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Since the introduction of the first diesel emission standards in the 1970s, and even more so after the introduction of the first particulate matter emission standard in 1988 (0.6 g/bhp-hr) and the subsequent increasingly stringent standards for all regulated pollutants, the need to lower emissions became the prime driving factor in the development of the diesel engine. The evolution of NOx and PM standards for heavy-duty engines in the USA and in European Union is shown in Figure 1.
Figure 1. Emission Standards for Heavy-Duty Diesel Engines
Note: The US 2007 limits shown reflect a common phase-in option of US 2010 emission standards.
As a result of these standards, diesel engine emission levels decreased dramatically in the period from the late 1980s to the early 2000s. For example, PM emissions from heavy duty diesel engines in the USA decreased by a factor of 10, from about 1 - 2 g/bhp-hr in the 1970s to below 0.1 g/bhp-hr in 1994. Reductions of similar magnitude were achieved for emissions of nitrogen oxides and hydrocarbons. This spectacular clean-up of the diesel engine was achieved through engine design methods in conjunction with better fuels. The US 1994 PM standard of 0.1 g/bhp-hr was accompanied by the introduction of low sulfur diesel fuel of 500 ppm (0.05%) S, down from the previous 0.5%. Similar sulfur levels were legislated in the EU, with a 500 ppm S cap introduced in 1996 and a 350 ppm S limit in 2000.
Diesel emission reductions in this time period—up to and including the US 2004 standards—were achieved without relying on exhaust gas aftertreatment (post-combustion) technologies. Only emission standards that took effect later forced the use of advanced aftertreatment devices such as diesel particulate filters and NOx reduction catalysts by setting extremely low emission limits. From this perspective, the 1980s and 1990s represent a transitional “pre-aftertreatment” period in the evolution of diesel emission control technology.
It should be noted that a number of diesel engines in this transitional, pre-aftertreatment period used exhaust gas aftertreatment devices. However, aftertreatment was typically not the critical technology used for compliance in these engines, especially for the most troublesome emissions of NOx and PM. For example, many US 1994 mechanically controlled engines used diesel oxidation catalysts to support PM emission reduction. Electronically controlled diesel engines were able to meet the 1994 0.1 g/bhp-hr PM standard with no exhaust aftertreatment. As the mechanically controlled engines were phased out, so was the use of catalytic converters on highway trucks in the USA.
The widespread use of diesel oxidation catalysts was seen on Euro 2/Euro 3 (1996/2000) diesel cars. An important reason for introducing these catalysts was the required reduction of CO and HC emissions. The PM emission reduction and in some cases the lean NOx functionality provided by these catalysts was limited and cannot be considered prime strategies for meeting emission standards. In some cases car manufacturers introduced aftertreatment voluntarily, to promote the clean image of the diesel engine or for other political reasons. Two noteworthy examples of such voluntary use of aftertreatment include the catalyst-equipped Volkswagen Golf “Umwelt” in 1989 and the particulate filter-equipped Peugeot 607 in 2000.
Advanced aftertreatment became an integral part of the diesel engine to satisfy stringent emission limits imposed by “aftertreatment-forcing” standards. Examples include the US 2007/2010, the Euro V (2008) and Euro VI (2013), as well as the US Tier 2 (2004-2009 phase-in), California LEV II and Euro 5 (2009) legislation for light-duty vehicles. The stringency of these regulations may be illustrated by the US 2007 PM limit of 0.01 g/bhp-hr which required another 10-fold emission reduction from the 1994 levels. Meeting these standards generally requires a combination of integrated engine and aftertreatment strategies, the latter being enabled by ultra low sulfur (10-15 ppm S) diesel fuel.
It should be emphasized that in-cylinder emission control continues to play a very important role in engines with aftertreatment. Aftertreatment technologies come with an added cost. Furthermore, some of them involve fuel economy penalties, while others have servicing requirements, such as the need to replenish urea in engines using selective catalytic reduction (SCR) for NOx control, that may inconvenience vehicle users. Through in-cylinder emission control methods, the required emission reductions may often be achieved at a lower cost. If aftertreatment is still necessary, cost savings may be possible on cleaner engines, for example by enabling smaller catalyst sizes. Several manufacturers continue to develop ultra clean diesel combustion systems that can meet the most stringent emission standards without the use, or with only a limited use, of aftertreatment. Examples of such engines include heavy-duty Euro V and US 2010 engines without NOx aftertreatment.
Fuel economy and greenhouse gas emissions, in addition to the “conventional” pollutant emissions, are becoming major drivers in the development of future internal combustion engines. The first CO2 emission standards have already emerged for light-duty vehicles, Figure 2 . Due to the large engine and vehicle sizes, passenger cars in North America historically had the highest CO2 emissions. However, compared to other regions, the United States is expected to require the highest CO2 emission reductions starting from 2011-2012. Fuel economy and GHG regulations are also being developed for heavy-duty engines. Hence, increased thermal efficiency while meeting near-zero emissions is now the focus in engine development.
Figure 2. GHG Emission Standards for New Passenger Vehicles
Standards converted to the NEDC test. Dashed lines indicate proposed regulations.
New technologies, as well as improvements to existing technologies are required for the diesel engine to meet the emissions and engine efficiency challenges. The diesel engine industry has implemented and is further investigating several approaches to achieve the NOx and PM emission reductions required by emission standards:
In-cylinder and engine-based design changes—the subject of this paper—cover a number of the engine subsystems including the fuel injection, induction, combustion, valve train, engine cooling, the electrical system, as well as accessories. With the advent of electronics and their massive use in the automotive technology, more of the emerging technologies are based on the capabilities afforded by electronic controls. From intelligent sensors to sophisticated control algorithms, controls technologies are opening new opportunities for automotive systems.
Following the increased stringency of emission standards, diesel engines gradually adopted a number of emission control techniques. The integration of the various technologies required several design choices by engine manufacturers to accommodate emission and efficiency requirements while controlling production cost increases in a competitive business environment. The evolution of emission control systems is discussed in detail elsewhere.