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
GATE AG - 2026,
Answer: B
P is volatility and Q is cetane number.
An SI engine must vaporise enough fuel in a cold intake to form an ignitable mixture. Hard or cold starting is therefore governed by the volatility of the fuel, in particular its front end (the temperature at which the first 10% distils).
In a CI engine the fuel ignites on its own in hot compressed air, and the time between injection and the start of combustion is the ignition delay. A high cetane number means a short delay, so the cetane number is the governing property.
Octane number measures resistance to knock in SI engines, not starting. Calorific value sets the energy per kilogram, flash point is a handling and safety property, and viscosity affects atomisation, none of which is the predominant control here.
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