Question

Fuel property that predominantly governs 'hard / cold starting' in SI engine is P; and the property that predominantly governs 'ignition delay' in CI engine is Q. Identify the correct option for P and Q combination.

• 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

Hint:

Think about which fuel property helps vaporization at low temperature and which one rates self-ignition readiness.

Answer 1

The Correct Option is B

Step 1: Look at what happens during SI cold starting.
At low temperature the fuel does not evaporate well, so a rich, easily vaporized fuel is needed to form a combustible charge.
The property that controls this is volatility: a more volatile fuel starts easier in the cold.

Step 2: Look at what controls ignition delay in a CI engine.
In a CI engine fuel is sprayed into hot compressed air and must self-ignite after a short delay.
Cetane number measures how readily the fuel self-ignites, so a low cetane number means a long ignition delay and rough running.

Final Answer:
P is volatility (SI cold starting) and Q is cetane number (CI ignition delay), so the pair matches option (B). \[ \text{P = Volatility}, \ \text{Q = Cetane number} \]