Combined diesel and gas
Combined diesel and gas (CODAG) is a type of
propulsion system for
ships which need a maximum speed that is considerably faster than their cruise speed, particularly
warships like modern
frigates or
corvettes.
|
Principle of a CODAG system, with two speed diesel gearboxes |
It 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, that
controllable pitch propellers can limit the rotations so that the diesels can continue to operate without changing the
gear ratios of their
transmissions. Because of that, special multi-speed gearboxes are needed. This contrasts to
CODOG systems, which couple the diesels with a simple, fixed ratio gearboxes to the shaft and disengage them, when the turbine is switched on.
E.g. for the new CODAG propulsed
Fridtjof Nansen class frigate of the
Royal Norwegian Navy, the gear ratio for the diesel engine is changed from about 1:7.7 (engine:propeller) 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.
Such a propulsion system has a smaller footprint than a diesel-only power plant with the same maximal power output, since smaller engines can be used and the gas turbine and gearbox don't need that much additional space. Still it retains the high
fuel efficiency of diesel engines when cruising, allowing greater range and lower fuel costs than with gas turbines alone. On the other hand, a more complex, heavy and troublesome gearing is needed.
Typical cruising speed of CODAG warships on diesel-power is 20
kts and typical maximal speed with switched on turbine is 30 kts.
CODAG has been pioneered by
Germany with the
Köln class frigate.
Sometimes the engine arrangement of diesel engine and gas turbine with each system using its own shafts and propellers is also called CODAG. Such installations avoid the use of a complicated switching gearbox but have some disadvantages compared to real CODAG systems:
* Since more propellers have to be used, they have to be smaller and thus less efficient.
* The propellers of the ideling systems cause drag.
CODAG WARP (CODAG Water jet And Refined Propeller), a system developed by
Blohm + Voss as option for their
MEKO line of ships, also falls in this categegory but avoids the above mentioned problems. CODAG WARP uses two diesel engines to drive two propellers in a
CODAD arrangement, i.e. both shafts can also be powered by any single engine, and a centerline
water jet powered by a gas turbine. The ideling water jet doesn't cause drag and since its nozzle can be placed further aft and higher it doesn't affect the size of the propellers.
*
CODAG WARP @ naval-technology.com
