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If I remember correctly from the old days in petroleum college (Азербайджанский Институт Нефти и Химии имени Азизбекова - which i never finished!!!)...
AKI - anti knock index...
most BMW engines, (as well as most other sport engines) are high compression engines. High compression engines achieve higher horsepower (power rating) per volume of cylinder displacement.
High compression engines have pistons that travel inside the cylinders a bit further and as a result, compress the air/fuel mixture to higher level of pressure.
In accordance with isothermic, isobaric and isovolumetric (isometric) laws, in a constant volume (the size of the cylinder), when the pressure builds up, it also changes the temperature of the gas mixture... higher pressure (further travel of the piston), higher temperature... Higher temperature, more chances for the gas/air mixture to self-ignite before the spark from the spark plug is introduced... If the mixture is self-ignited, the resulting combustion expands the gas, and pushes the piston out, as the internal pressure inside the chamber increases and overpowers the external pressure excerted onto the piston (from crank-shaft), thus resulting in mechanical KNOCK, which essentially having two powerful forces working against each other - one is pushing the piston INTO the chamber, the other one is pushing the piston OUT of the chamber... This is the main principle of a diesel engine, that has no spark plugs (glow plugs that provide additional source of heat inside chamber) - the diesel fuel ignites as a result of pressure, and as a side effect, the diesels have huge torque...
The special additives added to the fuel make the gas less "flammable" by increasing the "flash point" of self-igniting to certain level...
when a manufacturer specifies a certain AKI in its fuel, it has tested the engine to perform adequately using that fuel without "knocking" or, having the "knocking" managed by the special hardware/software.
Using a higher octane fuel, than specified, (93, instead of 91) will result in less intervention by the said software, and using minimally qualified AKI rated fuel will provide normal operation with accassional intervention of the "knock" software.
the higher octane fuels do NOT pack more energy per liter/gallon/ounce of weight or volume, they just have higher self-igniting point.
the oil producers charge more for this type of fuel because it geared towards sports and high end vehicles, and the notion is, if you got that car, you gotta pay for it. Besides, since the profit margin is much better on these fuels, the oil producers add as a bonus some detergents and other chemicals (some of them are not added but rather not "removed", thus making it even more profitable) to entice the buyers into buying their products...
so, M5james was right, if it says use minimum of AKI91 - then gotta use that...
Last edited by TerminatorX5; 08-26-2014 at 03:43 PM.
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