A tale of two policies

It was 1998. The EPA had just settled the largest Clean Air Act enforcement action in its history, for one billion dollars, with seven manufacturers of heavy-duty diesel engines who were found to have cheated the emissions regulation. The manufacturers had used software defeat devices that allowed an engine to pass certification tests in the laboratory but then run in more polluting but more fuel efficient ways during highway driving. As part of the settlement, the manufacturers agreed to additional engine emissions testing, both in the lab and on the road, in order to prove compliance.

Buried in Appendix C of the Consent Decree between the federal government and Caterpillar, Cummins, Detroit Diesel, Volvo, Mack Trucks/Renault and Navistar was a definition of the test that would be used for that additional compliance testing: the Not to Exceed (NTE) test. In 2005 the NTE was incorporated into the EPA’s “Final Rule for Control of Emissions of Air Pollution From New Motor Vehicles: In-Use Testing for Heavy-Duty Diesel Engines and Vehicles,” the first regulation in the world to mandate in-use testing of heavy-duty vehicles. And U.S. federal heavy-duty vehicle emissions regulation still relies on the NTE today, and will continue to do so until at least 2027.

It bears repeating: the test used to monitor and certify emissions performance of heavy-duty engines in the United States was devised somewhat hastily, under the pressure of negotiating a settlement to a legal case involving a specific type of cheating during highway operation in which a billion dollars was at stake (real money in 1998). It’s no wonder that it doesn’t work the way it might have if it had had a different starting point.

What is the NTE test? A technical description can be found here, but in brief, the NTE test places limits on exhaust emissions during real-world highway-type driving. The NTE test does not place any limits on emissions during other kinds of driving, such as low-speed or low-load operation, urban operation, or transient driving (lots of different speeds and accelerations and decelerations). And because U.S. regulations rely so heavily on the NTE test, they don’t effectively address emissions in those conditions. Unsurprisingly, heavy-duty vehicles on the road in the United States today have very low emissions during highway driving and very high emissions during urban driving. In fact, as diesel trucks exit a freeway to enter into a heavily populated area, the harmful emissions in their engine exhaust increase by an order of magnitude.

Compare this with the approach taken in Europe. The European Commission also wanted to include in-use testing designed to limit real-world emissions as part of a heavy-duty vehicle regulation (Euro VI) finalized about six years after the EPA’s 2005 rule. But the EU opted for a different approach to evaluating compliance. Instead of a method that completely discounts all urban driving, the EU test incorporates some of it. The approach is the Moving Average Window (MAW) test. While the MAW doesn’t give equal weight to highway emissions and urban emissions, it also doesn’t discard the latter entirely.

Both the NTE and the MAW collect and analyze second-by-second data on exhaust pollutants to determine whether an engine passes or fails to comply. But there are significant and crucial differences in how they define the conditions that have to be met for collected data to be considered valid for purposes of the test—and therefore in what data each test excludes or discards. A summary of how these two test cycles work appears in our just-released analysis of NOx emissions from heavy trucks in the United States and Europe. The key point here is that the NTE excludes all emissions data when the engine is operating as it would during urban driving, or other low-load, low-power operation.

Is the EU’s approach more effective? Yes. As we show in that just-published study, heavy trucks in the U.S. emit twice as much nitrogen oxides (NOx) as European trucks in urban driving. This is significant, because the EU heavy truck NOx emissions standard is more relaxed than the U.S. limit—that is, if trucks in both places were complying with the emissions limits defined in the regulations, EU trucks would be polluting much more, not much less. NOx emissions can be more challenging to control in low-load and transient driving, so the fact that EU trucks urban emissions are half those of US trucks demonstrates that EU regulations are compelling manufacturers to employ better emissions control in real-world operation.

NOx emissions by speed bin for European and U.S. HDVs. Dotted lines represent engine emission NOx limits for U.S. and European HDVs. Error bars show 95% confidence interval.

This is not to say that the current EU regulations don’t need improvement – in fact, we have a laundry list of improvements we would like to see. But there’s now conclusive evidence that the NTE test doesn’t work as well as it should as a tool for monitoring and enforcing pollutant emissions standards for heavy-duty engines, and it’s high time the U.S. replaced it with a better alternative.

Hence the timing of this post. Right now, the California Air Resources Board and the U.S. EPA are both working on updates to their heavy-duty NOx regulations, and both have indicated a willingness to move away from the NTE. They should. Pollution from diesel trucks remains a significant cause of poor air quality in U.S. cities, particularly in areas where industry and related transportation activity have concentrated. The people who live in those areas are frequently poor, and suffer disproportionately from the kinds of health impacts—like asthma, cardiopulmonary disease—air pollution brings with it. A better, more realistic test cycle for measuring pollutant emissions from heavy vehicles would be a big step toward lifting that burden.