Operational Information

The 4 Stroke Dual Fuel Engine

 
 

 

INTRODUCTION

 

LNG (Liquefied Natural Gas) carriers range in size from small carriers of about 10 000m3  up to 150 000m3 with larger ships of up to 230 000m3 in the design stage.

 

The liquified gas is carried at atmospheric pressure at a temperature of -161C. Although the tanks are well insulated, some of the gas, which comprises mainly of methane, will boil off. This is known in the industry as N-BOG or Natural Boil Off Gas. Although the amount of N-BOG will vary, it is usually accepted as being between 0.1% and 0.15% of the cargo per day.

 

If a 74 000m3 vessel is considered, then the boil off per day would be 74m3. Because the expansion rate from liquid to gas is about 600:1, this equates to 44 400m3 of gas at atmospheric pressure. The Lower Calorific Value of the gas will vary according to the amount of nitrogen that is present in the BOG, which can be up to 30% at the start of a voyage because it has a lower boiling point than methane, but a ball park figure of 28MJ/m3 will be used for the calculation.

 

44 400 28 = 1 243 200MJ of energy per day. To allow this gas to vent to atmosphere is not permitted. This leaves three options:

  1. The gas can be burnt off.

  2. The gas can be reliquified and returned to the tanks. (reliquefaction)

  3. The gas can be used as a fuel in the propulsion plant.

Option 1. is not energy efficient and is only used in an emergency, or if more gas than can be used in option 3 is being produced.

 

Option 2 is an option but the initial investment is high and the fuel consumption will increase because of the power requirements of the plant.

 

Option 3 is the one generally used. Up until recently all LNG vessels were powered by steam turbines, because the main boilers could be adapted fairly easily to burn the boil off gas and could be changed between HFO and gas without difficulty. Of course special precautions in the piping of the gas into the engine room have to be observed.

 

A steam turbine plant is not very efficient: about 29% for main propulsion and about 25% for electrical generation.

 

Four stroke diesel engines which can burn gas or diesel fuel (known as Duel Fuel or DF engines) have been around in shore based installations for some time. These engines are now being adapted for use in merchant vessels, notably LNG carriers, although some gasfield supply vessels have been built with dual fuel engines. MAN B&W have developed a two stroke engine which will run on gas.

 

The efficiency of the diesel engine in the dual fuel concept approaches 50%. When installed as part of the diesel electric propulsion plant (for economic, practical and redundancy purposes) the efficiency overall is about 43% due to losses in alternators, transformers, converters, motors and shafting. However the efficiency far exceeds that of the steam turbine plant.

 

The principle of the plant is outlined above. The gas boil off is pressurised to about 5.5 bar by compressors and is then heated to about 30C. The gas is then piped to the engines, where it is injected into the intake air before the air enters the cylinders. Ignition is by pilot injection of diesel fuel.

 

The first diesel electric LNG carrier is the Gaz de France at 74 000 m3 She is powered by 4 Wartsila 6L50DF engines each developing 5700kW at 514rpm. Under normal operation she runs 3 out of the 4 engines giving a service speed of 16 knots, although with all 4 engines running she can achieve 28.5 knots.

 

The energy consumption of the engines when burning gas is quoted as the Brake Specific Energy Consumption (BSEC) and for the Wartsila 50DF engine  it is given as 7410kJ/kWh (about 48.5% efficiency).  This means that each engine will consume 1 013 688MJ of energy in 24 hours. This means that at the lower boil off figure of 0.1%/day given above, there is only enough gas to meet the requirements for 1 engine. To meet the need to run all the engines, some of the cargo is forced to boil off (F-BOG) This is more economical than running the engines on diesel fuel. On ballast passage the engines can either be run on diesel fuel or LNG retained on board specifically for burning in the engines. It is possible to modify the engines to burn heavy fuel oil by adjustments to the fuel pump timing and fuel injection equipment, although the use of heavy fuel will require a change in the type of lubricating oil to a High Duty oil.

 

FUEL SYSTEM

 

Diesel Supply

There are two fuel supply systems: one for pilot fuel when the engine is running on gas, the other for back up operation on diesel fuel. The pilot fuel is supplied at 900 bar from a common rail supplied by an engine driven variable delivery radial piston type pump. The timing and duration of the pilot injection is electronically controlled.

 

For operation  on diesel fuel, standard type cam driven fuel injection pumps are used injecting through standard design spring loaded fuel injectors.

 

The injectors are twin needle valve units. The smaller needle is used for the electronically controlled pilot injection, and the larger needle for when running on diesel fuel.

 

Gas Supply

 

 

The photos opposite and below shows the gas regulating unit.

In the foreground of the picture opposite is the manual Isolating valve (with the red handle)

 

No 1 is the filter

No 2 is the flow meter

No 3 is the gas regulator unit

No 4 are the shut off valves

No 5 is a venting valve (1 of 3)

 

 

Gas Regulating Unit

 

The gas supply is filtered and then goes through a pressure regulator, the output of which is dependent on engine load and energy content of the gas, but is a maximum of 4 bar. The system also incorporates necessary shut of and venting valves  for safety purposes. The gas is then piped to the engine by a large diameter double wall common rail system with each cylinder having an individual feed to a gas admission valve.

 

 

PRINCIPLE OF OPERATION

 

The engine runs on the lean burn principle. This means that the air to fuel ratio is high (about 2.1:1). An advantage is that the engine will produce low NOx emissions (<1g/kWh) because the heat energy released by  the burning fuel is use to heat this extra air, limiting combustion temperatures. It is important that the air fuel ratio is kept within a relatively small window: Too rich (below 1.9:1) and knocking will occur; too weak (above2.2:1 and there is a danger of misfiring.

 

The engine is started using diesel fuel using both pilot and main injection. When combustion is stable, the engine is changed to gas supply. This takes about one minute, during which the fuel oil is gradually substituted by gas.

 

On the inlet stroke gas is admitted through the gas admission valve and mixes with the inlet air.

The mixture will not ignite on compression because the gas has a high self ignition temperature.

As the piston approaches TDC a small amount of diesel (1%) is injected through the pilot nozzle. Ignition of this pilot fuel then ignites the gas air mixture.

 

Maintaining the correct air fuel ratio over the operating range of the engine is essential to prevent knocking and misfiring and to keep emissions low.

 

This is achieved by an engine controlled waste gate which by-passes some of the exhaust gas around the turbocharger.

View of Engine Showing Waste Gate

 

SAFETY

 

In case the boil off gas cannot be used by the engines, and there is no reliquefaction plant, there must be an alternative way of disposing of it. To do this  a Gas Combustion Unit or Thermal Oxidiser is installed in the ships funnel.

 

View of V12 engine showing gas manifold and emergency gas venting valve actuator

 

 

The safety considerations are similar to those for an LNG steamship. A leak of methane into the machinery space could cause a devastating explosion. To prevent this, the gas is led through a double skinned pipe fitted with a flame arrestor at the inlet to the supply manifold, an extraction system and gas detector. There must be sufficient flexibility in the pipes to prevent fatigue failure due to oscilliation of the engine.  An extraction fan from a hood above the engine is also fitted with a gas detection system. Leakage of gas  will trip the gas master valve (the engine will automatically change over to diesel operation) and the BOG will be diverted to and burnt in the Thermal Oxidiser. The gas pipe system will also be fitted with a nitrogen inerting system which will operate to purge through if gas is detected.

 

IACS require that the following safety requirements must be met:

  • Only oil fuel is to be used when starting the engine.

  • Only oil fuel is, in principle, to be used when the operation of an engine is unstable, and/or during manoeuvring and port operations.

  • In case of shut-off of the gas fuel supply, the engines are to be capable of continuous operation by oil fuel only.

  • Crankcase relief valves are to be fitted in way of each crankthrow. The construction and operating pressure of the relief valves are to be determined considering explosions due to gas leaks.

  • Explosion relief valves or other appropriate protection system against explosion are to be provided in the exhaust, scavenge and air inlet manifolds.

  • The exhaust gas pipes from Dual Fuel engines are not to be connected to the exhaust pipes of other engines or systems.

  • Starting air branch pipes to each cylinder are to be provided with effective flame arresters.

  • Flame arresters are to be provided at the inlet to the gas supply manifold for the engine.
    Arrangements are to be made so that the gas supply to the engine can be shut-off manually from the starting platform or any other control position.

If a trunk piston type engine is used as as Dual Fuel engine, the crankcase is to be protected by the following measures:

  • Ventilation is to be provided to prevent the accumulation of leaked gas, the outlet for which is to be led to a safe location in the open through flame arrester.

  • Gas detecting or equivalent equipment. (It is recommended that means for automatic injection of inert gas are to be provided).

  • Oil mist detector.

The engine must be designed to stop before the gas concentration detected by the gas detectors specified reaches 60% of lower flammable limit.


If a cross-head type engine is used as a Dual Fuel engine:

  • The crankcase is to be protected by oil mist detector or bearing temperature detector.

  • Gas detecting or equivalent equipment is to be provided for piston underside space of cross-head type engine.

The engine must be designed to stop before the gas concentration detected by the gas detectors specified reaches 60% of lower flammable limit.

 

See Also The 2 Stroke Dual Fuel Engine

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