In diesel engines, lambda (λ) is the excess air ratio, which is the ratio of the actual air-fuel ratio to the stoichiometric air-fuel ratio.
Understanding Lambda (Excess Air Ratio)
Lambda is a crucial parameter in combustion analysis, particularly in internal combustion engines like diesel engines. It quantifies the amount of air present during combustion relative to the theoretically ideal amount needed to burn the fuel completely.
Here's a breakdown:
- Actual Air-Fuel Ratio: This is the real-world ratio of the mass of air entering the engine cylinder to the mass of fuel injected.
- Stoichiometric Air-Fuel Ratio: This is the ideal theoretical ratio of air to fuel required for complete combustion, where all the fuel is burned using all the available oxygen. For diesel fuel, the stoichiometric air-fuel ratio is typically around 14.5:1 to 14.6:1.
The formula for lambda is:
λ = (Actual Air-Fuel Ratio) / (Stoichiometric Air-Fuel Ratio)
Lambda in Diesel Engines
Unlike gasoline engines which often run near the stoichiometric ratio (λ ≈ 1), diesel engines typically operate with a significant amount of excess air. This means the actual air-fuel ratio is much higher than the stoichiometric ratio.
- Lean Combustion (λ > 1): Diesel engines always run lean, meaning lambda is always greater than 1. This is because diesel engines control power by varying the amount of fuel injected into a relatively constant volume of air drawn into the cylinder. Plenty of excess air is needed to ensure the fuel droplets can find enough oxygen to burn efficiently within the limited time available.
- Significance: The high excess air helps to reduce the formation of soot (particulate matter) compared to rich combustion (λ < 1), which is common in gasoline engines under certain conditions. However, high temperatures and pressures in lean combustion can contribute to the formation of nitrogen oxides (NOx).
Characterizing Lambda in Dual-Fuel Engines
As highlighted in the provided reference, lambda characterization is important when analyzing engine performance, especially in unconventional setups like diesel-CNG dual-fuel engines.
- In research, the value of lambda has been calculated for each percentage substitution of diesel with fuels like CNG in dual-fuel configurations.
- Methods like the Brettschneider equation can be used for such calculations, particularly in exhaust gas analysis to determine the air-fuel ratio and subsequently lambda.
Analyzing lambda helps researchers and engineers understand combustion efficiency, emissions characteristics, and optimize engine control strategies under various operating conditions and fuel mixes.
