The knock rating of an SI (spark-ignition) engine refers to its ability to resist knock, which is an undesirable phenomenon that occurs when the air-fuel mixture in the combustion chamber ignites spontaneously instead of being ignited by the spark plug. Knock can lead to engine damage and reduced performance.

To determine the knock rating of an SI engine, a mixture of two compounds is used. The compounds chosen for this mixture should have similar performance characteristics to the fuel being used in the engine. In this case, the mixture consists of cetane and alpha-methylnaphthalene.


Cetane is a compound that is used to measure the ignition quality of diesel fuel. It has a high cetane number, which indicates that it ignites easily under compression. However, in the case of an SI engine, cetane is used to simulate the knocking behavior of the fuel. It has a tendency to knock when exposed to high pressures and temperatures, making it a suitable compound for evaluating the knock resistance of an SI engine.


Alpha-methylnaphthalene is a compound that is used as a reference fuel in knock rating tests. It has a low propensity for knock, meaning it resists knocking under high pressures and temperatures. By mixing alpha-methylnaphthalene with cetane, the performance of the engine can be matched, and the knock rating can be determined.


Among the given options, option C (cetane and alpha-methylnaphthalene) is the correct answer. This is because cetane is used to simulate knocking behavior, and alpha-methylnaphthalene is used as a reference fuel with a low propensity for knock. By matching the performance of the engine with this mixture, the knock rating can be determined accurately.


The knock rating of an SI engine is determined by matching the engine's performance with a mixture of cetane and alpha-methylnaphthalene. This combination allows for accurate evaluation of the engine's knock resistance, which is crucial for optimizing performance and preventing damage.