HD Diesel Engines with EGR Technology

Hannu Jääskeläinen

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Abstract: Most heavy-duty engines used exhaust gas recirculation technology for meeting US EPA 2004 emission standards. However, other technology platforms were used as well, for instance Miller valve timing and multiple injection strategies. Some engines used DOC aftertreatment, while the use of DPFs was generally limited to bus applications. In Europe, the strategies for meeting Euro IV standards were different—the use of EGR remained limited and most engine models used urea SCR aftertreatment.


The 2004 EPA limits for heavy-duty diesel engines required further NOx reductions from those achieved during the 1980s and 1990s. The limit of NOx+HC became 2.5 g/bhp-hr. While PM limits remained at the 0.1 g/bhp/hr set in 1994, further efforts to control PM emissions were required because of the NOx/PM trade-off. Also, due to consent decrees reached with a number of engine manufacturers, most engine manufacturers had to comply with the 2004 limits 15 months ahead of time in October 2002. The consent decrees also introduced additional testing requirements, including SET and NTE testing.

Prior to US EPA 2004 heavy-duty emission limits, retarded injection timing had been the primary means of controlling NOx. Improvements in fuel injection and combustion systems along with reductions in oil consumption as well as DOCs in LHDD and MHDD engines were the primary tools used for PM reduction during the same period. Further reductions in NOx via injection timing retard would have been possible to meet 2004 limits but would have resulted in a significant fuel consumption penalty, Figure 1. Most engine manufacturers opted to used high pressure cooled EGR to meet the NOx limit because the fuel consumption penalty was considerably lower than with retarded injection timing with the technology available at the time. This did, however, make the control of PM emissions more challenging. To meet the 0.1 g/bhp-hr PM limit, further improvements in combustion and fuel injection systems were required as well as reductions in oil consumption and increased use of DOC in some engine categories.

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Figure 1. Effect of Injection Timing Retard Versus EGR on BSFC, NOx and PM Emissions

FTP test cycle. Heavy-duty diesel engine produced for mid-1990s emission standards.

One alternative to cooled high pressure EGR used variable intake valve timing to produce a Miller Cycle effect that also had the potential to control NOx emissions. However, this option also had the potential to reduce fuel consumption and PM emissions, Figure 2 [3203]. Note that Figure 1 and Figure 2 are not directly comparable; one represents data collected over a drive cycle while the other, data collected at full load.

Figure 2. Potential of Miller Cycle to Reduce Fuel Consumption and Emissions

Full load. Miller cycle intake valve timing and dual stage turbocharging.

While the above two strategies were arguably the most important to meet EPA 2004 emissions limits for heavy-duty applications, other approaches such as multiple injections were also used.

Meanwhile in Europe, Euro IV standards for heavy-duty on-road diesel engine came into effect in 2005. While the Euro IV NOx limit of 3.5 g/kWh (2.6 g/bhp-hr) was comparable to the US EPA 2004 HC+NOx limit of 2.5 g/bhp-hr, the Euro IV PM standard of 0.02 g/kWh (0.015 g/bhp-hr) was almost an order of magnitude lower than the EPA limit of 0.1 g/bhp-hr. This, combined with other factors such as higher fuel costs and differences in other regulatory requirements in Europe, meant that the emissions solutions for Euro IV were considerably different than those for EPA 2004—urea-SCR was widely used for NOx control, while EGR was used only on a limited number of engine models.