SCR Systems for Diesel Engines

W. Addy Majewski

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Abstract: Urea-SCR technology has been adopted as a NOx reduction strategy from most types of mobile diesel engines. Urea-SCR systems typically include an SCR catalyst, an auxiliary oxidation catalyst, and urea injection system which supplies an aqueous urea solution upstream of the SCR catalyst. High NOx reductions depend on the catalyst temperature window and on complex, electronically controlled urea injection control strategy.


NOx reduction systems based on the selective catalytic reduction (SCR) technology have been developed and commercialized for a number of mobile diesel engine applications in jurisdictions with stringent diesel emission standards, beginning with the EU (Euro IV/V, 2005/2008), Japan (JP 2005), and the United States (US EPA 2010). The use of ammonia has been practically ruled out, due to safety concerns, and urea (in water solution) has been commonly used as the preferred reductant.

SCR-only NOx Control. In applications with emission limits of moderate stringency, the most attractive SCR emission strategy involves a calibration of the engine for low PM (injection timing, high injection pressures), and using the SCR catalyst to reduce the increased NOx, Figure 1. This approach can also meet some applicable PM emission limits—as was the case with many SCR applications on Euro IV/V engines or on EU Stage IV and US Tier 4 nonroad engines. Due to the advanced injection timing a fuel economy improvement could be realized, making urea-SCR more attractive than the competing EGR technology that brings a fuel economy penalty. Urea-SCR may also have a fuel economy advantage over NOx adsorber catalysts—another competing NOx reduction technology—due to the fuel economy penalty resulting from adsorber regeneration. However, any fuel savings in SCR engines are offset to some degree by the cost of urea.

Figure 1. SCR-only NOx emission control strategy

SCR aftertreatment without EGR and DPF

The SCR-only strategy was widely used in Europe to simultaneously meet the Euro IV/V limits for both NOx (3.5/2 g/kWh, respectively) and for PM (0.02 g/kWh) [623]. The engines were calibrated for low PM emission levels, below 0.02 g/kWh, while engine-out NOx was elevated to about 9-11 g/kWh. SCR aftertreatment was then used to bring down NOx emissions to below 2 g/kWh. The required NOx conversion efficiency of the SCR system was about 80-85% for Euro V and only about 65% at the Euro IV stage. The need for a diesel particulate filter was eliminated, resulting in smaller size, complexity, and cost of the emission aftertreatment system. The Euro V calibration could provide fuel savings of some 3-5%.

SCR systems had also been considered a potential solution for meeting the US 2007-2010 heavy-duty NOx fleet average standards of about 1.1 g/bhp-hr without the use of EGR [980]. In this case, the emission control system combined an SCR catalyst and a diesel particulate filter (DPF) to meet the US 2007 PM limit of 0.01 g/bhp-hr. According to cost analyses, SCR aftertreatment presented the most cost-effective technology for meeting the US 2007 emission standards. This is illustrated in Figure 2, which presents the results of an analysis by DaimlerChrysler [977]. In terms of fuel economy, 2007-compliant SCR+DPF package could provide a 6% advantage over the MY 2004 baseline, comparing very favorably to the EGR+DPF alternative. Also, the total life cycle cost change for SCR compared favorably with the competing EGR and NOx adsorber paths over the entire analyzed range of cost for urea solutions. Notwithstanding the apparent cost benefit, EGR technology was used for NOx control in US 2007-2009 engines, while SCR was adopted three years later, in 2010.

Figure 2. Fuel consumption (left) and relative cost increase (right) for US 2007 emission strategies

DPF = catalyzed diesel particulate filter; NAC = NOx adsorber catalyst
Direct cost only, no overhead or profit

In light-duty application, a comparison between urea-SCR and the NOx adsorber paths to meet the US Tier 2 Bin 2 emission standards conducted by Ford concluded that urea-SCR could provide a cost advantage, both in terms of system cost and operating costs [1067].

SCR+EGR NOx Control. Applications with more stringent emission standards require simultaneous use of SCR and EGR for NOx control. The SCR+EGR configuration has been used in US 2010 engines to meet the NOx standard of 0.2 g/bhp-hr. Relative to the 2004 standard, the 2010 limit required over 90% NOx reduction over the transient FTP cycle. In applications with such high NOx reduction requirements the SCR technology has no extra capacity to handle any increased engine-out NOx levels resulting from fuel efficient engine calibration [622] and it is necessary to combine SCR with EGR to meet these more demanding NOx limits.

Further benefits of using EGR in SCR engines include better NOx control at low temperatures, such as in applications tested over low temperature test cycles [1160], and a more robust configuration for meeting OBD requirements.

SCR Issues. The application of SCR technology to mobile engines requires solving a number of technical, regulatory, and urea distribution infrastructure problems. The following are the most important issues: