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.
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