COOLING SYSTEM

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COOLING SYSTEM

THE PURPOSE OF COOLING SYSTEM IS TO KEEP THE ENGINE AT ITS MOST EFFICIENT OPERATING TEMPERATURE AT ALL SPEEDS, AND UNDER ALL OPERATING CONDITIONS. DURING COMBUSTION OF AIR-FUEL MIXTURE IN THE ENGINE CYLINDER, TEMPERATURE AS HIGH AS 2200C MAY BE REACHED BY THE DURING GASES. ALTHOUGH THE CYLINDER WALL, CYLINDER HEAD, ABSORBS SOME OF THIS HEAT VALVES, PISTON RINGS AND THE PISTONS; EVEN THOUGH THE AVERAGE TEMPERATURE IN THE INTERNAL VICINITY OF THE CYLINDERS STANDS AT ABOUT 14000C. IF THIS HEAT IS NOT DISSIPATED, THE SURFACE OF THE COMBUSTION CHAMBER WILL BECOME RED HOT, THE WALL WILL BURN AND WRAP, AND VARIOUS PARTS OF THE ENGINE WILL EXPAND EXCESSIVELY RESULTING IN SEIZURE OF PISTON AND BEARINGS.
CYLINDER WALL TEMPERATURE MUST NOT GO HIGHER THAN ABOUT (250 TO 2600C). TEMPERATURE HIGHER THAN THIS CAUSES THE LUBRICATING-OIL FILM TO BREAK DOWN AND LOOSE ITS LUBRICATING PROPERTIES. HOWEVER, THE ENGINE OPERATES BEST AT TEMPERATURE AS CLOSE AS POSSIBLE TO THE LIMITS IMPOSED BY OIL PROPERTIES. REMOVING TOO MUCH HEAT THROUGH THE CYLINDER WALLS AND THE HEAD LOWERS THE THERMAL EFFICIENCY OF THE ENGINE.
COOLING SYSTEMS ARE DESIGNED TO REMOVE ABOUT ONE-THIRD (30 TO 35 PERCENT) OF THE HEAT PRODUCED IN THE COMBUSTION CHAMBER BY BURNING OF AIR-FUEL MIXTURE. ONE-THIRD OF THE HEAT LEAVES THE ENGINE THROUGH THE EXHAUST SYSTEM. GENERALLY, ONLY ABOUT ONE-THIRD OF THE HEAT OF COMBUSTION CAN BE UTILIZED TO CREATE PRESSURE ON TOP OF THE PISTON FOR DOING USEFUL WORK.
THE ENGINE IS VERY INEFFICIENT WHILE COLD. THEREFORE, THE COOLING SYSTEM INCLUDES DEVICES THAT PREVENT NORMAL COOLING ACTION DURING ENGINE WARM-UP. THESE DEVICES ALLOW THE ENGINE PARTS TO REACH THEIR NORMAL OPERATING TEMPERATURE MORE QUICKLY. THIS SHORTENS THE EFFICIENT COLD-OPERATING TIME. WHEN THE ENGINE REACHES ITS NORMAL OPERATING TEMPERATURE, THE COOLING SYSTEM BEGINS TO FUNCTION. THE COOLING SYSTEM COOLS THE ENGINE RAPIDLY WHEN IT IS HOT, AND SLOWLY OR NOT AT ALL WHEN THE ENGINE IS COLD OR WARMING UP.

METHODS OF COOLING
1.      AIR COOLING:-
·         AIR JACKET COOLING
·         FIN OR NATURAL AIR-COOLING
·         FORCED AIR COOLING

2.      WATER COOLING
·         THERMO-SIPHON COOLING
·         PUMPED CIRCULATION (OR PRESSURIZED) COOLING

OF THESE METHODS, AIR-COOLING IS PREFERRED FOR 2-WHEELERS AND ON THOSE VEHICLES WHERE SPACE NEEDED TO INSTALL A RADIATOR IS LIMITED. WATER-COOLING IS VERY COMMON ON 4-WHEELARS, 6-AND MORE WHEELERS. COMBINED AIR AND WATER-COOLING SYSTEM IS EMPLOYED ON VEHICLES, WHICH HAVE TO RUN THROUGH DESERT AND HILLY TERRAINS, OR THE REGIONS WHERE THE WATER AVAILABILITY MAY BE SCARCE. IN THAT CASE, THE AIR-COOLING SYSTEM SERVES THE PURPOSE INSTEAD OF WATER-COOLING SYSTEM INSTALLED ON IT. COMPLETELY SEALED COOLING SYSTEM IS SIMILAR TO WATER-COOLING BUT ADOPTS ARRANGEMENT FOR LIFETIME OF THE VEHICLES, AND THERE IS NO NEED OF FILLING WATER AT SPECIFIED INTERVALS.

AIR COOLING SYSTEM
THIS SYSTEM EMPLOYS AIR AS THE COOLING MEDIUM. THE AIR REMAINS IN DIRECT CONTACT WITH THE EXTERIOR OF ENGINE SURFACE, AND TRANSFERS HEAT FROM CYLINDER TO THE ATMOSPHERE. AIR-COOLING SYSTEM IS SIMPLE IN LAYOUT, OCCUPIES SMALL AREA ON THE VEHICLE, CONTAINS MUCH LESS SPARE PARTS AND HAS ALMOST NO MOVING COMPONENTS. THERE ARE TWO DIFFERENT ARRANGEMENTS BY WHICH AIR IS IT UTILIZED IN THIS SYSTEM. THESE ARE:
                    I.            AIR FLOWING THROUGH THE AIR JACKET
                  II.            AIR FLOWING OVER THE FINS, BUILT-IN OR MOUNTED ON THE ENGINE.

AIR JACKET COOLING: - IN THIS COOLING ARRANGEMENT, THE AIR IS MADE TO PASS THROUGH THE SMALL PASSAGES FORMED IN THE CYLINDER BLOCK AND THE CYLINDER HEAD. THE CONSTRUCTION IS SUCH THAT A NETWORK OF PASSAGES IS MADE ALL AROUND THE CYLINDER DURING ITS CASTING. THE AIR IS PASSED THROUGH A SINGLE OR MULTI-PASSAGES, EITHER NATURALLY OR BY ARTIFICIAL MEANS. IF THE AIRFLOW IS NATURAL, AN AIR-DUCT IS PROVIDED IN FRONT OF THE ENGINE; AND A FAN OR A BLOWER IS USED IN CASE OF ARTIFICIAL FLOW OF AIR.
SOMETIMES A SHEET OF ALUMINIUM OR COPPER SURROUNDS THE ENGINE TO DISSIPATE THE HEAT MORE READILY. ADJUSTABLE DOORS ARE ALSO PROVIDED AT THE ENTRANCE OF AIR DUCTS TO CONTROL THE CIRCULATION OF AIR THROUGH THE JACKETS. AIR JACKET COOLING IS PREFERRED WHERE MODERATE COOLING IS DESIRED. HENCE, THIS SYSTEM IS EMPLOYED ON AUTO VEHICLES HAVING REAR-MOUNTED ENGINES, OR THE VEHICLES, WHICH ARE TO BE OPERATED IN COMPARATIVELY COLDER REGIONS.

NATURAL AIR COOLING: - IN NORMAL CAUSE, LARGER PARTS OF AN ENGINE REMAINS EXPOSED TO THE ATMOSPHERIC AIR. WHEN THE VEHICLES RUN, THE AIR AT CERTAIN RELATIVE VELOCITY IMPINGES UPON THE ENGINE, AND SWEEP AWAY ITS HEAT. THE HEAT CARRIED-AWAY BY THE AIR IS DUE TO NATURAL CONVECTION, THEREFORE THIS METHOD IS KNOWN AS NATURAL AIR-COOLING. ENGINES MOUNTED ON 2-WHEELERS ARE MOSTLY COOLED BY NATURAL AIR.
AS THE HEAT DISSIPATION IS A FUNCTION OF FRONTAL CROSS-SECTIONAL AREA OF THE ENGINE, THEREFORE THERE EXISTS A NEED TO ENLARGE THIS AREA. AN ENGINE WITH ENLARGE AREA WILL BECOMES BULKY AND IN TURN WILL ALSO REDUCE THE POWER BY WEIGHT RATIO. HENCE, AS AN ALTERNATIVE ARRANGEMENT, FINS ARE CONSTRUCTED TO ENHANCE THE FRONTAL CROSS-SECTIONAL AREA OF THE ENGINE.
FINS (OR RIBS) ARE SHARP PROJECTIONS PROVIDED ON THE SURFACES OF CYLINDER BLOCK AND CYLINDER HEAD. THEY INCREASE THE OUTER CONTACT AREA BETWEEN A CYLINDER AND THE AIR. FINS ARE, GENERALLY, CASTED INTEGRALLY WITH THE CYLINDER. THEY MAY ALSO BE MOUNTED ON THE CYLINDER.
FORCED AIR COOLING: - IT IS A WELL-KNOWN FACT THAT THE RATE OF HEAT TRANSFER DUE TO CONVECTION INCREASES WITH AN INCREASE IN THE VELOCITY OF THE AIR FLOWING OVER A HOT BODY. THIS INCREASE MAY BE 55% TO 60% WHEN THE AIR VELOCITY IS DOUBLED. TO UTILIZE GAINS OF THIS FACT, ARTIFICIAL MEANS ARE ADOPTED TO SUPPLY AIR AT HIGHER VELOCITY ON THE ENGINE. A FAN OR BLOWER IS USED FOR THIS PURPOSE, AND THE SYSTEM IS THEN KNOWN AS FAN AIR-COOLING.
ENGINE MOUNTED ON SUNNY ZIP, BAJAJ AUTORIKSHA FE AND RE, LML’S TRENDY, KINETIC HONDA, VANGUARD CAR ETC. EMPLOY THIS KIND OF COOLING.

SPECIAL PROVISIONS IN AIR-COOLED ENGINES
THE HEAT TRANSFER COEFFICIENT BETWEEN ‘A METAL AND THE AIR’ IS LOWER THAN THAT BETWEEN ‘WATER AND THE AIR’. THEREFORE, HEAT TRANSFER DUE TO AIR IS LESS THAN THE HEAT TRANSFER DUE TO LIQUID ON THE SAME METAL. HENCE, AIR- COOLED ENGINES REMAINS AT A HIGHER TEMPERATURE THAN THE WATER-COOLED ENGINES. TO KEEP THE TEMPERATURE OF AIR-COOLED ENGINES DOWN, SOME SPECIALIZED TREATMENTS ARE IMPARTED TO THEM. IMPORTANT TREATMENTS ARE GIVEN AS FOLLOWS.

·         INNER SURFACE OF THE CYLINDER IS SUPER-FINISHED AND POLISHED SO AS TO ABSORB MINIMUM AND REFLECT MAXIMUM HEAT.
·         EXHAUST VALVE AND VALVE SEAT ARE MADE WIDER, AND VALVE STEM AND VALVE GUIDE ARE MADE LONGER IN SIZE.
·         THE CROWN (OR TOP) OF THE PISTON AND THE VALVES ARE POLISHED SUCH THAT THE CARBON PARTICLES DO NOT STICK ON THEM AND THUS THE EXTRA SOURCE OF HEAT ABSORPTION IS AVOIDED.
·         THE EXHAUST VALVE IS MADE TO OPEN QUICKLY SO THAT MINIMUM HEAT IS TRANSFERRED TO IT FROM THE BURNT GASES.

FINS (OR SPINES):-
FOR QUICKLY AND EFFICIENT HEAT REMOVAL FROM THE ENGINE, ITS EFFECTIVE SURFACE AREA SHOULD BE LARGE. THIS IS ACCOMPLISHED BY MOUNTING A CONFIGURATION OVER THE ENGINE WHICH HAS AN ARRANGEMENT OF THIN AND TAPER SPINES. THESE ARE KNOWN AS FINS. THEY PROVIDED GREATER AREA OF CONTACT WITH AIR WITHOUT APPRECIABLE INCREASE IN WEIGHT OF THE ENGINE. FINS MAY BE CASTED, FORGED, WELDED OR SHRINK-FITTED ON THE ENGINE. THEY CAN BE ARRANGED CIRCUMFERENTIAL OR LONGITUDINALLY. USUALLY THEY ARE PROVIDED IN CIRCUMFERENTIAL DIRECTION ON THE CYLINDER BLOCK AND IN LONGITUDINAL DIRECTION ON THE CYLINDER HEAD.

HEAT FLOW CHARACTERISTICS OF FINS: - HEAT DISSIPATING CAPACITY OF FINS DEPENDS ON THEIR LENGTH AND THE CROSS-SECTION. TEMPERATURE AT THE BASE (ROOT) OF THE FINS REMAINS HIGHEST, WHICH GOES ON DECREASING ALONG THEIR LENGTHS AND BECOMES THE LEAST AT THEIR TIPS.

WATER COOLING SYSTEM
AIR-COOLING OF AN ENGINE TENDS TO BECOME LESS EFFECTIVE WHEN NUMBER OF CYLINDER INCREASES IN AN ENGINE. THIS IS BECAUSE OF REDUCED EFFECTIVE SURFACE AREA AROUND EACH CYLINDER. THIS INEFFICIENCY CAN BE OVER COMMAND BY USE OF WATER-COOLING SYSTEM ON MULTI CYLINDER ENGINES. UNLIKE AIR-COOLING SYSTEM, THE HEAT TRANSFER IN WATER-COOLING SYSTEM OCCURS MAINLY THROUGH WATER (A LIQUID) IN ADDITION TO AIR (A GAS). THUS ONE MORE STAGE IN TRANSFER OF HEAT IS ADDED. IN THIS WAY, WATER-COOLING OF AN ENGINE OCCURS IN TWO STAGES, VIZ.

·         HEAT TRANSFER FROM SOLID CYLINDER TO THE WATER
·         HEAT TRANSFER FROM WATER TO THE AIR.

WATER-COOLING PROVIDED A BETTER MEANS THAN AIR-COOLING, ALTHOUGH THE NUMBER OF SPARES AND ACCESSORIES INCREASES IN A WATER-COOLED ENGINE.
IN A WATER-COOLING SYSTEM THE HEAT IS TRANSFERRED TO THE LIQUID CONTAINED IN JACKETS SURROUNDING THE CYLINDERS. THIS LIQUID IS PASSED THROUGH A RADIATOR DESIGNED TO EXPOSE AS LARGE AN AREA AS POSSIBLE TO THE AIR. AIR IS CIRCULATED OVER THE RADIATING SURFACE BY MEANS OF AN ENGINE DRIVEN FAN AND BY THE FORWARD MOTION OF THE VEHICLE. THE HEAT IS CARRIED AWAY BY THE AIR STREAM. THE WATER COOLING SYSTEM ARE CLASSIFIED INTO TWO TYPES: 
        I.            FORCED (PUMPED) COOLING SYSTEM
      II.            THERMO-SIPHON COOLING SYSTEM.


1 comment:

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