överskottsluftförbrukning

Överskottsluft, or excess air, is the amount of air supplied to a combustion process beyond the stoichiometric requirement for complete combustion. In practice, engineers describe it with the lambda value: lambda equals actual air divided by the theoretical stoichiometric air. Lambda greater than 1 means excess air; lambda equal to 1 is stoichiometric.

Excess air is deliberate in most burners to ensure reliable ignition and complete combustion despite fluctuations in fuel quality, mixing, and flow. It helps prevent fuel-rich pockets that would produce carbon monoxide and unburned hydrocarbons and supports flame stability in varying operating loads. However, adding excess air increases the mass of exhaust gases and lowers the average flame temperature, reducing thermal efficiency and increasing heat losses.

The effects on emissions depend on conditions. Moderate excess air typically lowers CO and unburned hydrocarbons; NOx formation may be reduced due to lower peak temperatures, but the overall impact is range dependent. In engines and furnaces, practical excess air percentages vary by fuel and technology; common targets range from a few percent to a few tens of percent.

Measurement and control are achieved with flue-gas analysis and oxygen sensors. Control strategies aim to maintain a desired lambda under changing loads, adjusting air supply while monitoring exhaust composition. In engines, the air–fuel ratio is tuned for power, fuel economy, and emission goals; in boilers and furnaces, excess air is a key parameter for efficiency and combustion quality.