Fuel property that predominantly governs ‘hard / cold starting’ in an SI engine is P; and the property that predominantly governs ‘ignition delay’ in a CI engine is Q. Identify the correct option for the P and Q combination.
Options:
(A) P: Octane number, Q: Calorific value
(B) P: Volatility, Q: Cetane number
(C) P: Flash point, Q: Viscosity
(D) P: Calorific value, Q: Octane number
Official Answer Key:
B
Step-by-Step Solution & Explanation
P — SI engine cold starting: At low ambient temperature, only the lightest fractions of petrol vaporise. A combustible vapour–air mixture must form near the spark plug for ignition to occur. This is governed by the fuel’s volatility (front-end distillation characteristics / Reid vapour pressure). Poor volatility ⇒ hard or cold starting. Octane number governs knock resistance, not starting.
Q — CI engine ignition delay: After injection, the fuel must self-ignite by compression heat. The period between injection and ignition is the ignition delay, governed by the fuel’s self-ignition quality, measured by the cetane number. Higher cetane number ⇒ shorter delay ⇒ less diesel knock, smoother running and easier starting.
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