Question

Explain the term ‘knocking’. What is the relationship between the structure of hydrocarbons and knocking?

Answer 1

Sharp sounds produced by early burning of a portion of the compressed air-fuel mixture in the cylinder of an internal-combustion engine. The charge burns in a fully operating engine, with the flame front moving smoothly across the combustion chamber from the moment of ignition. However, depending on the composition of the fuel, at high compression ratios, some of the charge may spontaneously ignite ahead of the flame front and burn in an uncontrolled manner, resulting in powerful high-frequency pressure waves. The engine parts vibrate as a result of the pressure waves, resulting in an audible knock.

Complete answer:
Overheating of the spark-plug points, erosion of the combustion chamber surface, and rough, inefficient operation can all result from knocking. It can be avoided by modifying engine design and operation variables such as compression ratio and burn time; however, the most common method is to burn gasoline with a higher octane level.
Straight-chain alkanes burn more evenly than branched and cycloalkanes. Long alkanes (${C_7}{H_{16}}$) burn unevenly, but short alkanes (${C_4}{H_{10}}$) burn uniformly. When compared to alkanes, alkenes burn more evenly.

Note:
The common internal combustion engine burns the fuel/air mixture in the cylinder in an orderly and regulated manner under ideal conditions. The spark plug initiates combustion 10 to 40 crankshaft degrees before top dead centre (TDC), depending on a variety of conditions such as engine speed and load. This ignition advance gives the combustion process enough time to reach peak pressure at the optimal time for maximal work recovery from expanding gases.