🔍 Different Air-Fuel Ratios (AFRs) for Various Operating Conditions of a Gasoline Engine
The air-fuel ratio (AFR) is the ratio of the mass of air to the mass of fuel in the combustion mixture. For gasoline engines, the stoichiometric AFR (ideal for complete combustion) is approximately:
🔸 Stoichiometric AFR = 14.7:1 (14.7 parts air to 1 part fuel by mass)
However, this ratio is adjusted based on engine load, speed, temperature, and emissions control requirements.
📋 Typical AFRs for Gasoline Engine Conditions:
| Operating Condition | AFR Range | Purpose / Effect |
|---|---|---|
| 🔹 Cold Start | 11.5:1 – 13.0:1 | Rich mixture improves ignition and combustion at low temp |
| 🔹 Idle | 13.5:1 – 14.7:1 | Slightly rich for smooth running and stability |
| 🔹 Cruising / Light Load | 15.5:1 – 17.0:1 | Lean mixture improves fuel economy and reduces emissions |
| 🔹 Moderate Acceleration | 13.0:1 – 14.0:1 | Balanced for power and efficiency |
| 🔹 Full Throttle (WOT) | 11.5:1 – 13.0:1 | Rich mixture protects engine from knocking and overheating |
| 🔹 Deceleration (Fuel Cut-Off) | ∞ (Air only) | Fuel injectors may shut off completely |
| 🔹 Stoichiometric (Normal running) | 14.7:1 | Ideal for 3-way catalytic converter operation |
| 🔹 Lean Burn Mode (some engines) | 17:1 – 22:1 | Used in lean burn engines for maximum fuel economy |
🧠 Key Points:
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Richer mixtures (lower AFR) are used for cold starts, high loads, and power demands.
-
Leaner mixtures (higher AFR) are used during cruising, light load, and fuel economy modes.
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Modern ECUs constantly adjust AFR based on sensor inputs like O₂ sensor, MAF, MAP, TPS, and engine temperature.
📌 Conclusion:
Different engine operating conditions require different AFRs for optimal performance, fuel efficiency, and emission control. Managing these ratios precisely is the job of the engine control unit (ECU) in modern electronic fuel injection (EFI) systems.
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