Could LNG kill the two-stroke?

Could four-stroke propulsion with PTO be more flexible and cost efficient than two-stroke engines for LNG-fuelled bulkers? Could four-stroke propulsion with PTO be more flexible and cost efficient than two-stroke engines for LNG-fuelled bulkers?

Manufacturers have been hailing new four-stroke engines as ‘two-stroke killers’ for years, but a case study from Wärtsilä suggests that LNG as fuel could make four-stroke propulsion viable for some bigger ships.

The paper, examining the case for an 82,000dwt bulk carrier fuelled by LNG, notes that standard bulk carrier designs feature a single two-stroke main engine, directly connected to a fixed pitch propeller, with three auxiliary gensets. To date, LNG-fuelled bulkers have simply installed dual-fuel main and auxiliary engines.

Instead, Wärtsilä argues for a solution with two four-stroke engines catering simultaneously for propulsion and hotel load, connected to a controllable pitched propeller via a reduction gear. Instead of auxiliary gensets, power-take out generators connected to the gearbox would generate electricity. By connecting the PTOs to a frequency converter, the main engines could operate at variable speeds – offering a lower fuel oil consumption and increasing propeller efficiency at part loads.

The solution is based on the design concept proposed by partners in the Project Forward LNG bulker initiative, which include Arista Shipping, Wärtsilä, ABS and GTT. According to the paper it solves three issues: a static optimisation point for one two-stroke engine that does not reflect the full operational profile; insufficient redundancy with single engine providing propulsion; and the cost of installing a de-rated two-stroke engine compared to two four-stroke engines.

The paper suggest that these issues are being considered for LNG-fuelled ship designs given the technological maturity of smaller, gas-burning engines. “Four-stroke dual-fuel engine technology has developed enormously since the first engines were delivered in 1996. It is, therefore, an already fully proven technology compared to two-stroke dual-fuel engines that have very recently only entered the market.”

Flexibility is further improved by use of the controllable pitch propeller, which improves manoeuvrability and enables the efficiency of the engine and propeller to be optimised depending on operating conditions.

The authors concluded: “Two-stroke main diesel engines have for a long time been the standard in shipping. However, [these] might not be the optimum solution for all LNG fuelled ships.”

Download the paper here.

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