What is stoichiometric air–fuel ratio?

Short Answer:

The stoichiometric air–fuel ratio is the perfect ratio of air to fuel that allows complete combustion of the fuel with no leftover air or fuel. In this condition, all the fuel is burned using all the available oxygen, producing maximum efficiency and minimum emissions.

For petrol engines, the stoichiometric air–fuel ratio is approximately 14.7:1, meaning 14.7 parts of air are needed for every 1 part of fuel by weight. This ratio ensures ideal engine performance and is used as a reference point for designing carburetors and fuel-injection systems.

Detailed Explanation :

Stoichiometric Air–Fuel Ratio

The stoichiometric air–fuel ratio represents the chemically correct proportion of air and fuel that allows for complete combustion in an internal combustion engine. It is the ideal condition where the exact amount of oxygen in the air reacts with the exact amount of fuel, leaving no unburned fuel or unused oxygen after the combustion process. This ensures that all the chemical energy in the fuel is efficiently converted into heat energy, which can be used to perform useful work.

For petrol (gasoline) engines, the stoichiometric air–fuel ratio is around 14.7:1 by weight. This means 14.7 kilograms of air are required to completely burn 1 kilogram of petrol. For diesel engines, the stoichiometric ratio is usually around 14.5:1 to 15:1, depending on the type of fuel. However, diesel engines often operate with a much leaner mixture (more air) to ensure better efficiency and lower emissions.

The stoichiometric ratio is based on chemical equations that represent the combustion of hydrocarbons. For example, when octane (C₈H₁₈), a typical component of petrol, burns completely in oxygen, the chemical reaction is:

C₈H₁₈ + 12.5O₂ → 8CO₂ + 9H₂O

From this reaction, we can calculate the exact amount of oxygen required for complete combustion, which helps in determining the stoichiometric air–fuel ratio. Since air contains only about 21% oxygen, more air is needed to provide the required oxygen for the reaction.

Importance of Stoichiometric Air–Fuel Ratio

The stoichiometric ratio is a key factor in engine design and performance. It acts as a reference for adjusting the mixture strength depending on operating conditions. The correct air–fuel ratio ensures:

  1. Complete Combustion: All the fuel is burned, utilizing all available oxygen.
  2. Maximum Efficiency: Energy output from the fuel is maximized.
  3. Minimum Emissions: Pollutants such as carbon monoxide (CO) and unburned hydrocarbons (HC) are minimized.
  4. Smooth Engine Operation: The engine runs steadily with minimal knocking or vibration.

In real-world engine operation, the air–fuel ratio often varies depending on speed, load, and driving conditions. For instance, during cold starting or acceleration, a rich mixture (less air, more fuel) is used, while during cruising or light load, a lean mixture (more air, less fuel) improves economy.

Stoichiometric Ratio in Petrol and Diesel Engines

  1. In Petrol Engines:
    The stoichiometric air–fuel ratio is about 14.7:1. Petrol engines use a spark plug to ignite the mixture, so maintaining the correct ratio is important for proper flame propagation. If the mixture is too rich, combustion becomes incomplete, leading to carbon deposits and smoke. If it is too lean, the mixture may misfire or cause overheating.
  2. In Diesel Engines:
    Diesel engines compress only air during the compression stroke. Fuel is then injected into the hot air at high pressure. Although the stoichiometric ratio is around 14.5:1, diesel engines usually run on a leaner mixture (excess air) to improve combustion and reduce soot formation.

Rich and Lean Mixtures

While the stoichiometric ratio represents the ideal condition, in practice, engines may operate with different air–fuel ratios depending on requirements:

  • Rich Mixture (less air, more fuel):
    Used during starting or acceleration to produce more power. However, it increases fuel consumption and exhaust emissions.
  • Lean Mixture (more air, less fuel):
    Used during cruising to improve fuel economy. If the mixture becomes too lean, combustion may become unstable, causing engine knocking or misfiring.

The stoichiometric point serves as the balance between these two extremes, ensuring a compromise between power, efficiency, and emission control.

Control of Stoichiometric Air–Fuel Ratio

Modern engines use Electronic Fuel Injection (EFI) systems combined with oxygen sensors and Electronic Control Units (ECUs) to maintain the air–fuel ratio close to stoichiometric under various conditions. The oxygen sensor measures the oxygen content in the exhaust gases and sends feedback to the ECU. The ECU then adjusts the fuel supply to maintain the correct ratio.

Maintaining the stoichiometric ratio helps the catalytic converter operate efficiently, reducing harmful emissions such as CO, HC, and NOx. The catalytic converter works best when the exhaust gases are produced from combustion near the stoichiometric point.

Effect of Deviations from Stoichiometric Ratio

  1. Rich Mixture (Air–Fuel Ratio < 14.7:1):
    • Leads to incomplete combustion.
    • Produces black smoke and unburned fuel.
    • Causes poor fuel economy.
    • Increases carbon monoxide emissions.
  2. Lean Mixture (Air–Fuel Ratio > 14.7:1):
    • Causes slow combustion or misfiring.
    • Leads to higher NOx emissions.
    • May result in rough engine running.
    • Reduces engine power.

Thus, maintaining the stoichiometric ratio ensures that the engine operates efficiently, cleanly, and safely under all conditions.

Conclusion:

The stoichiometric air–fuel ratio is the chemically perfect ratio that ensures complete combustion of fuel in an engine. It plays a vital role in achieving high efficiency, smooth performance, and low emissions. While actual operating conditions may require rich or lean mixtures, the stoichiometric ratio remains the fundamental reference point for all engine tuning and fuel control systems. Maintaining this balance ensures better performance and reduced environmental impact.