The three main NOx control technologies currently on the market are exhaust gas recirculation (EGR); lean-burn NO_x traps (LNT); and selective catalytic reduction (SCR).

• EGR systems recirculate a portion of exhaust gas to the combustion chamber, lowering the combustion temperature and the production of engine-out NO_x. EGR use has been widespread from Euro 4 to Euro 6 since the 1990s and can be used alone or in combination with LNT and SCR. One of the limitations of relying solely on EGR is the difficulty of controlling NO_x emissions during high-load operation, the researchers noted.

• LNT systems adsorb NO_x to a catalyst during lean engine operation. The stored NO_x is periodically reduced during short periods of fuel-rich operation (LNT regeneration events). Because LNT systems do not require a separate tank of reductant, as do SCR systems, they are lighter and more compact than SCR systems. LNT systems have a high incremental cost per liter of engine displacement associated with increased used of platinum-group metals (PGMs). Small LNTs are generally more economical than SCR systems for passenger vehicles with displacements below 2 liters.

• SCR systems use a catalyst and an aqueous urea solution commercially known as diesel exhaust fluid (DEF) or AdBlue to reduce NO_x to gaseous nitrogen and water. The (current) third commercial generation of SCR systems can approach 95% NO_x reduction efficiency. SCR technology can also improve fuel economy by allowing engine operation to be tuned to higher efficiency and higher engine-out NO_x emissions (which are dealt with by the aftertreatment system), but it is limited by poor catalyst activity at low exhaust temperatures, especially during cold engine start events.

LNT and SCR dominate the Euro 6 diesel passenger car market (54% for LNT and 40% for SCR in 2014) in the European Union.

For the study, chassis dynamometer testing was conducted by the ADAC (Allgemeiner Deutscher Automobil-Club), the largest car club in Europe. Emissions were measured over both NEDC (cold-start) and WLTC 2.0 (hot-start) driving cycles. Compared with NEDC, WLTC has a higher maximum velocity (131.3 vs. 120.0 km/h, or 81.50 vs. 74.56 mph); more frequent and harder accelerations; and a smaller share of idling time (13.0% vs 23.7% of total cycle time. WLTC is considered more representative of real-world conditions.

Results by vehicle manufacturer for the WLTC testing. The 21 vehicles from BMW performed especially well over NEDC; despite a 6-fold increase in average emissions over WLTC, they were still better than the overall average over WLTC. Nine SCR-equipped Mercedes-Benz vehicles also had better than average performance over both NEDC and WLTC. One vehicle each from Volvo, Jeep, and Renault and two from Hyundai, all equipped with LNT, had very high NOx emissions over the WLTC. Credit: ACS, Yang et al. Click to enlarge.

Among their findings:

• Average NO_x emissions over WLTC 2.0 were roughly 5 times the average over NEDC, even though cold-start emissions were avoided on the WLTC 2.0 by running the hot-start version of the cycle.

• 64 out of 73 test vehicles (88%) met the Euro 6 type-approval limit of 0.08 g/km of NO_x over the NEDC. The remaining 12% exceeded the limit only moderately (between 0.001 and 0.016 g/km). Only 27% of the vehicles tested met the 0.08 g/km limit over the WLTC.

• Most EGR-only and SCR vehicles performed better than average over the WLTC, but average NO_x performance was still much higher than the average over NEDC.

• LNT-equipped vehicles had the best average performance over NEDC but the worst over WLTC. The researchers highlighted this finding as “a clear indication that, in some cases, LNT technology is tuned in such a way that it is almost completely ineffective except when conducting the NEDC certification test.”

• Vehicles of larger size (E and F segments) tend to perform better over both NEDC and WLTC. The researchers suggested this is likely due to the fact that larger cars tend to employ SCR for NO_x emissions control because SCR systems are more economical than LNT and may provide better fuel economy for engines bigger than 2.0 L, and they pose fewer packaging problems in larger vehicles.

… results indicate that the Euro 6 limit value of 0.08 g/km is not as stringent as it appears on paper, because it applies to an outdated emissions certification driving cycle (NEDC) that should soon be replaced by a more realistic one (WLTC).

However, in all likelihood, the biggest challenge for diesel passenger car manufacturers will arise not from the certification test (be it under NEDC or WLTC), but from the impending real-driving emissions (RDE) test that is scheduled to become a mandatory step for the type approval of passenger cars in the European Union in 2016. Under this new testing framework, diesel passenger cars will have to prove they can keep NO_x emissions at reasonably low levels during a test that more closely represents real-world driving situations.

—Yang et al.

Resources

• Liuhanzi Yang, Vicente Franco, Peter Mock, Reinhard Kolke, Shaojun Zhang, Ye Wu, and John German (2015) “Experimental Assessment of NO_x Emissions from 73 Euro 6 Diesel Passenger Cars” Environmental Science & Technology doi: 10.1021/acs.est.5b04242