Wärtsilä 2-speed gearbox explained by Dr. Kjell Haugland at Wärtsilä Marine Solutions

Wärtsilä 2-speed gearboxes

The Wärtsilä 2-speed gearbox is an enabler for significant fuel savings, with a corresponding reduction in the environmental footprint. With its low technical complexity, it offers advance functionality to comparatively low investment cost.

Text: Kjell Haugland Photo: Wärtsilä

Basic principles behind the fuel savings

Ships are normally equipped with a propulsion system designed for a certain maximum speed or towing requirement. However, many ships might only need this maximum propulsion power occasionally and are working in off-conditions at lower power and with less efficiency. For a controllable pitch propeller (CPP) installation, it is rather simple to reduce speed or thrust by reducing the propeller pitch. However, it is much more efficient to reduce the propeller speed at part load as indicated in Figure 1.

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Figure 1: Basic propulsion characteristics.

The propulsion system is designed for Max vessel speed (condition A). This requires a propeller speed of 130 rpm (Propeller speed 1) and a power of 5250 kW. However, the ship will frequently operate at the lower Vessel speed, 12 knots. This vessel speed can be achieved by one of the following:

  1. Reducing the propeller pitch and keeping the propeller speed constant at 130 rpm (Condition B, Propeller speed 1)

  2. Reducing the propeller speed from 130 rpm to 91 rpm with a corresponding increase in propeller pitch towards the propeller’s optimal pitch (Condition C, Propeller speed 2)

The required propulsion power at condition B is 2450 kW, while for condition C it is only 1900 kW.  The 550 kW reduction in the power requirement for the 12 knots sailing mode corresponds to 22% reduced fuel burned.

The condition C, Propeller speed 2 can be reached by reducing the engine speed as well by running along the propeller curve. However, this solution has certain drawbacks, such as the following:

  • Reduced engine efficiency at lower engine speeds

  • Slower engine power-ramp-up capability at lower engine speeds

  • Requirement for electric power frequency conversion for Power Take Off or Power Take In through gearbox

  • Limitations in optimizing main engine power for both propulsion and electric power production 

Some power savings can be achieved by allowing limited frequency variations, but similar savings can be obtained for a 2-speed system as well.

The total fuel saving for a ship depends on the operation profile of the vessel and the time it actually can be operated at the lower propeller speed. The AHTS vessel in Figure 2 has an operation profile allowing a 10%/750-metric-tonne reduction in annual fuel consumption. With a fuel price of 400 USD/mton (Marine Diesel Oil /June 2016), the annual savings is around USD 300,000. This means that the additional capex for the 2-speed functionality is payed back in less than one year.

The contribution of the fuel savings to the environment is significant as well, with reduction of NOx and CO2 emissions in the same order. This is just one example. For other vessel types, the potential savings can be even higher, depending solely on their specific operation profile.

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Figure 2: Wärtsilä 2-speed installation – Operation modes and potential savings compared to a single speed installation.

The Wärtsilä 2-speed gearbox design

Our 2-speed gearboxes are not new in the market. The first Wärtsilä 2-speed gearbox, the EVC84/2-P55, was designed in 1994. This was actually the first Wärtsilä gearbox ever. It was designed in cooperation with Valmet in Finland.

The second generation of Wärtsilä 2-speed gearboxes was introduced in 2003 with the SCV95/2 gearbox.

The development of the third generation of Wärtsilä 2-speed gearboxes started in October 2011. It was triggered by increasing attention to environmental emissions and fuel costs.

The design of the first size, the SCV90/2-P58, was released during the first half of 2012, and the first gearbox was delivered to the customer in December the same year. The lead time for design and production of this pilot was less than 15 months.

The latest design is characterized by the following:

  • 35% less installation length (between main engine and generator)
  • 20% lower weight
  • Robust design for machining and assembly, supporting a high quality standard
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Figure 3: The Wärtsilä 2-speed gearbox design evolution.

The new Wärtsilä 2-speed gearbox family

The third generation of Wärtsilä 2-speed gearboxes has now expanded to a large family of gearboxes. It covers a power range from 2 MW up to 13 MW. It is available in five basic sizes 80, 90, 100, 112 and 118 (gearbox size = offset between input- and output shaft in cm) with both vertical (V) and horizontal (H) executions.

The “family tree” for these gearboxes is shown in Figure 4. Due to the large power span for the gearboxes, the different sizes could not be designed by scaling alone.  On a more detailed level, there are significant differences in the design. However, the designs have the same basic features:

  • Two selectable propeller speeds 70-85% (Speed 2) and 100% (Speed 1)
  • Uninterrupted electric power generation from the PTO (Power Take Off) during speed change
  • PTO line dimensioned for loads up to 100% main engine load
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Figure 4: the Wärtsilä 2-speed gearbox family

The operation principles of the Wärtsilä 2-speed gearbox

The operation principle for a 2-Speed gearbox for a ship with five operation modes are shown in Figure 5:

  1. Propeller speed 1 – Diesel Mechanic mode
  2. Propeller speed 1 – Booster mode
  3. Propeller speed 2 – Diesel Mechanic mode
  4. Propeller speed 2 – Diesel Electric mode
  5. Generator mode
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Figure 5 (Click on the picture above to view the full-size figure)

The Wärtsilä 2-speed hybrid gearbox – The enabler

The basic features of the 2-speed gearboxes enable a number of additional features, such as the following:

  1. A significant reduction in fuel consumption and CO2/NOx emissions, as described
  2. Optimization of engine power for propulsion and electric power production
  3. Power boost functionality – adding electric power to the propeller through the PTO/PTI shaft
  4. Reduction in total installed engine power, main and auxiliaries, due to flexibility in combining power from the two sources
  5. Electric drive as one of the defined operation modes or as a Take-Me-Home functionality
  6. Redundancy due to both Diesel Mechanic and Electric drive – the E-drive can be powered from batteries or from conventional generator sets.
  7. Low noise operation: By reducing the propeller speed there is a significant reduction in the underwater radiated noise. This is of great importance for fishing vessels and seismic vessels, but it is also highly appreciated by the crew due to less noise and vibration in the living quarters of the vessel. Measurements show reduction of sound pressure at maximum 20 dB when the propeller speed is decreased from 125 rpm to 100 rpm. Note that 10 dB reduction in the noise level is perceived as 50% reduction in the noise.
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Figure 6: Measured underwater radiated propeller noise at propeller speeds 125 rpm and 100 rpm.

About the author: Dr. Kjell Haugland, Propulsion Technology, Gearboxes, Wärtsilä Marine Solutions, Email: kjell.haugland@wartsila.com

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