Exam Question

(a) The Boil Off Gas (BOG) is passed through a heater and compressed to 250-300 Bar in a HP gas compressor. Then it is fed to the engine through double walled stainless steel pipe and via gas control system containing pressure accumulator. The gas leaves the accumulator, passing through filters, regulators, and an isolating valve and goes to the engine via gas injection valves. There are two fuel oil injectors and two gas injection valves per cylinder, located in the cylinder head along with the usual air-start valve, relief valve, and exhaust valve. The engine is started and operated on fuel oil whilst manoeuvring, and once “full away" has been given from the bridge, gas is admitted along with the small amount of pilot fuel oil. On the compression stroke a calibrated amount of gas and diesel as pilot fuel is injected together just before TDC and compression combustion process (Diesel cycle / Auto ignition) takes place. The gas injection valve is hydraulically operated and electronically controlled. Gas leakage around the gas injection valve is avoided by using a high pressure oil seal arrangement (about 25 bar above gas pressure). To prevent gas leakage, a small amount of this oil is lost by being injected into the engine along with the gas.
(b) The advantage of the system that utilizes a gas injector is that the fuel can be stored in liquid form. Another advantage is that scavenge space is free of gas and has less chance of methane slip. This also results in good and complete combustion. Such a system does not have a problem with the quality (liquid + vapor) of fuel.