Gasoline Octane: Gasolines are most commonly rated based on their   Antiknock Index (AKI),
    a measure of octane quality. The octane is a measure of gasoline's ability to resist knock,   or
    auto-ignition. The fuel-air mixture in the cylinder of a spark ignition engine will,   under certain
    conditions, auto-ignite.This pre-firing may cause an audible "ping"or knock and may also cause
    an engine to continue running when turned off. Loss of power and damage to an engine can  oc-
    cur when knocking is severe and prolonged.

    The AKI of a motor fuel is the average of the   Research Octane Number (RON) and  Motor
    Octane Number (MON) or (R + M)/2. This is also the number displayed on the black    and
    yellow octane decal posted on the gasoline pump (U. S. only). Otimum performance and fuel
    economy is achieved when the AKI is adequate for the engine in which it is combusted. There is
    no advantage in using gasoline with a higher AKI than the engine requires to operate knock-free
    (Note: See also my explanation on how to detect knock-free operation in the "fuels" page).

    With lead (Pb) practically removed from gasoline (except for the Philippines until a few months
    from now), engine manufacturers have turned to other types of octane enhancers that do not
    pollute the air or the environment.

       Ethanol as an Octane Booster

    Ethanol is a grain alcohol extracted from corn and other grains. Fuel ethanol is blended with
    base gasoline to 10% by volume to increase the former's octane and additionally, to reduce
    pollution. This is  a favored approach by many mid-western states in the U.S. where   corn
    and wheat are grown. A 10% blend of ethanol is the optimum as an excess of it will actually
    increase, rather than decrease, harmful emissions.

    Another type of alcohol, methanol, has been used as an octane boosting additive.

       MTBE and Reformulated Gasoline

    MTBE, or methyl tertiary butyl ether, is an ether that has gained a lot of approval from petro-
    leum refiners for many years now, not only because it enhances the octane of base gasoline but
    also because it oxygenates gasoline. Oxygenated additives have the ability to reduce    carbon
    monoxide emissions and ozone/smog. MTBE contains oxygen atoms and  therefore    partially
    oxidizes gasoline especially during the winter months when combustion is incomplete,  thereby
    producing CO.

    MTBE by 2% volume plus 10% ethanol  has been approved by the US-EPA for oxygenated or,
    reformulated gasoline. Reformulated gasoline typically contains between 2.7% to 4.1%   total
    oxygen content. A fuel's total oxygen content is defined as the weight percent of    chemically
    bonded oxygen in alcohols and ethers.
 

Diesel Fuel Cetane:The cetane rating is a measure of an important combustion characteristic
    of a fuel in a compression ignition (diesel) engine. A long ignition delay (low cetane) in a die-
    sel engine will result in rapid pressure rise that can cause undesirable audible knock,    high
    stresses and severe engine vibration. Also, difficult staring in cold weather, misfiring and ex-
    cessive white smoke often result from too low cetane. The official cetane rating is determined
    in a special engine by ASTM D 913, Standard Test Method for Cetane Number of     Diesel
    Fuel Oil.

    The Cetane Index (which is differentiated from cetane number) is derived from a formula
    for estimating the cetane number of diesel fuel, utilising API Gravity (American Petroleum
    Institute's specific gravity of liquids expressed in whole numbers) and distillation tempera-
    tures.