What is octane number?

Short Answer:

The octane number is a measure of a fuel’s ability to resist knocking or premature ignition in a spark ignition (petrol) engine. It indicates how smoothly the fuel burns in the engine cylinder. A higher octane number means the fuel can withstand higher compression before detonating, which leads to better engine performance.

In simple terms, fuels with high octane numbers provide smoother engine operation, higher efficiency, and reduced knocking. The octane number is usually measured by comparing the fuel with standard reference fuels like iso-octane and n-heptane under specific conditions.

Detailed Explanation:

Octane Number

The octane number, also called the octane rating, is a numerical value that represents the anti-knocking property of a petrol (gasoline) fuel. Knocking occurs when the air-fuel mixture in the engine cylinder ignites before the spark plug fires or burns unevenly after ignition. This produces a knocking or pinging sound, reduces power output, and can damage the engine. The octane number helps in determining how effectively a fuel can resist this knocking phenomenon.

  1. Meaning of Octane Number

The octane number is defined as the percentage of iso-octane present in a mixture of iso-octane and n-heptane that has the same knocking characteristics as the test fuel.

  • Iso-octane (C₈H₁₈): A high-quality fuel that resists knocking and is assigned an octane number of 100.
  • n-Heptane (C₇H₁₆): A poor fuel that knocks easily and is assigned an octane number of 0.

For example, if a fuel has the same knocking tendency as a mixture of 90% iso-octane and 10% n-heptane, its octane number is 90. This means it performs nearly as well as iso-octane in preventing engine knocking.

  1. Need for Octane Number

In a spark ignition (SI) engine, the air-fuel mixture is compressed before the spark plug ignites it. If the fuel ignites too early due to high pressure and temperature, it causes knocking. Knocking reduces efficiency, increases engine wear, and may damage the piston or valves. Therefore, the fuel used must have a high resistance to knocking, which is represented by a high octane number.

The octane number helps engineers and manufacturers choose or design fuels suitable for specific engine compression ratios and performance levels.

  1. Factors Affecting Octane Number

Several factors influence the octane rating of a fuel:

  • Chemical Structure:
    • Branched-chain hydrocarbons (like iso-octane) have higher octane numbers.
    • Straight-chain hydrocarbons (like n-heptane) have lower octane numbers.
  • Aromatic Compounds: Aromatics such as benzene and toluene increase the octane rating.
  • Additives: Certain additives like tetraethyl lead (TEL) were used earlier to increase octane number, but due to pollution concerns, unleaded additives like ethanol and MTBE are now preferred.
  • Refining Process: Advanced refining methods, such as catalytic reforming, help in improving the octane number of fuels.
  1. Measurement of Octane Number

The octane number of a fuel is measured using a CFR (Cooperative Fuel Research) engine under controlled laboratory conditions. The steps include:

  1. Running the test engine with the sample fuel.
  2. Comparing its knocking intensity with that of standard mixtures of iso-octane and n-heptane.
  3. Adjusting the compression ratio until the knocking behavior of both fuels matches.
  4. The percentage of iso-octane in the standard mixture gives the octane number of the sample fuel.
  1. Types of Octane Numbers

There are mainly three types of octane ratings used for fuels:

  • Research Octane Number (RON): Measured under mild operating conditions, representing normal driving situations.
  • Motor Octane Number (MON): Measured under more severe conditions, representing high-speed or load conditions.
  • Anti-Knock Index (AKI): It is the average of RON and MON, used in commercial fuel rating as:

This value is usually displayed at fuel stations.

  1. Importance of Octane Number
  • Prevents Knocking: Fuels with higher octane numbers ignite only when the spark plug fires, preventing premature combustion.
  • Improves Engine Efficiency: Proper combustion leads to smooth engine operation and better fuel economy.
  • Allows Higher Compression Ratio: High octane fuels can be used in engines with high compression ratios, resulting in more power output.
  • Protects Engine Components: It reduces stress and vibration caused by knocking, extending engine life.
  • Enhances Performance: Sports and high-performance vehicles often use fuels with very high octane numbers for maximum efficiency.
  1. Typical Octane Ratings of Fuels
  • Regular Petrol: 87–90
  • Premium Petrol: 91–95
  • Aviation Fuel: Around 100 or more
    Modern engines designed for better efficiency often require fuels with higher octane ratings.
  1. Methods to Improve Octane Number
  • Use of Antiknock Additives: Adding compounds like ethanol, MTBE (Methyl Tertiary Butyl Ether), or iso-octane improves octane value.
  • Refining Techniques: Catalytic reforming and cracking help increase the proportion of branched and aromatic hydrocarbons, thus improving octane quality.
  • Blending Fuels: Mixing fuels of different octane ratings to achieve the desired performance.
Conclusion:

The octane number is a crucial indicator of fuel quality for spark ignition engines. It represents the fuel’s resistance to knocking and determines its suitability for different engine compression ratios. A higher octane rating ensures smooth, efficient, and reliable engine operation. With growing demand for cleaner and efficient fuels, modern refining and additive technologies are continuously improving octane values to meet performance and environmental standards.