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Diesel Digest: Stricter regulations helped develop new diesel engine pollutant treatments

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Diesel Digest: by Capt. Jeff Werner

Diesel engines are designed for two broad application categories: on-road (such as trucks, cars and buses) and off-road. Marine diesel engine designs are a special application of off-road engines, a category that includes engines for railroads, agriculture, mining, construction and emergency power-generating equipment.

The maritime sector is divided into two segments: yachting and commercial. The yachting end includes high-speed engines for both propulsion and generating electricity, while the commercial segment covers medium- and low-speed engines for propulsion of ships. High-speed engines operate at greater than 1,000 rpm while under load and use the same high quality diesel fuel used in on-road vehicles, although it is taxed differently. Medium- and low-speed engines operate in a range from less than 400 rpm to a maximum of 1,000 rpm under load and are powered by heavy fuel oil.

For the past 20 years, stricter air pollution regulations by the United States, the European Union and the International Maritime Organization have led engine manufacturers to develop a variety of methods to meet new diesel exhaust standards. These mandated regulations have required the development of Tier 4 diesel engines, which is now the legal standard for the reduction of air pollution. Tier 4 engines use both “in-engine” techniques and  “after-engine” treatments to meet the worldwide clean diesel exhaust targets.

In-engine technology – such as high-pressure common rail fuel injection, advanced turbocharging, microprocessor and electronic-controlled engine management – along with ultra low-sulfur diesel fuel (ULSD) is very effective for controlling levels of sulfur oxides (SOx), which produce acid rain, among other harmful environmental effects.

After-engine treatment takes the remaining exhaust gases and scrubs them to remove additional pollutants. These after-engine methods are designed to remove particulate matter (PM) and nitrogen oxides (NOx). Nitrogen oxides contribute to unhealthy smog in urban areas, such as that experienced in Beijing. Diesel particulate matter is a complex mixture of smoke, soot and unburned chemical compounds and metals. If small particles of soot enter the lungs, they can cause detrimental health effects.

There are three major after-engine treatment techniques on the market:

  1. Selective catalytic reduction (SCR) douses the exhaust gas with urea, a solution of ammonia and water, to convert NOx into nitrogen and carbon dioxide. The urea solution is known as diesel exhaust fluid (DEF), and it is available for diesel vehicles at most truck stops.
  2. Diesel particulate filters (DPF) capture soot and inorganic particles as the exhaust gas flows through fine-pore ceramic filter elements. These filters must be cleaned at regular intervals to regulate the backpressure on the engine and keep it at acceptable levels. This filter maintenance is called regeneration, which is another term for burning off the soot on the ceramics. It can be done passively, using the heat of the exhaust to continuously burn off the soot, or actively, using burners or electric heaters to raise the temperature in the exhaust to burn off the soot when needed.
  3. Exhaust gas recirculation (EGR) involves directing a portion of the engine’s exhaust gas back to the engine cylinders. This feeds the air intake with a lower oxygen level, which helps reduce the formation of NOx. The downside of EGR is that it also lowers the combustion temperature in the cylinder, and that compromises economy and power.

In practice, the Tier 4 engines use a combination of the three methods to meet their pollution reduction targets. Large yachts and tugboats are beginning to use SCR+DPF solutions, while ocean-going ships are experimenting with EGR+DPF technology.

The drawback of after-engine treatments is that carbon dioxide is a byproduct of all three techniques. And carbon dioxide is the greenhouse gas that is the major contributor to global warming. This is part of the conflict that allows industrialized countries, like Britain and France, to ban diesel cars and vans by the year 2040, while developing nations clamor for the power provided by diesel engines to continue the growth of their countries.

Capt. Jeff Werner is a 25-year veteran of the yachting industry as a captain on private and charter yachts, both sail and power, and a certified instructor for the RYA, MCA, USCG and US Sailing. He also owns Diesel Doctor (MyDieselDoctor.com). Comments are welcome below.

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2 thoughts on “Diesel Digest: Stricter regulations helped develop new diesel engine pollutant treatments

  1. Richard Boggs

    While much of the information published in your January Diesel Digest column “Treatments scrub diesel exhaust but contribute to global warming” is generally accurate and informative, the headline and premise of the article is not only extremely misleading, it misses the point of exhaust treatment completely. The amount of carbon dioxide produced by a diesel engine fitted with an exhaust aftertreatment system is no more than would be produced by a perfectly running engine without such a system.
    Carbon dioxide (CO2) is a “greenhouse gas” (GHG) and is one of the constituents of diesel exhaust. CO2 contributes to global warming by slowing the rate at which heat is transferred from the atmosphere to space. Because of the length of time CO2 persists in the atmosphere it is used as the baseline against which the impact of other greenhouse gases is measured.
    Environmental scientists use a scale called the Global Warming Potential or GWP to compare the impact of various gases. GWP measures the amount of heat one ton of a gas will absorb over a given period of time. The larger the number, the greater the amount of heat is retained over a given period of time, usually 100 years. Because CO2 is the major contributor to global warming it has been assigned a GWP of 1.
    Combustion of diesel fuel in the cylinder of an engine is never perfect and the exhaust contains a mixture of CO2, unburned hydrocarbons, carbon monoxide (CO), and nitrogen compounds such as nitrous oxide (N2O), nitrogen dioxide (NO2), and nitric oxide NO. The oxides of nitrogen are described as NOx and have a Global Warming Potential around 300 times that of CO2.
    Exhaust aftertreatment systems comprised of diesel particulate filters (DPFs) selective catalytic reduction systems (SCR) and combinations of both exist because they dramatically reduce the impact of diesel exhaust emissions on global warming and human health. A DPF captures and holds inorganic particulates that are not only a direct health hazard but form condensation nuclei that can contribute to low level air pollution. Unburned hydrocarbons, black soot and nitrous oxide are converted to water vapor and carbon dioxide within the filter itself.
    An engine fitted with a combination of DPF and SCR not only performs all the work of the DPF but converts NOx to inert nitrogen gas and water vapor. Therein lies the core of the reason engine aftertreatment has become the primary means of meeting regulatory requirements for reducing greenhouse gas emissions. NOx has a GWP 300 times that of CO2 and is far more damaging than the nanoscale increase in CO2 produced through the elimination of soot and unburned hydrocarbons.
    The title of the article should have been “Diesel exhaust aftertreament systems drastically reduce yachting’s contribution to global warming” – which is microscopic in any event.

    Richard Boggs is the owner of EnerYacht, a Fort Lauderdale based manufacturer of the SeaClean
    generator exhaust treatment system which he invented and holds multiple patents. He holds an
    unlimited chief engineer license endorsed for steam, motor, and gas turbine propulsion systems and
    was previously employed as Technical Superintendent for Camper & Nicholsons.

  2. Jeff Werner

    Thank you for your comments, Richard. As you correctly note, CO2 has a GWP of 1, and diesel after-engine treatment technologies do not reduce CO2 emissions. Since diesel engine manufacturers have no additional methods to remove CO2 as diesel fuel is burned, they cannot change public perception about the future viability of diesel engines. Therefore, countries like Britain and France have no option but to ban diesel cars and vans since they produce large amounts of CO2.
    The headline submitted with my article was “Diesel Engine Pollutant Treatment Technologies,” so my closing paragraph, when read in the context of my original headline, makes total sense. My editor at The Triton rewrites the headlines that appear on my columns to meet their publishing needs, and I don’t see those headlines until I pick up my copy in Fort Lauderdale at a local marina. I do agree with you that this headline is misleading. “After-engine treatments scrub diesel engine exhaust, but with limited effect on reducing global warming” would have been more to the point.
    Please let me know if you have any questions.
    Thanks,
    Capt. Jeff Werner

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