Diesel exhaust

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Class 55 Deltic diesel locomotive with their characteristic dense exhaust when starting a train

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 control of the air/fuel mixture as in conventional gasoline engines. When a diesel engine runs at idle, enough oxygen is present to burn the fuel completely. Diesel engines only make significant amounts of soot[1] [2] when running without enough oxygen. This is usually mitigated in a turbocharged diesel engine.

It is reported that emissions from diesel vehicles are significantly more harmful than those from petrol ones.[3] Diesel exhaust contains toxic air contaminants and is listed as carcinogen for humans by the IARC (part of the World Health Organization of the United Nations) in group 1.[4] Diesel exhaust contains fine particles which are harmful. 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.[5]

Diesel exhaust has been known for its characteristic smell, which largely disappears when the sulfur content of diesel fuel is reduced.

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 provide a unique challenge in reducing these compounds. Modern on-road diesel engines typically use selective non-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 in the exhaust fumes (sometimes visible as opaque dark-colored smoke) has traditionally been of greater concern, as it presents different health concerns and is rarely produced in significant quantities by spark-ignition engines. It is reported in 2012 that although total nitrogen oxides from petrol cars have decreased by around 96% through adoption of exhaust catalyst, diesel cars still produce nitrogen oxides at a similar level to those bought a decade and a half ago under real world tests, resulting in diesel cars emit around 20 times more nitrogen oxides than petrol cars.[6][7]

Diesel engines produce very little carbon monoxide as they burn the fuel in excess air even at full load,[8] at which point the quantity of fuel injected per cycle is still about 50 percent less than the stoichiometric requirement[citation needed].

Occupational health effects[edit]

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.[9][10]

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.[11] In 2013, the Environmental Protection Agency (EPA) established the Clean School Bus USA initiative in an effort to unite private and public organizations in curbing student exposures.[12]

Effect of particles on health[edit]

Diesel combustion exhaust is a source of atmospheric soot and fine particles, which is a fraction of air pollution implicated in human cancer,[13][14] heart and lung damage,[15] and mental functioning.[16] Diesel exhaust also contains nanoparticles, which have additional health effects, and are as yet poorly understood.[citation needed]

Particulate matter[edit]

Heavy truck, with visible particulate soot

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.[17]

Particulate matter (PM) emissions from transit buses running on ULSD and soybean biodiesel (B20) were investigated by Omidvarborna et al.[18] The results of the PM emission analysis showed that PM emissions were dependent on the engine model, cold and hot idle modes, and fuel type. The emission characteristics of biodiesel were analyzed in terms of total PM, elemental analyses, and organic carbon to elemental carbon ratio (OC/EC) analyses. The results showed that the total PM was related to idle conditions. The number of heavy metals in PM emitted during hot idling was greater than those from cold idling. Elemental analysis was used to find the sources of PM formation. Four factors were obtained and identified as the possible sources: fuel, oil and lubricants, engine parts, and ambient air. The most-repeatable elements in all cases were calcium, iron, and sulphur. The use of biodiesel could effectively reduce EC and increase the portion of OC emission by 10% and even by 90% in some cases. The main reasons for PM reduction in biodiesel emissions are thought to be due to the oxygenated structure of biodiesel fuel, engine technology, and the presence of catalytic converter in the system.[18]

Health effects[edit]

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[citation needed]. Long-term exposures can lead to chronic, more serious health problems such as cardiovascular disease, cardiopulmonary disease, and lung cancer.[13][14][19] Elemental carbon attributable to traffic was significantly associated with wheezing at age 1 and persistent wheezing at age 3 in the Cincinnati Childhood Allergy and Air Pollution Study birth cohort study.[20]

The NERC-HPA funded Traffic Pollution and Health in London project at King's College London is currently[when?] seeking to refine understanding of the health effects of traffic pollution.[21] Ambient traffic-related air pollution was associated with decreased cognitive function in older men.[16]

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).[citation needed]

The study of nanoparticles and nanotoxicology is in its infancy, and health effects from nanoparticles produced by all types of diesel engines are still being uncovered. It is clear, that diesel 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 extrapulmonary effects, effects that are correlated with an increase in cardiovascular disease,[22] a 2011 study in The Lancet concluded that traffic exposure is the single most serious preventable trigger of heart attack in the general public, as the cause of 7.4% of all attacks.[15] 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.[citation needed]

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-fuelled vehicles despite the wide uncertainties.[23]

Variation with engine conditions[edit]

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 95% of the volatile component of diesel nanoparticles is unburned lubricating oil.[24] 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 have not burned because of local low temperatures where the fuel is not fully atomized. These local low temperatures occur at the cylinder walls, and at the surface 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 burned. 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 high performance applications 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 burned 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.

Chemical components[edit]

This is a list of chemical components that have been found in diesel exhaust.

Contaminant Note Particulate extract mass concentration
acetaldehyde IARC Group 2B carcinogens
acrolein IARC Group 3 carcinogens
aniline IARC Group 3 carcinogens
antimony compounds Toxicity similar to arsenic poisoning
arsenic IARC Group 1 Carcinogens, endocrine disruptor
benzene IARC Group 1 Carcinogens
beryllium compounds IARC Group 1 Carcinogens
biphenyl It has mild toxicity.
bis(2-ethylhexyl) phthalate endocrine disruptor
1,3-Butadiene IARC Group 2A carcinogens
cadmium IARC Group 1 Carcinogens, endocrine disruptor
chlorine Byproduct of Urea injection
chlorobenzene It has "low to moderate" toxicity.
chromium compounds IARC Group 3 carcinogens
cobalt compounds
cresol isomers
cyanide compounds
dibutyl phthalate endocrine disruptor
1,8-dinitropyrene Carcinogen[citation needed]
dioxins and dibenzofurans
formaldehyde IARC Group 1 Carcinogens
inorganic lead endocrine disruptor
manganese compounds
mercury compounds IARC Group 3 carcinogens
methyl ethyl ketone
naphthalene IARC Group 2B carcinogens
nickel IARC Group 2B carcinogens
3-Nitrobenzanthrone One of the strongest carcinogens known 0.6 to 6.6 ppm
4-nitrobiphenyl 2.2 ppm
nitrogen oxides
polycyclic organic matter, including polycyclic aromatic hydrocarbons (PAHs)
Pyrene 3532–8002 ppm
Benzo(e)pyrene 487–946 ppm
Benzo(a)pyrene IARC Group 1 carcinogen 208–558 ppm
Fluoranthene 3399–7321 ppm
selenium compounds
styrene IARC Group 2B carcinogens
sulfur compounds
toluene IARC Group 3 carcinogens
xylene isomers and mixtures: o-xylenes, m-xylenes, p-xylenes IARC Group 3 carcinogens


Some problems associated with the exhaust gases (nitrogen oxides, sulfur oxides) can be mitigated with further investment and equipment; some diesel cars now have catalytic converters in the 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.


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 non-catalytic reduction (SNCR).

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.[27]

Remedial measures include:

Diesel particulate filter[edit]

Diesel particulate filters (DPF).

Selective non-catalytic reduction[edit]

Selective non-catalytic reduction (SNCR) 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. SNCR can reduce 90% of the NOx in the exhaust system. SNCR systems do not need a PM (particulate matter) filter, but when the SNCR and PM filters are combined, the engine is 3% to 5% more fuel efficient. One disadvantage of the SNCR system is the need to refill the DEF tank, which varies with the miles driven, load factors, and the hours used.[28] The SNCR system is not as efficient at higher revolutions per minute. SNCR is being optimized to have higher efficiency with broader temperatures, be more durable and more precise in its application[27]

Exhaust gas recirculation[edit]

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.[29] 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.[29] 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 SNCR system and a PM filter and another oxidation catalyst.

All these systems are controlled by a central engine control unit for the optimized minimum of pollutants released in the exhaust gas.[30]

Other fuels[edit]

Using other fuels, such as dimethyl ether.

Other effects[edit]

Experiments in 2013 showed that diesel exhaust impaired bees' ability to detect the scent of oilseed rape flowers.[31]

See also[edit]



  1. ^ Omidvarborna; et al. "Recent studies on soot modeling for diesel combustion". Renewable and Sustainable Energy Reviews 48: 635–647. doi:10.1016/j.rser.2015.04.019. 
  2. ^ Omidvarborna; et al. "Characterization of particulate matter emitted from transit buses fueled with B20 in idle modes". Journal of Environmental Chemical Engineering 2 (4): 2335–2342. doi:10.1016/j.jece.2014.09.020. 
  3. ^ Vidal, John (Jan 27, 2013). "Diesel fumes more damaging to health than petrol engines". The Guardian. Retrieved 5 June 2015. 
  4. ^ "IARC: DIESEL ENGINE EXHAUST CARCINOGENIC" (Press release). International Agency for Research on Cancer (IARC). Retrieved June 12, 2012. June 12, 2012 ‐‐ After a week-long meeting of international experts, the International Agency for Research on Cancer (IARC), which is part of the World Health Organization (WHO), today classified diesel exhaust as probably carcinogenic to humans (Group 1), based on enough evidence that exposure is associated with an increased risk of lung cancer 
  5. ^ Health Concerns Associated with Excessive Idling North Central Texas Council of Governments, 2008
  6. ^ Fuller, Gary (Jul 8, 2012). "Diesel cars emit more nitrogen oxides than petrol cars". The Guardian. Retrieved 5 June 2015. 
  7. ^ Lean, Geoffrey (Jul 19, 2013). "Why is killer diesel still poisoning our air?". The Telegraph. Retrieved 5 June 2015. 
  8. ^ Majewski, W. Addy (2012). "What Are Diesel Emissions". Ecopoint Inc. Retrieved 5 June 2015. 
  9. ^ Attfield, M. D.; Schleiff, P. L.; Lubin, J. H.; Blair, A.; Stewart, P. A.; Vermeulen, R.; Coble, J. B.; Silverman, D. T. (5 March 2012). "The Diesel Exhaust in Miners Study: A Cohort Mortality Study With Emphasis on Lung Cancer". JNCI Journal of the National Cancer Institute. doi:10.1093/jnci/djs035. 
  10. ^ Silverman, D. T.; Samanic, C. M.; Lubin, J. H.; Blair, A. E.; Stewart, P. A.; Vermeulen, R.; Coble, J. B.; Rothman, N.; Schleiff, P. L.; Travis, W. D.; Ziegler, R. G.; Wacholder, S.; Attfield, M. D. (5 March 2012). "The Diesel Exhaust in Miners Study: A Nested Case-Control Study of Lung Cancer and Diesel Exhaust". JNCI Journal of the National Cancer Institute. doi:10.1093/jnci/djs034. 
  11. ^ Solomon, Gina; Campbell, Todd (January 2001). "No Breathing in the Aisles. Diesel Exhaust Inside School Buses". NRDC.org. Natural Resources Defense Council. Retrieved 19 October 2013. 
  12. ^ "Clean School Bus". EPA.gov. United States Government. Retrieved 19 October 2013. 
  13. ^ a b bbc.co.uk - Diesel exhausts do cause cancer, says WHO, 2012-06-12
  14. ^ a b medpagetoday.com - WHO: Diesel Exhaust Causes Lung Cancer, 2012-06-12
  15. ^ a b Nawrot, Perez, Künzli, Munters, Nemery. Public health importance of triggers of myocardial infarction: comparative risk assessment The Lancet, Volume 377, Issue 9767, Pages 732 - 740, 26 February 2011 t doi:10.1016/S0140-6736(10)62296-9: "Taking into account the OR and the prevalences of exposure, the highest PAF was estimated for traffic exposure (7.4%)... "
    "... [O]dds ratios and frequencies of each trigger were used to compute population-attributable fractions (PAFs), which estimate the proportion of cases that could be avoided if a risk factor were removed. PAFs depend not only on the risk factor strength at the individual level but also on its frequency in the community. ... [T]he exposure prevalence for triggers in the relevant control time window ranged from 0.04% for cocaine use to 100% for air pollution. ... Taking into account the OR and the prevalences of exposure, the highest PAF was estimated for traffic exposure (7.4%) ...
  16. ^ a b Power; Weisskopf; Alexeeff; Coull; Spiro; Schwartz (May 2011). "Traffic-related air pollution and cognitive function in a cohort of older men" 119 (5). pp. 682–7. doi:10.1289/ehp.1002767. PMC 3094421. PMID 21172758. 
  17. ^ Omidvarborna; et al. "Characterization of particulate matter emitted from transit buses fueled with B20 in idle modes". Journal of Environmental Chemical Engineering 2 (4): 2335–2342. doi:10.1016/j.jece.2014.09.020. 
  18. ^ a b Omidvarborna; et al. "Characterization of particulate matter emitted from transit buses fueled with B20 in idle modes". Journal of Environmental Chemical Engineering 2 (4): 2335–2342. doi:10.1016/j.jece.2014.09.020. 
  19. ^ Ole Raaschou-Nielsen; et al. (July 10, 2013). "Air pollution and lung cancer incidence in 17 European cohorts: prospective analyses from the European Study of Cohorts for Air Pollution Effects (ESCAPE)". The Lancet Oncology 14 (9): 813–22. doi:10.1016/S1470-2045(13)70279-1. PMID 23849838. Retrieved July 10, 2013. Particulate matter air pollution contributes to lung cancer incidence in Europe. 
  20. ^ David I. Bernstein,Diesel Exhaust Exposure, Wheezing and Sneezing. Allergy Asthma Immunol Res. 2012 Jul; 4(4): 178–183. doi: 10.4168/aair.2012.4.4.178. PMCID: PMC3378923
  21. ^ '[1]
  22. ^ http://www.blackwellpublishing.com/isth2005/abstract.asp?id=46528 Exposure to Diesel Nanoparticles Does Not Induce Blood Hypercoagulability in an at-Risk Population (Abstract)
  23. ^ Int Panis, L; Rabl, A; De Nocker, L; Torfs, R (2002). "Diesel or Petrol ? An environmental comparison hampered by uncertainty". Mitteilungen Institut für Verbrennungskraftmaschinen und Thermodynamik, Publisher: Institut für Verbrennungskraftmaschinen und Thermodynamik 81 (1): 48–54. 
  24. ^ On-line measurements of diesel nanoparticle composition and volatility
  25. ^ "EPA Report on diesel emissions" (PDF). EPA. 2002. p. 113. Retrieved 19 August 2013. ]
  26. ^ Lippmann, Morton, ed. (2009). "Environmental Toxicants" (PDF). doi:10.1002/9780470442890. ISBN 9780470442890. 
  27. ^ a b Guan, B; Zhan, R; Lin, H; Huang, Z. (2014) Review of state of the art technologies of selective catalytic reduction of NOx from diesel engine exhaust Review Article Applied Thermal Engineering, Volume 66, Issues 1–2, Pages 395-414.
  28. ^ DieselForum.org (2014) About Clean Diesel: What is SNCR? http://www.dieselforum.org/about-clean-diesel/what-is-scr-.
  29. ^ a b Bennett, Sean (2004). Medium/Heavy Duty Truck Engines, Fuel & Computerized Management Systems 2nd Edition, ISBN 1401814999.
  30. ^ Technology to Reduce Emissions in Large Engines. http://www.deere.com/en_US/docs/pdfs/emissions/large_engine_technology_final.pdf
  31. ^ http://www.nature.com/srep/2013/131003/srep02779/full/srep02779.html , Diesel exhaust rapidly degrades floral odours used by honeybees, by Robbie D. Girling et al., Scientific Reports 3, article number 2779, received 26 June 2013, accepted 28 August 2013, published 03 October 2013.


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