Operational Information

The 2 Stroke Dual Fuel Engine

 
 

 

INTRODUCTION

 

With the two stroke engine the gas cannot be mixed with the inlet air as it would turn the scavenge space into an enormous bomb!

There are two possible methods of introducing the gas into the cylinder:

The first method introduces low pressure gas through a gas admission valve in the cylinder head when the exhaust valve has closed and pressure in the cylinder is low. The gas is compressed and mixed with the air and ignited by pilot injection of fuel oil.

 

1. The exhaust valve is closed and the gas admission valve is open. Gas at a higher pressure than cylinder pressure flows into the cylinder

 

2. The gas admission valve is closed and the piston moves up the cylinder compressing the gas/air mixture.

 

3. At just before TDC a pilot injector injects a small amount of fuel oil into the cylinder which starts to burn and ignites the gas/air mixture.

 

4. The piston is forced down the cylinder by the burning expanding gas.

 

5. The exhaust valve opens followed by the scavenge ports. Air enters the cylinder pushing out the exhaust gas.

 

 

The disadvantage of this system although simple, is that combustion is subject to knock and the efficiency is low

 

The second method is to compress the gas to a high pressure (250 - 300 bar) and inject it into the cylinder through special gas injectors at the same time as the fuel oil. This is the method which has been developed by MAN B&W in their ME GI engine which is basically the electronic camshaftless engine fitted with the extra equipment to inject the gas into the engine. The engine is fitted with gas injection valves in addition to the fuel valves and because it is computer controlled can burn  any ratio of gas and fuel with a preset minimum amount of fuel.

 

INJECTION CONTROL

 

The injection control system is modified from the standard electronic engine as shown.

 

The gas is pressurised by reciprocating compressors to 250 - 300 bar. It is then cooled and led to the valve blocks on each cylinder. Each valve block incorporates an accumulator which has a volume corresponding to about 20 times the amount of gas injected at full load operation. The purpose of the accumulator is to minimise any pressure drop during gas injection, and to monitor that small pressure drop as it forms an important part of the engines safety system.

 

The engine can run in various modes depending on circumstances.

  1. Normal dual fuel whereby the pilot fuel injected is about 6% of the load. If the gas supply is limited, then extra fuel oil will be injected to maintain power.

  2. If a limited, but constant gas supply is available, the engine can be run in what is known as “specified gas mode”. When in this mode, the same amount of gas is injected each time, with the amount of fuel oil varying, depending on engine load.

  3. Fuel only mode, for manoeuvring or when no gas is available.

The gas supply pipes are double walled with an air seal being supplied between the two pipes. Pressurised air, supplied by fans is then monitored for gas leakage. Because a failure of the inner gas pipe, could lead to very low temperatures as the high pressure gas expands as it leaks out, the outer pipes are made of a stainless steel able to withstand these low temperatures. The flow of air through the air seal is also monitored. Lack of flow would lead to shut down of the gas supply and purging of the lines with inert gas.

 

To prevent leakage of gas at the injectors and to lubricate the moving parts, the gas injectors are supplied with sealing oil which is pressurised to 25 - 50 bar above the gas injection pressure. The small amount of oil which leaks through to the gas is then burnt in the engine. Consumption is low (about 0.13g/kWh). The sealing oil system comprises of two pumps for redundancy purposes and a spring loaded accumulator to maintain the pressure if a pump fails, whilst the standby pump comes up to pressure.

 

Should a gas injector jam open then a pressure drop will occur in the accumulator in the valve block and the system will shut down, and the gas lines will be purged with inert gas. Should the pressure drop not be detected, the excess gas entering the cylinder will continue to burn as it leaves leading to a high exhaust temperature on that cylinder, leading to slow down, again shutting off the gas supply. In the unlikely event of late ignition of the gas occurring in the exhaust manifold, leading to a rapid pressure rise, the receiver is designed to withstand a pressure of 15 bar.

 

Any failure of injection of pilot fuel leading to non combustion of the injected gas will lead to the gas supply being shut off and the gas lines purged with inert gas.

 

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 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 4 Stroke Dual Fuel Engine

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