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

This question checks which fuel property controls cold starting in a spark ignition (SI) engine and which one controls ignition delay in a compression ignition (CI) engine.

1. Octane number and Calorific value: Octane number measures knock resistance, not cold starting ability. Calorific value gives energy content, not ignition delay. This pairing does not fit.
2. Volatility and Cetane number: A fuel with low volatility does not vaporize well at low temperature, so the SI engine cranks hard when cold. Cetane number rates how fast a CI fuel self-ignites after injection, so a low cetane number means a longer ignition delay. Both parts match the physics.
3. Flash point and Viscosity: Flash point relates to fire safety during storage, not starting. Viscosity affects spray atomization, not the ignition delay period directly. This pairing does not fit.
4. Calorific value and Octane number: Neither property controls the named phenomena. Calorific value sets power output and octane number resists knock, so this pairing is wrong.

Volatility governs hard and cold starting in an SI engine because a fuel that vaporizes easily forms a combustible mixture even at low cranking speed and temperature. Cetane number governs ignition delay in a CI engine because a higher cetane number shortens the delay between injection and self-ignition. So option (B) is correct.