Air-Fuel Ratio (A/F Ratio) Requirements in SI Engine:
(SI = Spark Ignition engine, like petrol engines)
The air-fuel ratio is the amount of air compared to fuel (by weight) in the mixture that enters the engine for combustion.
The ideal (stoichiometric) air-fuel ratio for petrol is about 14.7:1 —
➔ This means 14.7 parts of air for every 1 part of fuel.
But depending on engine operating conditions, the required air-fuel ratio changes:
Different Air-Fuel Ratios in SI Engines:
-
Idle Condition:
➔ Engine running at very low speed without load.
➔ Requires a richer mixture (A/F ratio around 12:1 to 14:1) for smooth operation. -
Acceleration (High Power Demand):
➔ When you press the accelerator for fast speed or overtaking.
➔ Needs a rich mixture (around 11:1 to 13:1) to produce more power and prevent knocking. -
Cruising / Steady Driving:
➔ When driving at constant speed (like highway driving).
➔ Can use a lean mixture (around 15:1 to 16:1) for better fuel economy. -
Deceleration (Throttle Off):
➔ When slowing down or engine braking.
➔ Mixture can be made very lean or fuel can be almost cut off. -
Cold Start:
➔ Engine starting at cold temperatures.
➔ Needs a very rich mixture (even richer than 11:1) to ensure combustion because fuel vaporizes poorly when cold.
Summary Table:
| Condition | Air-Fuel Ratio | Type of Mixture |
|---|---|---|
| Idle | 12:1 to 14:1 | Rich |
| Acceleration | 11:1 to 13:1 | Rich |
| Cruising/Steady | 15:1 to 16:1 | Lean |
| Deceleration | Very Lean | Very Lean |
| Cold Start | Very Rich | Very Rich |
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