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