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The first important US federal emission standards for heavy-duty onroad engines, introduced in 1974, included a HC+NOx limit of 16 g/bhp-hr and smoke limits of 20/15/50% during acceleration/lug/peak conditions. NOx emissions from unregulated engines prior to this ranged from 11 to 20 g/bhp/hr. Around 1980, the US EPA had determined that a 1.7 g/bhp-hr heavy-duty NOx standard would be required by the Clean Air Act. A proposed rule was expected in late 1980. Instead, it published an advance notice of proposed rulemaking on January 18, 1981 that stated the opinion that such a limit could not be attained. At the time, it was determined by the National Research Council that a 4-6 g/bhp-hr NOx limit was achievable but with a fuel consumption penalty between 2.5-12% and particulates no lower than 0.5 g/bhp-hr. EPA intended to propose a NOx standard of around 4 g/bhp-hr and a PM limit of 0.25 g/bhp-hr by 1986 . Instead, EPA settled for a NOx limit of 10.7 g/bhp-hr introduced in 1985—not substantially different from the 1979 NOx+HC limit of 10 g/bhp-hr.
The first US limits on PM were introduced in 1988, 12 years after heavy-duty engines emissions started to be regulated federally. This first PM limit of 0.6 g/bhp-hr PM was comparable to the actual emissions of many heavy-duty engines in use at the time. However, the 1990s saw a relatively rapid decrease in NOx and PM limits for heavy-duty applications. By 1994, the PM limit had been decreased to 0.1 g/bhp-hr where it would remain for 12 years—until 2006. The limit on NOx would decline more gradually until it reached 4.0 g/bhp-hr starting in 1998.
During this time, California was more aggressive than the US federal government in regulating vehicle emissions. For heavy-duty onroad engines, California introduced the 16 g/bhp-hr HC+NOx limit in 1973 and decreased it to 10 g/bhp-hr in 1975. In 1977, they switched to a 7.5 g/bhp-hr NOx limit. In 1980, they switched back to HC+NOx bringing in a 6 g/bhp-hr limit and decreasing to 4.5/5.1 g/bhp-hr in 1984.
Prior to 1974, limits on the concentration of unburned hydrocarbons and CO as well as smoke opacity standards were introduced federally in 1970 and in California in 1969.
Several reviews of technology to meet emission standards for heavy-duty engines from this era are available. One, published in 1981, discusses the challenges of meeting the 1986 standards  while another discusses the technologies used in the 1990s .
Figure 1 to Figure 3 illustrate the primary approach to reduce NOx emissions from the ~10 g/bhp-hr common in the 1980s to the ~ 4.0-6.0 g/bhp-hr required in the 1990s. Injection timing was the most effective tool by which NOx emissions were reduced during this era . Figure 1 shows the effect of injection timing on NOx emissions typical for an engine from the 1980s . Note that injection timing in Figure 1 (a) is given as dynamic timing versus Figure 1 (b) where injection timing is shown as static timing. Dynamic timing for the base engines in these studies was about 10° crank angle. Both examples are for engines that were being developed to meet the US 1991 heavy-duty emission standards.
Cooling charge air is another important measure that can provide additional NOx reductions. However, considering the practical range of intake manifold temperatures that are achievable, its impact is smaller than timing retard, Figure 2 .