Not Just a Number: The Chemical Factors Determining Fuel RON

JAKARTA, KOMPAS.com – The octane number or Research Octane Number (RON) is often used as the main reference in selecting fuel. However, many people mistakenly believe that RON is just a number “added” to petrol to make it appear higher. In reality, behind that number lies a complex chemical factor that determines the quality of combustion inside the engine. “RON is determined by the composition of chemical compounds in the fuel, not just by adding something to increase the number,” said Tri to Kompas.com recently. Tri explained that in measuring RON, the fuel is compared to a reference mixture of isooctane and n-heptane. For example, RON 90 means the fuel’s characteristics are equivalent to a mixture of 90 per cent isooctane and 10 per cent n-heptane. Nevertheless, he emphasised that the petrol at fuel stations does not actually contain that direct mixture. The RON number only serves as an indicator of the fuel’s ability to resist detonation, which is influenced by its chemical structure. Furthermore, Tri stated that the main factor determining RON is the composition of compounds known as PIONA, namely paraffins, isoparaffins, olefins, naphthenes, and aromatics. “The combination of these compounds determines the fuel’s properties, including resistance to detonation and combustion stability,” he said. This PIONA composition also affects the formation of deposits in the engine, such as on the intake valve. If deposits accumulate excessively, the flow of the air-fuel mixture to the combustion chamber can be disrupted. Not only that, inefficient combustion also leads to higher exhaust emissions. Therefore, in international standards like the Worldwide Fuel Charter, the limit for deposits in the engine is regulated so as not to exceed a certain threshold. To maintain engine cleanliness, fuel producers usually add detergent additives. However, Tri emphasised that additives primarily function to keep deposits from becoming excessive, not as the main determinant of the octane number. “Additives are more for ensuring deposits do not become excessive, not for determining RON. RON is still determined by the chemical composition of the fuel,” said Tri.