MAN Energy Solutions takes pride in having an engine solution for whatever its customers need, but in the past few years it has been overshadowed when catering for the fast-growing LNG carrier segment. But the engine maker is confident that could be about to change following the unveiling of its MAN B&W ME-GA engine at a live-streamed demonstration event at its Copenhagen R&D facility in March.
The new engine is an Otto-cycle variant of its existing ME-GI model but specifically designed for LNG carriers, with low capex, although with some scope for wider application in vessels such as Aframax tankers. First announced nearly two years ago, the ME-GA has been undergoing rigorous testing, drawing upon the experience gained from the ME-GI, particularly with regard to the use of MAN’s proprietary exhaust gas recirculation (EGR) system.
Whereas the ME-GI applies the Diesel principle of non-premixed combustion, whereby the fuel is injected at high pressure and burned directly, Otto cycle combustion pre-mixes a combination of fuel and scavenging air during the compression stroke, which is then ignited with the aid of a small amount of a VLSFO or diesel pilot fuel. Otto cycle admits the gas at a much lower pressure and allows for a more economic gas supply system, particularly with LNG carriers that can utilise boil-off gas from their cargo.
Gas admission system
The ME-GA uses an efficient ignition concept and unique gas-admission system that the company says delivers safe and reliable operation. Rather than have one for supplying gas to the engine and another for purging, a single double-walled bidirectional pipe is used, reducing capex and maintenance. Meanwhile the gas valve unit and gas regulating unit are separated. This allows the gas valve unit to be located on the shipside, away from the gas safe area, and giving shipyards greater freedom in how they integrate it into their design. There are also regulatory advantages since the gas valve unit is not then subject to the additional safety requirements.
Another notable feature is the Safe Gas Admission Valve (SGAV) developed to enhance engine operation, a feature also found in the ME-GI. Each SGAV, found on the side of every cylinder, has a window valve incorporated consisting of two mechanical valves controlled by individual electronic circuits. In order for gas to enter the engine then both valves must open, meaning that no single failure can cause an accident.
Because Otto cycle engines are susceptible to knocking, particular care needs to be taken with cylinder condition. To mitigate this, MAN says it has based its piston ring pack on the running hours of its G70 ME-GI engines, which have shown such good performance that the time between overhauls of the piston rings for its gas engines has been increased by 50%.
Tier III compliance
While the ME-GA will be compliant with NOx Tier III requirements without the use of abating technologies when in gas mode, for fuel mode there will also be the option of EGR or selective catalytic reduction (SCR) systems. The former has become a particular specialism of MAN, who style themselves as “world champions” when it comes to two-stroke EGR, with proven design that has been installed on more than 120 vessels currently in service and a further 250 on order.
Use of the EGR is calculated to save around 3% in specific gas consumption and more than 5% in its fuel oil equivalent. This increased efficiency also translates into reduced methane slip, while the very process of recirculation gives methane a second chance to be combusted. However, by its very nature, an Otto cycle engine will always leak more methane than the ME-GI, which is reckoned to be 10 times more efficient.
MAN Energy Solutions aims to start testing the first commercial ME-GA design by the end of this year, with the first actual delivery coming in early 2022. Initially the ME-GA engine was launched in a 70 bore version, however its latest programme also includes a 60 bore version to cater for LR1 and LR2 tankers, as well as Panamax bulkers and feeder container ships. An S50 version is also expected to be added.
