SCAVENGING (Two & Four Stroke Engines)

SCAVENGING

(Two & Four Stroke Engines)

Scavenging is the removal of exhaust gases from an engine cylinder.

Scavenging is the process by which exhaust gases are expelled from the combustion chamber and fresh charge is introduced.

That is, in an internal combustion engines, 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.

Efficient scavenging is necessary for good combustion of fuel inside the engine cylinder. The passage of scavenge air will also assist cooling of the cylinder, piston and valves.

The three main types of scavenging for two stroke engines are UNIFLOW, CROSS FLOW and LOOP FLOW.

Scavenging is equally important for both two-stroke and four-stroke engines. Most modern four stroke engines use CROSS-FLOW cylinder heads and valve timing overlap to scavenge the cylinders. Modern two stroke  engines use either LOOP scavenging or UNI-FLOW scavenging.

UNI-FLOW SCAVENGING is a design in which the fresh intake charge and exhaust gases flow in the same direction. This requires that the intake and exhaust ports be at opposite ends of the cylinder. As used by some two-stroke engines, the fresh charge enters through piston-controlled ports near the bottom of the cylinder and flows upward, pushing the exhaust gases out through poppet valves located in the cylinder head. 

CROSS-FLOW SCAVENGING 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.

LOOP SCAVENGING (also called as "Schnuerle scavenging" or "reverse scavenging") is a design used by most modern valveless two-stroke engines. The key difference compared to crossflow scavenging is that the transfer ports are located either side of the exhaust port and aimed at the opposite cylinder wall. As the fuel/air mixture enters the combustion chamber, it travels across the cylinder then up the cylinder wall opposite the exhaust port before looping over at the cylinder head and back down to the exhaust port.

Scavenge efficiency can be defined as the ratio of the volume of air in the cylinder at the start of the compression to the volume swept by the piston from the top edge of the ports to the top of the strokes.

Refer the following link:

https://www.youtube.com/watch?v=bUqc6OuUjG0&t=38s

Composed By:                                                                  E-Mail:

R.Satheesh, M.E., Asso Prof.,                rsatheeshemail@gmail.com.