.:Fuel Tech

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Fuel
The main type of fuel used in automobiles of the present is petrol (also known as gasoline). Petrol is distilled from Petroleum (also known as crude oil). This process of distilling petroleum started in the early 1900�s in Oil Company�s. The reaction that is taking place in the internal combustion engine is an exothermic combustion reaction. The chemical equation for a complete combustion of octane (figure 1) and oxygen is; 2 C₈H₁₈ + 25 O₂ = 18 H₂O + 16 CO_2. This says that the intake includes octane and oxygen, and the emissions of this product are water and carbon dioxide. Large amounts of energy are released from this reaction in the form of heat which increases pressure and which drives the piston. Because of this release in heat, this reaction is classified as exothermic. Because this reaction includes an organic molecule and oxygen, this means that the reaction is also classified as combustion.

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Figure 1 -�Octane molecule

Intake

Petrol is essentially made up of over a hundred hydrocarbons that have between 5 and 12 carbons in structure. These range from the compounds pentane to dodecane. Petrol is changed a lot to suit the engines needs whether it be for lubrication or for emission purposes. In the 1910�s, a law was created that prohibited people from storing petrol on property. Because of this Charles F. Kettering was forced to modify his internal combustion engine so it would intake kerosene instead of petrol. This did not work properly as it created a �knocking� (as seen in figure 2) and cracking of the pistons. This was due to �detonation.� This is when the remaining fuel and air mixture is combusted after ignition due to the heat and pressure. This then increases the pressure of the piston and can have bad effects on the engine. Today the above complications are handled using octane, which is known as �high octane fuel.� The difference between high octane fuel and normal petrol is the percentage of octane compared to n-heptane in the petrol. Obviously, high octane fuel has a higher percentage of octane and thus, lower percentage of n-heptane. N-heptane is the general compound that produces knocking and octane is knock resistant. High octane fuels are commonly more powerful then normal petrol. This means that when you let the same amount of both high octane fuel and normal petrol into the pistons (energy stored as chemical energy), you will get more energy from the high octane fuel (energy transformed into kinetic energy). The high octane fuel has a high output rating compared to the normal petrol and therefore has a higher efficiency. This is all possible because the high octane fuel lets the engine run at a higher compression ratio.

Figure 2 -�Consequences of long term knocking

Emissions
A few main compounds are emitted from internal combustion engines. Some are more dangerous then others and some are not as dangerous to the environment but still so damage. If we increase the efficiency of the engine, it decreases the carbon dioxide (CO2) emissions, but increases the nitrogen oxide emissions.� Nitrogen Oxide can represent a number of compounds that contain only nitrogen and oxygen (eg. N2O). Nitrogen oxide is a major source of air pollution (photochemical smog). If we decrease the amount of nitrogen oxide within emissions, we get a large amount of carbon monoxide (CO). Carbon monoxide is a dangerous compound. If only a 0.1% mixture of carbon monoxide is inhaled, it can be fatal, but because it is less dense then air, it will always rise to the higher parts of the atmosphere. Being high in the atmosphere and because of its chemical structure, it adds to the greenhouse effect and global warming. Exhaust catalysts can change what emits from the exhaust of a car using stoichiometry. The following reactions occur when the exhaust catalyst is present:�

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2H2 + O2� ->� 2H2O

2CO + O2� ->� 2CO2

CxHy + (x + (y/4))O2� ->� xCO2 + (y/2)H2O

2CO + 2NO� ->� N2 + 2CO2

CxHy + 2(x + (y/4))NO� ->� (x + (y/4))N2 + (y/2)H2O + xCO2

2H2 + 2NO� ->� N2 + 2H2O

CO + H20� ->� CO2 + H2

CxHy + xH2O� ->� xCO + (x + (y/2))H2�

The soul purpose of exhausts catalysts, is to make chemical reactions occur where environmentally dangerous compounds (such as hydrocarbons) are reacted together to create less harmful compounds. Sometimes more harmful compounds are produced by the exhaust catalyst but the overall result shows a decrease in compounds dangerous to the environment.

Last Modified 4/13/05 8:44 AM

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