Why is it more efficient for large ships to be powered by Diesel-Electric or similar two stage powerplants? I thought changing energy types and systems always involved losses.


For example, aren’t you losing a small % of energy converting the diesel in your car into power to move your car?
So then, wouldn’t you lose more % if you used diesel to then make electricity and THEN power a ship?
IE: The longer the chain of conversions, the less efficient your energy transfer will have. But I know large ships use an extra step compared to cars or a regular engine, why is that better for large ships?

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7 Answers

Anonymous 0 Comments

Because the alternative would be hooking the diesel engine directly to the motor of the ship which is less efficient and leaves you without a source of power for anything electric on the ship.

ICEs and diesel engines have an optimal performance window for RPMs. You can make a diesel engine really efficient by running it at one set rpm and powering a generator and using the electricity to actually do the work you need done.

This is the reasoning behind diesel electric locomotives as well.

Anonymous 0 Comments

ICEs, and diesels especially, have an ideal range of RPM where they produce the most power. They also have an ideal range where they are most efficient, which isn’t necessarily the same – usually a bit lower.

Connecting the diesel engine to a generator allows you to keep it in that ideal efficiency or power band independent of the speed of the propellers, the ship speed, or even the load on the electrical system.

Then you drive the propeller shafts with large electric motors. Electric motors are very efficient at pretty much any speed and can deliver very high torque at low rotational speeds compared to ICEs.

You can also use the electric power you generate to power other things on the ship, from “hotel” loads (lighting, heating, washing, cooking) to bow thrusters and navigational equipment. If you use a direct drive you need to either have a power take-off driving a generator or you need auxiliary diesel generators.

You can also locate the main engines anywhere in the ship rather than having them constrained to be co-linear with the prop shafts. You can also use alternative propulsion methods like azimuth thrusters or Voith-Schneider drives.

Anonymous 0 Comments

To add some additional info to this

electric motors are beasts and far more efficient than ice, if you could find a electric supply with enough energy density and the right size/weigh ratio then it would make sense to ditch the generator and make it pure electric, as it is currently diesel power density (27 times that of Lithium batteries) and weight/size ratio of diesel is higher,( so higher autonomy, range and better use of space/weight) if one day we find better electric sources that may change

Anonymous 0 Comments

Straight diesel (or more likely bunker fuel) is usually more efficient, which is why most ships use it with engines directly connected to propellors.

There are some use cases where diesel electric is better. The main one is azimuth thrusters which greatly improve maneuverability of the ship. It’s much harder to route a series of drive shafts into the thruster than a big electrical cable. So ships like cruise ships (docking almost every day), tug boats, ice breakers, etc use them. Tanker ships that only dock once a week or less may not care about maneuverability as much.


Anonymous 0 Comments

Cargo ships that are on the smaller size can have a large diesel directly connected to a reduction and onto the shaft. When they need to reverse, they stop the engine and actually reverse the engine. This is crude, because the propeller is shaped to be the most efficient in the forward direction.

In larger sizes of cargoship, nobody makes an engine big enough that’s also reversible, so you either need a propeller with adjustable blades (super expensive) or drive the shaft with an electric motor, which is easily reversible.

After a while, it becomes and chicken or egg thing, nobody buys “X” because nobody makes them, and nobody makes them because nobody buys them.

Anonymous 0 Comments

Navigational officer of major container carrier here

Our vessels are direct powered diesel engines to shaft on slow speed (26-100 rpm). No conversion, inline cylinder design. Very large hight requirement for the motor design

On vessels like cruise ships where you want to save upward space for passengers, you use V design engines to gen sets for electric power, typical medium speed (400-700 rpm). This make is possible to have several engines in the full length of the vessel. The smaller the less heights requirement. This power is than used both for habitat (AC, light etc) and propulsion via large electric motors to shaft or azimuth propellers

There is a loss in power every time you convert the power and also the diesel/electric is a more complex overall system as you have more parts and normally also more engines. But you can fit the engine room to only the lower decks, where a Maersk E class engine (Wartsila RT14-Flex96c) is 5 stores tall as world largest engine and the engine room is 9 stores deep

Anonymous 0 Comments

Large container and other heavy haul ships actually have 1 or more propulsion engines that drive the propeller. They are low speed engines and can run in reverse to turn the propeller backwards. They may have a small reduction gearbox but not much of one. The ship will then have multiple diesel generators to provide power to the ships systems.