Average NOx emissions of test vehicles over the five test routes compared to US-EPA Tier2-Bin5 emissions standard; repeat test variation intervals are presented as ±1σ; Route 1 for Vehicle A includes rush-hour/non rush-hour driving, ‘R’ designates routes including a test with DPF regeneration event, ‘nd’ - no data available. Click to enlarge.

The objective of the testing—particularly in the context of the recent identification by the Joint Research Centre (JRC) in Europe of off-cycle NO_x emissions from light-duty diesel vehicles (LDV) substantially exceeding the Euro 3-5 emissions standards on average by a factor of 4 to 7 over specific test routes—was to gain insight into real-world emissions of NO_x and other regulated gaseous pollutants from diesel LDVs certified to US-EPA Tier2-Bin5 and CARB LEV-II ULEV (CA) standards.

The researchers acquired two cars from a rental agency and one from a private owner. The test plan covered a wide variety of topological, road and ambient conditions as well as traffic densities over three major urban areas along the West coast—San Diego, Los Angeles, and San Francisco. Additionally, one of the urea SCR vehicles was operated over a total distance of ~4,000 km (~2,485 miles) between Los Angeles, CA and Seattle, WA to investigate emissions reduction characteristics over extended highway driving conditions.

The researchers used a Horiba OBS-2200 portable emissions measurement system (PEMS) to measure gaseous exhaust emissions, including NO_x, carbon monoxide (CO), carbon dioxide (CO₂) and total hydrocarbons (THC) on a continuous basis. Particle number concentrations and particulate mass emissions were inferred from real-time measurements performed using a Pegasor particle sensor.

Average CO2 emissions of test vehicles over the five test routes compared to EPA advertised CO2 values for each vehicle; repeat test variation intervals are presented as ±1σ; Route 1 for Vehicle A includes rush-hour/non rush-hour driving, ‘R’ designates routes including a test with DPF regeneration event, ‘nd’ - no data available. Click to enlarge.

In summary, the researchers found:

• Real-world NO_x emissions exceeded the US-EPA Tier2-Bin5 (at full useful life) standard by a factor of 15 to 35 for the LNT-equipped vehicle; by a factor of 5 to 20 for one urea-SCR vehicle; and at or below the standard for the second urea-SCR fitted vehicle over five pre-defined routes categorized based on their predominant driving conditions, namely, i) highway, ii) urban/suburban, and iii) rural-up/downhill driving. The second urea-SCR equipped vehicle exceeded the standard only during rural-up/downhill operating conditions by a factor of ~10.

• Distance-specific NO_x emissions for the two high-emitting vehicles were below the US-EPA Tier2-Bin5 standard for the weighted average over the FTP-75 certification cycle during chassis dynamometer testing. On-road emissions testing was performed with the engine and after-treatment in warmed-up condition (i.e. warm/hot start). Increased NO_x emissions are usually expected for cold-start, however, not for hot, running conditions.

• Generally, distance-specific NO_x emissions were observed to be highest for rural-up/downhill and lowest for high-speed highway driving conditions with relatively flat terrain. The LNT after-treatment based vehicle was observed to emit significantly (> 19% to 90%) more NO_x during diesel particulate filter (DPF) regeneration events.

• Even though exceeding the US-EPA Tier2-Bin5 standard on average by a factor of 6 during extended highway driving between California and Washington State, Vehicle B, the urea-SCR equipped vehicle, was found to have NO_x emissions below the regulatory standard for portions of the route characterized by low or negligible changes in altitude (i.e. near zero road grade), and with the vehicle operated in cruise-control mode at highway speeds.

• In general, CO and THC emissions were observed to be well below the regulatory level for all three test vehicles and driving conditions, with exception of two routes for the LNT-equipped vehicle where THC emissions were observed at slightly elevated levels.

• Particulate number emissions, inferred from PPS measurements, were observed below the Euro 5b/b+ standard except during vehicle operation exhibiting DPF regeneration events where PN emissions significantly increased by two to three orders of magnitude, thereby exceeding the Euro 5b/b+ standard under all driving conditions for the LNT and first urea-SCR vehicles.

• Highway driving showed lowest CO₂, whereas urban/suburban driving conditions lead to highest CO₂ emissions factors for all vehicles. Overall, urban/suburban driving leads to a 32-39% reduction in fuel economy over highway driving.

…only three vehicles were tested as part of this measurement campaign with each vehicle being a different after-treatment technology or vehicle manufacturer; conclusions drawn from the data presented herein are confined to these three vehicles. The limited data set does not necessarily permit drawing more generalized conclusions for a specific vehicle category or after-treatment technology.

—Thompson et al.

Resources

• Gregory J. Thompson, Daniel K. Carder, Marc C. Besch, Arvind Thiruvengadam, Hemanth K. Kappanna (2014) “In-use emissions testing of light-duty diesel vehicles in the US”