4‐STROKE AND 2‐STROKE TECHNOLOGY

Currently there are two options available whenconsidering Main Engines or Main Generators able toburn compressed GAS vapor.

» 4‐stroke engines» 2‐stroke engines

Wartsila and MDT are the two major makers offeringpackages in both categories.

Wartsila has the advantage in the 4‐stroke technologyas the most preferred and most experienced makerwhile MDT has the lead in 2‐stroke technology withthe vast majority of New Building orders.

WartsilaThe 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 twoconfigurations. In LNG carriers, usually four engines are used asgenerator engines connected to a HV grid (electro‐propulsion) andin smaller vessels are direct coupled to the propeller and oftenpaired via a gearbox or in a twin skeg design.

» The 2‐stroke X‐DF engine, which belongs to the new X‐generationengines being available from 2015. First order, in the LNG carrierssector, has already been placed in Samsung by SK Shipping andMarubeni and the second by Gaslog in HHI. Wartsila has alsoreceived orders for their W‐RT‐flex50DF, fitted in small size tankers,containers and LNGCs.

Man Diesel & TurboThe Danish maker has also two available options:» The 4‐stroke MAN 51/60DF Engine available for both

electrical and mechanical propulsion configurations with thelater concept leading to a higher total efficiency and lowerinvestment costs of the ship’s propulsion train.

» The 2‐stroke ME‐GI engine, which is actually a G‐typeequipped for gas burning. Even though the engine wasintroduced in 2011 (been in development for several year),has received so far the majority preference, in the LNGC NBsector, with approx. 25 vessels scheduled to be delivereduntil 2019. Orders has also been placed for containershipsand tankers.

Otto vs Diesel cycleBoth makers have developed their engine technology on adifferent basis principle which differentiates the engines inkey aspects like efficiency, operating characteristics andemissions:

» Wartsila has selected the Otto cycle in gas burning forboth their 4‐stroke and 2‐stroke engines

» MDT has selected the Diesel cycle in gas burning fortheir 2‐stroke engines and the Otto for their 4‐strokeengines

Before we go into the plus and minuses of the two typeengines, let’s first understand the characteristics of theOtto and Diesel cycle.

Typical configurations – Electro Propulsion

Typical configurations – Mechanical Propulsion

Typical configurations ‐MEGI

Typical configurations – X‐DF

Otto cycleThe Otto cycle consists of isentropiccompression, heat addition at constantvolume, isentropic expansion, andrejection of heat at constant volume. Inthe case of a four‐stroke Otto cycle,technically there are two additionalprocesses: one for the exhaust of wasteheat and combustion products atconstant pressure (isobaric), and one forthe intake of cool oxygen‐rich air also atconstant pressure.

The Wartsila dual‐fuel engines utilize a “lean‐burn” Otto combustionprocess when operating on gas. The gas is mixed with air before theintake 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 thecylinder is emptied of exhaust gases.

Otto cycle

Diesel cycleProcess 1 to 2 is isentropic compressionof the fluid (blue). Process 2 to 3is reversible constant pressure heating(red). Process 3 to 4 is isentropicexpansion (yellow).Process 4 to 1 isreversible constant volume cooling(green).The diesel internal combustion enginediffers from the Otto cycle by using ahigher compression in order to ignitethe fuel rather than using a pilot fuel.

In the diesel engine, air is compressed adiabatically with a compressionratio typically between 15 & 20. This compression raises the temperatureto the ignition temperature of the fuel mixture which is formed byinjecting fuel once the air is compressed.The MDT ME‐GI engine by using the high‐pressure gas injection allows itto maintain the numerous positive attributes the ME‐B and ME‐C low‐speed engines.

Diesel cycle

Thermal EfficiencyIn the below graph, we can clearly see the big advantage of the 2 strokeengine in terms of efficiency. For instance at a 19.5kts laden voyage a ME‐GI engine is 16% more efficient compared to a DFDE.

In the table below, we make a direct comparison of the consumptions ofeach configuration while keeping the ME‐GI engines as a base. It is worthnoticing that the operation profile of the vessel plays a key factor to thedecision of the most energy efficient engines.

Consumption

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 consThe 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 consThe 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 consThe 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

In the table below, we summarize the differences of the DFDE engines tothe two type of 2‐troke engines offered today, in the most importantfactors that mainly contribute to the final choice of the propulsionconfiguration.

4‐stroke vs 2‐stroke

DFDE / TFDE MEGI X‐DF

EFFICIENCY GOOD BEST CLOSE TO ME‐GI

RELIABILITY HIGH REDUNDANCY, MORE COMPLICATED CONTROL SYSTEM

HIGH RELIABILITY HIGH RELIABILITY

CAPEX SAME SAME SAME

OPEX MORE LESS LESS

EMISSIONS REGULATIONS NOX

COMPLIES ON GAS NEED SCR COMPLIES ON GAS

EXPERIENCE SUFFICIENT VERY SMALL NO EXPERIENCE

FLEXIBILITY IN OPERATIONS

MORE LESS  ‐ but reliquefactionplant provide more commercial flexibility

LESS – but complies with Tier III Nox on Gas.

Small sized 4‐stroke enginesDual fuel small generator engines are offered by:

» Wartsila» MDT» Himsen» Yanmar

Most commonly used are the Wartsila 34DF while Himsen has justentered the market with its 35DF.

4‐STROKE AND 2‐STROKE TECHNOLOGY