NOx and CO2 emissions from trucks: What trade-off?

The European Union and the United States are in the midst of setting a new round of pollutant emission standards for trucks and buses. The standards aim to address the prevailing air quality issues that many European and U.S. urban centers face from pollution by nitrogen oxides (NOx). Opposition to this regulatory initiative, wrapped in the urgency of climate change mitigation, has some lobbyist pushing a false dichotomy.

Specifically, any additional requirements to reduce NOx emissions, they argue, will invariably translate into higher CO2 emissions. A hand-drawn plot, like the one below used by Daimler Trucks at a conference last year, is sometimes used as an attempt to illustrate the claim.

Figure: HDV NOX CO2

To understand the origins of this either-or fallacy, it is necessary to delve into the technological evolution of NOx emissions control systems over the past decades. NOx is formed in the engine at very high temperatures when the molecular nitrogen and oxygen from air break apart and recombine to form NOx molecules. In the early ages of NOx emissions control efforts, the reductions mandated by the EPA 2004 and Euro III regulations could be met through a combination of enhanced injection control—enabling late combustion—and exhaust gas recirculation (EGR). While both strategies reduce peak combustion temperatures, thus limiting NOx formation, they also invariable increase fuel consumption. And so, the “low-NOx-means-high-CO2” narrative was born.

The NOx limits set by the newer EPA 2010 and Euro VI emission standards, however, cannot be met with engine measures alone and require the use of exhaust aftertreatment. The introduction of selective catalytic reduction (SCR) systems enabled significant reductions in the NOx emissions of trucks and provided, at the same time, an additional degree of freedom to optimize fuel consumption. However, SCR systems are miniaturized chemical plants that require high temperatures to function optimally. These high exhaust temperatures are typically only achieved when the vehicle is operating at high-power, such as on the highway. As a result, as our recent analysis highlights, SCR systems do not work well during cold-start and urban operation. Trucks are emitting high amounts of NOx in urban centers.

The future emission standards being developed in the United States and Europe aim to mandate adequate NOx control under ALL driving conditions, with a focus on cold-start and urban operation. The challenge in these operating conditions is to quickly warm-up the SCR system and to keep it above 200°C, even under low-load and low-speed operation. Simple logic would suggest that to heat something up, one needs to invest energy, that is, to burn more fuel. This fuel consumption penalty, manufacturers argue, would jeopardize their ability to comply with CO2 regulations such as the EU’s HDV CO2 standards or U.S. EPA’s GHG Phase 2 standards. This is the crux of the false dilemma noted above.

To assess the validity of the claim, we used vehicle and engine simulation to model a worst-case scenario for the active thermal management of the SCR system. That is, we only modeled the use of late- and post-combustion injection strategies in the low-load and low-speed areas of the engine map, without introducing additional engine technologies.

The simulations show that, even in this worst-case thermal management strategy, there is only a marginal fuel consumption impact over the cycles used for assessing compliance with CO2 regulations. Achieving a 25°C increase in the average exhaust temperature over California Air Resources Board’s proposed Low Load Cycle—deemed sufficient to achieve a faster light-off of the SCR system—increases the fuel consumption is around 0.9%, over the combined cycles used in U.S. EPA’s GHG Phase 2 standards. Over the combined cycles used in the EU’s HDV CO2 standards, our simulations indicate an increase in fuel consumption of about 0.7%.

However, such a brute force approach to thermal management of the aftertreatment is not the only option. Several technologies are commercially available to simultaneously reduce NOx and CO2 emissions, or to at least eliminate the small fuel consumption penalty from thermal management. Such technologies include cylinder deactivation, closed-coupled SCR systems, heated urea injectors, and mild hybridization. As our recently published cost-assessment shows, the deployment of such a technology package would increase the price of heavy-duty trucks by approximately 2%, while achieving a 90% reduction in NOx emissions and even having a slightly positive impact on CO2 emissions.

Tighter NOx emission limits do not jeopardize compliance with truck CO2 regulations in the United States or the European Union, despite the claims of some lobbyists. Quite the contrary. Future low NOx standards will instead incentivize the adoption of technologies that simultaneously reduce NOx and CO2 emissions, creating synergies between both regulations.