How octane number and cetane number are related with ic engine

Octane number:-

The hydrocarbon fuels used in spark ignition(S.I) engines have a tendency to causes engine knock when the engine operation conditions became severe. The knocking tendency of fuel in S.I engines is generally expressed by its octane number. The percentage, by volume, of iso-octane in a mixture of iso-octane and normal heptane, which exactly matches the knocking intensity of a given fuel, in a standard engine, under given standard operating conditions,  is termed as the octane number rating of the fuel. Thus, if a mixture of 50% iso-octane and 50% normal heptane matches the fuel under test, then this fuel is assigned an octane number rating of 50. If a fuel matches in knocking intensity a mixture of 75% iso-octane and 25% normal heptane, then this fuel would be assigned an octane number rating of 75. This octane number rating is an expression that indicates the ability of a fuel resists knock in an S.I engine.

Since, isooctane is a very good antiknock fuel, therefore it is assigned an A rating of 100 Octane number. On the other hand, normal heptane has very poor anti-knocking qualities, therefore it is given a rating of zero Octane number. These two fuels are Octane and normal heptane are known as primary reference fuels. It may be noted that the higher the Octane number rating of fuel the greater will be its resistance to knock and the higher will be the compression ratio, since the power output and specific fuel consumption are functions of compression ratio, therefore we may say that these are also Octane number rating. This fact indicates the stream importance of the Octane number rating in fuel for SI engines.

Note- the Octane number of petrol generally available is 80 to 100.

Cetane number:- the knocking tendency is also found in compression ignition(C.I) engines with an effect similar to that of S.I engines, but it is due to different phenomena. The knocking is C. I engine due to sudden ignition and abnormal rapid combustion of accumulated fuel in the combustion chamber. Such a situation occurs because of an ignition lag in the combustion of the fuel between the time of injection and the actual burning.

the property of ignition lag is generally measured in terms of Octane number. It is defined as the percentage by volume of Octane in a mixture of cetane and Alpha- methyl- naphthalene that produces the same ignition lag as the fuel being tested in the same engine and under the same operating conditions. For example, a fuel of cetane number 50 has the same ignition quality as a mixture of 50% cetane and 50% Alpha- methyl- naphthalene.

the city which is a straight-chain where good in ignition quality is assigned A certain number of 100 and Alpha methyl naphthalene which is a hydrocarbon with 4 ignition quality is assigned a zero cetane number.

Notes-
i) the knocking in CI in genes may be controlled by decreasing ignition lag. The shorter the ignition lag the laser is the tendency to knock.
ii) the cetane number of diesel oil, generally available is 40 to 55.

Testing of IC engines:-  the purpose of testing an internal combustion engine are

1) to determine the information which cannot be obtained by calculations.
2) to confirm the data used in design the validity of which is doubtful
3) to satisfy the customer regarding the performance of the engine.

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