Combined diesel and gas

From Wikipedia, the free encyclopedia

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.

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.

Turbines and diesels on separate shafts

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

Combined marine

propulsion

CODOG

CODAG

CODLAG

CODAD

COSAG

COGOG

COGAG

COGAS

CONAS

Principle of a CODAG system, with two speed diesel

gearboxes

Contents

� 1 Turbines and diesels on separate shafts � 2 CODAG WARP � 3 CODAG-electric � 4 External links

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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 idling 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 idling

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-electric

Another solution to combining the two types of engines is to have engine connected to a generator, and

then drive the propeller with an electric motor, much like a diesel-electric. The RMS Queen Mary 2

features such a design. Such a design also simplifies the use of propeller pods with the propulsion

motors mounted inside the pods. A further CODAG innovation on the QM2 is that the turbines are

mounted not in the engine room but directly under the funnel, thereby simplifying the supply of fresh air

to the turbines.

External links

� CODAG WARP @ naval-technology.com

Retrieved from "http://en.wikipedia.org/wiki/Combined_diesel_and_gas"

Category: Marine propulsion

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