Oxidation and Storage Stability

The fuel may remain in the storage and transportation systems for several weeks. During storage before it is consumed, fuel undergoes slow oxidation under the atmospheric conditions producing gummy substances.  Low temperature oxidation stability of gasoline is measured in terms of induction period (ASTM D 525) and existent gum test (ASTM D381). Induction period measures oxidation characteristics at a specified temperature and pressure in presence of oxygen denoting the time period when sharp drop in oxygen pressure occurs, which indicates the beginning of oxidation. The existent gum is a resin like substance that is  formed due to slow oxidation of gasoline during transportation, handling and storage which is present in the fuel sample. To improve oxidation stability of gasoline chemical additives known as antioxidants are used.

 The gum formed in gasoline during storage, leads to formation of deposits in carburettor, fuel injectors, intake manifold, on intake ports and intake valves, and in the combustion chamber. The gums clog fuel metering orifices, result in sticking of intake valves and form carbon deposits in the combustion chamber. Deposit formation in fuel system and combustion chamber lead to loss in fuel efficiency and

increase in carbon monoxide and unburned hydrocarbon emissions. Plugging of the fuel injector holes reduces fuel flow rate that disturbs the calibration of feedback controlled engine fuel management system.  A 30 percent flow reduction in port fuel injectors has been seen to increase HC emissions by 10 to 30 %.
The formation of deposits in the combustion chamber increases the effective engine compression    ratio.    These deposits being porous in nature  act like small crevices adsorbing hydrocarbons during compression. The hydrocarbons released from the combustion chamber deposits during expansion stroke escape combustion resulting in higher HC emissions. The combustion chamber deposits can increase HC and NO_x emissions by up to 10 to 15%. To prevent deposit formation in the engine intake, fuel system and combustion chamber, detergent/ dispersant type additives are added to gasoline.

Hydrocarbon Composition

Olefins and aromatic hydrocarbons are high octane gasoline components but affect engine emissions in several ways;

• Olefins have poor oxidation and storage stability and their presence leads to formation of higher existent gum.
• Reduction in olefin content lowers emissions of the air toxic 1, 3-butadiene.
• Lowering the content of smaller molecular weight olefins in gasoline, reduces ozone forming potential of evaporative hydrocarbons.
• Aromatics have higher adiabatic flame temperatures and hence, a tendency for higher NO formation.
• Combustion of aromatics leads to higher deposit formation in the combustion chamber.
• Benzene is a natural constituent of gasoline and also is formed in the combustion chamber due to dealkylation of alkylbenzenes. Lower benzene and aromatic content of fuel results in lower exhaust and evaporative benzene emissions, Benzene being carcinogenic in nature its content in gasoline is now controlled to less than 1% by volume in many countries.