Scavenging is a critical process in internal combustion engines, particularly in two-stroke engines. The process involves removing the spent exhaust gases from the combustion chamber and replacing them with fresh air or a fuel-air mixture, which is necessary for efficient combustion and power generation.
In a two-stroke engine, the scavenging process is typically achieved through the use of ports or valves that open and close at specific intervals to allow the fresh charge to enter and the exhaust gases to exit. The timing and duration of these ports or valves are carefully controlled to ensure that the maximum amount of exhaust gas is removed and replaced with fresh charge in the shortest possible time.
The efficiency of the scavenging process is critical to the overall performance of the engine, as it directly affects the power output, fuel efficiency, and emissions of the engine. A poorly designed scavenging system can result in incomplete combustion, reduced power output, and increased fuel consumption and emissions.
Therefore, engine designers must carefully consider the design and operation of the scavenging system to ensure that it is optimized for the specific requirements of the engine and its application. This may involve the use of advanced modeling and simulation techniques, as well as extensive testing and validation to ensure that the scavenging system meets the required performance and efficiency targets.
Types of Scavenging:
Loop Scavenging:
In this method, the fresh charge enters the cylinder through ports in the cylinder wall near the top. The incoming charge flows down and across the cylinder head, pushing out the exhaust gases through an exhaust port located at the bottom of the cylinder.
Uniflow Scavenging:
This method involves directing the fresh charge into the cylinder through a port located near the top of the cylinder, while the exhaust gases exit through a port located at the opposite end of the cylinder, typically at the bottom. The incoming charge flows in a unidirectional manner, which allows for efficient removal of the exhaust gases and improved combustion.
Advantages of Uniflow Scavenging:
- Highest scavenging efficiency
- No intermixing of exhaust and fresh air
- Reduced working temperatures
- Lower cylinder lube oil consumption (0.3-0.6 gm/bhp/hr in crosshead engines)
- Minimal residual exhaust gases
- No air loss during exhaust and scavenging
- Simpler liner design, allowing for shorter piston skirts
- Lower thermal stresses compared to other scavenging methods
Methods of Uniflow Scavenging:
- Poppet Valve Method: Uses a single exhaust valve at the cylinder top, providing fast exhaust gas release for improved efficiency. This is widely employed in modern 2-stroke engines.
- Opposed Piston Method: Used in older engines, such as Doxford, where two pistons control air inlet and exhaust ports separately.
Crossflow Scavenging:
In crossflow scavenging, the incoming charge is directed through ports located on one side of the cylinder, while the exhaust gases exit through ports located on the opposite side of the cylinder. The incoming charge and exhaust gases flow in a crosswise manner, which helps to minimize mixing and allows for more efficient removal of the exhaust gases.
Loop scavenging is the most commonly used method in marine engines, where fresh charge enters the cylinder through ports located at the top of the cylinder, while exhaust gases are pushed out through an exhaust port at the bottom of the cylinder. This method is preferred for its simplicity, efficiency, and reliability.
Marine engines may also use a variation of the uniflow scavenging method called uniflow scavenging with loop charging, which uses a separate charging air blower to direct the incoming charge through an air cooler and into the cylinder through the intake valve.
Turbocharging or supercharging may also be used to enhance the scavenging process in some marine engines. These methods increase the pressure of the incoming charge, improving the scavenging process and engine performance.
Scavenging: Pushing Out Exhaust Gases
During scavenging, fresh air enters the cylinder once the scavenge ports open. The higher pressure in the scavenge box pushes residual exhaust gases out, filling the cylinder with fresh air.
Post-Scavenging: Finalizing the Process
Post-Scavenging ensures the cylinder is fully refilled with fresh air and no fresh air escapes through the exhaust valve. The exhaust valve must close at the precise moment to prevent contamination of fresh air by residual exhaust gases. This process cools the cylinder, lowering temperatures for optimal engine performance.
Engine size, operating speed, power requirements, and specific application requirements all factor into the selection of the scavenging method in marine engines. Marine engine designers must carefully consider these factors to select the most suitable scavenging method for optimal engine performance and efficiency.
