Q1: Which of the following performance characteristics of a S.I engine is not affected by the front end volatility of the petrol used ?

ANS:C - Spark plug fouling.

Spark plug fouling occurs when deposits accumulate on the electrodes or insulator of a spark plug, hindering its ability to produce a strong, consistent spark. These deposits can be composed of various substances, including unburned fuel, oil, carbon, and other contaminants present in the combustion chamber. Here's how spark plug fouling occurs and its effects:

1. Combustion Process: During the combustion process in the engine's cylinders, fuel and air mix together and are ignited by the spark plug to produce power. However, under certain conditions, incomplete combustion can occur, leading to the formation of by-products such as carbon, oil, and other residues.
2. Deposits Formation: Some of these by-products can adhere to the surface of the spark plug electrodes and insulator over time, forming deposits. Factors that contribute to deposit formation include:
   • Poor fuel quality or improper fuel-air mixture: Low-quality fuel or an overly rich fuel-air mixture can result in incomplete combustion, leading to increased deposit formation.
   • Engine wear or malfunction: Engine issues such as worn piston rings, leaky valve seals, or malfunctioning sensors can contribute to deposit formation by altering combustion efficiency.
   • Driving conditions: Stop-and-go driving, short trips, and idling can exacerbate deposit formation by preventing the engine from reaching optimal operating temperatures.
3. Effects of Fouling: As deposits accumulate on the spark plug, they can interfere with the spark gap, insulator, or electrode surfaces, leading to several potential problems:
   • Reduced spark intensity: Deposits can act as insulators, preventing the spark from effectively jumping across the spark plug gap. This can result in weak or inconsistent spark, leading to misfires and poor engine performance.
   • Increased voltage requirements: To overcome the resistance caused by deposits, the ignition system may need to produce higher voltage levels to maintain spark intensity, placing additional strain on ignition components.
   • Engine hesitation and rough idle: Spark plug fouling can cause engine hesitation, rough idling, and poor acceleration, especially under load or during cold starts.
   • Increased emissions: Poor combustion due to spark plug fouling can result in increased emissions of unburned hydrocarbons (HC), carbon monoxide (CO), and other pollutants.
4. Prevention and Remediation: Spark plug fouling can be prevented or mitigated through regular maintenance practices, including:
   • Using high-quality fuel and following recommended fuel octane ratings.
   • Ensuring proper engine maintenance, including regular oil changes, air filter replacements, and tune-ups.
   • Following manufacturer recommendations for spark plug replacement intervals and using appropriate spark plug types.
   • Addressing underlying engine issues promptly to prevent excessive deposit formation.