The evolution of marine propulsion technology has led to the development of dual-fuel engines, which offer the capability to operate on both natural gas and conventional fuel oils. These engines are widely used in LNG carriers and other vessels due to their fuel flexibility, operational efficiency, and lower emissions. Since their inception, dual-fuel engines have undergone significant advancements to improve efficiency, reduce emissions, and meet stringent regulatory requirements.
Fundamentals of Dual-Fuel Engines
Dual-fuel marine engines are designed to run on multiple types of fuel, primarily natural gas and fuel oil. These engines can seamlessly transition between fuels without any interruption in power output. The primary motivation behind using dual-fuel engines is to optimize fuel consumption, reduce environmental impact, and ensure operational flexibility in different maritime conditions.
Methods of Introducing Natural Gas into the Engine
Two main methods are employed to introduce natural gas into the combustion chamber of dual-fuel engines:
1. Low-Pressure Gas Admission Method
In this method, natural gas is introduced into the combustion chamber at low pressure when the exhaust valve is closed and the cylinder pressure is low.
The gas is compressed along with the air, and the resulting mixture is ignited by a pilot injector that introduces a small amount of fuel oil into the combustion space.
This method is widely used but has challenges, such as engine knocking and lower efficiency.
2. High-Pressure Direct Injection Method
Here, natural gas is compressed to approximately 250-300 bar and directly injected into the combustion chamber simultaneously with the pilot fuel injection.
This method has demonstrated better efficiency, reduced knocking tendencies, and improved combustion characteristics, making it preferable in modern dual-fuel engine designs.