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
- Deposit modifiers, to reduce deposits, spark-plug fouling and
- 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
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.
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.
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
Gasoline is full of toxic chemicals, but the biggest danger remains the
flammability, and relative hazards to human health should be kept in
- 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
- 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.
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 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