The MON is determined using a similar method but with a heated engine and variable ignition timing. The MON better represents the behavior of gasoline under high-temperature conditions, such as city driving or heavy loads.
The overall octane number of a fuel is typically reported as the average of the RON and MON, known as the Anti-Knock Index (AKI) or Octane Index (OI).
Here are the steps involved in measuring octane using the RON method:
Engine Setup:
- Use a standard test engine specified for octane number testing, such as a Cooperative Fuel Research (CFR) engine.
Fuel Preparation:
- Prepare a sample of the fuel to be tested.
- Ensure the fuel sample is free of water, sediment, and other contaminants.
- Preheat the fuel to a specified temperature (typically 40°C or 104°F) to ensure vaporization.
Reference Fuel:
- Select reference fuels with known octane numbers, such as those standardized by the American Society for Testing and Materials (ASTM).
Engine Variables:
- Set the engine's operating parameters as per the test standard, including engine speed, air-fuel ratio, and ignition timing.
- Adjust the engine's compression ratio to a known value.
Testing Procedure:
- Start the engine and allow it to reach a stable operating temperature.
- Introduce the test fuel into the engine.
- Operate the engine at the specified conditions while gradually increasing the compression ratio.
- Listen for the onset of knocking or pre-ignition.
Knock Detection:
- Use an appropriate knock detection system, such as an accelerometer or pressure sensor, to determine the occurrence of knocking.
- Record the compression ratio at which knocking starts.
Reference Fuel Comparison:
- Repeat the test with the reference fuel with a known octane number.
- Record the compression ratio at which knocking occurs for the reference fuel.
Octane Calculation:
- Calculate the octane number of the test fuel using a mathematical formula that relates the compression ratios at which knocking occurs for the test fuel and the reference fuel.
The calculated octane number represents the fuel's resistance to knocking compared to the reference fuels. Higher octane values indicate greater knock resistance, which is important for higher-performance engines or vehicles operating in demanding conditions.
