World class efficiency: a pure gas engine for the marine market

Based on the Wärtsilä 31 product platform, the diesel version of the engine has been recognised by Guinness World Records as being the world’s most efficient 4-stroke gas engine and will possibly pave the way for using alternative fuels such as LPG.

“The highly efficient Wärtsilä 31SG moves the industry forward by providing owners and operators with a practical means for lowering costs and enhancing environmental sustainability,” said Rasmus Teir, product manager, Wärtsilä Marine.

“The gas-only focus and lean-burn spark ignition technology allows for further optimisation of thermal efficiency, while lowering greenhouse gas emissions and facilitating adaptations for alternative heavier gas fuels, such as LPG,” he added.

Technical developments

The Wärtsilä 31SG design incorporates the 31 platform’s hydraulic valve actuation system, which helps ensure the fuel-air ratio in the cylinder is optimised at all times, along with digital control of fuel injection and ignition and real-time closed-loop combustion control.

The development of smart software features like active control of the engine operating point, triggered either by real-time cylinder air-fuel ratio calculation or boost pressure, have improved the engine’s dynamic capabilities.

Valve timing

The engine makes use of two valve timing strategies in order to improve air availability during fast ramp ups. By modulating the inlet valves before bottom dead centre, the engine can simply reduce temporarily the Miller amount in the loading phase. Such a Variable Inlet valve Closure (VIC) strategy boosts the pressure, but also results in higher compression end temperature, which can increase the risk of auto ignition and knocking at higher temperatures. The Wärtsilä 31SG avoids this risk by applying the VIC strategy at the low and medium load range.

The engine also includes more effective cleaning of the combustion residual gases by increasing valve overlap. This improves the scavenging of the combustion chamber. Wärtsilä has introduced Variable Exhaust valve Closing (VEC) technology to the Wärtsilä 31SG. The valve overlap strategy also lowers the compression end temperature, thanks to the improved scavenging of the hot residual gases.

Markets

Wartsila intends to initially bring out pure gas gensets by late 2021, and subsequently as main propulsion engines in 2022.

The Wärtsilä 31SG has achieved efficiencies of over 50% in the stationary market, compared with efficiencies of around 40% for simple-cycle gas turbines, which had gained wide adoption in the FRSU market. The proven reliability and operating record of the pure-gas engine in the stationary energy market, and the significant opex savings potential substituting turbines with pure-gas engines, means it is likely to attract interest from the FRSU market.

In addition, the pure-gas engines are likely to attract interest from ship owners in the cruise and ferry segments, which have seen growth in LNG-fuelled vessel purchases.

With five cylinder configurations – 8V, 10V, 12V, 14V and 16V – the engines have a mechanical output of 4240 – 8800kW and a cylinder output of 530/550kW. The engine also boasts a specific energy consumption is 7243kj/kWh.

Comparison with DF engines

Speaking at Gastech in Houston, Mr Teir noted that a comparison of three W12V31DF engines with three W12V31SG engines produced greenhouse gas savings of 18% for an FRSU, and that similar savings were seen for a RoPax. Combining the three pure gas engines with a battery hybrid configuration produced 22% savings in greenhouse gases as well as 9% saving in operating expenses from a lower fuel consumption and maintenance cost.

Wärtsilä 31SG specifications