Introduction
In today's world, the efficiency and health of internal combustion engines directly depend on the quality of the consumed fuel. One of the most important parameters determining gasoline quality is the octane number. This number indicates the fuel’s resistance to premature ignition in the engine’s combustion chamber. This article provides a specialized examination of the concept of octane number, its chemical structure, methods to increase the octane number, equipment used for its measurement, and its importance in the automotive and refining industries.
In combustion engines, fuel quality plays a decisive role in engine performance, efficiency, and longevity. The octane number, as an indicator of fuel resistance to auto-ignition, is a key factor in preventing potential engine damage. With advances in new engine technology and increased compression ratios, the need for fuels with higher octane numbers is more critical than ever.
What Is Octane? (Chemical and Physical Structure)
Octane (C₈H₁₈) is a member of the alkane (paraffin) family, consisting of eight carbon atoms and eighteen hydrogen atoms. Octanes have 18 structural isomers, each with different physical properties and combustion behaviors in engines.
- n-Octane (Normal Octane): Has a straight-chain structure. This isomer has lower thermal stability and is prone to premature ignition; therefore, its octane number is low (around 0).
- 2,2,4-Trimethylpentane (Iso-Octane): A branched isomer used as the reference for octane number 100 because it has very low tendency to auto-ignite.
The branched structure of molecules leads to greater stability and more controlled combustion in the engine, which is key to understanding octane differences.
What Is Octane Number?
The Octane Number or Octane Rating is a dimensionless value that measures a fuel’s ability to resist auto-ignition (knocking) inside the engine cylinder. The higher the octane number, the greater the fuel’s resistance to uncontrolled explosions.
The octane number of real gasoline is determined by comparing it to standard mixtures of iso-octane (octane number 100) and n-octane (octane number 0). For example, if a gasoline behaves like a mixture of 90% iso-octane and 10% n-octane, its octane number is 90.
Two main types of octane numbers:
- Research Octane Number (RON): Measured under mild laboratory conditions (low temperature, low engine speed), reflecting performance in city driving.
- Motor Octane Number (MON): Measured under more severe conditions (higher temperature, higher engine speed), reflecting performance under heavy load and high speed.
Anti-Knock Index (AKI) = (RON + MON) / 2
In many countries, such as the USA, this formula is used to display the octane number on fuel pumps.
Importance of Octane Number
- Preventing Knocking
Knocking occurs when fuel ignites prematurely and explosively before the proper time. This causes:
- Engine vibrations and unusual noise
- Severe damage to pistons, connecting rods, and cylinder heads
- Increased internal heat and pressure
High octane gasoline enables more controlled and smoother combustion.
2. Improved Thermal Efficiency
In engines with high compression ratios, using fuel with a high octane number improves thermal efficiency and reduces fuel consumption.
3. Reduced Emissions
High octane fuels have compounds that allow more complete combustion, resulting in lower pollutant emissions such as CO and HC in exhaust gases.
Factors Affecting Octane Number
- Molecular Structure
• Branched hydrocarbons: high octane number
• Linear hydrocarbons: low octane number
• Aromatic compounds: high octane number
• Oxygenated compounds: high octane number
Octane Number Increasing Processes in Refineries
Refineries use advanced processes to increase the octane number:
- Catalytic Reforming
Converts linear n-paraffins into aromatic compounds (like benzene, toluene, xylene) and iso-paraffins with higher octane numbers, using platinum catalysts. - Alkylation
Combines isobutane with light olefins (propylene, butylene) to produce iso-paraffins with very high octane numbers (about 98 to 100). This occurs in alkylation units using sulfuric acid or HF catalysts. - Isomerization
Converts n-butane and n-pentane into isopentane and other isomers with higher octane numbers under pressure using zeolite or aluminum chloride catalysts.
Octane Boosters (Additives)
Sometimes produced gasoline does not meet specific standards and requires octane enhancing additives:
Common additives:
- MTBE: high-octane ether added in refineries.
- Ethanol: bio-alcohol with octane number around 110; effective booster but with corrosion and water solubility concerns.
- MMT: manganese-based metallic additive (restricted in some countries).
- Commercial octane boosters (Grade A.A): product by Artin Azma Mehr, designed for high-performance vehicles.
Methods of Measuring Octane Number
Special devices called octane meters are used to accurately determine octane number. These simulate engine combustion and operate according to international standards.
- CFR Engine Method (Cooperative Fuel Research):
Considered the most accurate and reliable method based on ASTM standards D2699 (RON) and D2700 (MON). Uses a single-cylinder test engine with adjustable compression ratio to simulate fuel behavior under controlled conditions.
The portable OCTANE-IM octane and cetane meter is compact, easy to use with a USB interface, suitable for industrial quality control and rapid product identification. It supports both RON and MON measurements per ASTM D2699 and ASTM D2700 standards, capable of measuring all grades of unleaded gasoline, providing excellent repeatability and long-term stability, designed for fuel quality control labs and refineries.
- Spectroscopic Methods:
- Near-Infrared Spectroscopy (NIR)
- Raman Spectroscopy
- FTIR Spectroscopy
International Standards and Regulations
- ASTM D2699: RON determination
- ASTM D2700: MON determination
- ASTM D4814: Gasoline fuel specifications
- EN 228: European gasoline quality standard
These standards define precise laboratory methods, test conditions, temperatures, and reference blend ratios.
Octane Number in Global Markets
Octane numbers vary by country and common engine types:
Country | Common Gasoline Octane Numbers |
Iran | 87, 91 |
USA | 87 (Regular), 91 (Premium) |
Germany | 95 (Super), 98 (Super Plus) |
Japan | 95, 98 |
Russia | 92, 95, 98 |
In Iran, super gasoline is often produced using octane-enhancing additives.
Future of Octane Number and New Fuels
- Biofuels: Use of ethanol and other oxygenated bio-compounds.
- Synthetic Fuels (E-fuels): Production of renewable, high-octane fuels.
- Development of Environmentally Friendly Additives: Replacement of toxic compounds like MTBE.
Conclusion
Octane number is a key parameter in determining the quality and performance of gasoline fuels and plays a vital role in the proper operation of internal combustion engines. It indicates fuel resistance to premature ignition, and choosing fuel with an appropriate octane number not only enhances engine efficiency and prevents knocking but also reduces emissions and extends engine life.
This article reviewed various aspects of the octane number, including chemical structure, refinery methods to increase octane, booster additives, measuring equipment, and relevant standards. It also highlighted that due to advances in modern engine technology and environmental requirements, the use of high-octane fuels has become increasingly important.
Recommended Products by Artin Azma Mehr
✅ Octane Booster Grade A.A
Specially designed for domestic and imported vehicles – usable in all seasons – increases octane number by at least 4 to 6 points.
✅ OCTANE-IM Octane and Cetane Meter by TERMEX
The OCTANE-IM meter results comply with ASTM D2699, ASTM D2700, ASTM D613, ISO 5163, ISO 5164, EN 25163, EN 25164 standards and are traceable through official laboratories (knock engine testing).