Different types of engine produce different exhaust emissions. This can be due to the fuel: air mixture or the fuel used in combustion. Conventional engines use a higher fuel: air mixture which, when combusted, produce more pollutants.
Lean burn engines require a much lower fuel to air mixture. This results in their being a lower concentration of pollutants. However, after most of the fuel has been combusted, the concentration is too low and some of the fuel goes out the exhaust, increasing hydrocarbon concentrations in the atmosphere. Due to the high air content in the mixture, there is a lot of oxygen released into the atmosphere, and due to the lower temperatures created in the engine, lower NOx emissions.
Diesel engines produce lower emissions for Hydrocarbons, CO, CO2 and NOx, and still produce the highest oxygen levels. This is because diesel engines use a different fuel to the conventional and lean burn engines.
The table below shows the levels of pollutants:
Atmospheric nitrogen can lead to the production of acid rain. NO emissions leave the exhaust and remain in the lower atmosphere where they are oxidized to NO2.
2NO + O2 → 2NO2
Or...
NO + O3 → NO2 + O2.
This NO2 can then react with OH radicals in the environment to form nitric acid which
mixes with water vapour and returns to Earth as acid rain:
NO2 + OH → HNO3
Heterogenous catalysts are now used to reduce the levels of air pollution. These work by adsorbing the reactants onto the surface of the catalyst. This weakens the bonds between atoms and the molecules break. This allows new bonds to form between the reactants to form the products. The products then diffuse away from the catalyst, leaving it intact.
Noxer blocks are an example of Heterogenous catalysts. They contain a 5-7mm thick coating of titanium dioxide, which acts as a photocatalyst, using UV rays from the sun to render NOx into a harmless nitrate.
As the titanium dioxide absorbs UV rays, electrons are excited to higher energy levels which react with oxygen, to form a superoxide ion
-
O2 + e- → O2-
-
H2O → H+ + OH + e-
-
Overall reaction is therefore: H2O + O2 → H+ + O2- +OH
Once the OH radical has been formed, the electron on the superoxide ion returns to the titanium. Once the OH radical has been formed it can react with the NOx to form a hydrogen ion and a nitrate ion.
NO2 + OH → H+ + NO3-
The superoxide formed from step 2 is also capable of forming nitrate ions through reacting with NO:
NO + O2- → NO3-
Once it rains, all the nitrate ions are washed away or soak into the concrete block the titanium dioxide is mounted on.
3 Way catalytic converters are designed to prevent the emissions of dangerous chemicals, by removing them from the exhaust gasses. 3 way catalytic converters remove carbon monoxide, hydrocarbons and nitrogen oxides.
To remove carbon monoxide, a reaction occurs between CO and oxygen which creates carbon dioxide.
2CO + O2 → 2CO2
To remove unburnt hydrocarbons, a reaction with oxygen occurs, to produce water vapour and carbon dioxide.
C7H16 + 11O2 → 8H2O + 7CO2
To remove nitrogen oxides, a reaction with carbon monoxide occurs to produce carbon dioxide and nitrogen.
2CO + 2NO → 2CO2 + N2
Under a “stoichiometric” fuel mixture, optimum conversion of the pollutants occurs. There is also compound called Ceria present on many catalytic converters, which improves the performance of the catalyst, by storing oxygen when the exhaust mixture is lean and releasing oxygen when the mixture is rich. This allows the oxidation of pollutants even when there is very little oxygen in the exhaust mixture.
