Further, CO₂ emissions—which are proportional to fuel consumption—for heavy-duty vehicles are roughly five times those of cars. In other words, on an engine-load basis, heavy-duty vehicles are about 10 times better than light-duty diesels at reducing NO_x.

The differences are attributable in large part to differences in how light-duty and heavy-duty vehicle emissions are regulated, according to Rachel Muncrief, ICCT researcher and author of the paper. Significant real-world emissions and conformity-factor reductions were accomplished in the transition from Euro V to Euro VI heavy-duty vehicle standards.

Top: Average real world NOx and CO2 emissions of Euro V/VI/6 heavy-duty and light-duty vehicles. Bottom: Average real-world NOx of Euro V and Euro VI heavy-duty and Euro 5 and Euro 6 light-duty vehicles under real-world driving conditions (in g/km). For comparison, emissions of a Euro 6 gasoline passenger car. Source: ICCT. Click to enlarge.

Among the significant changes between Euro IV/V and Euro VI that likely contributed to that improvement were:

1. Addition of an off-cycle test during type approval.

2. Improved type-approval test cycle that includes cold start and lower load conditions as well as transient and high-load conditions.

3. PEMS test for in-service conformity testing, with limited restrictions on the boundary conditions used during the test and subsequent data processing.

In its current form, the Euro 6 regulation for diesel passenger cars makes none of the three changes (aside from cold-start and lower-load conditions), which is likely contributing to the high in-use emissions of these cars. The current Euro 6 regulation does not include transient or high-load test, does not include an off-cycle test, and does not include a PEMS in-service conformity test.

—“NO_x emissions from heavy-duty and light-duty diesel vehicles in the EU”

The contrasting performance highlights the importance of an upcoming decision on strengthening the real-driving emissions (RDE) test for passenger cars in the EU.

The significantly lower NO_x emission levels of trucks and buses are most likely a result of differences in regulation. Official testing requirements of light-duty vehicles remain limited to laboratory measurements of carefully prepared prototype vehicles.

In contrast, for measurement of NO_x emissions from trucks and buses, mobile testing devices became mandatory in 2013. As a consequence, randomly selected vehicles can be tested under real-world driving .

— Dr. Peter Mock, Managing Director of ICCT in Europe

Similar tests with portable emissions measurement systems (PEMS) will be introduced for passenger cars beginning in September 2017, as part of the European Real-Driving Emissions (RDE) regulation. That should bring about a significant improvement in the NO_x emission levels of diesel cars, the ICCT said. But further improvements in the light-duty vehicle testing protocols will be needed to truly measure and control NO_x emissions.

According to the current status of the RDE regulation, vehicle manufacturers will still be allowed to carefully select special prototype cars for emissions testing. Instead, it would be much better to measure the emissions of ordinary mass-production vehicles, obtained from customers who have had been driving them in an ordinary way, and not from the manufacturers.

—Peter Mock

The European Commission, despite resistance from some vehicle manufacturers and EU member states, plans to bring forward for discussion a package of possible revisions to the existing RDE regulation as part of a working group meeting on 17 January in Brussels. The package would include, among other changes, provisions for testing in-use, privately owned cars by independent third parties.

Resources

• NO_x emissions from heavy-duty and light-duty diesel vehicles in the EU: Comparison of real-world performance and current type-approval requirements