Combined diesel and gas
Combined diesel and gas (CODAG) is a type of propulsion
Marine propulsion
Marine propulsion is the mechanism or system used to generate thrust to move a ship or boat across water. While paddles and sails are still used on some smaller boats, most modern ships are propelled by mechanical systems consisting a motor or engine turning a propeller, or less frequently, in jet...

 system for ship
Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

s which need a maximum speed that is considerably faster than their cruise speed, particularly warship
A warship is a ship that is built and primarily intended for combat. Warships are usually built in a completely different way from merchant ships. As well as being armed, warships are designed to withstand damage and are usually faster and more maneuvrable than merchant ships...

s like modern frigate
A frigate is any of several types of warship, the term having been used for ships of various sizes and roles over the last few centuries.In the 17th century, the term was used for any warship built for speed and maneuverability, the description often used being "frigate-built"...

s or corvette
A corvette is a small, maneuverable, lightly armed warship, originally smaller than a frigate and larger than a coastal patrol craft or fast attack craft , although many recent designs resemble frigates in size and role...


It consists of diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

s for cruising and gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

s 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 propeller
Controllable pitch propeller
A controllable pitch propeller or variable pitch propeller is a type of propeller with blades that can be rotated around their long axis to change their pitch...

s to limit the rotations so that the diesels can continue to operate without changing the gear ratio
Gear ratio
The gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear...

s of their transmission
Transmission (mechanics)
A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...

s. Because of that, special multi-speed gearboxes are needed. This contrasts to CODOG
Combined diesel or gas
Combined diesel or 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....

 systems, which couple the diesels with a simple, fixed ratio gearbox to the shaft, but disengages when the turbine is switched on.

E.g. for the new CODAG propulsed Fridtjof Nansen class frigate
Fridtjof Nansen class frigate
The Fridtjof Nansen-class frigates are the main surface combatant units of the Royal Norwegian Navy. The ships are named after famous Norwegian explorers, with the lead ship of the class bearing the name of Fridtjof Nansen, the Norwegian scientist, explorer and humanitarian.Five ships were ordered...

 of the Royal Norwegian Navy
Royal Norwegian Navy
The Royal Norwegian Navy is the branch of the Norwegian Defence Force responsible for naval operations. , the RNoN consists of approximately 3,700 personnel and 70 vessels, including 5 heavy frigates, 6 submarines, 14 patrol boats, 4 minesweepers, 4 minehunters, 1 mine detection vessel, 4 support...

, 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
Fuel efficiency
Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is...

 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
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

 with the Köln class frigate
Köln class frigate
The ships of the F120 Köln class of frigates were the first major warships built in Germany after World War II.They were the world's first ships to feature a combined diesel and gas propulsion system. The ships received numerous refits during their long careers with new electronics and torpedo tubes...


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 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 Water jet And Refined Propeller, a system developed by Blohm + Voss as option for their MEKO
The MEKO family of warships was developed by the German company Blohm + Voss.MEKO is a registered trademark. The portmanteau stands for "Mehrzweck-Kombination" . It is a concept in modern naval shipbuilding based on modularity of armament, electronics and other equipment, aiming at ease of...

 line of ships, also falls in this category but avoids the above mentioned problems. CODAG WARP uses two diesel engines to drive two propellers in a CODAD
Combined diesel and diesel
Combined diesel and diesel is a propulsion system for ships using two diesel engines to power a single propeller shaft. A gearbox and clutches enable either of the engines or both of them together to drive the shaft....

 arrangement, i.e. both shafts can also be powered by any single engine, and a centerline water jet
A pump-jet, hydrojet, or water jet, is a marine system that creates a jet of water for propulsion. The mechanical arrangement may be a ducted propeller with nozzle, or a centrifugal pump and nozzle...

 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.


Another way to combine the two types of engines is to connect them to generators and drive the propellers electrically as in a diesel-electric (See
  1. REDIRECT CODLAG, Combined Diesel-Electric and Gas). The RMS Queen Mary 2
    RMS Queen Mary 2
    RMS Queen Mary 2 is a transatlantic ocean liner. She was the first major ocean liner built since in 1969, the vessel she succeeded as flagship of the Cunard Line....

     uses this design. This also permits propeller pods
    Azimuth thruster
    An azimuth thruster is a configuration of ship propellers placed in pods that can be rotated in any horizontal direction, making a rudder unnecessary...

     with the propulsion motors inside the pods. A further CODAG innovation on the QM2 is to mount the turbines not in the engine room
    Engine room
    On a ship, the engine room, or ER, commonly refers to the machinery spaces of a vessel. To increase the safety and damage survivability of a vessel, the machinery necessary for operations may be segregated into various spaces, the engine room is one of these spaces, and is generally the largest...

    but directly under the funnel, simplifying the fresh air supply.

CODAS (Combined diesel-electric and steam)

This is a Diesel engine with a boiler to extract heat from the cylinder and the exhaust gases, generating steam at a rate proportional to the fuel flow. The steam is separately injected into the cylinder close to top dead center, actively modifying combustion and afterburning. The steam to fuel mass ratio is preferably 1:1 to 3:1 and the steam temperature and pressure are as high as engine materials permit. This system, adaptable to large Diesel engines, strongly reduces fuel consumption and emissions.

External links

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