Dual-fuel diesel electric was the industry’s answer to a question steam had settled by brute tolerance: how do you run a ship on a fuel supply you do not control, without accepting thirty per cent efficiency?
The architecture
Several medium-speed four-stroke engines, each driving an alternator. The alternators feed a common electrical bus. The bus feeds electric propulsion motors connected to the shaft, and everything else on the ship.
There is no mechanical link between the engines and the propeller. That is the whole idea.
The engines are dual-fuel: they burn gas ignited by a small pilot injection of liquid fuel, or run entirely on liquid fuel if gas is unavailable. Tri-fuel — TFDE — adds heavy fuel oil to the menu, which mattered a great deal before sulphur regulation and matters less now.
Why electric transmission helps
Three reasons, and they compound.
Load matching. A ship at sea needs full power; a ship manoeuvring needs a fraction of it. With four engines on a bus you run one, or two, or all four, each at or near its efficient load, and shut down the rest. A single directly coupled engine has to run at whatever load the propeller demands, efficient or not.
Redundancy. Losing one engine of four costs a quarter of the power. Losing a single main engine costs the ship.
Freedom of layout. Engines can sit anywhere convenient because they are connected by cables rather than a shaft line. On a vessel already awkward to arrange — huge cargo tanks, a superstructure aft, restricted visibility over the deck — that is worth real money.
The cost is conversion losses. Every step from engine to alternator to switchboard to motor to shaft loses a little, and the chain gives back perhaps three or four percentage points against a directly coupled engine of the same thermal efficiency. Against steam it was still a rout.
Handling the boil-off supply
The problem steam solved by tolerance is solved here by numbers. Boil-off is distributed across whichever engines are running; if it exceeds what they can use, a gas combustion unit disposes of the surplus; if it falls short, the engines make up the difference with liquid fuel.
Changeover is managed automatically and is undramatic. The engines can transfer between fuels under load, which is what makes the arrangement practical for a cargo that produces vapour on its own schedule.
The weakness: methane slip
Dual-fuel medium-speed engines burn gas on the Otto cycle: gas and air are premixed and the charge is ignited by a pilot spray of liquid fuel. Lean premixed combustion is efficient and produces very little nitrogen oxide, which is why it was chosen.
It also leaves some methane unburned. A small fraction of the charge sits in crevices — around the piston ring pack, in the corners of the combustion chamber — where the flame does not reach, and it leaves with the exhaust.
The quantities are small as a share of fuel and large as a climate matter, because methane traps far more heat than the carbon dioxide it would have become. Methane slip has consequently gone from a footnote to a central argument about whether gas-fuelled shipping delivers the emissions benefit it claims, and it is the main reason the industry moved on again.
Where it stands now
Conventional carrier newbuildings have largely gone to two-stroke gas engines, which are more efficient again and, in the high-pressure variant, slip far less.
The dual-fuel electric architecture has not disappeared, though. It remains a strong fit wherever a vessel needs large electrical loads for something other than propulsion: FSRUs running regasification, FLNG units running a liquefaction plant, and vessels that spend much of their life moored rather than sailing. On those, the ability to redirect generated power between propulsion and process is the point rather than a side effect.