SCAVENGING AND ITS TYPES

                            SCAVENGING

·     IT IS A PROCESS OF REMOVING EXHAUST GAS IN THE ENGINE CYLINDER BY MEANS OF INCOMING CHARGE AIR.

·     AS A RESULT CYLINDER IS FILLED WITH FRESH PRESSURIZED AIR FOR NEXT COMPRESSION STROKE.    

·     THE PASSAGE OF SCAVENGE AIR WILL ALSO ASSIST COOLING OF THE CYLINDER, PISTON AND VALVES.

·     TIME AVAILABLE FOR SCAVENGING PROCESS IN 2 STROKE ENGINE IS LESS THAN 4 STROKE ENGINES.

SATISFACTORY SCAVENGING DEPENDS ON-

•   EFFICIENT EVACUATION OF EXHAUST GASES
•   EXHAUST PORTS OR VALVES OPEN BEFORE SCAVENGE PORTS
•   SCAVENGE PORTS CLOSE AFTER THE PROCESS OF GAS EXCHANGE
•    LOSS OF FRESH AIR THROUGH EXHAUST PASSAGE TO BE MINIMUM

IMPORTANCE OF SCAVENGING

SCAVENGING AFFECTS –

•         OVERALL EFFICIENCY
•         POWER OUTPUT PER UNIT WEIGHT
•         FUEL CONSUMPTION PER UNIT POWER
•         INCOMPLETE SCAVENGING RESULTS IN POOR COMBUSTION
•         LUBRICATING OIL CONTAMINATION
•         WEAR PISTON RINGS AND LINER
•         MEAN TEMPERATURE OF CYLINDER HIGH         

           TYPES OF SCAVENGING

1.          CROSS SCAVENGING
2.          LOOP SCAVENGING
3.          UNIFLOW SCAVENGING

        UNIFLOW SCAVENGING

      

1.       THE AIR INLET AND EXHAUST FLOW IN THE SAME DIRECTION.
2. ·      IT IS THE BEST SCAVENGING PROCESS BECAUSE OF HIGH SCAVENGING           EFFICIENCY (MORE THEN .9)
3. ·      INTERMIXING IS MINIMUM BECAUSE THERE IS NO DIRECTION CHANGE    IN THE FLOW (INLET AND OUTLET).

ADVANTAGES

• HIGHEST SCAVENGE EFFICIENCY AT ALL SCAVENGE RATIOS
• DROP IN SCAVENGE EFFICIENCY LOWEST WITH HIGH SCAVENGE PRESSURE
• MOST SUITABLE FOR PRESSURE CHARGING
• GREATER AREA FOR AIR AND GAS FLOW
• POSSIBILITY OF SHORT CIRCUITING AND MIXING LESS

DISADVANTAGES

• COMPLICATED CYLINDER HEAD AND INCREASED MAINTENANCE                                       

LOOP SCAVENGING

• LOOP SCAVENGING IS THE GAS EXCHANGE PROCESS IN WHICH THE CHARGE AIR PASSES OVER THE PISTON CROWN AND RISES TO FORM A LOOP WITHIN THE CYLINDER, EXPELLING EXHAUST GASES THROUGH EXHAUST PORTS CUT IN THE SAME SIDE OF THE LINER ABOVE THE SCAVENGE PORTS. 

MAJOR REQUIREMENT FOR THE LOOP SYSTEM

• THE INLET AIR PORTS ARE ANGLED TO GIVE ROTARY SWIRLING EFFECT TO THE  INCOMING AIR.
• IT ALSO REQUIRE LONG PISTON SKIRT OR EXHAUST TIMING V/V TO PREVENT  SCAVENGING AIR LEAK INTO EXHAUST WHILE THE PISTON IS AT TOP OF IT’S  STROKE.

ADVANTAGES

• SIMPLICITY, RELIABILITY AND CYLINDER COVER IS SIMPLIFIED.

DISADVANTAGES

• TEMPERATURE GRADIENT EXHAUST TO SCAVENGE PORTS IS HIGH
• EXHAUST BACK PRESSURE HIGHER WITH DEPOSITS
• POSSIBILITY OF INTERMIXING AIR AND GASES
• PISTON AND LINER DISTORTION AND UNEVEN PISTON RING WEAR DUE TO PORTS
• SCAVENGE EFFICIENCY IS LOWER THAN UNIFLOW SCAVENGE EFFICIENCY AT ALL SCAVENGE RATIOS

CROSS SCAVENGING

·       1.  CROSS FLOW SCAVENGING IS THE GAS EXCHANGE PROCESS IN WHICH THE    2. CHARGE AIR PASSES IS DIRECTED UPWARDS, PASSING UNDER THE CYLINDER    3. COVER AND DOWN THE OPPOSITE SIDE, EXPELLING EXHAUST GAS THROUGH  4.  EXHAUST PORTS ON THAT SIDE.
        THIS SYSTEM IS OBSOLETE NOW.