Vehicle NOx emissions: The basics

Nitrogen and oxygen are present in the ambient air, which means they’re present in the air-fuel mixture combusted in all gasoline and diesel engines. During combustion, these elements combine to form NOx. It’s not possible to design an internal combustion engine that does not produce NOx when it burns fuel.

The amount of NOx formed during combustion varies with peak combustion temperature: as temperature rises, so does the rate of NOx formation. Combustion temperature tends to increase as the load on the engine increases—for example, when accelerating rapidly or driving uphill. Diesel cars will sometimes feature lower combustion temperatures than gasoline engines because diesel combustion is much leaner (i.e., the air-fuel mixture contains a higher proportion of air). Some of the oxygen will act as a heat sink, and the engine therefore will produce less NOx in all operating modes than gasoline engines do.

Vehicle NOx, represented here by thermal NO, increases exponentially as combustion temperature increases. (UN-ECE, Task Force on Techno-Economic Issues)

NOx reacts with atmospheric chemicals to form secondary fine particulate matter (PM_2.5), or soot. Exposure to PM_2.5 can cause stroke, ischemic heart disease, chronic obstructive pulmonary disease, lung cancer, and lower respiratory infections. PM_2.5 caused 4.2 million premature deaths worldwide in 2015. When combined with volatile organic compounds and sunlight, NOx helps form ground-level ozone, a major component of smog. Ozone can cause or exacerbate chronic lung diseases like asthma, chronic obstructive pulmonary disease, or emphysema, especially among vulnerable populations like children and the elderly, for whom it may prove deadly. Researchers attribute 254,000 premature deaths to ozone pollution in 2015.

NOx emissions also affect ecosystems and agricultural crops. Ozone pollution is toxic to plants and contributes to loss of biomass, crop yields, and forest productivity. PM_2.5 pollution reduces solar irradiation, decreasing photosynthesis in plants and reducing their biomass. The loss in biomass means less carbon is sequestered in plants, leaving more CO₂ in the atmosphere. Both ozone and PM_2.5 pollution can directly change the way ecosystems work by affecting the exchange of CO2 and water vapor across the surface of leaves, which can have significant effects on hydrology—even changing stream flows.

Susan Anenberg of Environmental Health Analytics, coauthor of “Impacts and mitigation of excess diesel NOx emissions in 11 major vehicle markets,” on the effects of vehicle NOx.

Creation of some amount of NOx in the combustion process is unavoidable. The basic problem with NOx emissions from vehicles is, therefore, first to minimize the amount created, and second to remove NOx from the exhaust. The first task is mainly accomplished by lowering combustion temperature. The second is accomplished using an aftertreatment device to cause a chemical reaction reducing NOx in the exhaust to nitrogen and water and/or CO₂.

Too much oxygen present in the vehicle exhaust makes it more difficult for that chemical reaction to occur. Problematically, too little oxygen makes it more difficult to get rid of other pollutants in the exhaust, unburned hydrocarbons and carbon monoxide. Gasoline engine exhaust typically has a good balance of oxygen from this perspective, so a relatively simple and inexpensive three-way catalytic converter can reduce NOx very effectively.

But a diesel engine, because of its compression-ignition design, uses much more combustion air, and diesel engine exhaust consequently contains much more oxygen than gasoline engine exhaust (more oxygen in, more oxygen out). That is an unfavorable environment for the chemical reaction reducing NOx to take place in. It’s an inherently hard problem, which requires more complex solutions than a gasoline-engine vehicle does.

ICCT program director Rachel Muncrief on the difficulty of controlling NOx in diesel vehicles.

All modern gasoline-engine vehicles are equipped with a three-way catalytic converter as part of the exhaust system. It’s called a three-way catalytic converter because it controls three pollutants: carbon monoxide (CO), which combines with oxygen in the converter to become carbon dioxide (CO₂); unburned hydrocarbons, which combine with oxygen to produce CO₂ and water vapor (H₂O); and NOx, which is reduced over the catalyst to nitrogen and water and/or CO₂. The three-way catalyst, invented in the 1970s, is inexpensive and poses little or no penalty to fuel economy, performance, drivability, or maintenance. And it is very effective: a new 2017 gasoline-engine passenger car, properly tuned and operating in normal conditions, has only a negligible amount of NOx present in the exhaust as it exits the tailpipe. (This does not mean that the NOx problem is fully solved for gasoline engines; a hundred thousand cars stuck in traffic still add up to a health hazard and a pollution problem.)

Because the problem of controlling NOx in diesel exhaust is more complicated, diesel vehicles require different approaches. To begin with, most modern diesel vehicles incorporate exhaust-gas recirculation (EGR) into their design. EGR systems recycle a portion of the exhaust gas back into the combustion chamber, where it combines with “fresh” intake air. This reduces the oxygen content and increases the water vapor content of the combustion mixture. That has the effect of reducing peak combustion temperature. Because more NOx is created as peak combustion temperature rises, EGR effectively reduces the amount of NOx produced by the engine. Why only recycle a portion of the exhaust gas? Using too much would increase PM_2.5 and reduce fuel efficiency, so proper design entails a delicate balance.

EGR addresses the problem of controlling NOx emissions inside the engine cylinder, at the point where NOx forms. Two methods are used in diesel vehicles to control NOx after the exhaust has permanently exited the engine (hence the term “aftertreatment”). A lean NOx trap (LNT) uses a catalyst to temporarily store NOx from the exhaust. At intervals (ranging from seconds to minutes, depending on operating conditions), the engine controller briefly increases the proportion of fuel in the air-fuel mixture being combusted. The exhaust from burning the richer air-fuel mixture contains proportionally less oxygen and more unburned hydrocarbons, and the stored NOx at the catalyst reacts with hydrocarbons in the exhaust to produce nitrogen and water and/or CO2. Selective catalytic reduction (SCR) reduces NOx over a catalyst using ammonia as the reductant. The ammonia is typically supplied in the form of urea, which must be stored in solution in a tank on the vehicle. For reasons relating to engine size, operating characteristics, and the cost of raw materials for the catalyst, as a practical matter heavy-duty vehicles being produced today use only SCR systems and light-duty vehicles can use either SCR or LNT.

EGR, LNT, and SCR are active systems, in contrast to the three-way catalytic converter. Their operation is controlled by the vehicle’s engine control unit (which determines, for example, the intervals at which urea solution is injected into the exhaust for SCR, or the air-fuel mixture is enriched to regenerate the LNT) and they come with maintenance requirements and costs both direct (e.g., a service charge to refill a urea tank) and indirect (slightly reduced fuel economy from running the engine rich periodically or from recirculating exhaust gas). The costs can provide a motive for shutting down one of these aftertreatment systems during some vehicle operating modes, and the digital electronic control offers a means.

The technical challenges related to NOx control presented by light-duty and heavy-duty diesel vehicles differ. The relative lack of physical space in which to install emissions-control equipment is a key challenge for cars, especially small cars. In the passenger-car market diesels compete with gasoline/petrol vehicles, which can control NOx emissions easily and cheaply using a three-way catalyst and do not need additional aftertreatment devices. In contrast, the heavy-duty market is completely dominated by diesel; in the European Union more than 99% of new heavy-duty vehicle registrations are diesels. Consequently, the incremental cost of emission controls is a far more important issue for diesel cars than trucks, and the technology required to meet Euro 6 emissions regulations imposes an appreciable cost premium on diesel cars relative to comparable gasoline vehicles.

Still, effective technologies to control NOx emissions in diesel vehicles do exist. Nevertheless, many diesel vehicles emit NOx at levels far in excess of legal limits. Over half of emissions from light-duty diesel vehicles and approximately one-third of emissions from heavy-duty diesel vehicles around the world are “excess” emissions in that sense. Eliminating that “excess” is a different sort of challenge than, for example, the engineering design problem of finding space for a big enough urea tank on a passenger car.

There are a number of possible explanations for why a vehicle that is certified as meeting NOx standards might exceed the standards in everyday operation:

• Poor design or defects in emission-control system components in the manufactured fleet — i.e., problems that only crop up after certification testing, once the vehicle has gone into mass production
• Deterioration of emission-control system components over time
• Deliberate cheating on vehicle certification tests—e.g., using the engine controller to sense when a vehicle is not driving a cert test and turning off the aftertreatment system(s) when it’s not
• Poor vehicle maintenance, or failure to maintain the SCR system
• Removing or tampering with components of the emission-control system
• A certification test that is so unreflective of operating conditions encountered in real on-road driving that it’s possible for the vehicle to meet emissions limits over the test cycle without cheating or manipulating the test and still not be engineered to meet them in normal operation

To start with, better vehicle emissions regulations are essential.

• Certification tests should more adequately represent real-world driving. The European Union’s Real Driving Emissions test is the primary example of regulators addressing this issue at the moment. Additional changes to the RDE could bring emissions down to near the legal limit.
• Compliance monitoring and enforcement measures must be certain and effective. Monitoring should include testing and real-world measurement by both regulators and independent third parties. Ultimately, to ensure that pollutant emissions limits are met, the cost of compliance for manufacturers must be less than the cost of noncompliance. Important steps include improving test procedures and instituting emission recall programs.
• Inspection of maintenance programs must be in place to ensure that vehicle owners maintain vehicles and don’t tamper with emission controls.

Beyond direct emissions regulations, there are policy options that can affect the makeup and operation of the vehicle fleet in ways that would effectively reduce NOx pollution. Scrappage programs to get older vehicles off the road accelerate the benefits from direct tailpipe emission regulations that only affect new vehicles. Some countries have already used scrappage programs, such as China, Germany, Japan, and the United States. Policymakers can also incentivize the adoption of electric vehicles that have zero tailpipe emissions. These incentives are becoming more and more popular as governments around the world see the public health and climate benefits of electric vehicles.

Low-emissions zones can also reduce the number of high-emitting vehicles operating in cities, where ambient NOx is a particularly urgent problem. A number of cities around the world are already pursuing this strategy. London, for example, has instituted a Low Emission Zone around most of its metropolitan area in which most larger vehicles that do not meet certain emission standards must pay a daily charge. In February 2017, London Mayor Sadiq Khan announced the city was pursuing an Ultra Low Emission Zone in central London in which all vehicles that do not meet tighter emission standards must pay a daily charge as well. Mexico City and Paris have gone even farther, announcing that they will ban all diesel vehicles from their cities by 2025. This is quite a blunt instrument, but these major cities are taking steps in the absence of federal or supranational action.

Edited by Stephen Naimoli and Joe Schultz, with Rachel Muncrief, John German, and Oscar Delgado

Sources

State of Global Air, “Current Health Impact”

U.S. EPA, “Health effects of ozone pollution”

Reuters, “China to scrap millions of cars in anti-pollution push”

Der Spiegel, “Cash for Clunkers: Car-Scrapping Plans — Germany’s Lessons”

Reuters, “Japan to offer cash to scrap old cars, buy new ones”

U.S. Department of Transportation (archived), “CARS: Car Allowance Rebate System”

Transport for London, “About the LEZ”

City of London, “Mayor plans to introduce Ultra Low Emission Zone in April 2019”

C40 Cities, “Daring Cities Make Bold Air Quality Commitment To Remove All Diesel Vehicles By 2025”

Further reading

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

Impacts and mitigation of excess diesel NOx emissions in 11 major vehicle markets

Impact of improved regulation of real-world NOx emissions from diesel passenger cars in the EU, 2015−2030

A historical review of the U.S. vehicle emission compliance program and emission recall cases

Overview of the heavy-duty vehicle market and CO2 emissions in the European Union

Estimated cost of emission reduction technologies for LDVs

Electric vehicle capitals of the world: Demonstrating the path to electric drive