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Formation of the exhaust emissions components

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IC Engine Emissions

 

HC – Hydrocarbons

Hydrocarbons in the exhaust are due to incomplete combustion (unburned and partially burned fuel and also some lubricating oil) due to non-homogeneous combustion conditions within the combustion chamber. Hydrocarbons will tend to be lower in the exhaust than the concentration in the cylinder during expansion due to oxidisation which can continue during the exhaust process. HC’s are higher from SI engines than CI engines by a factor of about five.

Hydrocarbons are produced:

  • Misfires and partial burns will produce very large amounts of hydrocarbons. Avoided via ensuring that the AFR (Air to Fuel Ratio) in each cylinder is slightly rich of the lean misfire limit under all operating conditions (including transients, idle and cold starts).
  • Flame quenching in crevice volumes is the major source of HC’s. Crevice volumes are regions where the mixture can get into but into which the flame cannot propagate. Some regions are the annulus above the top piston ring and around the spark plug central electrode.
  • Wall quenching and deposits. The flame extinguishes close to the combustion chamber walls (due to drop in temperature) and a thin layer of charge of perhaps 0.lmm remains unburned. However, most of  this layer is diffused into the bulk charge and undergoes oxidisation. The volume of unburned charge in the quench layer increased by the presence of deposits (smooth surface finish and good quality engine oils reduce these deposits) which roughen the surface.
  • Oil absorption. The oil layer left in the combustion chamber walls absorbs HC’s during compression and combustion and releases them during expansion.

 

HC’s can be reduced by a number of measures:

  • Minimising the combustion chamber surface area.
  • Reducing crevices volumes as much as possible.
  • Maintaining high wall and exhaust temperatures.
  • Positioning of the exhaust valve.
  • Minimising oil consumption.

 

Toxicity: HC’s can cause eye irritation, coughing and drowsiness and some species are considered carcinogenic. HC’s react with NOx in the presence of sunlight to form oxidants (which are irritants, odorants and participate in the production of photo-chemical smog) and some species are known greenhouse and/or ozone-depleting gases. Ethylene can cause damage to fruits and plants.

 

 

CO –Carbon Monoxide

It is primarily formed in the bulk of the charge due to incomplete combustion and is dependent on the air to fuel ratio. At lean mixtures, there is little CO generated. At rich mixtures, a large amount of CO will be present due to the partial oxidisation of the carbon. At extremely rich mixtures, a higher proportion of CO will be present and some of the carbon will produce soot. CO is mainly emitted by SI engines, the concentrations for CI engines being very low.

Toxicity: CO is a colourless, odourless and tasteless highly toxic gas. It interferes with absorption of oxygen by red blood cells, impairs perception, slows reflexes, causes drowsiness etc. Inhalation of 0.3% by volume can cause death within 30 minutes.

 

 

NOX – Oxides of Nitrogen

NOX emissions are produced at temperatures higher than 2000K in the combustion chamber when the dissociation of nitrogen and oxygen occur. A rapid increase in NOX occurs as the temperature is increased further. The concentration of NOX is dependent on the availability of O2 and therefore NOX declines when the mixture is rich. In SI engines, the proportion of NOx is less than 10%. It is higher in CI engines, especially at part load.

The production of NOx is highly temperature dependent and NOx concentration gradients occur across the combustion chamber due to temperature gradients. The highest values correspond to the charge near to the spark plug which is burned first and the lowest values to the last portions to burn.

NOx can be reduced by a number of measures:

  • Positioning of the spark plug well away from the exhaust value should reduce port emissions since more of the higher concentration charge will be retained in the cylinder.
  • Ignition timing significantly influences the level of NOx emissions. Increasing the advance, so that combustion occurs earlier in the cycle, increases the peak cylinder pressure and higher peak temperatures for burned gas are achieved. Also, the burned gases remain longer at high temperature. These two conditions promote the formation of NOx. An advance reduction of 10 crankshaft degrees can cut NOx emissions at constant power by about 20-30%.
  • Rate of change of charge temperature and the time that the burned gases are held at high temperature. Engine speed and combustion rate affect the times and the charge temperature. The resulting effect is therefore complicated with no simple general trends being apparent.
  • Using a diluent, such as EGR, to lower peak temperature is one of the most effective ways of reducing NOx emissions. A 50% reduction in NOX can be achieved with 10% EGR. The value of the EGR depends on the load, the valve settings, and particularly on the valve overlap. A higher overlap increases the dilution of the feed and decreases the NO emissions

 

Toxicity: NO is a colourless, tasteless gas which is rapidly converted to NO2 in the presence of oxygen. NO2 is a reddish brown, poisonous gas with a strong odour which can cause irritation in mucous membranes and destroy lung tissue. It increases susceptibility to viral infections such as influenza, causes bronchitis and pneumonia and increases sensitivity to pollen and dust in asthmatics.

 

Particulates – Solid Carbon Particulates

Solid carbon particles (i.e. soot) emissions are abnormal in properly-adjusted SI engines because they occur only with excessively rich carburetted mixtures. The exhaust of CI diesel engine contains solid carbon (soot) particles that are generated in the fuel-rich zones within the cylinder during combustion. Maximum density of particulate emissions occurs when the engine is under load at WOT. At this condition maximum fuel is injected to supply maximum power, resulting in a rich mixture and poor fuel economy.

Soot particles are clusters of solid carbon spheres. These spheres have diameters from 10-80nm (nm = 10-9m), with most within the range of 15-30nm. Up to about 25% of the carbon in soot comes from lubricating oil components which vaporise and then react during combustion. The rest comes from the fuel.

Toxicity: Particulates can easily be breathed into the lungs due to their size (approximately 0.1?m).  They can also produce unpleasant odours, reduce visibility (smoke) and soil buildings, materials and fabrics. Particulates are regulated and the legislation has become very severe in recent years.

 

CO2 – Carbon dioxides

Although no legislation exists at the present time for CO2, there is growing concern about this due to its greenhouse effect. Carbon dioxide accounts for about 90% of the total effective greenhouse gases emitted from a gasoline powered car exhaust (other greenhouse products include HC, CO, N2O and CH4).

CO2 can only be reduced by either reducing the fuel consumption and/or changing the fuel (e.g. hydrogen).

 

SO2/SO3 – Sulphur Dioxide/Trioxide

A small amount of sulphur is present in all hydrocarbon fuels (gasoline < 0.06%, diesel < 0.5% by mass). Also, some SO2 and SO3 is produced in the engine.

Toxicity:  SO2 and SO3 can then combine with water to produce sulphuric acid (H2SO4) and sulfuronsacid (H2SO3)  (ingredients of the acid rain). Many countries have laws restricting the amount of sulphur allowed in fuel, and these are continuously being made more stringent.

 

Lead

Lead was traditionally added to gasoline fuels to improve the octane rating and improve the engine durability. This is now regulated and can only be present in trace amounts in unleaded gasoline.

Toxicity: Lead Oxide accumulates in plant and animal tissue and is ingested in humans through the lungs and the gastrointestinal tract. It affects circulatory, reproductive, nervous and kidney systems and affects the learning ability of children. Lead has a major detrimental effect on exhaust catalyst performance, acting as a “poison”, therefore, only unleaded gasoline can be used with catalyst equipped cars.

 

Oxidants

Includes ozone, organic peroxides etc. These are produced when exhaust gas is exposed to sunlight.

Toxicity: Oxidants cause damage to forests and crops, cracking of rubber products and severely affects visibility and air quality. In humans, it causes eye irritation, physical discomfort and reduces resistance to colds and phenomena.

 

 


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