4‐STROKE AND 2‐STROKE TECHNOLOGY
Currently there are two options available when
considering Main Engines or Main Generators able to
burn compressed GAS vapor.
» 4‐stroke engines
» 2‐stroke engines
Wartsila and MDT are the two major makers offering
packages in both categories.
Wartsila has the advantage in the 4‐stroke technology
as the most preferred and most experienced maker
while MDT has the lead in 2‐stroke technology with
the vast majority of New Building orders.
Wartsila
The Finnish maker has two available options:
» The 4‐stroke 50DF engine, which is actually an oversized and de‐
rated 46 bore able for gas burning, that can be installed in two
configurations. In LNG carriers, usually four engines are used as
generator engines connected to a HV grid (electro‐propulsion) and
in smaller vessels are direct coupled to the propeller and often
paired via a gearbox or in a twin skeg design.
» The 2‐stroke X‐DF engine, which belongs to the new X‐generation
engines being available from 2015. First order, in the LNG carriers
sector, has already been placed in Samsung by SK Shipping and
Marubeni and the second by Gaslog in HHI. Wartsila has also
received orders for their W‐RT‐flex50DF, fitted in small size tankers,
containers and LNGCs.
Man Diesel & Turbo
The Danish maker has also two available options:
» The 4‐stroke MAN 51/60DF Engine available for both
electrical and mechanical propulsion configurations with the
later concept leading to a higher total efficiency and lower
investment costs of the ship’s propulsion train.
» The 2‐stroke ME‐GI engine, which is actually a G‐type
equipped for gas burning. Even though the engine was
introduced in 2011 (been in development for several year),
has received so far the majority preference, in the LNGC NB
sector, with approx. 25 vessels scheduled to be delivered
until 2019. Orders has also been placed for containerships
and tankers.
Otto vs Diesel cycle
Both makers have developed their engine technology on a
different basis principle which differentiates the engines in
key aspects like efficiency, operating characteristics and
emissions:
» Wartsila has selected the Otto cycle in gas burning for
both their 4‐stroke and 2‐stroke engines
» MDT has selected the Diesel cycle in gas burning for
their 2‐stroke engines and the Otto for their 4‐stroke
engines
Before we go into the plus and minuses of the two type
engines, let’s first understand the characteristics of the
Otto and Diesel cycle.
Typical configurations – Electro Propulsion
Typical configurations – Mechanical Propulsion
Typical configurations ‐ MEGI
Typical configurations – X‐DF
Otto cycle
The Otto cycle consists of isentropic
compression, heat addition at constant
volume, isentropic expansion, and
rejection of heat at constant volume. In
the case of a four‐stroke Otto cycle,
technically there are two additional
processes: one for the exhaust of waste
heat and combustion products at
constant pressure (isobaric), and one for
the intake of cool oxygen‐rich air also at
constant pressure.
The Wartsila dual‐fuel engines utilize a “lean‐burn” Otto combustion
process when operating on gas. The gas is mixed with air before the
intake valves during the air intake period. After the compression phase,
the gas/air mixture is ignited by a small amount of liquid pilot fuel
(LFO). After the working phase the exhaust gas valves open and the
cylinder is emptied of exhaust gases.
Otto cycle
Diesel cycle
Process 1 to 2 is isentropic compression
of the fluid (blue). Process 2 to 3
is reversible constant pressure heating
(red). Process 3 to 4 is isentropic
expansion (yellow).Process 4 to 1 is
reversible constant volume cooling
(green).
The diesel internal combustion engine
differs from the Otto cycle by using a
higher compression in order to ignite
the fuel rather than using a pilot fuel.
In the diesel engine, air is compressed adiabatically with a compression
ratio typically between 15 & 20. This compression raises the temperature
to the ignition temperature of the fuel mixture which is formed by
injecting fuel once the air is compressed.
The MDT ME‐GI engine by using the high‐pressure gas injection allows it
to maintain the numerous positive attributes the ME‐B and ME‐C low‐
speed engines.
Diesel cycle
Thermal Efficiency
In the below graph, we can clearly see the big advantage of the 2 stroke
engine in terms of efficiency. For instance at a 19.5kts laden voyage a ME‐
GI engine is 16% more efficient compared to a DFDE.
Consumption
In the table below, we make a direct comparison of the consumptions of
each configuration while keeping the ME‐GI engines as a base. It is worth
noticing that the operation profile of the vessel plays a key factor to the
decision of the most energy efficient engines.
ENGINES MODE
ME-GI
TFDE
X-DF
HFO @ 19.5kn
100%
122%
105%
HFO @ 16kn
100%
113%
106%
HFO @ 12kn
100%
112%
105%
However an LNG vessel will very seldom sail on HFO only mode, even in ballast condition.
ENGINES MODE
ME-GI
TFDE
X-DF
MIX MODE @ 19.5kn
100%
114%
105%
MIX MODE @ 16kn
100%
96%
103%
MIX MODE @ 12kn
100%
97%
101%
ENGINES MODE
ME-GI
TFDE
X-DF
GAS ONLY @ 19.5kn
100%
104%
102%
GAS ONLY @ 16kn
100%
104%
103%
GAS ONLY @ 12kn
100%
92%
101%
The pros and cons
The ME‐GI engines are superior in terms of:
» Lower SFOC
» Avoiding knocking
» No methane slip
» Proven reliability (of the G‐type engines)
» First to develop the two stroke dual fuel concept
while lacking in:
» No field tested
» High Capex – fuel gas management
» SCR or EGR to satisfy IMO Tier III on gas
» Power consumption in Auxiliaries
» Reliability of the high pressure gas fuel system
The pros and cons
The X‐DF engines are superior in terms of:
» Simple fuel gas management design
» Satisfying IMO Tier III on gas
» Less power consumption
» Capex is less
» Low pressure (16 bar easier to handle)
while lacking in:
» No field tested
» 6‐stage compressor reliability
» Higher SFOC
» Methane slip
The pros and cons
The DFDE engines are superior in terms of:
» Experience gather so far
» Flexibility
» Redundancy
» Simple fuel gas management
» IMO Tier III compliant on Gas
while lacking in:
» Capex / Opex
» Reliability
» Highest SFOC
4‐stroke vs 2‐stroke
In the table below, we summarize the differences of the DFDE engines to
the two type of 2‐troke engines offered today, in the most important
factors that mainly contribute to the final choice of the propulsion
configuration.
DFDE / TFDE
MEGI
X‐DF
EFFICIENCY
GOOD
BEST
CLOSE TO ME‐GI
RELIABILITY
HIGH RELIABILITY
HIGH RELIABILITY
CAPEX
HIGH REDUNDANCY, MORE COMPLICATED CONTROL SYSTEM
SAME
SAME
SAME
OPEX
MORE
LESS
LESS
EMISSIONS REGULATIONS NOX
EXPERIENCE
COMPLIES ON GAS
NEED SCR
COMPLIES ON GAS
SUFFICIENT
VERY SMALL
NO EXPERIENCE
FLEXIBILITY IN OPERATIONS
MORE
LESS ‐ but reliquefaction
plant provide more commercial flexibility
LESS – but complies with Tier III Nox on Gas.
Small sized 4‐stroke engines
Dual fuel small generator engines are offered by:
»
»
»
»
Wartsila
MDT
Himsen
Yanmar
Most commonly used are the Wartsila 34DF while Himsen has just
entered the market with its 35DF.
4‐STROKE AND 2‐STROKE TECHNOLOGY