When I toured the USS Carl Vincent (a nuclear air craft carrier) at Alameda I was surprised at how "small" its engines were relative to large container ships. It helps that they are electric of course (no big gas expansion chambers) but I suspect if it weren't for the proliferation concerns cargo ships of this size could be economically run on nuclear power plants.
Indeed, Nimitz-class carriers have far more powerful engines,[1] as do as do Russian-made icebreakers, which are civilian ships,[2] as did Savannah, an American civilian ship.[3]
Edit: Carl Vinson, like most nuclear-powered vessels to date, is direct-drive. The Ohio Replacement Submarine is slated to have electric drive.[4]
Generally speaking, torque increases are associated with increased stroke where horsepower increases are associated with increased piston surface area.
Running a ship engine with a long stroke (to increase torque) serves to reduce stress on the parts and increase lifespan since these engines have very long life expectancies.
Also, you are correct that the lower rpm engine would deliver more efficiency but not necessarily because it is larger/runs slower. It is more efficient because a smaller engine running at a higher rpm would have to work harder to generate the same power to move the ship and its load and would have the undesirable side effect of sacrificing longevity.
(speaking just to size, not RPM:) The square-cube law causes the burning gasses in a larger cylinder to bleed less heat away into the water jacket compared to a smaller cylinder. This is the primary reason why the largest diesel engines in the world are also the most efficient.
Theoretically, the 14-cylinder Wärtsilä-Sulzer RTA96-C could be even more efficient if it were instead a gigantic single cylinder. There must be some other engineering or economic limitation which prevents us from manufacturing 25,000 liter cylinders.
edit: example: http://en.wikipedia.org/wiki/Square-cube_law#Engineering_exa... "Watt recognized the problem as being related to the square-cube law, in that the surface area of the model's cylinder surface to volume ratio was greater than the much larger commercial engines, leading to excessive heat loss"
edit2: There is also a square-cube law with aerodynamic losses related to piston speed (how fast the air gets sucked into the engine: "pumping losses"). The slower you spin the engine, the lower your pumping losses are. However, I believe this plays a smaller role in affecting efficiency than the heat losses. (also, this engine is turbocharged, which changes things a bit).
Like what Tyler said, balance shafts to counter a single cylinder would be huge but also they'd add unnecessary complexity that could otherwise be overcome by adding additional cylinders instead.
Furthermore, in order to obtain the same level of torque in a single-cylinder that a multi-cylinder engine creates you'd have to account for the power generated per cylinder.
In the Wartsila RTA96C you have HP up to 109,000 (rounded) and torque up to 5,600,000ft-lbs [1]. These #s are for all cylinders. Assuming it is the 14-cylinder variant generating this power and you want to reduce this down to a single-cylinder package then you can easily deduce that the displacement would be around 25,480 liters (mentioned elsewhere in this thread).
Assuming a stroke to liter ratio of 1.374mm/liter (2500mm stroke for each 1,820 liter cylinder) you'd end up with a 35,009mm stroke for a single-cylinder engine vs. a 14-cylinder engine. Since there are 25.4mm in an inch that's a 98-inch stroke for 14-cylinders vs. a 1,378-inch stroke for a single-cylinder or an 8-foot stroke vs. a 114-ft stroke.
Finally, since you'll need a piston many times larger than the current setup for a single-cylinder vs. a 14-cylinder engine then you can imagine how much mass you have moving which can do some serious damage if it becomes unbalanced. Also you'd have an engine that instead of being long and relatively low is now very short and extremely tall (not something you want in a ship where capsizing is a very real concern).
What's the minimum number of cylinders required for harmonic balance in a two-stroke inline engine? I'm just curious what the biggest engine you could build would be by cylinder volume.
I'm sure someone will correct me if I'm wrong, but I've heard it has to do with the mass of the moving parts. Since each increase in unit velocity increases the kinetic energy exponentially (squared), the connecting rods, etc would have to be so huge to withstand the force that it's easier to optimize for low RPMs.
It's amazing to think how slow the pistons move on this engine. At 87 RPM, it would take a little less than a second for the piston to complete a cycle. Compare that to your average automobile where the piston completes 50 cycles every second (at 3000 RPM).
The other reason is that the screw speed for a large vessel is about 100 RPM. It is better if the engine can drive the screw without a reduction gear that would decrease efficiency.
I think that's backwards reasoning. If the engine ran more efficiently at 200 RPM, they wouldn't install a transmission, they'd just use a smaller screw.
At the full-rated (as opposed to down-rated operational) output specification provided, that translates (for used to US units) to:
93,496 HP @ 84 RPM
5,845,726 lb-ft @ 84 RPM
The most mind boggling number for me is the torque figure. Almost six million lb-ft of torque. Power is an important figure when determining how much work you can do, but torque is important when engineering the components that will go in to your drive line. For example, transmissions typically specify a maximum torque input, as well as a maximum rotational speed. Independently, neither can be exceeded, regardless of power input. For example, the transmission in my car is good for about 450 lb ft and 9,000 RPM. I'd love to read over some of the engineering specs for the driveline components attached to this thing.
A 2 stroke diesel is different from a 2 stroke gasoline engine. It doesn't have the same environmental issues because there is no oil in the intake. The only downside is it needs a supercharger to work so it's not practical in small engines. Most(all?) large diesels like in trains and ships are 2 stroke because of the better power/weight ratio.
2-stroke gasoline engines such as dirtbike engines are run with oil in the gas (thus oil in the intake) for economic and spatial reasons (removing or shrinking lots of parts like oil pumps, oil pans and ports.
It doesn't really lubricate like a 4-stroke where oil is pumped up and then scraped back down into the pan. It is still pumped up but it is stored in devices inside the cylinder liner where it is only dispersed as necessary. Each cylinder instead has oil squirters (receptacles) in the cylinder liners and essentially when the pressure under the piston is low enough that the system has determined there is insufficient oil, spring pressure from the oil squirters forces out enough oil until the pressure in the cylinder (under the piston) is enough to overcome the spring pressure and thus the oil squirters stay closed [1].
Compares one ship to another, first engine is described in terms of kilowatts and the next in horsepower. Talks about cargo capacity in terms of TEUs (shipping containers, I guess) rather than gross tonnage, which is the way we've always talked about ships forever.
I know nothing about ships but I suspect this is the shipping equivalent of Information Week.
>Talks about cargo capacity in terms of TEUs (shipping containers, I guess) rather than gross tonnage, which is the way we've always talked about ships forever.
Gross tonnage has little to do with cargo capacity, as it covers weight of hull, machinery, fuel etc. Net tonnage is more relevant, but it is not a good basis for comparing the capacity of different types of cargo ships. TEUs are a good way to compare container ships to each other, so at least in that respect the article is fine.
I wanted to read the site's "About" page (to figure out if they have some relationship with Maersk) but since it's infinitely high I didn't have the patience to scroll there.
Does nobody care that the concepts "footer" and "infinite scroll" don't mix?
It seems it would fail the most trivial of testing, when every page element isn't even possible to click?! Gaah.
> The two-stroke engine is rated at 69,720 kW @ 84 rpm, although has been de-rated to 56,800 kW, and stands a whopping 17.2 meters tall (that’s over 56 feet!).
Is it really the "largest engine ever"? The Saturn V was 110 meters tall; even discounting the payload that's pretty damned big.
Surely most of the 110m of the Saturn V was fuel tanks? Perhaps they don't count towards the size of the engine. If they did, then I imagine include the fuel for this ship would make it pretty big.
There were lots of physically huge reciprocating steam engines in power houses and pumping stations just before turbines came in. The power ratings were low by modern standards.
I know, but the original article measures the size of the engine in meters, not Watts. So, the question is whether this thing is larger or not, not whether it is more powerful.
How is the block on one of these engines fabricated? I would imagine it is not a one piece casting, so are a few smaller castings bolted together? Are they made of stacks of plate? Do they have to build a gantry mill over it for machining?
Gas turbines are more efficient in terms of size and weight for a given power level but burn more fuel. Merchant ships mostly care about fuel efficiency.
Do they? I thought most fossil-fueled power plants use coal, which they burn to power a steam turbine. Natural gas plants often use gas turbines, but this is very different from the gas turbine used by a ship: the latter burns kerosene, not natural gas. (Natural gas is uneconomical to power a moving vessel; because it's a gas, you need space to store it, and that space can be orders of magnitude bigger than an equivalent energy-density of diesel fuel or kerosene.) Diesel engines or oil-powered gas turbines are very rare for power plants, outside of backup generators for hospitals etc.
Peaking power. For the relatively short peaks when e.g. everyone gets off from work it's most efficient to fire up some gas turbines for a little while. Or on the relatively infrequent hottest days of the year.
Gas turbines are in fact used for direct electrical generation. In a combined cycle plant, the waste heat from the gas turbine exhaust is used to generate steam for a steam turbine.
Simple cycle gas turbines have efficiencies of 30-35%, whereas a large diesel engine can reach ~50%.
A combination of gas turbine, steam boiler running off GT exhaust and a steam turbine can reach efficiencies of ~55%. However, the system is complex and expensive, and not well suited for driving ship's propellers. Once additional kit like gearboxes are introduced, the drive system efficiency is no better than a direct-drive low speed diesel.
Finally, the diesel engines burn viscous, high-sulfur fuel... not really sure if a GT could handle the crap those engines use.
