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Correct air-fuel ratio (during idling, starting, acceleration and other conditions)


It is important to maintain the correct air-fuel ratio to get the best performance and fuel economy from your engine. When this ratio is incorrect, your engine will run rich or lean and can cause damage. In this article, we will review typical ideal air-fuel ratios under various conditions.

What is the air-fuel ratio?

The air-fuel ratio (AFR) is the mass ratio between the amount of air and the amount of fuel mixed in the combustion chamber of a vehicle. This relationship must be corrected so that the fuel burns properly and efficiently.

If the ratio is too rich or too lean, the engine will not burn the air-fuel mixture optimally, which can cause performance problems or consume too much fuel. 14.7:1 is the ideal air-fuel ratio that burns all the fuel without excess air. The “stoichiometric” was mixed. In this case, you need 14.7 parts air to 1 part fuel.

However, under certain conditions, not all fuels can be mixed and vaporized with air. Some of the different conditions will be explained in the next section.

Before discussing the air-fuel ratio under different conditions, let me first explain the different types of air-fuel ratios in the vehicle.

  • RICA air-fuel ratio: There is less air than the ideal AFR. This may be good for power, but bad for fuel economy and emissions. (example: 13:1)
  • POOR air-fuel ratio: There is more air than the ideal AFR. This can be good for fuel economy and emissions, but bad for power. (example: 16:1)
  • Ideal air-fuel ratio: It has the appropriate air-fuel mixture for good combustion. (for example: 14.7:1)

Good air-fuel ratio in various conditions.

Now that you understand what the air-fuel ratio is and how it can affect the internal combustion process, here we will look at what the best air-fuel ratios are for different conditions.


the car doesn't start

When you start your car, all of the engine components, such as the cylinder head, cylinder block, and intake manifold, are cold. In this case, some additional fuel is needed to start the engine, so a rich fuel mixture is temporarily needed.

A simpler way to describe this is that the choke on older cars with carburetors was used to block air and therefore draw more fuel into the engine to start the car.

When you start your engine, the air-fuel ratio can be as low as 9:1making it very rich.

Warm-up (slow)

irregular engine

After it starts and the engine is still running, the coolant temperature is still low and requires more fuel than normal until the vehicle reaches operating temperature. So in this case AFR is rich about 12:1; it is necessary.

See also: How much gasoline does idling really consume?


When you depress the accelerator pedal to gain speed, more air enters the cylinder to meet the additional power requirements, so naturally, more fuel is needed. At full speed, the air-fuel ratio can be around 11:1 (very rich) although moderate throttle can mean about a 13:1 (rich) air-fuel ratio.

Cruise (constant speed)

the car shakes when accelerating

In this condition, the engine is already warmed up and the air-fuel mixture is close to the stoichiometric ratio. about 14.7:1. This provides the best combination of fuel economy, emissions, and power.

heavy loads

the steering wheel makes noise when turning

Under heavy loads, such as going up a hill or if you are towing a trailer, the vehicle requires the engine to produce more power. This means that a rich air-fuel ratio is needed like the throttle for excessive demands under heavy loads. The AFR will be somewhere about 12:1.


brake pedal mount

In this condition, the accelerator pedal is released, meaning that no power output is required from the engine other than to keep it running. Air-fuel ratio is around 17:1 (poor) at this point because fuel demands are very low at this point. At this stage, the exhaust gases are also evacuated.

Air-fuel ratio table

Condition Common air/fuel ratio
Started 9:1
Warm-up (slow) 12:1
Density 11:1 to 13:1
Cruise (constant speed) 14.7:1
Heavy load (pulling/climbing) 12:1
Acceleration (foot gas) 17:1

Signs of an Improper Air-Fuel Ratio

Here are some common signs that your air-fuel ratio is too rich or too lean:

The air-fuel ratio is too high.

The air-fuel ratio is too lean

  • Engine stutters or jerks
  • Bad acceleration
  • Hard idle (car vibrates)

Causes of incorrect air-fuel ratio



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