NYK Buys World's First Vessels With Variable Compression Ratio Engines

The technology could also be retrofitted onto some existing ships, according to WinGD (File image courtesy NYK)

Published Jun 28, 2023 7:30 PM by The Maritime Executive

NYK is about to acquire a series of LNG dual-fuel ships featuring one of the first genuinely new fundamental technologies in two-stroke design in years. China State Shipbuilding Corporation's engine design bureau, WinGD, has worked with Mitsui E&S DU Company (MESDU) to develop the world's first production design for a variable-compression marine diesel engine, which will be installed on two NYK newbuild coal carriers. 

Compression ratio is a core design parameter for any internal combustion engine, and it has profound effects on efficiency, maximum available power, and the magnitude of the forces exerted on engine components. Virtually every production engine in the world has a constant compression ratio, selected at the time of design, printed on the spec sheet and built into the dimensions of the crankshaft, rods and cylinders. 

That began to change in 2019 when Nissan shipped the world's first mass-production car with a variable compression ratio (VCR) engine. Electronically-controlled VCR adds complexity and more moving parts, but for the first time, it allowed Nissan's engineers to select different compression ratios at different times. The idea was to wring both more power and more fuel efficiency out of a small engine, and this drivetrain can be found on the road today.  

VCR has different uses in a marine engine, but fuel economy is still a top goal. According to WinGD, the new design was created with dual-fuel diesels in mind. By mechanically adjusting the length of the piston's stroke, the system can increase or decrease the engine's compression ratio to optimize for LNG combustion or for fuel oil. Historically, dual-fuel engine designers had to select a single compression ratio that works well enough for both fuels, without optimizing for either one. With VCR, for the first time, the engine can be adjusted to the right compression ratio for the right fuel. This could be useful not just for LNG, but also for biofuels and e-fuels, according to WinGD. 

In the first shoreside trials at MESDU's plant in Japan, VCR cut the engine's fuel consumption by six percent when running in diesel mode (compared to the same engine in its stock configuration). In LNG mode, it cut fuel consumption by three percent. These are small improvements, but they are a rare example of a new efficiency gain inside a marine two-stroke, one of the world's most efficient and mature power technologies.  

The first examples will be installed on NYK newbuilds, but the system is also backwards-compatible with some existing WinGD engines, and it will eventually be made available for retrofits. The system is well-suited to NYK's decarbonization strategy, which starts with fuel efficiency and LNG/LPG fueling.