Gasoline Properties

Additives
Anti-Knock agents
Composition
Corrosiveness
Dangers
Requirements
Stability
Volatility

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), which have an adverse effect on emissions. 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
Knocking is caused by a rapid rise in pressure after ignition in a spark-ignited engine, and the premature combustion of fuel, that is, prior to firing of the spark plug. Premature combustion is caused by the heat of compression as the piston moves forward to compress the fuel. 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 is a fuel produced by refining crude oil and is a mixture of hundreds of individual hydrocarbons. 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 creates corrosion, and when combusted will form 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.

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

Requirements

Gasoline should:

  • 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.

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 driveability 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

 

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