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The diesel engine has been long established as the principal powerplant for heavy-duty trucks, buses, and off-road vehicles and machinery. Increased use of diesels in light-duty applications has followed the importance and public awareness of such issues as energy conservation and carbon (greenhouse gas) emissions. In view of growing concerns over global warming and depletion of fossil fuel resources, diesel engines will continue to play an increasingly important role. In the European Union, dieselization of vehicle fleets became one of the major tools to achieve long term fuel economy targets. This resulted in an increase in the proportion of diesels in the passenger car market, which exceeded 30% in the year 2000  and reached about 50% in 2005.
Historically, the issue of natural resources depletion was first recognized by the general public in the USA and in many other industrialized countries in the mid-1970s, during the Middle-East oil crisis . Alternative sources of energy became one of the most talked about subjects, alongside energy conservation, passenger car downsizing, car-pooling, etc. Motivated by the oil embargo, many engine manufacturers decided to offer a diesel option, not only to conserve energy, but also to allow their customers to capitalize on reduced fuel consumption and operating costs.
However, the common view of the public on diesel engines was rather negative, with the image of black smoke being the major obstacle to their wider acceptance. As the usage and importance of diesels gradually increased, so did public pressure to mandate increasingly stringent diesel emission limits. Compared to their gasoline counterparts, diesels initially enjoyed modest emission regulations. It was only in the 1990s when diesel emission standards became increasingly difficult to meet. The focus in diesel legislation has been on emissions of nitrogen oxides (NOx) and particulate matter (PM) due to their sizeable contributions to global emission inventories.
To accommodate the clean air requirements as well as energy conservation, engine manufacturers made several important design choices while being cognizant of their increased production costs and competition from other manufacturers. These design choices gradually transitioned the image of diesel—rooted in vehicle technologies dating back to the 1970s—as a slow, smoky, dirty, noisy, heavy, and smelly engine to the responsive, much cleaner, and exciting to drive engine that debuted in some European cars in the 1990s. The historical progress in diesel car emissions from the early designs through approximately Euro 2 technology is shown in Figure 1 .
Significant improvement in diesel emission levels, in both light- and heavy-duty engines, was achieved in the 1970 - 2000 period. PM, NOx, and HC emissions were cut by one order of magnitude. Most of that progress was achieved by emission-conscious engine design, such as through changes in the combustion chamber design, improved fuel systems, implementation of low temperature charge air cooling, and special attention to lube oil consumption. However, another look at Figure 1 tells us that more progress was still required, as the NOx and PM emissions from diesels remained higher than those from gasoline cars (to give diesels justice, their HC emissions—a major challenge in gasoline cars—are low and easily controlled).
A new wave of diesel emission regulations was developed with implementation dates around 2005-2010, which required the introduction of exhaust gas aftertreatment technologies for diesel engines, as well as fuel quality changes and additional engine improvements. As these “aftertreatment-forcing” regulations are being implemented, the diesel engine is increasingly coupled with aftertreatment devices such as particulate filters and NOx catalysts, just as the gasoline engine adopted the three-way catalyst in the 1980s. Some of the important emission standards designed to force these technologies include:
We should also mention that there is a certain cost for meeting these ambitious emission standards with future diesel engines. This cost consists of two components: (1) the cost of the emission control equipment and (2) a fuel economy penalty. The first component can vary greatly depending on the technology (some of which relies on expensive precious metal catalysts). The fuel economy penalty may be derived from a number of sources. First, traditionally there has been a correlation between engine-out NOx emissions and fuel consumption in the diesel engine, where higher engine efficiency and better fuel economy are associated with higher NOx. Second, exhaust aftertreatment devices are associated with a varying additional fuel economy penalty caused by such factors as increased pressure drop and energy consumption for the regeneration of filters and/or NOx adsorbers. In the case of SCR catalysts, while there may be no direct fuel economy penalty, operating costs are increased by the cost of urea. Increased costs for low emissions are also known from the gasoline engine, which is operated at a stoichiometric A/F ratio—with significant fuel economy penalty relative to lean-burn operation—in order to enable the three-way catalyst technology.