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PETROL ENGINE EMISSIONS
Exhaust emissions are a major contributor to air pollution due to the large number
of vehicles on the road. Initial refinements to engines and fuel systems were followed
up by the introduction of catalytic converters due to stricter global emissions
regulations. A catalytic converter treats the exhaust gas before it leaves the car
and removes about 90% of the pollutants. This is the main method of pollution control
in petrol engines.
The table shows the main emissions from a petrol engine.
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Non-toxic Emissions
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Nitrogen gas (N2)
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Air is 79% nitrogen gas, most of which passes straight through the engine
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Water vapour (H2O)
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Produced during combustion when the hydrogen in the fuel combines with oxygen in
the air
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Carbon Dioxide (CO2)
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Produced during combustion when carbon in the fuel combines with oxygen in the air
(a greenhouse gas which is the major contributor to global warming)
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Harmful Emissions
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Carbon Monoxide
(CO)
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Produced during combustion, this is a poisonous gas that is colourless and odourless
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Volatile Organic Compounds (VOC's)
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Consisting of unburned hydrocarbons and products of combustion reactions, these
can further react to form ground level Ozone
(O3), a major component of smog.
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Nitrogen Oxides (NOX)
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NO and NO2 contribute to smog and acid rain, and also cause irritation
to human mucus membranes.
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The harmful emissions are the three main regulated emissions, and also the ones
that catalytic converters are designed to reduce. Thus often referred to as a three-way
catalytic converter.
The converter uses two different types of catalysts. The reduction catalyst is the
first stage and uses platinum and rhodium to help reduce the NOx emissions by converting
harmful NO or NO2 molecules to harmless Nitrogen and Oxygen molecules.
The oxidation catalyst is the second stage and it completes the removal of unburned
hydrocarbons and carbon monoxide by burning (oxidising) them over a platinum and
palladium catalyst. This catalyst aids the reaction of the harmful carbon monoxide
and hydrocarbons with the remaining oxygen in the exhaust gas converting them into
non-toxic Carbon Dioxide and water vapour.
For the emission control system to work properly, the fuel/air ratio must be carefully
controlled so that all of the fuel is burnt using all of the oxygen in the air.
An essential part of the catalytic conversion process is a control system that monitors
the exhaust stream, and uses this information to control the fuel injection system.
An oxygen sensor tells the engine computer how much oxygen is in the exhaust. The
engine computer can then increase or decrease the amount of oxygen in the exhaust
by adjusting the air-to-fuel ratio. This control scheme allows the engine computer
to ensure that there is enough oxygen in the exhaust to allow the oxidation catalyst
to burn the unburned hydrocarbons and CO.
Since the late 1980's, three-way catalyst systems have been universally employed
on petrol-engined vehicles in the USA, Japan and Europe, enabling increasingly stringent
emission standards to be met. Their use in South Africa continues to increase, and
will by 2008 be compulsory for all new petrol vehicles.
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