Nitrogen Oxides (NOx)

NOx nitrogen oxides are formed during combustion by various chemical reactions, in particular:

  • between O2 and N2 in the comburent air (Thermal NOx);
  • by oxidation of the nitrogen contained in the fuel (Fuel NOx);
  • with a two-stage mechanism in which the nitrogen in the air reacts with the HC radicals during the formation of cyan- and amino-radicals which oxidize into NOx (Prompt NOx).

The first can be described, simplifying, from the “Zeldovich mechanism” with three reactions in the gas phase:

O + N2 → NO + N

N + O2 → NO + O

N + OH → NO + H

By acting on combustion temperature, quantity of oxygen in the cylinder and duration of combustion, the formation of NO can be reduced.

The second reaction depends on the content of N2 (0.2-0.5% by weight) in the fuel, where it is present in various organic substances which volatilize in combustion, emitting by pyrolysis other volatile compounds which react further by oxidation to form NOx.

In the formation of Fuel NOx, the main variable of combustion is the fuel / air ratio; the formation slowly grows as the excess air grows above the stoichiometric indication. Vice versa, it decreases rapidly with the increasing of the fuel. Unlike Thermal NOx, the reduction in temperature slightly reduces the Fuel NOx.

The formation of the Prompt NO occurs through the so-called "Fenimore mechanism" whose reactions, originating from the nitrogen of the air, occur over a wide range of temperatures, and can be described as follows:

CH + N2 → HCN + N

N + O2 → NO + O

N + OH → NO + H

C2 + N2 → 2 CN

CN + O → C + NO

CN + O2 → CO + NO

The production of NO2, compared to the total NOx emitted, is relatively low (5-10%) and is formed by the oxidation of NO partly at high temperature with H2O radicals, and partly at low temperatures with O2:

NO + H2O → NO2 + OH

2NO + O2 → 2 NO2

The parameters that influence the formation of NOx are:

  • the combustion temperature
  • the mixing degree between fuel and air
  • the permanence of the fuel in the cylinder

In particular, the amount of NOx produced is higher when the combustion temperature is higher and when the cooling of the gases produced is faster, the process of cooling could prevent the decomposition into nitrogen and oxygen.

In the case of "rich" mixtures (ie with a low amount air), emissions with low nitrogen monoxide content (but high emissions of hydrocarbons and carbon monoxide due to incomplete combustion) are obtained due to the low temperature reached in the combustion chamber. In the case of "poor" mixtures (i.e. with a high amount of air) there are still low NOx concentrations in the emissions, but the excess of air cools the combustion chamber and this prevents good engine performance. When the gases are mixed with air at the exit of the funnel, a significant amount of nitrogen dioxide is formed by oxidation of the nitrogen monoxide by oxygen.