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.

VALVE OVERLAPPING

Valve overlapping

(Four Stroke Engines)

   Valve overlap is the period during engine operation when both intake and exhaust valves are open at the same time.

  

  It is defined as the number of degrees between when the inlet valve starts to open before TDC and when the exhaust valve closes after TDC.

     

                                                    

   Valve overlap in an IC engine is used to improve the  performance of the engine.

    

     Valve overlap is very important for SCAVENGING purpose.

    

    At high speed, overlap takes advantage of the Scavenging effect.

   However, at idle and low speed, overlap will produce low  vacuum and a rough idle.

   

    The further away from TDC that valve overlap is present, the  more effect the piston motion will have on the airflow.    

  

    Early overlap may result in exhaust gasses being expelled into  the intake manifold and late overlap may result in exhaust  gasses being drawn back into the cylinder.

Refer the following links:

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

 Composed By:                                                            E-Mail:

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

CRANKCASE COMPRESSION

CRANKCASE COMPRESSION

CCC

(Two Stroke Engines)

A crankcase  is the housing for the crankshaft in IC engine.

In most modern engines, the crankcase is integrated into the engine block.

Refer the following link:

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

        Many 2 stroke engines use a crankcase-compression design, where a partial vacuum (suction) draws the air-fuel mixture into the Spark Ignition (SI) engine (or) air into the Compression Ignition/Injection (CI) engine as the piston moves upwards. 

        Then as the piston travels downward, the inlet port is uncovered and the compressed air-fuel mixture (or) air in SI (or) CI engine respectively is pushed from the crankcase (bottom side) into the combustion chamber (top side).

         Crankcase-compression designs are often used in small petrol (gasoline) engines for motorcycles. 

         This design has also been used in some small diesel engines, however it is less common.

          Both sides of the piston are used as working surfaces. 

          The upper side is the power piston, the lower side acts as a pump. Therefore an inlet valve is not required. Unlike other types of engines, there is no supply of oil to the crankcase, because it handles the fuel/air mixture. 

           Instead two stroke oil is mixed with the fuel used by the engine and burned in the combustion chamber.

Crankcase compression ratio can be determined by cylinder capacity (cylinder volume) and crankcase volume [the piston is at top dead center position]. 

      

                                Composed By:                                                             E-Mail:

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

Two Stroke Engine Vs Four Stroke Engine

Two Stroke Engine Vs Four Stroke Engine

Composed By:                                                                  E-Mail:

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

Petrol Engine Vs Diesel Engine

Petrol Engine Vs Diesel Engine

Composed By:                                                           E-Mail:

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

TWO STROKE COMPRESSION IGNITION / INJECTION ENGINES (CI ENGINES)

Two STROKE

compression IGNITION / injection ENGINES

(cI ENGINES)

Four working processes (additional one) done in 

two strokes and Ports (Inlet, Transfer and Exhaust ports) 

are there in 2 stroke engines.

• Upward Stroke:
• Suction  and Compression: Piston moves from BDC to TDC.                
• Downward Stroke:
• Power, Exhaust and Crankcase Compression: Piston moves from TDC to BDC.    

        

• Upward Stroke:
• Suction  and Compression:
• Inlet port opened and exhaust & transfer ports are closed.
• Downward Stroke:
• Power, Exhaust and Crankcase Compression:
• Inlet port closed and exhaust and transfer ports are opened.      

• Upward Stroke:
• Suction: Air entry into the engine crankcase side due to increase of crankcase volume by piston movement from BDC to TDC.

• Compression: Due to decrease of cylinder volume in combustion chamber side by piston movement, the air gets high pressure and temperature and its ready to combustion. At the end of compression, the fuel is injected from fuel injector and combustion takes place.

• Downward Stroke:
• Power: After combustion, the power gets (mechanical energy), due to power the piston moves from TDC to BDC.

• Exhaust: During the movement of piston towards down, the exhaust port open and leaves the exhaust gases through it  from engine cylinder to atmosphere.

• Crankcase Compression: During downward stroke, the piston compress the air in crankcase side and its transfer through transfer port and enter into combustion side for power next cycle.

Composed by:  R.Satheesh, M.E., Asso Prof., email: rsatheeshemail@gmail.com.

FOUR STROKE COMPRESSION IGNITION / INJECTION ENGINES (CI ENGINES)

FOUR STROKE compression IGNITION ENGINES

(cI ENGINES)

Four working processes done in four individual strokes and 

Valves (Inlet and Exhaust valves) are there in 4 stroke engines.

• Suction -------------- Piston moves from TDC to BDC.
• Compression ------- Piston moves from BDC to TDC.
• Power --------------- Piston moves from TDC to BDC.
• Exhaust ------------- Piston moves from BDC to TDC.

• Suction----------: Inlet valve open and exhaust valve closed.
• Compression---: Inlet valve and exhaust valve both are closed.
• Power-----------: Inlet valve and exhaust valve both are closed.
• Exhaust---------: Exhaust valve open and inlet valve closed.

• Suction : Air entry into the engine due to increase of cylinder volume by piston movement.

• Compression : Due to decrease of cylinder volume by piston  movement, the air gets high pressure and temperature and at the end of compression, the fuel is injected  from fuel injector  and combustion takes place.

• Power : During power the piston moves from TDC to BDC. Its nothing but working stroke.

• Exhaust : During this process, the exhaust gases leaves from engine cylinder.

Composed by:  R.Satheesh, M.E., Asso Prof., email: rsatheeshemail@gmail.com.

TWO STROKE SPARK IGNITION ENGINES (SI ENGINES)

TWO STROKE SPARK IGNITION ENGINES

(SI ENGINES)

Four working processes (additional one) done in 

two strokes and Ports (Inlet, Transfer and Exhaust ports) 

are there in 2 stroke engines.

• Upward Stroke:
• Suction  and Compression: Piston moves from BDC to TDC.                
• Downward Stroke:
• Power, Exhaust and Crankcase Compression: Piston moves from TDC to BDC.    

     

• Upward Stroke:
• Suction  and Compression:
• Inlet port opened and exhaust & transfer ports are closed.
• Downward Stroke:
• Power, Exhaust and Crankcase Compression:
• Inlet port closed and exhaust and transfer ports are opened.      

• Upward Stroke:
• Suction: Air-fuel (Petrol/gasoline) mixture entry into the    engine crankcase side due to increase of crankcase volume by piston movement from BDC to TDC.

• Compression: Due to decrease of cylinder volume in combustion chamber side by piston movement, the mixture gets high pressure and temperature and its ready to combustion. At the end of compression, the spark ignited from spark plug and combustion takes place.

• Downward Stroke:
• Power: After combustion, the power gets (mechanical energy), due to power the piston moves from TDC to BDC.

• Exhaust: During the movement of piston towards down, the exhaust port open and leaves the exhaust gases through it from engine cylinder to atmosphere.

• Crankcase Compression: During downward stroke, the piston compress the mixture in crankcase side and its transfer through transfer port and enter into combustion side for power next cycle.

                

Composed by:  R.Satheesh, M.E., Asso Prof., email: rsatheeshemail@gmail.com.

FOUR STROKE SPARK IGNITION ENGINES (SI ENGINES)

FOUR STROKE SPARK IGNITION ENGINES

(SI ENGINES)

Four working processes done in four individual strokes and 

Valves (Inlet and Exhaust valves) are there in 4 stroke engines.

• Suction -------------- Piston moves from TDC to BDC.
• Compression ------- Piston moves from BDC to TDC.
• Power --------------- Piston moves from TDC to BDC.
• Exhaust ------------- Piston moves from BDC to TDC.

• Suction----------: Inlet valve open and exhaust valve closed.
• Compression---: Inlet valve and exhaust valve both are closed.
• Power-----------: Inlet valve and exhaust valve both are closed.
• Exhaust---------: Exhaust valve open and inlet valve closed.

• Suction :Air-fuel (Petrol/gasoline) mixture entry into the engine due to increase of cylinder volume by piston movement.

• Compression: Due to decrease of cylinder volume by Piston movement, the mixture gets high pressure and temperature and ready to combustion. At the end of compression, the spark ignited and combustion takes place.

• Power: During power the piston moves from TDC to BDC. Its nothing but working stroke.

• Exhaust: During this process, the exhaust gases leaves from engine cylinder.

Composed by:  R.Satheesh, M.E., Asso Prof., email: rsatheeshemail@gmail.com.

CLASSIFICATIONS OF INTERNAL COMBUSTION ENGINES

CLASSIFICATIONs

OF

INTERNAL COMBUSTION ENGINES

        

IC engines are classified are based on:

• Number of strokes per cycle

          Four stroke engine               Two stroke engine

• Cycle of operations

          Otto cycle engine              Diesel cycle engine              Dual combustion cycle engine

• Types of fuel used

          Petrol engine                    Diesel engine               Gas engine

• Methods of charging

          Naturally aspirated engine             Supercharged engine

• Types of ignition

          Spark –ignition engine                 Compression ignition engine

• Types of cooling

          Air cooling                   Water cooling

• Speed of engines

           Low speed engine                Medium speed engine                  High speed engine

• Number of cylinders

           Single           Two             Four             Six               Eight            Twelve

• Arrangement of cylinders

           Straight or in line engine                  Horizontal engine                Radial engine   

                    V – engine                                     Opposed cylinder engine

• Method of governing

          Quality governing                           Quantity governing

• Valve arrangement

          L-head         I-head          F-head         T-head                  etc.....

Composed by:  R.Satheesh, M.E., Asso Prof., email: rsatheeshemail@gmail.com.