2. What is Scavenging?
โข Scavenging is the process of removing exhaust gases from the cylinder
after combustion and replenishing the cylinder with fresh air.
โข Efficient scavenging is necessary for good combustion of fuel inside
the engine cylinder.
โข The passage of scavenge air will also assist cooling of the cylinder,
piston and valves.
โข Time available for scavenging process in 2 stroke engine is less than 4
stroke engines.
4. Satisfactory Scavenging Depends on:
โข Efficient evacuation of exhaust gases
โข Exhaust ports or valves open before scavenge ports
โข Scavenge ports close after the process of gas exchange
โข Loss of fresh air through exhaust passage to be minimum
5. Importance of Scavenging
Scavenging affects โ
โข Overall efficiency
โข Power output per unit weight
โข Fuel consumption per unit power
โข Incomplete scavenging results in poor combustion
โข Lubricating oil contamination
โข Wear piston rings and liner
โข Mean temperature of cylinder high
6. Different Stages of Scavenging
Different Stages of Scavenging
1st Stage โ Blow Down Period
โข Sudden opening accelerates exhaust gases to exhaust manifold.
2nd Stage โ Scavenge Period
โข Begins after scavenge ports open
โข Scavenge air sweeps gases out
โข Pressure difference between the scavenge trunking and exhaust manifold is
important
3rd Stage
โข Effort made to contain air in the cylinder
7. Types of Scavenging
Based on the direction of flow of fresh charge (or air) and exhaust gases,
scavenging methods are classified
โข Cross scavenging
โข Loop or Reverse scavenging
๏Full Joop or MAN scavenging
๏Tangential loop or Schmuerle scavenging
๏Curtis scavenging
โข Uniflow scavenging
8. Cross Scavenging
This method of scavenging is widely used in low capacity (upto 8 kW)
two stroke S.I engine due to simplicity in construction
In the cross scavenging, inlet and exhaust ports are placed opposite to
each other.
The air or mixture moves up to combustion chamber one side of the
cylinder and then down on the other side of flow out of exhaust ports.
This is possible due to deflector provided on piston top. The deflector
guides the air flow and forces the exhaust to move outward.
9.
10. The main advantages are:
1. Simple in design and construction.
2. It has low cost.
3. Less maintenance due to absence of valve, pumps or any drive devices.
The cross scavenging is not very effective, however it is used in low
capacity petrol engine due to its simplicity and low cost. It is commonly
used with two wheeler engines.
11. Disadvantages of this system are:
1. Poor scavenging means more amount of exhaust remain inside
the cylinder which dilute the fresh charge.
2. Some of the fresh charge also goes directly to exhaust without
combustion means fresh charge loss.
3. Due to above two reasons, power output and thermal
efficiency reduces.
12. Loop or Reverse scavenging:
In this method flow of fresh charge makes a loop for better removal of
exhaust gases, hence the short circuiting of the cross scavenged engine
is reduced and improves scavenging efficiency.
In the loop scavenging, incoming charge is directed in such a way that
both charge loss and mixing are minimized.
There are three types of loop scavenging as follows
13.
14. MAN type loop scavenging method:
In this method, the exhaust and
inlet ports are placed on same side,
but exhaust port is placed slightly
above the inlet port
This arrangement is suitable for
large engine running at medium or
high speed.
15. Tangential loop type scavenging method:
In this system two pair of inclined inlet
ports are directed upward and exhaust
port is placed at center.
The fresh charge enters from an
inclined inlet port directed upward
forming "U" shape loop and forces the
exhaust which removes from exhaust
port.
It provides much better scavenging
than the simple loop system.
16. Curtis type loop scavenging method:
In this system number of inlet port having
with different upward inclination and their
inclination varies according to their
positions relative to exhaust port.
The inlet port near to the exhaust port have
smaller inclination compare to far inlet port
for better scavenging.
This promotes a displacement of exhaust
gases from the top and their expulsion from
exhaust ports.
17. Advantages of loop scavenging system are:
(1) Better scavenging compared to cross scavenging.
(2) Simple, reliable and less maintenance.
Disadvantages of loop scavenging system are:
(1) Better cooling is required to reduce the thermal stresses on piston
and cylinder liner.
18. Uniflow scavenging:
In Uniflow scavenging the air enters the
cylinder from one end and leaves as exhaust
from the other end.
The flow is from end to end called straight
flow. The straight flow reduces the
turbulence and hence the less mixing of the
fresh charge with burnt gases.
Thus, scavenging efficiency is increased.
Also, the direction of flow does not change
and hence, there is minimum possibility of
short circuiting of fresh charge and exhaust
so scavenging is best out of all three type of
scavenging.
There are mainly two types of uniflow
scavenging as
19. In both the types fresh charge enters
from bottom and flow is controlled
by movement of piston.
In poppet valve type, valve is fitted at
top which is mechanically operated
by cam and exhaust gases goes out of
cylinder when it opens.
20. In opposed piston type, inlet and
exhaust ports are controlled by
separate piston and this pistons will
be driven by separate crank
21. Advantages of uniflow scavenging:
1. Better scavenging because of minimum mixing of fresh charge and
exhaust.
2. More power and efficiency because of better combustion.
Disadvantages of uniflow scavenging :
1. System is complicated as separate crank shaft required for giving
motion to poppet valve or opposed piston.
2. high maintenance.
3.high intial cost.
