ICE maiden – the 31SG debuts at 50% efficiency

15 May 2018

The efficiency bar for what is possible from an internal combusion engine has been raised to a new level with the release of Wärtsilä’s 31SG engine. The basis for the new engine, the type 31, has been recognised by Guinness World Records as the most efficient 4-stroke diesel available. The gas version builds on that design.

Wärtsilä’s newest engine marks a step change in both flexibility and efficiency. The secret is a ground-breaking new design, conceived entirely from scratch, that makes it tough enough to withstand the heavy mechanical loading associated with two-stage turbocharging.

The new unit is a gas fuelled version of its successful type 31 engine, which has been recognised by Guinness World Records as the world’s most efficient 4-stroke diesel engine. It is designated 31SG. The development follows from positive acceptance of the diesel version in marine sector applications.

The defining feature of the new engine is its ability to achieve simple-cycle efficiency levels in excess of 50 % (compared to around 40 % with modern gas turbines), which represents a milestone achievement in the energy sector. It reduces emissions, and offers the potential for significant fuel cost savings to power producers.

A feature specifically aimed at during development is the increased flexibility which enables rapid response to continuously changing load patterns, especially within systems having a significant share of solar and wind energy. The engine can be continuously operated at 10 % load, and can reach full load in two minutes from the start command. This flexibility overcomes the challenge created by the daily stops and starts that are inevitable with output from renewable sources.

The 20-cylinder, spark-ignited Wärtsilä 31SG produces 12 MWe. The specially designed robust engine structure achieves an unprecedented brake mean effective pressure (BMEP) of 30 bar, allowing it to fully accommodate all the benefits of two- stage turbocharging.

The new engine is available in diesel, dual-fuel and gas variants. When the first 20V31SG rolled off the production line in 2017, it became the most efficient simple- cycle internal combustion engine ever built.

Starting point

Six years ago, engineers at Wärtsilä set out to do something that is quite rare among manufacturers of large engines. Rather than improving an existing design, they started from a completely blank slate to develop a new engine family that would be considerably more efficient than anything that had come before. 

Designing a new engine entirely from the bottom up gave Wärtsilä’s engineers the opportunity to examine all of the various factors that contribute to engine efficiency, making improvements in each to achieve the most possible gain. Combustion quality, engine parameters, heat and flow loss prevention, and internal friction were among the issues exhaustively explored and addressed to their fullest.

Engine structure

The most critical advance however, was an engine structure designed expressly to accommodate two-stage turbocharging. While the considerable boost in efficiency associated with two-stage turbocharging was well known – the industry had been experimenting with it for quite some time – no existing engines were capable of taking full advantage of the effect. They simply could not stand up to the loading and strain that results from the step change in firing pressure. To overcome this the entire engine structure was given a very robust design with, as already noted, an unparalleled brake mean effective pressure.

But having a design that incorporates efficiency-boosting concepts is one thing while bringing a new engine to life is another. Development was a long and complex process that involved extensive computer simulation, testing on a single- cylinder experimental engine then testing on a number of multi-cylinder engines. All this was carried out to guarantee that the efficiency gains would be realised and that the final product would meet the highest standards of reliability.


A fundamental advantage of the new engine is ability to start up quickly and maintain high efficiency throughout the entire load range. Conventional baseload is disappearing as power generators take on the new role of intermittently backing up the grid when the output from renewables dips. This shift represents a huge challenge. Combined cycle power plants cannot cope with the daily starts, stops, and continuously changing load patterns that are becoming the new norm. But an internal combustion engine is just the right tool for the job thanks to its dramatically more flexible operating profile. With this in mind the designers wanted to push the 31SG’s flexibility to the practical limit.

A major contributor to the engine’s increased flexibility is its completely redesigned valve actuation method. The new design replaced the mechanical, rocker arm-driven valve mechanism of previous models with a hydraulic system, similar to those used in the car industry. This feature allows very smooth and precise control of valve timing to ensure that the fuel-air ratio in the cylinder is optimised at all times. It’s in this way that the engine can take maximum advantage of the boost provided by two-stage turbocharging, specifically so on partial loads.

Optimised engine parameters and adjustable inlet valve timing combined with electronic ignition timing result in fewer unburned hydrocarbons and thus vastly improved efficiency and lower emissions irrespective of the load. An added bonus of the hydraulic valve actuation concept is that there’s no need for valve clearance adjustment.


Automation is a key element of any modern gas engine, underpinning efficiency, safety and flexibility, but there are particular requirements for a machine as advanced and complex as this one. With 20 cylinders each firing six times per second and so many variables coming into play, the ability to easily harmonise and control every aspect of the operation is vital. Accurate, cylinder- specific control algorithms are essential for a gas engine operating with firing pressures this high.

For this reason, as part of the development of this engine, Wärtsilä went to great lengths to create a next-generation automation system, which is integrated into the engine itself. The engine automation system was built entirely in-house based on new hardware and software.

The engine automation system is distinctive for its wide implementation of digital controls, which regulate valve timing, gas admission, ignition, coolant temperature and various aspects of the turbocharging. It’s this high degree of automation that allows the 31SG to account for differences in fuel as well as run at various loads without compromising efficiency.

The new automation hardware is modular and integrated. All sensors are connected directly to the control ports, without going through connectors or junction boxes – a change that reduces the likelihood of faults. The equipment was also subject to rigorous vibration and heat testing.

The long view

The world is likely to see a dramatic reduction in fossil fuel-burning power generation in the coming decades. This change will come about not only because of higher fuel prices, but also from political pressure leading to legal limits on emissions. The solutions that survive will be those that are most flexible and efficient. 

ICE Wärtsilä 31SG, the most efficient 4-stroke engine available
ICE Wärtsilä 31SG gas fired ICE

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