Dual-Fuel 2-Stroke engines
The BOG gas is passed through a heater before being compressed to 250-300 Bar. It is fed from the gas compression room to the engine room gas control system through double walled stainless steel pipe.
The normal 2-stroke operation is applied to the dual fuel engine, with a few modifications.
The compressed gas is supplied to the gas control system where it is contained in an accumulator. The gas leaves the accumulator, passing through filters, regulators, and an isolating valve (by-passing to vent or recirc) before being supplied to the engine gas admittance valves.
There are two normal fuel oil injectors and two gas admittance valves per cylinder, located in the cylinder head along with the usual air-start, relief valves, and exhaust valve.
My interpretation of the new designed 2-stroke dual fuel marine engine is shown below;
The gas valve is hydraulically/electronically operated and controlled. Gas leakage is avoided by using a high pressure oil seal arrangement (about 25 bar above gas pressure). Although preventing gas leakage, a small amount of the oil is lost by being injected into the engine along with the gas.
The engine is started and operated on fuel oil whilst maneuvering, and once “full away" has been communicated from the bridge, gas is admitted along with the fuel oil.
On the compression stroke a calibrated amount of gas and diesel is injected just before TDC and compression combustion follows, forcing the piston on its power stroke down the cylinder.
As the piston descends it uncovers the scavenge air inlet ports located about two thirds from the top of the cylinder liner, and a fresh charge of air is introduced to the cylinder. As the piston begins to rise back up the cylinder, the exhaust valve opens and discharges combustion gasses to the turbo-blowers. The exhaust valve shuts and compression combustion again occurs completing the 2-stroke cycle.
The operating principles of the two-stroke dual fuel engine are therefore:
- BOG gas is compressed to 250-300 Bar before being supplied to an accumulator
- There are two gas injection valves per cylinder, and two diesel / HFO fuel injectors per cylinder located on the cylinder head.
- Exhaust valve, fuel oil, and gas injectors are operated hydraulically.
- All gas piping in the engine room is double walled, stainless steel pipe.
- Engine can be run on gas, fuel oil or a combination of both.
- Fuel oil is used whilst maneuvering or on standby.
The two-stroke dual fuel marine engine is still the new kid on the block, but trials and installation of dual fuel two-stroke engines has been carried out by Wartsila and MAN/B&W manufacturers.