University of Michigan
ME 589: EcoDesign and Manufacturing
Home | Sources: Transportation | Fuel to Air Ratio
What is the difference between burning rich and burning lean?
- Burning rich is when the actual Fuel to Air ratio is greater than the stoicheometric Fuel to Air ratio (~14.6:1) , and burning lean is when the actual Fuel to Air ratio is less than the stoicheometric Fuel to Air ratio.
- It is important to distinguish rich and lean conditions of combustion because the exhaust species will be different.
- When there is lean combustion, fuel is the limiting reactant and there is excess oxygen. This means that ideally, there will be complete combustion of the fuel and air, which results in an exhaust mixture primarily of CO2, H2O, and N2. When the fuel and air mixture is extremely lean (equivalence ratio << 1), there is a chance of lean misfire because there is not enough fuel in the tank to ignite the mixture. The unburned fuel will exit the cylinder during the exhaust stroke, thus releasing hydrocarbons (VOCs) into the atmosphere.
- In rich combustion, however, there is an excess of fuel and air is the limiting reactant. Thus, there will be unburned hydrocarbons that are exhausted due to incomplete combustion. Another product of incomplete combustion is CO, which is formed primarily because there is not enough oxygen to create CO2, but also because of high temperature dissociation of CO2 during combustion. Dissociation also happens in slightly lean conditions because of the high cylinder temperatures at near-stoicheometric A/F ratios.