4 stroke cycle spark ingition engine

7/30/2019 4 Stroke Cycle Spark Ingition Engine

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The Four-stroke-cycle spark-ignition (petrol)engine

1.Induction Stroke: The inlet valve is open. The

movement of the piston creates suction pressure thatinduces (sucks in) fresh charge of air and atomisedpetrol.

2.Compression stroke: Both the inlet and exhaustvalves are closed. The piston moves upward. Thecharge is progressively compressed to 1/8 to 1/10 ofthe original volume, increasing the charge pressure

and temperature.3.Power stroke: Both the inlet and the exhaust valvesare closed. Just before the piston reaches the TDCduring the compression stroke, a spark plug ignitesthe charge. When the piston reaches the TDC, thecharge begins to burn, rapidly raising the pressureand temperature and forcing the piston to movedownward in the power stroke.

4.Exhaust Stroke: At the end of the power stroke, theexhaust valve opens. Because the cylinder pressureis much higher than atmospheric (about 4 bars). Theremaining burnt gases in the cylinder will be pushedby the movement of the piston upward in theexhaust stroke.

The Four-stroke-cycle compression-ignition(diesel) engine

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The induction and exhaust strokes are the same.3. Compression Stroke: Both the inlet and the exhaustvalves are closed. The piston moves upward. The chargeis progressively compressed to 1/12 to 1/24 of its original

volume, raising the pressure to 30-50 bars.4. Power Stroke: Just before the end of the compressionstroke, diesel fuel is injected, vaporised by the heatedcharge. The mixture is ignited. The burning of themixture raises the pressure inside the cylinder veryrapidly and forces the piston to move away from thecylinder head.

Comparison between the spark-ignition andcompression-ignition engines

Thermal efficiency: Petrol engines can have thermalefficiency ranging between 20% and 30%. Diesel engineshave improved efficiencies, between 30% and 40%.

Noise: Diesel engines are noisier. The combustionprocess is quieter in the petrol engine and it runssmoother than the diesel engine.Cost: Due to their heavy construction and injectionequipment, diesel engines are more expensive thanpetrol engines.

The two-stroke-cycle petrol engine

1.Induction and exhaust stroke: The piston movesdown the cylinder and initially uncover the exhaustport (E) releasing the burnt gases to the atmosphere.Such a movement also compresses the charge in thecrankcase. Further movement of the piston uncoverthe transfer port (T) allowing the compressed mixtureto be transferred to the inside of the cylinder pushingout any remaining of the burnt gases.

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2.Compression and power stroke: The piston moves inthe direction of the cylinder head, sealing off all portsand compressing the mixture. Further movement ofthe piston increases the volume in the crankcase,

creating suction so when the inlet port is uncoveredfresh charge is sucked in the crankcase. Just beforereaching the TDC, a spark plug ignites thecompressed mixture, raising the pressure andtemperature of the mixture very rapidly. The burntgases expand forcing the piston to move down thecylinder.

Comparison of the two and four-stroke cycle petrolengines

Theoretically, the two-stroke engine should develop twicethe power of the four-stroke engine for the same cylindersize but actually the factor is 1.3 because the induction

exhaust stroke of the two-stroke engine is less effective.The cooling load is greater for the two-stroke engine andit is thermally less efficient than the four-stroke engine.

The two-stroke engine has fewer working parts so it ischeaper to manufacture.

Two-stroke cycle diesel engine:

The piston moves away from the cylinder head and whenit is half way down its power stroke the exhaust valvesopen allowing the burnt gases to escape through theexhaust valves. When the piston moves down near theend of the power stroke, the inlet ports are uncovered,allowing fresh air from the blower to be admitted. Thepiston moves upward toward the cylinder head sealingoff the inlet ports and helping the fresh air to push anyremaining burnt gases through the exhaust valves andthe exhaust valves close. The piston continue to move

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upward compressing the charge of air and raising thetemperature and the pressure to about 30 to 40 bars.Before the piston reaches the TDC fuel is injected intothe charge. The heated charge vaporises the fuel and

ignites it. The rapidly burning mixture raises the pressureand temperature very rapidly inside the cylinder andforces the piston to move away from the cylinder head inthe power stroke.

Piston & Connecting-rod assemblies

Piston-Ring action:Piston ring can be divided into two groups:i) Compression rings, whose function is to seal the

space between the piston and the cylinder wall sothat gas can not escape.

ii) Oil-control rings, whose main purpose is to controlthe amount of lubricant passing up to the top of the

cylinder walls.

Compression-ring action:

The piston ring is designed to expand radially outwardwhen fitted in its groove so the ring will tend to springoutwards to apply pressure on the cylinder wall.On the upward compression stroke, the compressedcharge will move between the groove and the ring sidefaces, press the ring against the lower groove of thepiston. This provide a very effective compression sealwithout leakage.On the downward power stroke, piston acceleration isgreater than that of the ring so that the upper grooveand the ring side faces will be held firmly together toform the seal.

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Oil-Control-ring action

During the crankshaft rotation, oil is splashed from thebig end bearing on to the cylinder walls. The oil control

scraper ring performs two functions: first, it regulates theamount of oil passing to the combustion chamber andsecondly, it distributes a film of oil over the cylindersurface to lubricate the cylinder wall and thecompression rings.On the pistons upward stroke, the lower face of the ringwill be held firmly against the lower groove so that the

upper face of the ring will scrape a proportion of the oil.The excess oil will accumulate in the clearance space ofthe groove until it overflows through the drillings to thesump.On the piston downward movement the ring will snapover to the top of the ring groove. The sharp edge of theworking face of the ring will scrape the oil down the

cylinder wall. The surplus oil accumulates in the space ofthe groove and overflows through the drillings.

Piston and piston-ring working clearances

Piston-ring side clearance:Is the gap between the ring and land side faces. With

insufficient clearance, the expansion of the lands willwedge the rings in their grooves and could destroy the oilfilm and cause overheating. A lose fit will cause the ringto flutter. This hammers the ring against the groovefaces, producing rapid groove wear. Ring side clearancecan be checked by removing the ring from the piston androlling it around the outside of the piston in its groove,suitable size of feeler gauge can be slipped between thering and the groove to check the clearance.

Typical minimum ring-side clearances for pistonsbetween 6 and 12 cm in diameters are as follows:

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Compression ring: Petrol 0.05 mm, Diesel 0.06 mmOil control ring: Petrol 0.04 mm, Diesel 0.04 mm

Piston-ring joint clearance

A clearance must be allowed at the piston-ring joint tocompensate for the expansion, which takes place fromcold to hot working temperature. Insufficient clearancewill cause the ring ends to buckle, expanding the ringagainst the cylinder wall, affecting the oil film and cancause overheating. Rings with large gaps may cause a

loss of compression.Typical minimum ring-joint clearance are as follows:Water-cooled four-stroke engines 0.03 mm per cmdiameter. Air-cooled four-stroke engines 0.04 mm per cmdiameter.

Piston-skirt-to-bore clearance

The correct clearance between the skirt and the cylinderwall is necessary to eliminate piston slap when theengine is cold. To check the clearance, insert the feelerblade into the cylinder bore for its full length, then slidethe corresponding piston into the bore so that it traps thefeeler blade at its largest diameter then hold the pistonand pull the feeler.

Piston and connecting-rod Gudgeon pin Joints

The piston and connecting-rod are coupled together by aGudgeon pin , which is supported in holes bored in thepiston at right angles to the piston axis at about mid-height position, the central portion of the Gudgeon pinpasses through the connecting-rod small-end eye.

To secure the connecting-rod and Gudgeon-pin inposition, the connecting-rod small-end faces are polished

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with emery cloth and heated evenly with an Oxy-accetylene torch (230 320 C) then the Gudgeon-pin isforced through both the piston and the small-end eyeuntil it is centrally positioned. The small-end then cools

and shrinks tight over the pin.

Crankshaft Construction

Main Journals: are the parallel cylindrical portions, whichare supported by the plain bearings.Counterbalance Weights: are attached or integrated with

the crankshaft. Their function is to counteract thecentrifugal force created by each individual crankpin andits webs.Crank-webs: The cranked arms of the shaft, whichprovide the throws of the crankshaft are known as crank-webs. Their purpose is to support the big-end crankpin.

The flywheel: The flywheel serves three main purposes:

i) to support the clutch assembly and to transmit thedrive between the crankshaft and the gearbox bymeans of friction between the friction face of theflywheel and the clutch driven-plate.

ii) To provide a carrier wheel for the ring gear whenthe engine to be started.

iii) To store energy on the power stroke so that it willbe given out on the three idle strokes, this beingnecessary to reduce crankshaft speed fluctuationthroughout each cycle of operation.

Valve Timing Diagrams and Camshaft Drives:

The inlet valve opens before TDC for the followingreasons:

1.To prevent excessive cam-follower shock loads andspring vibration, the inlet valve is opened verygradually.

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2.Due to its inertia, it takes some time for thecharge to enter the cylinder after the inlet valveopens.

3.To make use of the partial depression in the

combustion chamber caused by the outgoing gases.

The inlet valve closes after BDC

To maximize the air charge entering the cylinder andmake use of the inertia of the incoming fresh chargeparticularly at medium to high engine speed. At low

speed, the inertia is insufficient to oppose the upwardmoving piston so that a portion of the newly arrivedcharge will actually be pushed back and return to theinduction manifold.

Exhaust valve opens before BDC for the followingreasons:

1) Toward the end of the power stroke the burningprocess slows down because the burnt gasessuffocate and prevent the mixing and burning of theunburnt charge so that the loss of power is small.

2) To take advantage of the kinetic and pressureenergy of the exhaust gases to clear the cylinderbefore the end of the power stroke.

Exhaust valve closes after TDC

To take advantage of the exhaust gases. The momentumof the outgoing exhaust gases leave a vacuum thatinduces fresh charge to enter the cylinder andsimultaneously push any remaining exhaust gas out.

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Camshaft with push-rod and rockers

That valve-mechanism is made up from the followingcomponents:

a) a camshaftb) a cam followerc)a push rodd) a rocker arme) a rocker shaftf) a return springg) a poppet-valve

Cooling Systems

Engine heat distribution and the necessity for a coolingsystem

The energy released from the combustion of fuel in thecylinder is dissipated in roughly three ways:

35-45% heat energy doing useful work on the piston.30-40% heat expelled with the exhaust gases22-28% heat carried away by heat transference

The importance of the cooling systems

If the cooling was not effective, the heat-flow ratethrough the metal will be low and the temperature of theinner surfaces will rise to a point where the heat destroysthe lubricating properties of the oil film on the cylinderwalls. Simultaneously, thermal stresses will beestablished, which may distort the cylinders.

Methods of Heat Transfer

1) By conduction through solids or stagnant fluids

( )21 TTK

AQcond =

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whereA is the area of heat transfer, Kis thethermal conductivity of the material, is the thicknessof the material, and T1-T2 is the temperature difference.2) By convection through the movement of fluids

)( 21 TTAhQconv=

whereA is the area of heat transfer, h is the heattransfer coefficient, and T1-T2 is the temperaturedifference.3) By radiation (no medium is required)

)( 424

11211 TTFAQrad =

whereA1 is the surface area of the body emitting theradiation, T1 is the temperature of the body emittingthe radiation, T2 is the temperature of the bodyreceiving the radiation, 1 is the emissivity of the bodyemitting the radiation, F1-2 is the view factor from body1 to body 2, and is Steven Boltezmann constant =5,67 10-8 W/m2 K4

Types of cooling systems

i) Direct air-cooling, where cool circulating air ismade to come in contact with the exposed andenlarged external surfaces of the cylinder andhead and thereby dissipate their heat to thesurrounding air.

ii) Indirect cooling (liquid cooling), where a liquidcoolant is used to transmit the heat from thecylinder and head to the radiator. Movement ofair through the radiator then extracts anddissipates the unwanted heat to thesurroundings.

Direct air-cooling system

If direct air-cooling is to be used, the surface area ofthe outside walls of both the cylinder and the head

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must somehow be enlarged to anything fromfive to fifteen times the plain cylindrical surface area.Fins are used to increase the external surface area ofthe cylinder. The length of these fins will be greatest

where the cylinder is hottest-near the cylinder headand will progressively reduce toward the cooler-operating crankcase.

Description of an air-cooled system

Air-cooled engines mounted on a motorcycle frame areusually exposed to the surrounding atmosphere. Theyrely on the natural air stream caused by the forwardmovement to circulate air around the cylinders, head,and crankcase.For multi-cylinder engines, controlled air-cooling is

usually achieved by incorporating a fan, which blowsfresh air over the external finned surfaces of theengine. To improve the effectiveness of the blown air,the sides of the finned cylinders and heads areenclosed by a sheet metal. The shape of the sheetmetal guides the forced convection current around allthe cylinders and provide a direct exit after the air hasextracted and absorbed the heat from the engine.

Heat transfer in an indirect liquid-cooled enginesystem

The heat released from the burning of the atomisedmixture of air and fuel is transferred in all directions tothe metal walls of the combustion chambers, cylinders,and pistons by direct radiation, by convection currentsand then by conduction through a stagnant boundarylayer of gas and a film of oil to the metal walls.

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Due the difference in temperature between theinner and outer cylinder walls, heat will be conductedthrough the metals. It is then further conductedthrough a thin stagnant boundary layer of liquid to the

coolant liquid in the passages around the cylinders.

Thermo-syphon liquid-cooling system

As the liquid around the cylinders receives heat itexpands and becomes less dense relative to liquid whichis not in contact with the hot metal walls; therefore, the

lighter hot coolant will rise to the highest point of thesystem, which is the header tank over the radiator tubes.At the same time, the liquid in the radiator will be cooledby the air stream passing around the tubes and over thefins, consequently the density of the liquid in the tubeswill increase so that the cooled liquid sinks to the bottomand replace the hot and less dense liquid in contact with

the cylinder walls; thus, a convection current flowsbetween the engine and the radiator and so forms anenclosed circulating loop known as the thermo-syphoncooling system.

Description of a liquid-cooled system

Radiator: the radiator transfers the heat absorbed by theliquid coolant to the surrounding air. The radiatorconsists of columns of spaced-out copper or aluminium-alloy tubes held in position at the ends by an upperheader tank and a bottom tank. Attached to these tubesare layers of horizontal copper or aluminium sheetsknown as fins. These sheets improve the effectiveness ofair-convection heat dissipation. The upper header tank

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distributes the collected hot liquid evenly among thevertical tubes. The bottom tank collects the cooled liquidcoolant from each tube and passes it to the enginescoolant passages surrounding the cylinders.

Flexible hosing: The flexible hoses are necessary toabsorb the relative movement between the radiator,which is bolted to the body and the suspended engine,which tends to vibrate while operating.Coolant Jackets: are passages for the coolant wateraround the combustion chamber walls, the inlet andexhaust ports and their valve seats, and the spark-plug

or injector holes.Fan: to provide a continuous air stream over the tubesand fins to dissipate the heat being circulated by thecoolant.

Limitations of the thermo-syphon cooling system

a) Under heavy load condition, the rate of thecoolant circulating cannot match (much less) the rateof heat transfer from the cylinder walls to thecoolant.

b) Without coolant-circulation control, the enginetends to be overcooled and very rarely reaches theoptimum operating temperature.

Forced-convection pump circulation

A centrifugal pump is incorporated to speed up the rateof coolant circulating and heat removal. With the forcedcirculation of coolant, the coolant is uniformly distributedamong all cylinders. This helps to prevent overheating ofindividual cylinders. Increasing the coolant flow rateenables the radiator to work more efficiently so it isreduced in size.

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The cooling process is controlled by placing athermostat valve in series with the top hose. When theengine is cold, the valve is closed-this prevents the bulkof the liquid circulating . Once normal working conditions

are reached, the thermostat automatically senses thedesired working temperature and opens the valve andbulk circulation begins.

To prevent excessive pressure build up in the enginecoolant passages, a bypass pipe circulate about onetenth of the liquid directly between the cylinder-headthermostat housing and the inlet side of the pump.

Comparison of air-and liquid-cooling systems

Air-cooling:Advantages:

a) Air-cooled engines operate extremely well in bothhot and cold climates

b) Air-cooled engines rapidly reach their workingtemperature from coldc)Air-cooled engines are lighter than similar-sized

liquid-cooled enginesd) Air-cooled engines have no coolant leakage or

freezing problems.

Disadvantagesa) A relatively large amount of power is required to

drive the cooling fanb) The large quantities of intake air passing into the

cooling system can make the engine noisyc)The cooling fins can under certain conditions vibrate

and amplify noised) The pitch between cylinder centers has to be

greater than in liquid-cooled engines to permit thefins to extend between cylinders

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e) Each cylinder has to be individually castwhereas a rigid monoblock construction is used byliquid-cooled engines

Liquid Cooling

Advantagesa) Liquid-cooled engines provide greater

temperature-uniformity around the cylinders

compared with air-cooled engines.b) The combined power consumption of the coolantpump and the fan in liquid-cooled units is far lessthan that of the sir-cooled engine fan.

c)The liquid-cooled engine cylinders are situated closertogether, providing a very rigid and compact unitcompared with the air-cooled engine.

d) Mechanical noise from the engine is damped byboth the coolant and the jackets.

Disadvantages

a) Liquid-coolant joints are subject to leakageb) Precautions must be taken to prevent coolant

freezingc)Liquid-cooled units take longer to warm upd) The coolant passages tend to scale, and the

hoses and radiator tubes deteriorate with time.

Thermostat-controlled cooling systems

The function of the thermostat is to regulate heatdissipation by controlling the rate of coolant flow throughthe radiator. Engines are designed to operate mostefficiently over a narrow temperature range (usually

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between 80 and 100 C for coolant) because of thefollowing reasons:

a) The working clearances of rubbing componentssuch as piston rings and cylinder, the journals and

bearings, the valves and guides, etc. will not be idealuntil the engine is fully warmed up, so that bothnoise and wear will be accelerated during theheating-up period.

b) Large amount of condensed corrosive vapour inthe cold crankcase may contaminate the lubricatingsystem

c)An improved uniform air-fuel mixture will be formedat the optimum working temperature, so more usefulwork will be done during combustion

d) The optimum operating temperature willmaintain the lubricating oil at the correct viscosity,so that the rubbing parts have the best lubricantprotection.

Operation of the bellows-type thermostat

The thermostat is usually situated in front of the cylinderhead in a coolant-outlet housing, therefore, all thecoolant flowing through the top hose from the engine tothe radiator has to pass through the thermostat-valveassembly. The thermostat consists of a brass flexiblebellows partially filled with a volatile fluid such asalcohol. A poppet valve is attached to one end, and theother end is mounted in a brass frame, which fits into thecoolant-passage housing.While the engine is cold, the bellows lobes will contracttogether so that the poppet valve is closed and onlysmall amount of coolant circulate in the engine through abypass passage.Once the engine has warmed up, the heat will act on thebellows and the liquid will rapidly expand, pushing apart

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the individual bellows lobes so that the valve will beginto open. If the temperature continues to rise, thesubstance inside will change its state into gas, andfurther expansion of the bellows will result until the valve

is fully open.

Engine Lubrication System

A portion of power is called friction power is lost toovercome the resistance to relative motion of all themoving parts of the engine. This includes the frictionbetween the piston rings, piston skirt, and cylinder wall;friction in the big end, crankshaft, and camshaftbearings. Friction in the valve mechanism, friction in thegears, or pulleys and belts, which drive the camshaft and

engine accessories.

The Importance of Lubrication

The lubricant and lubricating system perform thefollowing functions:

1.Reduce the friction resistance of the engine to aminimum to ensure maximum mechanicalefficiency.

2.Protect the engine against wear.3.Contribute to cooling the piston and regions of

the engine where friction work is dissipated.4.Remove all impurities from lubricated regions.5.Hold gas and oil leakage at an acceptable

minimum level.

Lubricating System

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In that system, oil is circulated through thesystem using an oil-pump, which may be driven directlyfrom the crankshaft or indirectly from the camshaft orany auxiliary shaft. When the camshaft rotates, oil from

the sump is drawn through the submerged strainer andpick-up pipe to the pump. The oil is then compressed anddischarged through a drilling to the lubrication system.Control of the oil pressure is achieved by a pressure-relief valve situated on the output side of the pump. Ifthe oil pressure becomes too high, the relief valve willopen, bleeding any surplus oil back to the sump. From

the oil-pump, all the oil flows through drillings in thecrankcase to a cylindrical filter unit. The oil circulatearound the filter bowl, forces its way through the centerand flows out to the main oil gallery (the main oilpassage). By various branch cross-drillings in thecrankcase, oil is distributed to the crankshaft main-

journal bearings and to the camshaft bearing.

Main-and big-end bearing lubrication:

Continuous oil feed to the big-end bearings from the oilgrooves is provided by diagonal drillings in thecrankshaft.

Cylinder and Piston Lubrication

One of the common methods for cylinder and pistonlubrication is connecting-rod big-end radial-hole oil spray.In this method, through a small radial drilling in eachconnecting rod, a spray of oil is directed to the thrustside of the cylinder bore once every revolution.

Valve rocker-arm-mechanism lubrication

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An oil drilling from one of the camshaft bearingssupplies oil to the tappet-follower gallery drillings. Oilfrom this gallery flows through the hollow push-rod andto the rocker-arm.

Petrol-engine Carburetion Fuel System

Layout of a Petrol-engine Fuel System:

A fuel system fro a carburetted engine includes:

a) A fuel tank, which stores the petrol and has afuel-gauge sensor unit incorporated to indicate theamount of petrol in the tank.

b) A feed pump, which transfers the petrol from thetank to the carburettor.

c)A feed filter, which prevents any contaminatingparticles from passing into the carburettor.

d) An air-silencer and filter unit, which quietens thefast-moving air intake and prevents dirt fromentering the engine.

e) A carburettor, which merges air and petroltogether so that they mixed in the correctproportions and the petrol is finely atomised.

f) An induction manifold, which collects the preparedair-fuel mixture and distributes it to the various inletports in the cylinder head.

g) Supply and return pipelines.

Carburetion

Air and petrol mixture strengths: According to chemicalcombination requirements, the air-fuel ratio, which givescomplete combustion is 15. Rich mixtures, which containmore than the optimum amount of petrol, produce morepower than optimum. The maximum power of the engine

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can be obtained when the mixture is about 15-20%rich (air-fuel ratio between 12 and 13). Prolonged runningwith very rich mixture will result in forming a blackpowder on the cylinder walls and on the spark-plugs.

Weak mixture, which contain less than the optimumamount of petrol produce less power than optimum, butfuel economy is much better than for other conditions.For minimum fuel consumption, the mixture can be 15 to20% weak (air fuel ratio is 17 to 18). Burning is generallyslow and requires sufficient ignition timing advance tocompensate for this prolonged combustion period.

Single-jet Fixed-choke Carburettor

It is a vertical tube that is connected to a petrol reservoirthat has a float and a valve assembly as shown in thefigure. Dividing the two sides of the U-tube carburettor atthe base of the bent is a restriction orifice known as the

petrol jet, its function being to meter the amount ofpetrol flowing into the venturi. As the air flow through therestriction its velocity increases and its pressuredecreases so that the atmospheric pressure acts on thepetrol in the float chamber, pushing the petrol into theventure. As the petrol enters the venturi it willimmediately torn apart into various sizes of droplets sothat the petrol will be finely distributed throughout theair stream as it is drawn into the engine cylinders.

To control the speed and load output of the engine, abutterfly throttle valve is placed on the downstream sideof the venturi. The spindle of the valve is connected tothe accelerator pedal by a cable or levers. The quantityof charge entering the engine can be varied by thedegree of depression acting at the discharge nozzle,which depends on the angular position of the throttlespindle opening the butterfly valve. The function of the

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float chamber is to provide a reservoir of petrol ofconstant depth under steady-running conditions.

Limitation of the Single-Jet Carburettor

The quantity of air consumed by an engine in unit time isdirectly proportional to the engine speed, but due to theinertia of liquid flow, the rate at which petrol is drawn outof the discharge nozzle into the air stream increasesalmost with the square of the engine speed. Therefore, ifthe engine is designed to have the correct air-fuel ratio

at the design speed (2000 rpm in the figure), the enginewill have weak mixtures at speeds lower than the designspeed so that the limitation of the single-jet carburettoris that it does not meet the engine requirement for thecorrect air-fuel ratio at speeds that are lower or higherthan the design speed.

Capacity-well Compensation

During initial acceleration when the throttle is open, thepressure drop created at the venturi will draw fuel fromthe discharge nozzle at a far greater rate than can besupplied by the petrol jet alone, but the capacity well willprovide the extra fuel demanded for rich accelerationmixture. The level of fuel in the capacity well will dropquickly until the well is emptied. Petrol dropletssuspended in air will be formed at the base of the welland will prevent any more enriching of the mixture. Anyfurther increase in speed will only result in a constantamount of fuel flow from the compensation petrol jetsince the air passage bleed reduces some of thedepression created across the petrol jet. The limitation ofthe capacity-well compensation is that it is not flexibleenough under varying operating condition.

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Air-bleed and capacity- well compensation

This system uses one fuel jet and suspension tubesituated in the center of the capacity well. Under no-load

conditions, the fuel level in the well will be the same as inthe float chamber. With initial throttle opening, the fuel inthe well will be consumed; thus, providing an enrichedmixture. As the level of fuel in the well drops, it exposesthe uppermost of the suspension tube holes. This allowsmore air to enter the well and mix with the fuel, thuspreventing any tendency towards undue richness. As the

petrol level in the well drops further it allow more airbleed correcting the composition of the mixture.

Coil Ignition System

The combustible mixture of air and petrol is ignited by aspark occurring between two electrodes in thecombustion chamber at the end of the compressionstroke just before TDC. It is the function of the ignitionsystem to periodically provide a spark of sufficient heatintensity to ignite the mixture at the predeterminedposition in the engines cycle under all speed and loadconditions.

The voltage necessary to ionise the air between theelectrodes so that the spark will bridge the air gap canvary from as little as 500 volts when the gap is small andthe engine is hot, to a value of 20,000 volts when the

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spark-plug electrodes are badly eroded, the air gap islarge and the engine is cold.

Ignition-system equipment

Battery: This is usually a 6 or 12-volt battery. It storeschemical energy, which can be converted into electricalenergy to supply the flow of current through the ignitionsystem when required.Ignition switch: This switch is connected in series in thecoil primary-winding circuit. It enables the driver to

switch on or off the electrical supply of the battery asrequired to operate the ignition system.Ignition coil: It is an electrical step-up transformer, whichconverts the relatively low battery voltage to a high-intensity voltage.If the ignition switch is closed and the contact-breakerpoints are together, current will flow from the battery

through the primary winding and the earth-return pathback to the battery so that a magnetic field is produced,which interlinks both the primary and secondary winding.When the rotating distributor cam opens the contactpoints, the primary current falls very rapidly to zero andthe magnetic field also decays rapidly. Self-induction actsso as to oppose these changes and a very large backe.m.f. is induced in the primary winding. By transformerstep-up action, an even larger e.m.f. (200 times larger) isthus induced in the secondary winding and is fed to thespark-plug gap to produce a spark.Capacitor: The capacitor is connected in parallel with thecontact-breaker points, the surge current in the primarywinding when the contacts open finds an easier paththrough the capacitor so that the primary-current flowstops instantly and the back e.m.f. that is induced in theprimary winding will be much greater. When the contactsclose again, the charge stored in the capacitor charges

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into the primary winding and so helps to acceleratethe build-up of a new magnetic field in the primarywinding.Spark-plug: It periodically provides a spark of sufficient

heat intensity to ignite the charge mixture.

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4 stroke cycle spark ingition engine

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