We don’t use diesel engines to create torque for the wheels on cargo and passenger trains. Instead, we use a diesel generator to create electrical power which then runs the traction motors on the train.
Considering how pollutant cargo ships are (and just how absurdly large those engines are!) why don’t they save on the fuel costs and size/expense of the engines, and instead use some sort of electric generation system and electric traction motors for the drive shaft to the propeller(s)?
I know why we don’t use nuclear reactors on cargo ships, but if we can run things like aircraft carriers and submarines on electric traction motors for their propulsion why can’t we do the same with cargo ships and save on fuel as well as reduce pollution? Is it that they are so large and have so much resistance that only the high torque of a big engine is enough? Or is it a collection of reasons like cost, etc?
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The main reason diesel electric makes sense is that it is good when the load or rotation speed changes a lot (a lot of acceleration and stopping). The generator part lets the diesel engine run at optimal rpm while the electric motors can handle a wide range of speed and provide good low end torque without gearboxes. Once you are at optimal running speed the diesel electric part is not more efficient than a straight diesel.
Cargo ships spend 99% of their time already running at a fixed speed so they don’t really need that kind of complicated arrangements. The propellers and ship in general are designed to be run at the optimal most efficient rotation speed of the diesel engine.
> Considering how pollutant cargo ships are..
One of, if not the, lowest pollution levels per mile per tonne of cargo carried on the planet. Far more efficient than land transport, and in a different league from air transport.
What makes you think it would save fuel and be more efficient? A good generator might be about 90% efficient, and likewise a good motor around that as well, for a system efficiency of about 81% of the diesel’s output power being used to drive the propeller.
On the other hand, a direct drive to the propeller from the diesel’s crank is very nearly 100%, only losing a tiny bit too friction within the support bearings.
Where is your imagined efficiency gain?
A train requires a ton of torque at almost 0 rpm and is correlated to train speed. Electric motors are good at this. Diesels are not. Plus trains need to start stop for coupling.
A boat prop does not have this requirement it’s can be spin up quickly and provides pushing force. Almost no correlation between boat speed and prop speed. Sort of like doing a continuous burnout. Diesel is much better here.
Edit: I did get a little crazy with my analogy but it’s still fair that a diesel boat can dead start better than a train.
I’m a ship deck officer, so if there are any marine engineers that want to correct me, I want to ask that they forgive me. I just drive.
Practically every new cruise ship is diesel electric as they need to maneuver daily. Using a diesel engine is more simple, which saves on the ship’s initial cost and manning costs, as with a diesel electric system both the chief engineer and chief electrician need special training.
Cargo ships are built for a specific speed and they’ll adjust their speed by a few RPM at most. I’ve been on a ship where we would put a box over the engine lever so that nobody would touch it during an ocean crossing. The engine is also optimized for these exact RPM. The engine is then directly connected to the propeller shaft. That’s as simple as you can make it. There’s no point in running the same (or 2 smaller) engines into dynamos that then power seperate electric engines to power the same propeller shaft if you already know what RPM the propeller will be spinning at 99% of the time.
Of course, the above also means that the correlation between a ship’s efficiency and speed through water is not linear and there is a speed you don’t want to be going. Marine engines also have so-called critical RPM that induce massive vibrations in the hull, so you want to pass through them as quickly as possible.
Cruise ships, on the other hand, need to follow a schedule, have to maneuver daily and aren’t that worried about fuel efficiency, so they are either fitted with controllable pitch propellers (to maintain optimal engine RPM and allow for easy maneuvering) or they’re diesel electric (the engine team is already a lot bigger, so there’s allowance for extra electricians).
Nuclear carriers and larger subs use steam turbines for propulsion, not electricity. The power electronics for a carrier’s propulsion system would be complex and expensive vs. steam.
Ships in general don’t have the 0RPM torque problem that wheeled transport has. Propellers (and water-jet systems) can easily “slip” in water. Wheels on road or track cannot (or should not) so water-based propulsion has a built-in clutch mechanism.
It’s kind of like traditional automatic transmissions in cars: there’s an engine-driven turbine in the torque converter of your standard automatic transmission that allows for some slip between the engine and transmission. Doesn’t matter if its trans oil or water, this slip allows the engine to transmit torque even if the vehicle/vessel is at a dead stop.
Rocket engines work the same way: fluid mechanics. High speed gas or low-speed water pumped through a nozzle (a propeller acts as a pump AND a nozzle in incompressible water; ducted props are just more efficient) produces thrust, assuming the nozzle is structurally connected to the vessel.
Diesel engines, like all piston engines, need to spin some to make power – they don’t make any power at all at 0 RPM. Meanwhile, vehicles which are powered by driving the wheels directly don’t like the wheels slipping against the ground. So there’s a mismatch there that prevents you from connecting this type of engine directly to wheels. You’ll need a transmission, with a mechanism to allow a rotating engine to apply enough power to a stationary axle to get the vehicle going, and usually also a few sets of gears to match the desired vehicle speed range with the engine’s speed range.
Most car engines use a torque converter for the stationary start. That has the engine spinning a small compressor (kind of like a propeller…) in hydraulic fluid with a turbine connected to the driveshaft. Works pretty well for getting cars going.
Making a good transmission for a train is hard. Lots of wheels to drive, lots of mass to get up to speed. So it’s often better to do the diesel-electric thing – use several smaller electric motors to drive all of the traction wheels.
On a ship, you’re driving a propeller, which is already a lot like that torque converter. You can drive it at any speed any time, no matter how fast the ship is moving. No need for anything between it and the engine, besides a single set of gears to match the speed.
Anything that you put between the engine and what it’s driving will inevitably cost you some efficiency. Locomotive engineers determined that the electric setup is more efficient than a mechanical transmission would be. On a ship, you can drive the propeller directly, so anything you put between your source of mechanical rotation and the propeller will inevitably be worse. I’m pretty sure the nuclear naval vessels connect their propellers directly to the steam turbines too instead of going through generators and motors.
The torque requirements for a locomotive vary wildly as do the rpms. Ocean vessels are a bit more predictable. The resistance from the water is pretty consistent. Also, generating electricity from an engine to drive an electric motor means you’re going to lose energy with the conversions and you need a giant generator and a giant electric motor vs JUST having a big engine, maybe with a transmission. Fewer components and better efficiency.
Nuclear reactors for commercial ships were considered, and one mixed use ship powered by a nuclear reactor was in service- NS Savannah.
One issue was that you needed highly skilled specialists to operate the reactor but what ultimately killed nuclear power on non-military ships is the risk of a catastrophic failure in port.
You can get a ship diesel engine serviced almost anywhere around the globe; not so for e-drives. Diesels are also simpler to operate, maintain, carry spare parts for, and train crew on. Simple and cheap, with pretty good fuel efficiency.
Cargo ships going to a nuclear power plant is a terrible idea because the maintenance is an order of magnitude more complicated, and at the end of its service life a diesel cargo ship is easily cut up for scrap. Nuclear contamination and defueling a reactor is more than the scrap companies can deal with. It would get mixed in with regular scrap or dumped into the ocean.
in addition to the other reasons mentioned is that in the fractional distillation of raw petroleum you’re gonna end up with heavy fuel oil so it shall be put to use. and for where you can use the different products, it makes sense to allocate heavy fuel oil to cargo ships.
Trains use diesel electric because accelerating from zero to full speed (fast rotation of the wheels) on a long train would require an immense transmission with a lot of gears. Using electric motors that have their torque starting at zero eliminates the need for the transmission.
Ships just need to turn the propeller, so they have one reduction gear to keep the propeller in an efficient rpm range, with reverse. The losses of converting from engine shaft torque to electricity to propeller shaft torque would also make it less efficient.
These engines also use something closer to crude oil than diesel. Basically it’s the leftovers after making diesel, gasoline, jet fuel, etc. It’s efficient from the standpoint of using everything we pull from the ground.
There are cargo ships that use diesel electric drives most of them are highly specialized such as shuttle tankers or cruise ships that require high manoeuvrability I’ve personally been on a tanker that was diesel electric so they do exist. Reason for it not being more common is cost iirc the pod (propulsion unit) had to basically be donated to the company in order for them to even consider doing it because it was too expensive. Another reason is crewing, most vessels have 440v and that doesn’t require any special certificate to work on diesel electric vessels generally have 1000-6.6kv which requires crew to be certified to work on it and again that costs the company money. That’s the only reasons not to do it from a engineering standpoint diesel electric is far better for the engines, environment and cargo capacity so let’s break this down quickly, it’s better for the engines because you run at a very very consistent load and diesel engines are most efficient and do the best around 85% load, at consultant loads you have very consistent emissions wich generally is good for the environment, cargo capacity you can move the engine room back slightly and therefore you can have slightly larger cargo holds on tankers at least
I’ve worked on a diesel electric tanker that was 1.2 million barrel capacity and it is a functional alternative. The fuel savings is, however, negligible in my experience. The ship I’m on right now hardly ever runs at max engine speed so generally our diesel generator burns MORE fuel powering the ship’s electronics and cargo than our main engine does. Modern, large, slow speed diesel engines are very fuel efficient and international regulations require emissions controls (which are getting more strict over time). I think, at the end of the day, a large slow speed is just cheaper and easier than an equivalent electric generation and propulsion system.
Edit: tanker capacity.
For ships that need speed and maneuverability diesel is often used. Like on cruise ships or military warships. They are better for higher speed and more dynamic speed.
Cargo ships however don’t need that. And it’s cheaper for them to build simple and big engines that run on fuel oil they can get for dirt cheap. Petroleum refining creates a lot of sub products. The oil that cargo ships use is one of the less desirable ones so its cheap to buy in abundance. Essentially it’s the left overs when the more popular gasoline, diesel, jet fuel, and kerosene is made.