The knock rating of an SI engine is determined by matching the performance of the engine with a mixture of:

Concept:According to standard practice, the antiknock value of an SI engine fuel is determined by comparing its antiknock PROPERTY with a mixture of two reference fuels, normal heptane (C7H18) and iso-octane (C8H18).Rating of SI engine fuel:The rating of SI engine fuels is based on their antilock property.Rating of a PARTICULAR fuel is ACCOMPLISHED by comparing its PERFORMANCE with that of a standard reference fuel which is usually a combination of iso-octane (C8H18) and normal heptane (C7H18) plus tetraethyl lead.Iso-octane being a very good anti-knock fuel is arbitrarily assigned a rating of 100 octane number.Normal heptane, on the other hand, has very poor anti-knock qualities and is given a rating of zero octane number.Octane number rating is an expression that indicates the ability of a fuel to resist knock in a SI engine. And the antiknock value of a CI engine fuel is determined by comparing its antiknock property with a mixture of two reference fuels, α-methyl naphthalene (C11H10) and cetane (C16H34).Rating of CI engine fuels:Knock rating of a CI engine fuel is found by comparing it with a reference fuel under prescribed working conditions.Reference fuel: normal Cetane (C16H34) which is assigned a Cetane number of 100 and α-methyl naphthalene (C11H10) which is assigned a Cetane number of zero.