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IGNITION

Initiation of a combustion reaction process i.e. Starting of flames in gaseous

phase.

Ignition of gaseous fuels is easier than liquid and liquid easier than solid fuels.

Example: Aircraft combustion chamber, flame extinction can result due to rain

droplet ingestion.

Understanding IGNITION phenomenon is crucial for the calculation of energy

required for ignition and location of igniting source.

Two basic methods of ignition: ‘Spark ignition (=10kV)’ and ‘Naked flame

ignition ~ 20J’.

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Octane Number

• The octane 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.

• ‘OCTANE’ is a hydrocarbon, a colourless liquid obtained during crude

refining.

• Higher the octane number, higher the compression ratio the fuel mixturecan withstand.

• One member of the octane family, isooctane, is used as a reference

standard to benchmark the tendency of gasoline/petrol or LPG fuels to

resist self-igniting.

• Heptane handles compression poorly.

• Thus, an 87 octane fuel has 87% Iso-octane and 13% Heptane.

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• It is possible for a fuel to have a Research Octane Number (RON) more

than 100, because ISO-octane is not the most knock-resistant substance

available.

• Racing fuels, avgas, LPG and alcohol fuels such as methanol may have

octane ratings of 110 or significantly higher.• Typical "octane booster" gasoline additives include MTBE, ETBE,

isooctane and toluene.

• Lead in the form of tetraethyl lead (TEL) was once a common additive,

but its use for fuels for road vehicles has been progressively phased-out

worldwide, beginning in the 1970s.

• Typical "octane booster" gasoline additives include Methyl tert-butyl ether

(MTBE), Methyl tert-butyl ether (ETBE), isooctane and toluene.

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Research Octane Number (RON)

The most common type of octane rating worldwide is the Research Octane

Number (RON).

RON is determined by running the fuel in a test engine with a variable

compression ratio under controlled conditions, and comparing the results withthose for mixtures of iso-octane and n-heptane.

Motor Octane Number (MON)

There is another type of octane rating, called Motor Octane Number (MON),which is a better measure of how the fuel behaves when under load, as it is

determined at 900 rpm engine speed, instead of the 600 rpm for RON.

MON testing uses a similar test engine to that used in RON testing, but with apreheated fuel mixture, higher engine speed, and variable ignition timing to

further stress the fuel's knock resistance.

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Depending on the composition of the fuel, the MON of a modern gasoline will

be about 8 to 10 points lower than the RON, however there is no direct linkbetween RON and MON.

Normally, fuel specifications require both a minimum RON and a minimum

MON.Anti-Knock Index (AKI)

In most countries, including Australia and all of those in Europe the "headline"

octane rating shown on the pump is the RON, but in Canada, the United States,Brazil, and some other countries, the headline number is the average of the

RON and the MON, called the Anti-Knock Index (AKI, and often written on

pumps as (R+M)/2).

It may also sometimes be called the Pump Octane Number (PON).