What is SCAVENGING?
In an engine, scavenging refers to the process of ejecting the exhaust gas and replenishing it with fresh air or air/fuel mixture for the next cycle. When done correctly, scavenging is massively effective to maximize efficiency and reduce the loss of fresh air through the exhaust valve. It plays a crucial role for not only efficient combustions inside the engine, but also to assist the cooling of piston, valves and cylinder. Therefore, it is important for two- stroke as well as four- stroke engines alike.
History of Scavenging:
In a two-stroke engine scavenging, that is the ejection of exhaust gases and admission of fresh air/fuel has been an important issue since the day two-stroke engines were invented, back in 1879 by German inventor, Karl Benz. But the first engines deliberately designed to encourage scavenging can be traced way back to the early 1890s in The United Kingdoms by Crossley Brothers Ltd.
These gas engines were called The Crossley Otto Scavenging Engines and were made possible by shifting to the Poppet valve, from the traditional Side valves, which allowed more control over the Valve Timing events. Valve Timings are basically the ideal timings to open and close Exhaust and Intake Valves to achieve optimum scavenging and maximum output from the engine. Given below is a diagram of Valve timing in a two-stroke engine.
Types of Scavenging
Broadly there are three types of scavenging which are listed as follows –
CROSS FLOW – In this type of scavenging, the fresh air or air/fuel mixture is pushed upwards through the inlet valve which pushes the exhaust gases before it. The exhaust gases are then ejected through the exhaust port. The mixture travels all across the cylinder in this process, hence the name crossflow scavenging. Usually there is a deflector on the piston crown to facilitate the upward movement of the Air/fuel mixture. It is mostly used in light weight two-stroke engines such as motorcycles, mopeds, scooters etc.
- It takes very low cost to manufacture.
- Low engine volume when used in multi cylinder arrangement.
- Provides with good scavenging at low speeds (part throttle).
- Poor scavenging at high speeds (full throttle).
- Has a heavy piston, resulting in high heat absorption.
- Requires a water-cooling system.
- High probability to get knocked.
LOOP FLOW– In this type of scavenging, the inlet and the exhaust are both on the same side of the cylinder. The inlet and exhaust ports are aligned in such a manner that the entrance of fresh air through the inlet causes them to move in loops in upward direction and then push the exhaust gases towards the exhaust port which is just above the inlet port. This is the most commonly used scavenging system.
- Low cost to manufacture and low maintenance.
- No water-cooling system required.
- Low surface area, hence lesser heat loss.
- Provides with good scavenging at full throttle.
- Poor scavenging at part throttle.
- Lesser scavenging time.
UNI FLOW– In this type of scavenging the exhaust port is above the inlet ports. They are designed in a way such that both the fresh air and exhaust gases move in the same direction, hence the name Uniflow Scavenging. The fresh air enters through the lower side of the cylinder and on its way, it pushes the exhaust gases towards the very top of the cylinder, where the exhaust valve is situated. It is used in large scale two stroke diesel engines.
- The most efficient among all three scavenging methods.
- Lower fuel consumption in comparison to others.
- High power output.
- Provides with good scavenging in all speeds and throttle positions.
- Expensive manufacturing and maintenance.
- Cooling of the piston gets tough.
Scavenging in Two-Stroke Engine
As it is pretty clear at this point, that in order to get the optimum results from a two-stroke engine, scavenging is required. But it is also important to understand which type of scavenging design is preferred for the given task. Using the correct type of scavenging method with the right type of scenario is also equally important.
In the beginning, crossflow type scavenging was widely used because of its easy design and low maintenance and low cost as well. But it got obsolete as soon as loop type scavenging was invented in the 1930s by the German inventor Adolf Schnürle. It was widely used in Germany, especially after World War II. This was not achieving better combustion results and preventing the air/fuel mixture to leave out of the exhaust ports but also due to this type of scavenging the piston head can be made of nearly domed or even flat. This resulted in making the overall structure to be less prone to uneven heating, expansion, piston seizures, dimensional changes, and compression losses.
Fun fact, Japanese companies like Kawasaki, Suzuki and Yamaha implemented these highly coordinated loop scavenging methods in their motorcycles and achieved great success in Grand Prix Motorcycle racing, in the 1960s due to the increased efficiency of loop scavenging over crossflow type.
Scavenging in Marine Engines
Marine engines require much higher efficiency than any other normal two-stroke engines. Not only that they themselves are massive, and they carry mammoth tons of cargo, these gargantuan machineries need to travel throughout the globe to get the job done. Therefore, it is very crucial to provide these ships with the heaviest the most efficient form of scavenging out there. That is where Uniflow scavenging comes in. This modern form of two-stroke design is assured to provide the most optimum power output at all speeds and throttle positions. Although it has some minor drawbacks, as of now this is the most fuel efficient, heavy duty type of scavenging which is ideal for ships and is used widely all across the planet.