Answer
1. Core Definitions
Octane Number (Gasoline/Spark-Ignition Engines)
The Octane Number is a measure of a fuel's ability to resist auto-ignition (also known as pre-ignition, knocking, or detonation) in a spark-ignition (SI) engine.
The Goal: The fuel-air mixture should ideally be compressed without igniting until the spark plug fires. It must burn smoothly, not explode prematurely.
Scale: It is based on a scale from 0 to 100:
Iso-octane (C8H18): Highly resistant to knocking. Assigned an octane rating of 100.
n-Heptane (C7H16): Knocks very easily. Assigned an octane rating of 0.
Cetane Number (Diesel/Compression-Ignition Engines)
The Cetane Number (CN) is a measure of a fuel's ignition delay—the time period between the start of fuel injection and the start of actual combustion in a compression-ignition (CI) engine.
The Goal: Marine diesel engines require a short ignition delay. If the delay is too long, unburnt fuel accumulates in the cylinder and ignites all at once, causing a rapid pressure rise, known as "diesel knock." A higher cetane number means a shorter ignition delay and smoother combustion.
Scale: It is based on a scale from 0 to 100:
n-Cetane (Hexadecane, C16H34): Ignites very easily under compression. Assigned a cetane rating of 100.
Heptamethylnonane (C16H34 structural isomer): Poor ignition quality. Assigned a cetane rating of 15 (historically, alpha-methylnaphthalene was used as the 0 reference).
2. Direct Comparison Table
| Feature | Octane Number | Cetane Number |
|---|---|---|
| Engine Type | Spark-Ignition (SI) / Petrol Engines | Compression-Ignition (CI) / Diesel Engines |
| Primary Property | Resistance to auto-ignition (Anti-knock). | Readiness to auto-ignite (Short ignition delay). |
| Desired Value | High (Prevents premature detonation). | High (Prevents excessive fuel accumulation before ignition). |
3. The Inverse Relationship
Chemically, octane and cetane numbers are inversely proportional. A fuel that has a high octane number will naturally have a low cetane number, and vice versa.
Residual Fuel Oils & CCAI
For heavy fuel oil (HFO), the standard Cetane Number test engine cannot handle the high viscosity and slow burning characteristics. Therefore, the industry uses the Calculated Carbon Aromaticity Index (CCAI) or Calculated Ignition Index (CII) under ISO 8217.
CCAI is derived from the fuel's density and kinematic viscosity.
A higher CCAI indicates a highly aromatic fuel with a lower ignition quality (equivalent to a low cetane number), which can lead to high peak pressures, cold-start difficulties, and prolonged combustion delays.
Dual-Fuel & Gas Engines (LNG, Methanol, Ammonia)
With the implementation of strict IMO Tier III NOx limits and decarbonization strategies, Class 1 engineers must manage dual-fuel engines operating on the Otto cycle (lean-burn spark-ignited or gas-admission engines).
Methane Number: When using LNG, the "Octane Number" equivalent used is the Methane Number (MN). A high methane number (e.g., pure methane = 100) means the gas resists auto-ignition/knocking. If the LNG has high levels of ethane, propane, or butane, the MN drops, risking engine knock and damaging the pistons/liner.
Distillate Fuels (MGO/MDO)
For Marine Gas Oil (MGO) and Marine Diesel Oil (MDO), ISO 8217 specifies a minimum Cetane Index/Number (typically 40 or 45).
Low Cetane Consequences in Aux/Main Engines: Difficult cold starting, excessive white smoke post-start (unburnt fuel vapor), misfiring at low loads, increased thermal and mechanical stress on the running gear due to sudden pressure spikes (dP/dθ), and rapid soot accumulation in the exhaust gas economizer (EGE).