Page 26 - 2024 - Q3 - Minerva in Focus

LNG Carriers A look into Minerva’s Newbuildings

Each vessel will be equipped with two 5G70ME-C10.5-GA-EGR
direct coupled main engines capable of producing a peak power
of 2 x 10.89 MW at 74 RPM, with a normal rating of 2 x 9.8 MW at
71.4 RPM. At this normal rating, the engines will allow the vessel
to achieve a service speed of 19.5 knots with a 20% sea margin.
MAN’s ME-GA engine is a dual-fuel engine that can utilise liquid
fuel, similar to traditional single-fuel engines. However, it can also
utilise gas as a fuel, which is essential for LNGCs to control the
pressure inside the cargo tanks.
It is worth mentioning that when the engine is utilising liquid fuel
(VLSFO or MGO), its operation follows the Diesel thermodynamic
cycle, while for its gas operation, it follows the Otto thermodynamic
cycle. The alternation between the two thermodynamic cycles,
although complicated, is necessary for allowing the gas to be
admitted into the engine with a relatively low pressure (14 bar),
resulting in a simpler and less CapEx-intensive fuel gas supply
system, compared to a high-pressure gas injection engine, like
the ME-GI.
MAN has designed the ME-GA engine to always run with exhaust
gas recirculation (EGR). This decision was made not only to com-
ply with NOx emissions regulations within controlled areas but
also to address the methane slip inherent in Otto gas engines.
Continuous EGR to the combustion chambers reduces methane
A deep dive slip, improves combustion control with fewer knocking events,
and results in lower fuel oil and gas consumption through better
engine running.
into the MAN Like all engines that compress an air/fuel mixture, ME-GA is sen-
sitive to ambient condition changes, which can create pre-ignition
or misfiring phenomena. These events have the undesired effect
ME-GA engines of a gas trip, which, while not a safety issue since the engines
continue running on liquid fuel, results in inefficiency. During a
gas trip, the main gas consumers no longer use the generated
for LNGCs gas in the cargo tanks for propulsion and instead rely on liquid
fuel. This is not only inefficient but can also become commercially
problematic if gas has to be wasted in the oxidiser to control
cargo tank pressure.
This is why MAN has applied two additional control schemes for
by Nikolaos Grivas, the ME-GA engines. The first one is called the Fuel Ratio Control
Technical Manager, (FRC), which allows a small quantity of liquid fuel (main, not pilot
Minerva Gas Inc. fuel) to be added to the combustion chambers during the gas
operation of the engine. This stabilises combustion under certain
conditions, enabling the engine to continue running primarily in
gas mode, with a small expenditure on liquid fuel.
The other control scheme is the ME-GA Opti-II, which affects the P
compression / P scavenge Ratio, the global air/fuel ratio, and the
gas admission timing in order to keep the engine in an optimum
operational range during gas operation without needing to activate
the FRC, which consumes extra fuel. For this to be achieved, the
Opti-II utilises the variable exhaust valve timing of each cylinder,
the variable gas admission timing of each individual gas admis-
sion valve, and the operation of the exhaust gas by-pass valve
for the turbocharger. Of course, there are limits to this continuous
parameter optimisation and once they are breached, the engine
will change its control strategy to FRC to maintain gas operation.
In conclusion, it’s worth noting that there are 46 ME-GA engines
Minerva Gas has two LNGCs under in operation onboard LNGCs worldwide, with a total of 120,000
construction, both of which running hours recorded up to August 2024, of which 87,000 hours
will be equipped with MAN ME-GA were in gas mode. While the engines are experiencing some
main engines, an air lubrication teething issues, MAN is actively supporting their product, and
system, and a sub-cooler to assist many updates have been released in a short period of time to help
with onboard gas management. operators run their engines more efficiently and reliably.

26 ISSUE 29 / Q3 2024