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Marine Propulsion & Auxiliary Machinery

Marine Propulsion & Auxiliary Machinery

Four-strokes: sucking, squeezing, banging and blowing their way into the future

Wed 11 Jul 2018 by Selwyn Parker

Four-strokes: sucking, squeezing, banging and blowing their way into the future
The latest Wärtsilä 20 coaxes another 200 rpm from the power unit

Four-stroke technology is advancing, but enginebuilders are honing their existing units, rather than starting from scratch, meaning shipowners need not invest in new maintenance equipment, writes Selwyn Parker

In the endless battle to extract more bang for the same hydrocarbon buck, Wärtsilä has unveiled a new version of one of its most faithful four-strokes, the Wärtsilä 20. It is hoped that the new power plant could keep the design going for many more years, simply by coaxing another 200 rpm out of it, as Wärtsilä product manager for small bore marine solutions Johan Kalax explains: “There are only two ways to gain higher output from an engine: the first is to increase the firing pressure at a fixed rotational speed, which is not usually possible without an extensive redesign of many parts. The second method – and the most economical – is to increase the rotations per minute (rpm) of the engine at a fixed firing pressure.”

Understandably, the Finland-based group has adopted the second method for the Wärtsilä 20, which is now in production. The engineering wizardly can increase maximum rpm from 1,000 to 1,200 in mechanical propulsion engines, and from 900 to 1,200 for 60 hz generating sets. The extra boost from the revamped engine is measured in an increased cylinder output of 220 kW at 1,200 rpm.  

This latest advance has been imported from Wärtsilä dual-fuel 20DF, which has been operating at 1,200 rpm since its introduction in 2011.

As Mr Kalax explained, the 10% extra power is achieved by improving the relationship between the turbocharger and the compressor: “The key to tweaking additional rpm and capacity on the upgraded Wärtsilä 20 stems from the turbocharger and compressor aspects, where a new cooling pipe arrangement is needed to reduce the internal temperatures of the turbocharger.”

Once this solution had been implemented, the rest was apparently relatively easy. A few new flanges and connecting holds were machined into the engine block, cylinder heads and camshaft to allow for cooling pipes and a new injection system was installed across the whole Wärtsilä 20 range, as well as a new and longer-lasting air waste gate, replacing the old and somewhat unreliable pneumatic system. And the updated power unit will come complete with Wärtsilä’s UNIC control and automation system that delivers all the vital operational data via an LED touch screen.  

While that sounds like quite a lot of tweaking, Wärtsilä confirmed that the basic design of the original engine “remains untouched”. Thus, the Wärtsilä 20 - a favourite of lower-powered merchant ships, special vessels and offshore supply ships - looks set for a new lease of life having been introduced in the early 1990s, since which time more than 6,000 of these engines have been delivered.

Although production of the Wärtsilä 20 started earlier this year, it will take about two years to develop all versions of the engine. The company sees the revamped unit as being most suitable for the 1.3 to 2.0 MW segment of the merchant fleet.  

Azipod power

Even the biggest four-stroke power units are being made more functional through extra, outside forms of propulsion. When Virgin Voyages’ three new cruise ships are launched progressively between 2020 and 2022, each of the 278 m-long ships will be driven by two of ABB’s Azipod XO units, each delivering up to 32 MW in power.  

Linked to Wärtsilä’s giant 46F four-stroke engines, the biggest of which weighs more than 230 tonnes, Azipod propulsion represents a clear advance in cruise ships. It is a gearless, steerable system with the electric motor included in a submerged pod located outside the ship’s hull.

According to ABB’s estimates, the system can reduce fuel consumption by up to 15% compared with standard shaft line systems. The Azipods will be boosted by ABB’s own TPL-C turbochargers during more demanding operations.

Silent steaming

There has been plenty of talk about slow steaming over the last few (recessionary) years, but silent steaming for four-strokes could be around the corner. Later this year Wisconsin-based Catalytic Combustion Corporation will start installing its Silent NOx system on vessels operating on America’s west coast. The main benefit of the system, according to the company, is that the reactor box containing the catalytic reduction system (SCR) is more compact and quieter than competing technology. “[Silent NOx] saves a tremendous amount of valuable space on a new or retrofit vessels,” it said, adding “The system fits right over the engine footprint, so great for refit applications and new installations.”

The system can be configured for four different sound levels: industrial, residential, critical and hospital grade. The last should be good enough for silent steaming.

Evolution not revolution

Upgraded power units, rather than completely redesigned ones, are popular in both the enginerooms and the head offices of shipping companies because they mean maintenance procedures remain largely unchanged. Upgrades also mean that operators have access to a large store of existing spare parts, rather than having to buy in expensive new ones.

To this end, Cummins has released a retooled version of its X15 four-stroke, aimed at the commercial marine market, from inland waters to passenger transport. The new X15 meets US EPA Tier III emissions standards and comes with the CM2350 engine control system that is designed to automatically shut down the engine to prevent a catastrophic failure. CM2350 monitors the fuel, gear pressure and temperature through a range of sensors.

Designed for retrofits as well as newbuilds, the engine is seen as a replacement for units such as Cummins’ N14s, K19s and QSK19s that are approaching the end of their functioning lives. According to the company, the first of the updated X15s will be installed shortly in a crew boat in Louisiana.

PEMS - Power when you need it

When the weather cuts up rough in the North Sea, the urgent, high-precision repair of vital power cables places huge demands on vessel engines. They must deliver just the right amount of power at the right time to hold a vessel steady. Fortunately, the propulsion system on the new 140 m-long NKT Victoria, one of the world’s most advanced cable-laying ships, was up to the job when the crew were called out for just such a mission.

The NKT Victoria uses a direct-current energy management system to maintain position in rough seas (image courtesy NKT)

Equipped with an ABB four-stroke power unit controlled by a direct-current energy management system (PEMS), the NKT Victoria was able to support the repair work in large part due to the technology that allowed the vessel to hold station. The PEMS system, which is designed specifically for new and alternative energy sources, marks a step forward on standard alternative-current systems, where generators run at a fixed maximum speed irrespective of the power demand on board.

“This leads to excessive engine wear and poor fuel efficiency at lower loads,” explained ABB global product manager for onboard DC grid in the marine and ports division John Olav Lindtjorn. Unlike standard alternative-current systems, direct current allows the generators to run at variable speeds.

In its first year of operation, the NKT Victoria has more than proved its value, having installed 335 km of cables for offshore wind and interconnector projects in Denmark, Belgium and Britain.

The direct-current technology is expected to be widely adopted in two- and four-stroke engines alike in coming years, offering advantages including low fuel consumption and reduced wear and tear on the engine.

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