Gaseous Emissions

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• Nitrogen Oxides
• Hydrocarbons
• Carbon Monoxide
• Sulfur Dioxide
• Nitrous Oxide
• Hydrogen

Nitrogen Oxides—NOx

Nitrogen oxides—as commonly defined by emission regulations and regulatory measurement protocols—include two gases: nitric oxide NO and nitrogen dioxide NO₂. Physical properties of both gases are listed in the table on the right [175]. From the chemical point of view, there are several other nitrogen oxides. One of them, nitrous oxide (N₂O), is discussed below.

NOx is considered one of the critical pollutants found in emissions from all types of internal combustion engines. Nitrogen oxides are highly active ozone precursors playing an important role in the smog chemistry. They can also form secondary nitrate particulates in the atmosphere.

Concentrations of NOx in untreated diesel exhaust are typically between 50 and 1000 ppm. If concentrations are given in mass units, NOx is usually expressed as NO₂ equivalent.

Nitric oxide (NO) is a colorless and odorless gas. In the laboratory, it may be synthesized directly from nitrogen and oxygen under high temperature and pressure:

N₂ + O₂ ↔ 2NO - 182.4 kJ/mole(1)

The negative heat effect represents an endothermic reaction. Equation (1) can also represent the overall reaction of NO formation from the elements in the engine cylinder, where temperature and pressure are high. The exact NOx formation mechanisms in the diesel engine, however, are much more complex; they were discussed under Emission Formation in Diesel Engines.

At low temperature and pressure, the chemical equilibrium moves to the left side of Equation (1). Thermodynamically, nitric oxide has a tendency to decompose to nitrogen and oxygen under conditions in diesel exhaust. The rate of decomposition, however, equals practically zero and NOx control from diesel engines requires sophisticated in-cylinder and aftertreatment techniques.

In older, naturally aspirated diesel engines, approximately 95% of nitrogen oxides were composed of NO and only 5% of NO₂. The proportion of NO₂ in total NOx in turbocharged diesel engines (without aftertreatment) is typically higher, reaching up to about 15%. According to British data, the fraction of NO₂ in vehicle NOx emissions (all fuels) increased from around 5-7% in 1996 to 15-16% in 2009 [2338]. NO can be oxidized by oxygen into nitrogen dioxide:

2NO + O₂ ↔ 2NO₂ + 113.8 kJ/mole(2)

NO₂ is a red-brown gas of an unpleasant irritating odor. NO₂ is extremely reactive, exhibits strong oxidation properties and is more toxic than nitric oxide. NO₂ chemistry also plays a role in several types of emission control catalysts, where NO₂ reactions may include oxidation of hydrocarbons, carbon monoxide as well as diesel particulates.

At ambient conditions, the equilibrium of the reaction is shifted to the right, but the reaction rate is slow, Figure 1 [3727]. At low NO concentrations on the order of a few ppm—which can be representative of exposures to engine emissions—a number of days may be required to achieve a substantial level of conversion to NO₂.

It should be noted that the kinetics of NO → NO₂ oxidation shown in Figure 1 is applicable in environments without ozone or UV radiation (sunlight), for instance in underground mining. In the presence of ozone, NO can be oxidized very fast according to Equation (3), while a number of further reactions involving ozone and various nitrogen oxides are catalyzed by UV radiation. Hence, the rate of NO oxidation in polluted urban air will also depend on O₃ concentration and other variables.

NO + O₃ → NO₂ + O₂(3)

With increasing temperatures, the equilibrium of Equation (2) is shifting to the left, but NO₂/NOx ratios of 70-80% are still possible at temperatures typical for diesel engine exhaust. The reaction rate of NO oxidation can be accelerated by catalysts. Therefore, the NO₂/NOx ratios are typically much higher in diesel engines equipped with catalysts or catalytic particulate filters than in engines without aftertreatment. The average NO₂/NOx ratio in US 2007 heavy-duty truck engines (with catalytic aftertreatment) tested by the ACES study was 68% [2245]. Further production of NO₂ according to Equation (2) occurs spontaneously—albeit at a slower rate—in the NO-air mixture after exhaust gases are discharged into the atmosphere.

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