Scavanging In Auto Engine

1. A. D. Patel Institute Of
Technology
AUTOMOBILE ENGINEERING DEPARTMENT
Prepared By:
140010102051 Shah Jainam H.
140010102053 Shah Parva C.

2. Scavenging
Definition:
“Scavenging is the process by which the burnt gases escape the cylinder.”
•During the power stroke the air-fuel mixture burns in the cylinder and the burnt gases are to be
exhausted from the cylinder so that the fresh charge may enter the cylinder during the suction
stroke.
•If some of the exhaust gases remain in the cylinder they will be mixed with the fresh charge and
dilute it.

3. • The dilute charge will develop less power.
• The fresh charge also should not go out, with the exhaust
gases.
• If the unburnt fresh charge goes out it will increase the fuel
consumption, fuel is wasted and less power is produced.
• Exhaust gases should escape the cylinder completely and
the fresh charge would not be mixed with the exhaust
gases.
• For this purpose the combustion chamber and piston head
are specially designed.

4. •The piston head of two stroke cycle engine is
specially designed for scavenging.
•The deflector at the piston head is designed in
such a form that it deflects the fresh charge
towards the combustion chamber and burnt
gases towards the exhaust port.
•Thus, the fresh charge does not escape the
cylinder without burning.

5. Types Of Scavenging
(1)Based on Theoretical process
(2)Based on Practical Process

6. (1)Based on Theoretical process:
(A)Perfect Scavenging:
•Ideally, the fresh fuel-air mixture should remain separated from the
residual combustion products w.r.t both mass and heat transfer during
the scavenging process.
•In short in this process the fresh charge do not mix with the
combustion products and it simply pushes the exhaust gases towards
the exhaust port.

7. (B) Perfect Mixing:
•The incoming fresh charge mixes completely and instantaneously with
the cylinder contents, and a portion of this mixture passes out of the
exhaust ports at a rate equal to that entering the cylinder.
•This homogeneous mixture consist initially of products of combustion
only and then gradually changes to pure air.

8. (C)Short Circuiting:
•The fresh charge coming from the scavenge manifold directly goes
out of the exhaust ports without removing any residual gas.
•This is a dead load and its occurrence must be avoided.

9. (2)Based on Practical process:
(A)Uniflow scavenging:
•In this, exhaust valve is fitted at the top of the cylinder and two inlet
ports are at the lower part of the cylinder.
•The fresh charge enter the cylinder from the two inlet ports, which
pushes the exhaust gases to go out through the exhaust valve.
•The direction of the flow of the fresh charge and exhaust gases is the
same, hence it is known as uniflow scavenging.

10. Uniflow Scavenging

11. (B)Loop Scavenging:
•In this, the inlet and exhaust port, both are situated at one side in the
lower part of the cylinder.
•The fresh charge flows in the cylinder in the form of a loop and pushes
the exhaust gases to go out through the exhaust port.

12. Loop Scavenging

13. (C)Cross flow scavenging:
•This type of scavenging is generally adopted in the two stroke cycle
engines.
•The transfer port and exhaust port are situated in front of each other
at the lower part of the cylinder.
•The fresh charge enters the cylinder through the transfer port and is
deflected upwards.
•Then it comes towards the exhaust port. Thus, it pushes the exhaust
gases towards the exhaust port to go out.

14. Cross flow Scavenging

15. Thank You