Fuel ignition quality is the ability of a fuel to ignite spontaneously when injected into the compressed air charge in an engine cylinder. This property is measured by the cetane number. A higher cetane number indicates a shorter ignition delay and better ignition quality.

A fuel's chemical structure significantly influences its ignition quality. The cetane number is determined by comparing the fuel's ignition performance to two primary reference fuels:

• n-hexadecane (cetane): A straight-chain paraffin with very good ignition quality and a cetane number of 100.
• 1-methylnaphthalene: A ring-structured aromatic with poor ignition quality and a cetane number of 0.

Generally, fuels with a higher proportion of straight-chain hydrocarbons (like paraffins) have better ignition quality because they are more reactive and ignite more easily under compression. In contrast, fuels with more branched or aromatic structures have a longer ignition delay and therefore a lower cetane number.

Operating an engine on a fuel with a lower ignition quality than it's timed for causes an increased ignition delay, which is the time between fuel injection and the start of combustion.

If the ignition delay is longer than what the engine timing is set for, the ignition will occur later than the optimal crank angle. This results in:

• Lower Peak Pressure (Pmax​): The peak combustion pressure will be lower than designed because the combustion is delayed, occurring when the piston is already moving down the cylinder. This reduces the force applied to the piston and thus, the engine's power output.
• Reduced Power Output: As a direct result of the lower peak pressure, the engine produces less power and is less efficient.
• Afterburning: Delayed combustion can continue into the expansion stroke, leading to combustion in the exhaust port. This phenomenon, known as afterburning, can cause thermal damage to engine components and increase exhaust emissions.

(i) Adjusting an Engine for Different Fuel Quality

An engine can be adjusted to accommodate fuels of different ignition qualities by modifying the fuel injection timing. In modern engines, systems like the Variable Injection Timing (VIT) or Super VIT automatically adjust timing for various loads but are typically fixed for a specific fuel quality.

To handle different ignition quality fuels, a specific Fuel Quality System can be installed. This system allows the operator to manually adjust the start of injection based on the fuel's cetane number. The system has a setting from 0 to 10:

• Setting 0: For fuels with good ignition quality.
• Setting 10: For fuels with poor ignition quality.

As the setting is increased from 0 to 10, the system advances the fuel injection timing to compensate for the longer ignition delay of lower-quality fuels. This ensures that combustion starts at the correct time, maintaining optimal peak pressure and efficiency.

(ii) Checks to Ensure Correct Engine Operation

To ensure correct engine operation after adjustments, the following checks are carried out:

• Monitoring Pmax and pressure diagrams: to confirm correct ignition timing and efficient combustion.
• Checking exhaust temperatures: to detect signs of afterburning or misfiring.
• Observing engine performance parameters: such as power output, fuel consumption, and smoothness of operation.