OC­TANE AND EN­GINE PER­FOR­MANCE

Car (South Africa) - - TECH -

To grasp the link be­tween the oc­tane num­ber and en­gine per­for­mance, it is im­por­tant to un­der­stand max­i­mum brake torque (MBT), which is re­alised at a spe­cific speed and load con­di­tion of the en­gine when the spark tim­ing is op­ti­mal. En­gine-cal­i­bra­tion en­gi­neers spend hours on an en­gine dy­namome­ter to tune the ig­ni­tion-tim­ing map of the en­tire grid of speed and load points to re­alise MBT.

PART-LOAD CON­DI­TION

Dur­ing day-to-day driv­ing, an en­gine spends most of its time in a part-load con­di­tion, where the driver does not de­press the throt­tle all the way to the stop. With a fixed air­fuel ra­tio at 14,7:1 to op­ti­mise fuel econ­omy and emis­sions, the MBT curve dur­ing part load is il­lus­trated be­low. Ad­vanc­ing ig­ni­tion tim­ing any fur­ther than MBT tim­ing sim­ply sees a re­duc­tion in en­gine torque. The fuel-oc­tane num­ber has lit­tle in­flu­ence in this con­di­tion be­cause in-cylin­der pres­sures are rel­a­tively low.

FULL-LOAD CON­DI­TION

When max­i­mum per­for­mance is de­manded from an en­gine, the driver de­presses the ac­cel­er­a­tor fully (WOT) to a full-load en­gine con­di­tion. As the in-cylin­der pres­sures and tem­per­a­tures are now at their high­est, it is usu­ally im­pos­si­ble to reach MBT by ad­vanc­ing the ig­ni­tion tim­ing be­cause auto-ig­ni­tion (knock) sets in (ex­plained in the graphic be­low). This is called knock-lim­ited per­for­mance. By us­ing a fuel with a higher-oc­tane num­ber, the knock limit is pushed out and more tim­ing ad­vance is al­lowed, re­sult­ing in a gain in per­for­mance. In this con­di­tion, many man­u­fac­tur­ers en­rich the fuel mix­ture to fur­ther com­bat the on­set of knock and help cool the ex­haust valves, but this ob­vi­ously has a neg­a­tive ef­fect on fuel econ­omy.

HIGHER-OC­TANE FUEL PUSHES OUT THE KNOCK LIMIT

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