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:
- 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.
- 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.
- 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.