What is octane all about or the purpose?

Answer 1

Almost all cars use four-stroke gasoline engines. One of the strokes is the compression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1. Theoctane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting. The compression ratio of your engine determines the octane rating of the gas you must use in the car. Can higher octane fuels give me more power? On modern engines with sophisticated engine management systems, the engine

can operate efficiently on fuels of a wider range of octane rating, but there

remains an optimum octane for the engine under specific driving conditions.

Older cars without such systems are more restricted in their choice of fuel,

as the engine can not automatically adjust to accommodate lower octane fuel.

Because knock is so destructive, owners of older cars must use fuel that will

not knock under the most demanding conditions they encounter, and must

continue to use that fuel, even if they only occasionally require the octane. If you are already using the proper octane fuel, you will not obtain more

power from higher octane fuels. The engine will be already operating at

optimum settings, and a higher octane should have no effect on the management

system. Your driveability and fuel economy will remain the same. The higher

octane fuel costs more, so you are just throwing money away. If you are

already using a fuel with an octane rating slightly below the optimum, then

using a higher octane fuel will cause the engine management system to move to

the optimum settings, possibly resulting in both increased power and improved

fuel economy. You may be able to change octanes between seasons ( reduce

octane in winter ) to obtain the most cost-effective fuel without loss of

driveability. Once you have identified the fuel that keeps the engine at optimum settings,

there is no advantage in moving to an even higher octane fuel. The

manufacturer's recommendation is conservative, so you may be able to

carefully reduce the fuel octane. The penalty for getting it badly wrong,

and not realising that you have, could be expensive engine damage. Does low octane fuel increase engine wear? Not if you are meeting the octane requirement of the engine. If you are not

meeting the octane requirement, the engine will rapidly suffer major damage

due to knock. You must not use fuels that produce sustained audible knock,

as engine damage will occur. If the octane is just sufficient, the engine

management system will move settings to a less optimal position, and the

only major penalty will be increased costs due to poor fuel economy.

Whenever possible, engines should be operated at the optimum position for

long-term reliability. Engine wear is mainly related to design,

manufacturing, maintenance and lubrication factors. Once the octane and

run-on requirements of the engine are satisfied, increased octane will have

no beneficial effect on the engine. Run-on is the tendency of an engine to

continue running after the ignition has been switched off, and is discussed

in more detail in Section 8.2. The quality of gasoline, and the additive

package used, would be more likely to affect the rate of engine wear, rather

than the octane rating. Can I mix different octane fuel grades? Yes, however attempts to blend in your fuel tank should be carefully

planned. You should not allow the tank to become empty, and then add 50% of

lower octane, followed by 50% of higher octane. The fuels may not completely

mix immediately, especially if there is a density difference. You may get a

slug of low octane that causes severe knock. You should refill when your

tank is half full. In general the octane response will be linear for most

hydrocarbon and oxygenated fuels eg 50:50 of 87 and 91 will give 89. Attempts to mix leaded high octane to unleaded high octane to obtain higher

octane are useless for most commercial gasolines. The lead response of the

unleaded fuel does not overcome the dilution effect, thus 50:50 of 96 leaded

and 91 unleaded will give 94. Some blends of oxygenated fuels with ordinary

gasoline can result in undesirable increases in volatility due to volatile

azeotropes, and some oxygenates can have negative lead responses. The octane

requirement of some engines is determined by the need to avoid run-on, not

to avoid knock. What happens if I use the wrong octane fuel?

If you use a fuel with an octane rating below the requirement of the engine,

the management system may move the engine settings into an area of less

efficient combustion, resulting in reduced power and reduced fuel economy.

You will be losing both money and driveability. If you use a fuel with an

octane rating higher than what the engine can use, you are just wasting

money by paying for octane that you can not utilise. The additive packages

are matched to the engines using the fuel, for example intake valve deposit

control additive concentrations may be increased in the premium octane grade.

If your vehicle does not have a knock sensor, then using a fuel with an

octane rating significantly below the octane requirement of the engine means

that the little men with hammers will gleefully pummel your engine to pieces. You should initially be guided by the vehicle manufacturer's recommendations,

however you can experiment, as the variations in vehicle tolerances can

mean that Octane Number Requirement for a given vehicle model can range

over 6 Octane Numbers. Caution should be used, and remember to compensate

if the conditions change, such as carrying more people or driving in

different ambient conditions. You can often reduce the octane of the fuel

you use in winter because the temperature decrease and possible humidity

changes may significantly reduce the octane requirement of the engine. Use the octane that provides cost-effective driveability and performance,

using anything more is waste of money, and anything less could result in

an unscheduled, expensive visit to your mechanic. Can I tune the engine to use another octane fuel? In general, modern engine management systems will compensate for fuel octane,

and once you have satisfied the optimum octane requirement, you are at the

optimum overall performance area of the engine map. Tuning changes to obtain

more power will probably adversely affect both fuel economy and emissions.

Unless you have access to good diagnostic equipment that can ensure

regulatory limits are complied with, it is likely that adjustments may be

regarded as illegal tampering by your local regulation enforcers. If you are

skilled, you will be able to legally wring slightly more performance from

your engine by using a dynamometer in conjunction with engine and exhaust gas

analyzers and a well-designed, retrofitted, performance engine management

chip.