a health standard in January 2001 designed to reduce exposure in underground metal and nonmetal mines, on September 7, 2005, MSHA published a notice in the Federal Register proposing to postpone the effective date from January 2006 until January 2011.
To rapidly reduce particulate matter from heavy-duty diesel engines in California, the California Air Resources Board created the Carl Moyer Program to provide funding for upgrading engines ahead of emissions regulations. In 2008 the California Air Resources Board also implemented the 2008 California Statewide Truck and Bus Rule which requires all heavy-duty diesel trucks and buses, with a few exceptions, that operate in California to either retrofit or replace engines in order to reduce diesel particulate matter.
Remedial measures include:
- Diesel particulate filters (DPF)
- Exhaust gas recirculation (EGR)
- Selective catalytic reduction (SCR).
- Using other fuels, such as dimethyl ether.
With emissions standards increasing, diesel engines are having to become more efficient and have less pollutants in their exhaust. As a result, engineers have come up with two separate systems to make the U.S. 2010 emissions criteria. One system is Exhaust Gas Recirculation (EGR) and the other system is Selective Catalytic Reduction (SCR).
Both systems are in the exhaust system of diesel engines to promote efficiency . Currently light duty truck must have NOx emissions less than .07 g/mile, and in the U.S. 2010 the proposed NOx emissions must be less than .03 g/mile.
In recent years the United States, Europe, and Japan have become more stringent and are extending the emissions control regulations from on road vehicles to include locomotive, marine, stationary generator applications and farm vehicles.
SCR (selective catalytic reduction) injects a water and urea mix, known as diesel exhaust fluid (DEF), into the exhaust of a diesel engine to change nitrogen oxide into nitrogen and water. SCR can reduce 90% of the NOx in the exhaust system. SCR systems do not need a PM (particulate matter) filter, but when the SCR and PM filters are combined the engine is 3% to 5% more fuel efficient. One disadvantage of the SCR system is the need to refill the DEF tank, which varies with the miles driven, load factors, and the hours used. The SCR system is not as efficient at higher revolutions per minute. SCR is being optimized to have higher efficiency with broader temperatures, be more durable and more precise in its application 
EGR (Exhaust gas recirculation) is recirculating exhaust gas from the exhaust manifold of a diesel engine into an EGR valve which is timed with the intake valves to allow some exhaust back into the cylinder for compression and the power stroke. Thus less fuel is used on the power stroke, thereby avoiding engine knock, letting the engine run on a much leaner fuel to air ratio, giving the engine better fuel economy and less gaseous emissions. However, with this system there is more particulate exhaust, which requires a Particulate Matter (PM) filter in the exhaust. EGR needs a pressure differential across the exhaust manifold and intake manifold. This needs a variable geometry turbocharger, which has inlet guide vanes on the turbine to build exhaust backpressure in the exhaust manifold directing exhaust gas to the intake manifold. It needs external piping and a valve that would need more maintenance.
Another way of achieving an EGR-like effect is cam overlap whereby the intake and exhaust valves remain open simultaneously for a period.
In new farm tractor diesel engines John Deere is implementing a 9 liter inline 6 diesel that has:
- Two turbochargers, the first on the exhaust manifold being variable geometry and containing the EGR system, and the second being a fixed geometry turbocharger. The recirculated exhaust gas and the compressed air from the turbochargers have separate coolers and then the air merges before entering the intake manifold.
- A SCR system and a PM filter and another oxidation catalyst.
- Automobile emissions control
- Carl Moyer Program
- National Emissions Standards for Hazardous Air Pollutants
- List of IARC Group 1 carcinogens
- List of IARC Group 2A carcinogens
- List of IARC Group 2B carcinogens
- List of IARC Group 3 carcinogens
- Diesel Retrofit in Europe.
- NIOSH Mining Safety and Health Topic: Diesel Exhaust
- Diesel Particulate Matter, a case study at www.defendingscience.org
- Clean School Bus USA, EPA Initiative
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- U.S. Department of Labor Occupational Safety & Health Administration: Safety and Health Topics: Diesel Exhaust
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- Impact of Fuel Metal Impurities on the Durability of a Light-Duty Diesel Aftetreatment System National Renewable Energy Laboratory
- Scientific Study of Harmful Effects of Diesel Exhaust: Acute Inflammatory Responses in the Airways and Peripheral Blood After Short-Term Exposure to Diesel Exhaust in Healthy Human Volunteers
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- Health Concerns Associated with Excessive Idling
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