6.2.3. Air-fuel ratio

AFR_stoich = Mass of air required / Mass of fuel $ext{AFR}_{ ext{stoich}} = \frac{\text{Mass of air required}}{\text{Mass of fuel}}$

The stoichiometric air-fuel ratio (AFR) is a crucial concept in combustion. It represents the precise amount of air needed to fully combust a specific mass of fuel. For complete combustion, we must have the right balance of air and fuel to ensure that all the fuel is used up and that no excess fuel remains. This balance is defined mathematically by the formula provided, where the mass of air is divided by the mass of the fuel. If we have the correct AFR, the combustion will produce the maximum amount of energy with minimal pollutants.

Imagine cooking with a recipe that requires a specific ratio of ingredients. If you add too much sugar (the fuel) without enough flour (the air), the dish won't turn out as intended. Similarly, in combustion, having the right air-to-fuel ratio ensures that the reaction occurs efficiently and effectively.

% Excess air = (Actual air / Stoichiometric air − 1) × 100% $ext{Excess air} = \left( \frac{\text{Actual air}}{\text{Stoichiometric air}} - 1 \right) \times 100 \%$

Excess air refers to the additional air supplied beyond the stoichiometric requirement. In combustion systems, it is common to use more air than the stoichiometric ratio suggests. This is quantified as a percentage of excess air, calculated using the formula provided. It is important because using excess air can improve combustion efficiency and help reduce emissions by ensuring complete combustion. However, too much excess air can also lead to energy losses and lower combustion temperatures.

Consider a car engine where more air is needed for complete combustion. If a mechanic suggests using an extra set of air filters to ensure more air flow to the engine, this is similar to adding excess air in combustion. While this ensures that the fuel burns efficiently, too many filters might choke the engine, highlighting the need for a balance.

ϕ = Stoichiometric AFR / Actual AFR $\phi = \frac{\text{Stoichiometric AFR}}{\text{Actual AFR}}$

The equivalence ratio (ϕ) is a measure that indicates whether the mixture of fuel and air is rich or lean. When ϕ is equal to 1, it means the air-fuel mixture is at the stoichiometric ratio, leading to optimal combustion. If ϕ is less than 1, the mixture is lean (more air than needed), and if it is greater than 1, the mixture is rich (too much fuel). Understanding the equivalence ratio is vital for tuning combustion systems to achieve desired performance and emission levels.

Think of a bicycle: if you have the right air pressure in the tires (similar to a stoichiometric mixture), the bike rides smoothly. If there’s too much air (lean), it’s hard to steer. Conversely, if there’s not enough air (rich), the tires might feel sluggish. The equivalence ratio helps balance the conditions for optimal operation.

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