Most variants of the CFM56 feature a single-annular combustor. An annular combustor is a continuous ring where fuel is injected into the airflow and ignited, raising the pressure and temperature of the flow. This contrasts with a can combustor, where each combustion chamber is separate, and a canannular combustor which is a hybrid of the two. Fuel injection is regulated by a Hydromechanical Unit (HMU), built by Honeywell. The HMU regulates the amount of fuel delivered to the engine by means of an electrohydraulic servo valve that, in turn, drives a fuel metering valve, that provides information to the full authority digital engine controller (FADEC).