Exhaust gas composition
In the complete combustion of a so-called CxHy fuel, consisting only of C and H atoms, the exhaust gas contains the components oxygen (O2), nitrogen (N2), carbon dioxide (CO2), and steam (H2O). In real, incomplete combustion, carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxide (NOx), and particulates also appear in addition to the above components. As opposed to these substances, which are detrimental to human health, CO2, which is partially responsible for the greenhouse effect, is not viewed as a pollutant, since it does not pose a direct health hazard and appears as the final product of every complete oxidation of a hydrocarbon. A reduction of CO2 in the exhaust gas is thus only to be achieved through a reduction in consumption or through an altered fuel having a smaller amount of carbon with reference to its heating value.
A cloudless hemisphere is mainly transparent to short-wave solar radiation but is quite opaque to long-wave infrared rays emitted from the surface of the earth. Carbon dioxide (CO2) has the greatest blocking effect of all; water vapor and synthetic CFCs also play important roles in blocking the direct escape of infrared energy. The phenomenon of transparency to incoming solar radiation and blanketing of outgoing infrared rays is called the greenhouse effect. The increase of the CO2, water vapor, CFCs, and other gases, often called greenhouse gases (GHGs), eventually will result in a rise in air temperature near the earth’s surface. This is known as the global warming effect.
The formation of CO, HC and NOx is primarily contingent on the air-fuel equivalence ratio λ and the combustion temperature coupled with it, see Fig. 14.1. While CO and HC rise as products of incomplete combustion in a rich mixture (λ < 1.0), NOx formation is favored by a high temperature at sufficient levels of oxygen (λ ≈ 1.1). With a lean mixture (λ > 1.2), the combustion temperature sinks, so that NOx emissions fall off and HC emissions increase.
In Fig. 14.2, the compositions of the exhaust gasses (without a catalytic converter) of SI and diesel engines are shown. From this we see that the amount of pollutants has, from the point of view of energy, no significance in the engine process, but rather only from the point of view of its potential to jeopardize human health and the environment. Although the diesel engine only emits about a fifth the amount of pollutant that SI engines do, the absolute NOx concentrations are not very different. While in the case of the diesel engine, particulate matter also represent a critical magnitude besides nitrogen oxides, CO is the dominate pollutant component in the SI engine.
Carbon monoxide (CO)
Under a local lack of air (λ < 1.0), as a rule CO develops as a product of incomplete combustion. The oxidation of CO proceeds varyingly depending on the air-fuel equivalence ratio λ. In the sub-stoichiometric range (λ < 1.0), CO oxidation progresses, due to a lack of O2, in competition with H2 oxidation
(1) CO + OH• ↔ CO + H• and
(2) H + OH• ↔ H O + H•
Whereby the hydroxyl radical • OH and atomic hydrogen • H function as chain propagators. While reaction (2) is found in practical equilibrium, reaction (1) is kinetically controlled and thus advances much more slowly in the sub-stoichiometric range. With a climbing air ratio and temperature, the deviation of the kinetics of the OHC equilibrium becomes smaller and CO concentration thus decreases with an increasing air ratio λ. In the stoichiometric range (λ ≈ 1.0), reactions (1) and (2) can be described with a very good approximation as a gross reaction via the water gas reaction
CO + H2O ↔ CO2 + H2
Unburned hydrocarbons (HC)
In the combustion of CxHy fuels, no measurable HC concentrations appear "behind" the flame front assuming that λ > 1. HC thus originates in zones that are not completely or not at all involved in combustion. The unburned hydrocarbons are thereby composed of a number of different components, which are either completely unburned or already partially oxidized. Legislators today restrict only the sum of all HC components, which are usually determined with a flame ionization detector. In this way, no statement is made about the composition of these unburned hydrocarbons. The particular hazardous potential of certain components is thus not considered.
Particulate matter emission in the diesel engine
As the particulate matter content in the exhaust gas is designated the quantity of all substances that are captured by a certain filter after the exhaust gas has been diluted according to a defined method and cooled down to ϑ < 52 °C. Diesel particles consist up to 95 % of organic (PAH and soot) and up to 5 % of inorganic components.
Nitrogen oxides
In the troposphere, nitrogen oxides (NOx) favor the formation of ozone close to the ground and photochemical smog. In engine combustion, mainly nitrogen monoxide (NO) develops, which however is converted after a longer period of time almost completely into nitrogen dioxide (NO2) under atmospheric conditions. In combustion, NO can be formed in three different ways. In this case, we distinguish between so-called thermal NO , which is formed among the combustion products at high temperatures according to the Zeldovich mechanism from atmospheric nitrogen, so-called prompt NO , which develops already in the flame front via the Fenimore mechanism from air nitrogen, and finally so-called fuel NO , which is produced by nitrogen portions in the fuel.
Prepared and presented by Dr. Samer Mohamed