The Wärtsilä dual-fuel engines are unique because they have two different injection systems. A micro pilot injection system injects a very small amount of liquid fuel when the engine is operating in gas mode. The micro pilot system is of the common rail type, which allows for very small injection amounts.
This makes it possible to meet very stringent emission regulations, which would be impossible if a normal injection system were used. A conventional injection system is used when the engine is run on liquid fuel such as heavy fuel oil or of course diesel oil if that is the fuel choice. Fuel flexibility and high efficiency are the main advantages of the dual-fuel technology. Two engine models are offered at the moment, the Wärtsilä 34DF and Wärtsilä 50DF. The latter has shown an efficiency of over 48%, which is probably a record at this time.
Wärtsilä gas-diesels were the first gas engines offered by the company. The first engines have now been in operation for almost 70,000 h. The concept was initially developed for the offshore market, where it has been successfully applied in a large number of floating production units. However, these engines have also been used in numerous power plants because of their fuel flexibility.
In the gas-diesel the gas is injected according to a diesel process at high pressure. A small amount of liquid fuel is pilot injected when the engine is operating in gas mode. A gas-diesel can operate on gas with pilot injection and also on liquid fuels such as diesel oil, heavy fuel oil or even crude oil. In gas mode the gas-diesel is more tolerant in terms of methane number than other gas engine concepts. Wärtsilä’s gas-diesel engines are the Wärtsilä 32GD and Wärtsilä 46GD.
Engine working principle of dual-fuel (DF) engines
The dual-fuel engine utilizes a “lean-burn” otto combustion process when operating on gas. Here, the gas is mixed with air before the intake valves during the air intake period. After the compression phase, the gas/air mixture is ignited by a small amount of liquid pilot fuel (LFO). After the working phase the exhaust gas valves open and the cylinder is emptied of exhaust gases. The inlet air valves open when the exhaust gas valves close, and the process starts again.
The dual-fuel engine is also equipped with a backup fuel system. In the event of a gas supply interruption, the engine transfers from gas to fuel oil operation (LFO, HFO) at any load instantaneously and automatically. Futhermore, the separate backup fuel system makes it possible to switch over from LFO to HFO without load reduction. During fuel oil operation the DF engine utilizes the conventional diesel process.
Engine working principle of gas-diesel (GD) engines
The GD engine utilizes the diesel combustion process in all operational modes. In gas mode, the gas is injected at high pressure after the pilot fuel and is ignited by the flame from the pilot fuel injection. The amount of pilot fuel is equivalent to approximately 5% of the fuel energy input at full engine load.
The gas-diesel engine can be switched over instantly to liquid fuel mode operation. The liquid fuel can be light fuel oil, heavy fuel oil or crude oil. In this case, the process is the same as the conventional diesel process.
In fuel sharing mode, the ratio between liquid and gas fuel amounts can be controlled and varied during operation. The operating window for the fuel sharing mode is 30 to 100% load and the gas/liquid fuel ratio can vary according to the fuel sharing window.
The gas-diesel process can tolerate big variations in the gas quality and is especially suitable for “non-pipeline quality gas”, such as associated gas in oil fields. Read more in Wärtsilä Oil & Gas Industry.
Read more about Wärtsilä dual-fuel power plants.