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.
