In an internal combustion engine, scavenging is the process of replacing the exhaust gas in a cylinder with the fresh air/fuel mixture (or fresh air, in the case of direct-injection engines) for the next cycle. If scavenging is incomplete, the remaining exhaust gases can cause improper combustion for the next cycle, leading to reduced power output.
Scavenging is equally important for both two-stroke and four-stroke engines. Most modern four-stroke engines use crossflow cylinder heads and valve timing overlap to scavenge the cylinders. Modern two-stroke engines can use Schnuerle scavenging (also known as "loop scavenging") or uniflow scavenging.
The first engines deliberately designed to encourage scavenging were gas engines built by Crossley Brothers Ltd in the United Kingdom in the early 1890s. These Crossley Otto Scavenging Engines were made possible by the recent change from slide valves to poppet valves, which allowed more flexible control over valve timing events. The closing of the exhaust valve on occurred more than 30 degrees later than on earlier engines, giving a long 'overlap' period (when both the intake and exhaust valves are open). As these were gas engines they did not require a long period of valve closure during the compression stroke. The exhaust gases were drawn from the engine by a partial vacuum following in the wake of a 'slug' of exhaust gas from the previous combustion cycle.
This method requires that the exhaust pipe is long enough to contain the gas slug for the entire duration of the stroke. As the Crossley engine was so slow-revving, this resulted in an exhaust pipe with a length of 65 feet (20 m) between the engine and its cast-iron 'pot' silencer.
Crossflow cylinder heads are used by most modern four-stroke engines, whereby the intake ports are located on one side of the combustion chamber and the exhaust ports are on the other side. The momentum of the gases assists in scavenging during the 'overlap' phase (when the intake and exhaust valves are simultaneously open).
For two-stroke engines, crossflow scavenging was used in early crankcase-compression engines, such as used by small motorcycles. The transfer port (where the fuel/air mixture enters the combustion chamber) and the exhaust port were located on opposite sides of the combustion chamber. A deflector piston was often used, where the piston shape directed the intake gases towards the spark plug at the top of the cylinder. However, the deflector piston was not very effective in practice, and the shape of the piston compromised the shape of the combustion chamber, with long flame paths and excessive surface area. Therefore, vertical loop scavenging is rarely used in modern two-stroke engines.
Schnuerle scavenging (sometimes called "loop scavenging" or "reverse scavenging") is a design used by some two-stroke engines. The key difference compared to crossflow scavenging is that the gases travel in a horizontal loop instead of a vertical loop. This is achieved by having two angled transfer ports that are located either side of the exhaust port and angled away from the exhaust port. As the fuel/air mixture enters the combustion chamber, the gases travel in a horizontal loop around the cylinder wall, finishing with the exhaust gases being pushed out through the exhaust port.
Uniflow scavenging is a design used by some two-stroke engines where the fresh charge enters near the bottom of the cylinder and the exhaust gases exit through a valve situated at the top of the cylinder. Alternatively, some uniflow engines (such as the Ricardo Dolphin marine engine) use a downwards flow direction, due to the fresh air/fuel mixture entering at the top of the cylinder and the exnaust gases exiting at towards the bottom of the cylinder. In all uniflow scavenging engines, both the fresh charge and burnt gases move in the same direction.
The uniflow method of scavenging has been often used for two-stroke diesel engines in motor vehicles, marine vessels, railway locomotives and as stationary engines.
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