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CODOGCODAGCODLADCODLAGCODADCOSAGCOGOGCOGAGCOGASCONASIEP or IFEP
Combined diesel and gas is a type of propulsion system for ships that need a maximum speed that is considerably faster than their cruise speed, particularly warships like modern frigates or corvettes.
Pioneered by Germany with the Köln-class frigate, a CODAG system consists of diesel engines for cruising and gas turbines that can be switched on for high-speed transits. In most cases the difference of power output from diesel engines alone to diesel and turbine power combined is too large for controllable-pitch propellers to limit the rotations so that the diesels cannot continue to operate without changing the gear ratios of their transmissions. Because of that, special multi-speed gearboxes are needed. This contrasts to combined diesel or gas systems, which couple the diesels with a simple, fixed ratio gearbox to the shaft, but disengage the diesel engines when the turbine is powered up.
For an example the new CODAG-propelled Fridtjof Nansen-class frigates of the Royal Norwegian Navy, the gear ratio for the diesel engine is changed from about 1:7.7 for diesel-only to 1:5.3 when in diesel-and-turbine mode. Some ships even have three different gear ratios for the diesel engines — one each for single-diesel and double-diesel cruises, and the third when the gas turbine is engaged.