Statement (I): In Automotive Petrol engines during idling operation a rich mixture is required (F/A ≃ 0.08)

Statement (II): Rich mixture is required because mixture is diluted by products of combustion.

Explanation:

• An idling operation engine operates under the no-load condition and with nearly closed throttle. Due to which exhaust gases remain in the combustion chamber.
• The mass of exhaust gases in the cylinder at the end of exhaust stroke remains fairly constant throughout the idling range.
• The amount of fresh charge that enters the cylinder is less compared to that during full throttling. So, exhaust gas remains mixed with fresh charges admitted to the cylinder.
• Thus, the engine requires a rich mixture (more fuel than a stoichiometric mixture) for starting at idling to compensate for the charge dilution due to exhaust gases from the combustion chamber. Exhaust gas dilution prevents efficient combustion by reducing the contact between the fuel and air particles. So, finally, combustion is poor.

Conclusion:

We can conclude that the richer fuel mixture is used (F/A) is used in the idling range due to exhaust gas dilution of fresh charge. Statement I) and Statement II) are correct and statement II) is the correct explanation of statement I).