Gasoline

Gasoline, which is also spelled gasoline, is a mixture of volatile, flammable liquid hydrocarbons derived from petroleum.

From chemical view, gasoline is a complex mixture of hundreds of different hydrocarbons. Most are saturated and contain 5 to 12 carbon atoms per molecule.
Gasoline used in automobiles boils mainly between 30° and 200° C (85° and 390° F), the blend being adjusted to altitude and season. Aviation gasoline contains smaller proportions of both the less-volatile and more-volatile components than automobile gasoline.


Extraction of Gasoline

Gasoline was at first produced by distillation, simply separating the volatile, more valuable fractions of crude petroleum.
Later processes are designed to raise the yield of gasoline from crude oil. Its mechanism depends on splitting large molecules into smaller ones; these processes are known as cracking.
Thermal cracking, employing heat and high pressures, was introduced in 1913 but was replaced after 1937 by catalytic cracking, in which catalyst is used. The application of catalysts facilitate chemical reactions producing more gasoline.
Other methods used to improve the quality of gasoline and increase its supply include polymerization, converting gaseous olefins, e.g. propylene and butylene, into larger molecules in the gasoline range; alkylation, a process combining an olefin and a paraffin such as isobutane; isomerization, the conversion of straight-chain hydrocarbons to branched-chain hydrocarbons; and reforming, using either heat or a catalyst to rearrange the molecular structure.


Properties of Gasoline (Advantages and drawbacks)

Additives

Some gasoline factors such as alcohols and olefins can cause deposits accumulating on intake valve surfaces. Gasoline manufacturers now routinely use additives to prevent intake valve deposits (IVDs). A quality gasoline additive package to provide clean engine operation and fuel economy would include:
- Octane enhancers, to improve octane ratings.
- Anti-oxidants and metal de-activators, to inhibit gum formation and improve stability.
- Deposit modifiers, to reduce deposits, spark-plug fouling and pre-ignition.
- Surfactants, to prevent icing, improve vaporization, inhibit deposits, and reduce NOX emissions.
- Freezing point depressants, to prevent icing.
- Corrosion inhibitors, to prevent gasoline corroding storage tanks.
- Dyes, to give product color for safety or regulation.

Anti-Knock agents

Any "knock" caused by a fuel will rapidly mechanically destroy an engine. Many cars have anti-knock sensors which compensate by slowing spark timing, but this can reduce power and acceleration.

Composition

Gasoline specifications impose controls on the physical and performance properties of gasoline's constituents. The art of correctly formulating a gasoline that does not cause engines to knock apart; does not cause vapor lock in summer but is easy to start in winter; does not form gums or deposits; burns cleanly without soot or residues; and does not dissolve or poison the car catalyst or owner; is based on knowledge of the gasoline composition.

Corrosiveness

Sulfur in gasoline causes corrosion, and when combusted, it forms corrosive gases that attack the engine, exhaust, and environment. The copper strip corrosion test (measuring active sulfur) and the sulfur content specification (measuring total sulfur present) are used to ensure fuel quality.

Stability

Motor gasoline may be stored for up to six months, but consequently may form gums. Antioxidants and metal deactivators are added to reduce gum-formation from reactions of unsaturated hydrocarbons.

Volatility

Volatility is the tendency of petrol to vaporize, and is critical to engine performance. Petrol must vaporize readily to allow easy starting and drivability when the engine is cold (to prevent carburetor icing), but not so much that it begins to evaporate in fuel lines when the engine is hot (called vapor lock, which impedes fuel flow). Fuel volatility must therefore change with location and season. Volatility also affects vapor emissions. For environmental and health reasons, petrol must not be so volatile that evaporation from the fuel tank is excessive in hot weather. Volatility is controlled by distillation and vapor pressure specifications.

Requirements

- Enable good engine performance - this is achieved by controlling octane number and volatility.
- Allow engines to start and run well in cold or hot weather - this is achieved by controlling volatility.
- Let engines run reliably and efficiently for a long time - this is achieved by controlling stability and corrosiveness, and the tendency of the gasoline to form residues and deposits in engines and fuel systems.
- Limit evaporative and exhaust emissions for environmental and health reasons - this is achieved by controlling volatility, total aromatics and olefins, and by efficient fuel vehicle technology.
- Be removed of unnecessary health risks - this is achieved by limiting benzene content, blending components like MTBE, and additives like lead or manganese.
- Able to benefit from new engine and emissions control technologies - this is achieved by limiting sulfur content and controlling octane number and volatility.


Uses of Gasoline

- Fuel for internal-combustion engines.
- A solvent for oils and fats.
- It became the preferred automobile fuel because of its high energy of combustion and capacity to mix readily with air in a carburetor.



Damages of Gasoline

Gasoline is full of toxic chemicals, but the biggest danger remains the flammability, and relative hazards to human health should be kept in perspective.


References:

- Fundamentals Petroleum Refining book, first edition 2010