In the process of internal combustion, numerous substances are released that are potentially of great damage to the environment. Here we shall be looking at > the processes in which the harmful outputs are formed, > effects of their output, often linked with > major global issues such as acid rain. When discussing such issues only the effects of exhaust gases from internal combustion are included.
Carbon Dioxide, CO2 is formed during internal combustion in engines using oil related fuel, especially when the three way catalyst oxidises Carbon Monoxide, CO, converting it into CO2. The source of carbon is the carbon-containing compounds from the fuel input, which fully oxidise.
Carbon Dioxide does not directly harm the human body. However, it has a great role in the process of global warming and is considered as the key “greenhouse gas” – its contribution to global warming is 57%.
This is because it is a great absorber of Infra Red radiation that is emitted towards the space from the earth surface.
Methods of reducing CO2 emissions are by either improving the heat efficiency of engines thus reducing the amount of fuel consumption, or by using alternative fuels not related to oil.
Carbon Monoxide, CO forms when, with rich fuel:air mixtures, there is insufficient oxygen to fully oxidise the carbon (the source being carbon-containing compounds from fuel input). In high temperature products, even with lean mixtures, dissociation ensures that there are quite significant levels of CO.
Times during combustion when the oxygen content is often insufficient for complete oxidation are upon starting the engine (when a high air:fuel mixture is necessary), during idling and during acceleration, when the temperature is low or reaction time is short during flame spread in air:fuel mixture (on the lean fuel side).
Carbon Monoxide is highly damaging to the human body. Instead of oxygen combining with red blood cells to form oxyhaemoglobin, essential for the oxygen to be transported around the body, the carbon monoxide combines with haemoglobin.
When CO is inhaled, it can be absorbed from the lung alveoli 300 times faster than oxygen. A higher concentration of Carbon Monoxide in the blood makes it increasingly difficult for the heart to pump blood through the arteries.
Hydrocarbons, formulaically represented in the form CxHy, are formed during the combustion process, though it is unburned hydrocarbons, formed by incomplete combustion of the carbon from the fuel that can cause problems.
The emissions of unburned hydrocarbons have numerous sources. During compression / combustion, increasing cylindrical pressure forces some gas into crevices, and much of this gas is unburned fuel:air mixture, much of it escapes the primary combustion process as the crevice is too narrow for the flame to enter – this gas which then leaves the crevices in the expansion / exhaust processes is a major source of unburned hydrocarbons. Hydrocarbons are an exhaust gas from spark ignition engines.
The walls of the combustion chamber are another source – a layer containing unburned / partially burned fuel:air mixture is left at the wall when the flame is extinguished – the porous deposits on the walls increase hydrocarbon emissions from the engine when the engine is operating.
Another source of unburned fuel is thought to be the engine oil left in a thin film on the cylinder wall, piston and maybe cylinder head – these oil layers can absorb and desorb hydrocarbon components of the fuel before and after combustion respectively, so a fraction of the fuel can escape the initial combustion process unburned.
A final source is from incomplete combustion due to bulk quenching of the flame in the fraction of the engine cycles where combustion is slow. These conditions are most likely to take place when the air:fuel ratio, spark timing and fraction of the exhaust recycled for emission control may not be well matched.
Problems of Unburned Hydrocarbons
Unburned hydrocarbons often result in great problems due to association with oxides of nitrogen, another exhaust gas of the internal combustion engine, discussed later. It can form ozone with oxides of nitrogen, which is a central nervous system depressant, and other hydrocarbons can cause the convulsion of CNS.
This association with oxides of nitrogen not only contributes to destruction of the ozone layer but also causes photochemical smog – which is a possible key cause of melanoma, a form of cancer.
Arguably the oxides of nitrogen, collectively represented formulaically as NOx, can be considered as the most harmful of the exhaust gases resulting from internal combustion because their affects can be seen in a variety of areas.
Nitric oxide, NO, forms throughout high temperature burned gases behind the flame through chemical reactions involving the 2 elements that do not achieve chemical equilibrium. The higher the temperature of burned gas, the higher the rate of nitric acid formation. So when combustion is done in the area closer to the theoretical air:fuel ratio, the Stoichiometric ratio, more nitric oxide is released as it is closer to complete combustion. Oxides of nitrogen are the greatest resultant exhaust gas from spark ignition engines.
In high temperature and combustion this reaction occurs to form nitric oxide: N2 + O2 –> 2NO
When this oxide cools in the presence of air it is further oxidised to nitrogen dioxide:
2NO + O2 –> 2NO2 – a high pressure is also necessary for such a reaction.
Oxides of nitrogen give a relatively minimal yet significant contribution to the greenhouse effect of 6% – this problem has been previously discussed.
Oxides of nitrogen have a rather short time persistence in the atmosphere and are an ozone precursor – the following equations show how NO and NO2 can break up ozone, O3…
(a) NO + O3 –> NO2 + O2 (b) NO2 + O –> NO + O2 (c) O + O3 –> 2O2
The effects of acid rain will be briefly discussed after this section. As previously discussed, oxides of nitrogen work with unburned hydrocarbon to produce photochemical smog.
Oxides of nitrogen can also be very harmful to the human body. It can cause dilation of air spaces in the lungs and nitrogen dioxide is also suspected to impair the respiratory system’s defence mechanism, with children being more susceptible.
However, arguably the key problem with NOx is its contribution to acid rain, along with sulphur dioxide, SO2. This will be discussed in more detail in the section titled Acid Rain and also briefly in the oxides of sulphur section.
Oxides of sulphur are formed by burning sulphur, a compound in the petroleum, with air, resulting in different oxides depending on the amount of oxygen available – compounds such as Sulphur Dioxide, SO2, and SO3 anhydrous sulphur / sulphur trioxide. As fuel in both light and heavy oil includes more sulphur, it is oxidised during combustion and then discharged.
Like oxides of nitrogen, the effects of oxides of sulphur are evident in a variety of forms.
In humans, Sulphur Dioxide, SO2, can irritate the throat, lungs and damage one’s respiratory syste, of there are fine dust particles of air. Oxides of sulphur combine with other substances of air to produce a visibility-reducing haze.
Sulphur Dioxide is also, along with oxides of nitrogen, the cause of acid rain that causes global devastation – the effects will be discussed in the next section. It can turn leaves yellow and decreases the growth rate of crops.
Lead, Pb, is sometimes an output of internal combustion – it is not a natural compound of fuel but is often added to petroleum for the molecules to act as branched chains as this is more cost effective (in the short term).
There are many problems associated with lead input so now Unleaded fuel is a more common alternative and the development of Lead Replacement Fuel is hoping to decrease the effects of lead.
Problems are mainly for humans – when there is a great proportion of lead, the body mistakes lead for calcium when ingested which can disrupt functioning enzymes. The effects of this include disabilities, heart and kidney disease, neurological deficits and strokes.
3 quarters of ingested lead is deposited in bones and tissues causing irreversible brain and kidney damage – particularly vulnerable are young children who have a growing nervous system.
In general, the amount of emissions from car engines is rising year by year as developing countries become more industrialised, and capitalist areas such as USA use private transport more and more.
However there are measures to control all of the global issues, such as United Nations Sulphur Protocol giving limits to a country’s sulphur emissions in a year.
Also the harmful gases such as Carbon Monoxide and Nitrogen Dioxide can be made harmless by reacting the two substances with use of a catalytic converter.