The burgeoning demand for energy, with consumption met primarily by fossil-
fuel combustion, has rendered atmospheric pollution one of the severest of our modern-
day societal challenges. The quality of the air we breathe is influenced by many different variables. Along with the emissions produced by industry, domestic households and power stations, the pollutants from road traffic are of major significance in this respect. We are left in consequence with two major requirements set to contribute much to the design of the combustion engines of today and tomorrow, namely eco-
compatibility and efficiency.
The internal combustion process produces water (H2O), carbon dioxide (CO2) and nitrogen (N2), plus the pollutants carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), sulphur dioxide (SO2) and – primarily in the case of diesel engines – particulate matter (PM).
As national and international regulations governing emissions and atmospheric pollution become ever more stringent, to the point at which further optimisation of in-engine and on-engine measures alone no longer suffices, post-combustion exhaust aftertreatment systems emerge as absolutely essential.
Diesel engines are the primary source of emission solids such as particulate matter (PM, also known as black carbon, BC). These solids are the result of incomplete combustion. Particulate matter has a verifiably carcinogenic effect on the human body, because the smaller these soot particles are, the easier it is for them to pass through our lungs and into our bloodstream and other organs. Science has now also demonstrated that PM emissions are partly responsible for global warming (climate change), because, on account of their dark/black colour, soot particles absorb the rays of the sun and then radiate this energy as heat back to the atmosphere in the immediate environment. In the Arctic, in particular, soot is condemned as being the second most powerful driver of climate change. This in turn means that particles of soot released to the atmosphere contribute significantly to the accelerated rate at which ice and glaciers are melting in Arctic latitudes, and also in the high mountain regions and glacial zones of the continental landmasses.
Sulphur dioxide (SO2
) is a colourless gas with a pungent odour and severely irritant effect that is released as the result of the sulphur content in automotive fuel. It reacts with water and sunlight to form a sulphurous acid, which, as one of the causes of acid rain, is responsible for forest dieback.
Nitrogen oxides (NOx) is the collective term for compounds of nitrogen and oxygen. All combustion processes release nitrogen oxides. Nitrogen monoxide (NO) and nitrogen dioxide (NO2
) are the two most common nitrogen oxides released by the processes that take place inside the internal-
combustion engine. NO is a colourless, odourless and tasteless gas that gradually converts in the ambient air to nitrogen dioxide or NO2
is a pungent and toxic gas. Nitrogen oxides figure among the root causes of forest dieback (acid rain); together with hydrocarbons they are responsible for causing smog.
As a collective term, "hydrocarbons" covers all chemical compounds of carbon (C) and hydrogen (H). HC emissions are due to incomplete combustion, although they are also released when fuel evaporates. Hydrocarbons are carcinogenic if exposure is prolonged. Partially oxidised hydrocarbons have an unpleasant odour and in sunlight they form secondary products that irritate mucous membranes and are among the primary causes of summer smog.
Insufficient intake of air results in incomplete combustion and the formation of carbon monoxide (CO). CO is a colourless, odourless and tasteless gas. When inhaled, carbon monoxide prevents the blood's absorption of oxygen and, depending on the concentration, causes headaches, nausea, or even carbon-
monoxide poisoning. Oxidation processes convert CO to CO2
Although proportionately the main constituent of the air inducted by the engine, nitrogen (N2
) has no part to play in the combustion of the fuel. However, it also accounts for the highest proportion of exhaust gas.
Carbon dioxide (CO2
) is present in atmospheric air and is partly responsible for the greenhouse effect and the associated phenomenon of global warming. The quantity of carbon monoxide released is directly proportionate to fuel consumption and consequently, the only way to reduce it is to minimise consumption.