Scavengine

In automotive usage, scavenging is the process of pushing exhausted gas-charge out of the
cylinder and drawing in a fresh draught of air or fuel/air mixture for the next cycle.
This process is essential in having a smooth-running internal combustion engine. If
scavenging is incomplete, the following stroke will begin with a mix of exhaust fumes rather
than clean air. This may be inadequate for proper combustion, leading to poor running
conditions such as four-stroking. If scavenging is not properly done it will affect the total
power output of the engine.
Scavenging is equally important for both two- and four-stroke engines. However it is more
difficult to achieve in two-stroke engines, owing to the proximity, or even overlap, of their
induction and exhaust strokes. Scavenging is also equally important to both petrol and diesel
engines.

There are three types of scavenging on the basis of the flow of air:
Direct, cross or loop
Reverse, using Schnuerle porting
Uniflow
Uniflow scavenging


Types
This method was used for early crankcase compression two-stroke engines, as used for small
motorcycles.
In this method, the transfer port (or inlet port for the engine cylinder) and exhaust port are
situated on the opposite sides of the engine cylinder, so that the burnt gases would be pushed
out by the cross flow.
The piston crown was usually shaped with a raised rib as a deflector piston, so that the fresh
charge was intended to move upwards into a vertical loop, then downwards with the exhaust
gas.
In practice, the gasflow failed to follow the idealised pattern. The rib of the deflector piston
also gave a poor shape for the combustion chamber, with long flame paths and excessive
surface area. This method of scavenging has now been almost entirely replaced by loop
scavenging.
Although obsolete for piston-ported two-stroke engines, crossflow scavenging is now very common in four stroke engines, where
their inlet and exhaust valves are mounted on opposite sides of the cylinder head. As the inlet and exhaust strokes of the four stroke
cycle happen on different strokes of the piston—the upward exhaust stroke, pushing exhaust gases out, is followed by the downward
inlet stroke allowing the fresh charge in—the flow is as two isolated single flows, rather than as an unstable loop.
Rather than the flow loop being vertical, the gases are encouraged to move in two horizontal loops.
In this method, the inlet and outlet ports are situated on the same side of the engine cylinder. The fresh charge, while entering into the
engine cylinder, forms a loop and pushes out the burnt gases.
In this method, the fresh charge, while entering from one side (or sometimes two sides) of the engine cylinder pushes out the gases
through the exit valve situated on the top of the cylinder. In uniflow scavenging, both the fresh charge and burnt gases move in the
same direction. Usually in diesel type engines the direction of flow is upward but with spark ignition engines such as the Ricardo
Dolphin the direction of flow is generally downward with fresh air / fuel entering at the top of the engine.
The uniflow method of scavenging has been widely used for two-stroke diesels, such as the Detroit Diesel Series 53,71,92,110 AND
149 and also the locomotive Electro-Motive Diesel and Cleveland Diesel and used in large 2 stroke ship propulsion engines such as
Wärtsilä engines.