The emissions output of large marine-diesel engines has been under scrutiny in recent years. These massive engines are expected to be more efficient and pollute less, which is not an easy task for a powerplant designed to hustle a cargo ship. One of the steps being implemented to reduce marine-diesel emissions is the use of natural gas, which promotes a more complete combustion burn that helps improve emissions performance.
The MaK VM 46 DF is a marine diesel engine developed by Caterpillar Motoren to power large commercial ships like cargo, ferry, and cruise vessels. The medium-speed diesel provides customers with the flexibility to run on traditional fuels like marine diesel oil (MDO), heavy fuel oil (HFO), and crude oil, then switch to natural gas (NG). The ability to use natural gas cuts NOx emissions by 80 percent, allowing the engine to meet the more stringent emissions requirements of the International Maritime Organization’s III code (IMO III). When not using NG, the powerplant only meets IMO II levels. Having this ability to select between fuel types allows the user to choose one that is most appropriate for their needs.
The switch from MDO to NG takes a mere half second and can be performed automatically or manually. A changeover works by slowly cutting back the fuel being used and balancing the reduction with the correct amount of replacement gas or liquid until the switch is complete. During the changeover, valve timing is adjusted along with the air/fuel ratio to accommodate the type of fuel that’s being introduced to the engine.
On its own, natural gas does not promote efficient combustion, which can cause a loss in power. So, to increase efficiency as much as possible, a small amount of a high-cetane fuel used as an ignition source is injected into the cylinder along with the gas.
MaK marine engines, like the VM 46 DF, feature individual nodular cast-iron heads that contribute greatly to their reliability and efficiency and make them easy to service and maintain. Each head has two intake valves and two water-cooled exhaust valves with valve rotators, as well as oil-cooled common-rail injection nozzles that are easily accessible.
The composite-type pistons feature oil-cooled, steel crowns and forged-steel iron skits that add to the engine’s robustness and serviceability. Each piston’s crown has two compression rings and one oil-scraper ring. The running surface of one compression ring is chromium diamond–plated while the other has only chromium plating on its running surface. The single oil ring is chromium diamond–plated. Further enhancing the pistons are three-piece connecting rods.
A one-piece, nodular cast-iron engine block gives the VM 46 DF a robust foundation. The block resists warping and is drilled for oil lines that are routed throughout to reduce the number of junction points that can possibly leak. The charge-air manifold is also incorporated in the casting to avoid vibration and leaks. The engine’s camshaft is sectionalized per each cylinder. This unique design allows cam segments to be removed from the block sideways, and use of an underslung crankshaft design facilitates extracting the one-piece crank without disassembling the entire engine.
There’s no cooling circuit in the engine block, eliminating chances of cross-contamination. Cooling is instead completed by a two-stage freshwater system that uses a high- and low-temperature side. The high-side coolant circulates through the high-temperature side of the two-stage, charge-air cooler, cylinder heads, and cylinder-liner water rings. Thermal readings are maintained at a preset level with a temperature-control valve. The low-temperature circuit goes to the low side of the staged charge-air cooler, lube-oil cooler, and diesel-oil coolers. The circuit’s design enables it to be used with gearbox and generator coolers as well.
The VM 46 DF uses two ABB TPL high-pressure turbochargers to optimize the fuel consumption and emissions. Multi-piece exhaust manifolds connect to the turbochargers through the engine valley. Using a flexible multi-application design enables the turbos to be mounted at either end of the oil-burner. With two turbos supplying air and a high-pressure common-rail system injecting fuel, the VM 46 DF is able to produce a staggering 20,000 hp.
SPECIFICATIONS
Engine: MaK VM 46 DF
Displacement: 1,595.2L (97,348 ci)
Engine Layout: V-16
Valvetrain: 64-valve
Bore x Stroke: 18.11 x 23.62 inches (460 x 610 mm)
Firing Pressure: 190 bar
Head Material: Nodular cast iron
Block Material: Nodular cast iron
Piston Material: Composite—steel crown/nodular cast-iron skirt
Power: 20,705 hp
Emissions: IMO II with diesel/IMO III with natural gas
Induction: Twin ABB high-pressure turbochargers
Exhaust: Insulated steel
Intercooler: Air-to-air
Cooling System: Liquid-cooled
Fuel System: High-pressure common-rail
Lubrication System: Dry sump
Lubrication Capacity: 23,036 quarts
Dry Weight: 440,000 pounds (220 tons)
Length: 466.54 inches (11,850 mm)
Width: 156.50 inches (3,975 mm)
Height: 157.36 inches (3,997 mm)
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Source: Caterpillar
