• Cold Starting Fuel-Air Ratio in Carbureted Engines:
• Combustible Limits: The air-fuel mixture entering the cylinder must be within the combustible limits of $25:1$ (lean limit) and $8:1$ (rich limit) by weight.
• Fuel Vaporization Fraction: At low ambient temperatures during cold starting, only $10\%$ ($0.10$) of the delivered fuel vaporizes in the intake manifold.
• Calculation:
• For the leanest vaporized mixture that will ignite: $(A/F_v) = 25:1 \implies (F_v/A) = 1/25 = 0.04$.
• Since only $10\%$ of the supplied fuel $F_s$ vaporizes ($F_v = 0.10 F_s$), the supplied fuel-to-air ratio must be:

• Conclusion: Carburetor must supply a rich fuel-to-air ratio of 0.4:1 (Option 4).

Correct option: (4) 0494

• Cold Smoke (White Smoke) Formation in Diesel Engines:
• Mechanism: Cold smoke (white smoke) consists of condensed droplets of unburned or partially vaporized liquid diesel fuel exiting the exhaust when combustion temperatures are too low to sustain complete autoignition.
• Conducive Factors:
• Low intake air / cylinder temperatures: Prevents injected fuel from reaching autoignition temperature.
• Light engine loads: Generates minimal cylinder heat and low residual wall temperatures.
• Low-cetane fuels: Exhibit prolonged ignition delays, allowing fuel to accumulate without igniting promptly.
• High air-to-fuel ratios (lean mixtures): Excess cold intake air quenches combustion reaction zones.
• Conclusion: Low air temperatures, light loads, low-cetane fuels, and high air-to-fuel ratios (Option 3).

Correct option: (3) Low air temperatures, light loads, low-cetane fuels, and high air-to-fuel ratios

• Motor Method (ASTM D2700) for Fuel Testing:
• Octane Number Testing: Evaluates knock resistance in spark-ignition engine fuels using standardized Cooperative Fuel Research (CFR) variable-compression engines.
• Standard Methods:
• Research Octane Number (RON) - ASTM D2699: Tested at milder conditions (engine speed $600\text{ rpm}$, unheated intake mixture temperature $52^\circ\text{C}$).
• Motor Octane Number (MON) - ASTM D2700: Tested under severe, high-speed, high-load conditions (engine speed $900\text{ rpm}$, preheated mixture temperature $149^\circ\text{C}$).
• Cetane Number: Measured via ASTM D613 for compression-ignition diesel fuels.
• Conclusion: Octane number (Option 2).

Correct option: (2) Octane number