air-fuel ratio explained: properties, formula and examples

The air-fuel ratio (AFR) is a critical parameter in combustion processes, particularly in internal combustion engines, as it influences engine performance, emissions, and fuel efficiency. Understanding both the stoichiometric and actual air-fuel ratios helps in optimizing the combustion process.

1. Stoichiometric Air-Fuel Ratio (AFR)

The stoichiometric air-fuel ratio is the ideal ratio at which a fuel completely reacts with oxygen, resulting in complete combustion without any excess oxygen or unburned fuel remaining. This ratio varies depending on the type of fuel used.

2. Actual Air-Fuel Ratio (AFR)

The actual air-fuel ratio is the ratio of the actual mass of air supplied to the mass of fuel consumed during combustion. It can differ from the stoichiometric AFR due to various operating conditions:

• Rich Mixture: When the actual AFR is lower than the stoichiometric ratio (more fuel than necessary), resulting in incomplete combustion and higher emissions of unburned hydrocarbons (HC) and carbon monoxide (CO). Typical applications include high-power demand scenarios, such as acceleration in vehicles.

• Lean Mixture: When the actual AFR is higher than the stoichiometric ratio (more air than necessary), resulting in more complete combustion and lower emissions but potentially leading to issues like engine knocking. Lean mixtures are often used in fuel-efficient fuel management strategies.

air-fuel ratio formulas

1. Stoichiometric Air-Fuel Ratio (AFR)

The stoichiometric air-fuel ratio is the ideal ratio of air to fuel that achieves complete combustion. This ratio varies for different fuels and is determined from the balanced combustion equation.

General Formula:
For a complete combustion reaction of a hydrocarbon fuel (CₙHₖ), the stoichiometric AFR can be calculated by:

$$\text{Stoichiometric AFR}=\frac{(4.76\cdot n+\frac{k}{2})}{1}$$

Where

• $n$ = number of carbon atoms,
• $k$ = number of hydrogen atoms in the hydrocarbon.

2. Actual Air-Fuel Ratio (AFR)

The actual air-fuel ratio is the ratio of the actual amount of air supplied to the fuel used during combustion.

General Formula:

$$\text{Actual AFR}=\frac{\text{Mass of Air}}{\text{Mass of Fuel}}$$

3. Percentage Excess Air

Percentage excess air indicates how much more air is supplied compared to the stoichiometric requirement. It describes the deviation from perfect combustion conditions.

Formula:

$$\text{Percentage Excess Air}=\left(\frac{\text{Actual AFR}-\text{Stoichiometric AFR}}{\text{Stoichiometric AFR}}\right)\times 100\mathrm{\%}$$

4. Mixture Strength (or Fuel-Air Ratio)

Mixture strength often refers to the fuel-air ratio, which indicates the strength of the fuel-air mixture. This ratio is the inverse of the air-fuel ratio.

Formula:

$$\text{Fuel-Air Ratio}=\frac{\text{Mass of Fuel}}{\text{Mass of Air}}=\frac{1}{\text{AFR}}$$

Alternatively, it can be expressed in terms of the stoichiometric ratio:

$$\text{Mixture Strength}=\frac{1}{\text{Stoichiometric AFR}}\text{ for stoichiometric combustion}$$

Summary of Key Formulas

• Stoichiometric AFR: $\text{Stoichiometric AFR}=\frac{(4.76\cdot n+\frac{k}{2})}{1}$
• Actual AFR: $\text{Actual AFR}=\frac{\text{Mass of Air}}{\text{Mass of Fuel}}$
• Percentage Excess Air: $\text{Percentage Excess Air}=\left(\frac{\text{Actual AFR}-\text{Stoichiometric AFR}}{\text{Stoichiometric AFR}}\right)\times 100\mathrm{\%}$
• Mixture Strength: $\text{Mixture Strength}=\frac{1}{\text{AFR}}$

Measuring and Importance

• The actual AFR is often measured or calculated during engine operation and is used to adjust the fuel injection and air intake systems to maintain optimal performance and compliance with emission standards.

Example Calculation

Given:

• For an engine using gasoline with a stoichiometric AFR of 14.7:1.
• If during operation, the engine measures an actual air-fuel ratio of 16:1.

Interpretation:

• The mixture is lean because the actual air-fuel ratio (16:1) is greater than the stoichiometric ratio (14.7:1). This indicates that the engine is using more air relative to the fuel, which can lead to better fuel efficiency and lower emissions of certain pollutants but might require adjustments to prevent misfire or performance issues.

Understanding and optimizing both the stoichiometric and actual air-fuel ratios are essential for improving combustion efficiency, engine performance, and minimizing emissions. If you need more specifics or examples regarding this topic, feel free to ask!