Presentation on theme: "Cylinder Liners Liners are cylindrical components that fit inside the cylinder bore."— Presentation transcript:
1Cylinder LinersLiners are cylindrical components that fit inside the cylinder bore.
2Purpose of liners:Liners are provided in order to increase the service life of the engine i.e., wear resistant surface for boreIt simplifies the production of cast iron enginesMaterial used:Liners are made of cast iron and special alloys of iron containing silicon, manganese, nickel and chromium.There are two types of liners Dry liner and Wet liner
3Dry linerIt is in the form of a barrel having flange at the top which fits in the grooves of block.It is not directly in contact with water. Hence it is called dry liner. It is machined from both the sides.Wet linerIt is in the form of barrel shape provided with flange at top which fits into the grooves of the cylinder block.Grooves are provided to cylinder or liner. Rubber packings are inserted into the bottom grooves.These are having direct contact with water. Hence called as wet liners.Machining from both the sides are not required because it does not bear against the cylinder block.
5Comparison of dry liners with wet liners Sl. No.DescriptionDry LinersWet Liners1.Contribution of rigidity of cylinder blockMore rigidityLess rigidity2.Introduction of thermal barrier at the adjoining surfacesMoreLess3.CoolingInferiorBetter4.Renewal after wearComparatively difficultEasy5.Chances of coolant leakage
9Functions of a PistonTo transmit the force of explosion to the crankshaftTo form a seal for high pressure gasses in the combustion chamber do not escape into the crankcaseTo serve as a guide and a bearing for small end of the connecting rod
10Piston must satisfy the following conditions Silent in operationRigidity for high pressureThe design should be such that seizure does not occurResistant to corrosion due to some products of combustion e.g. sulphur dioxideIt should have shortest possible length so as to decrease overall engine sizeLighter in weight so that inertia forces created by its reciprocating motion are minimizedHigh thermal conductivity for efficient heat transfer so that higher compression ratios may be obtained without the occurrence of detonation
12Top of the piston is called head or crown Some pistons head is provided valve relief.Pistons used in some high powered engines may have raised domes, which increases compression ratio as well as controls the Combustion.In some engines pistons may be specially dished to form the desired shape of the combustion chamber.
13Piston ‘Skirt’The part of the piston below the rings is called ‘Skirt’.Its function is to form a guide suitable for absorbing the side thrust produced on account of the inclination of the connecting rod.It must be of sufficient length to resist tilting of the piston under load.The combustion pressure from the piston crown is transmitted to the connecting rod through the ‘webs’ inside the piston.The webs also form heat path from the piston crown to the gudgeon pin bosses and the skirt. The ‘Bosses’ form a bearing surface for the rocking motion of the connecting rod.
14Piston MaterialsCommon materials used are cast iron, cast aluminium, forged aluminium, cast steel, forged steel and alloys of aluminium.Cast iron is the ideal material for rubbing surfaces of piston.Cast iron and steel have the high strength required for good wearing qualities at required temperature and low thermal expansions.But heavier pistons than aluminium and low thermal conductivityAluminium alloy pistons have the advantage of low weight and high thermal conductivity which makes it to run cool.But it is less strong and hence thicker sections have to be used.
15Since it is softer fine particles of lubricating oil get embedded on it. Piston life gets shortened due to abrasionExpansion is about 2.5 times that of the cast ironStrength also reduces as temperature risesDue to unequal coefficient of expansion from the cylinder if large cold clearance is kept the ‘piston slap’ would occur.
16To prevent heat from the piston skirt following methods are adopted By providing horizontal slotBy providing inclined slot in the oil ring grooveBy making a heat damBy vertical slotBy T-slotBy tapered pistonUsing special alloysWire wound pistonsAutothermic pistonBi-metal pistonAeconoguide piston
19Wire wound pistons: Autothermic piston: A band of steel wire under initial tension is put between thepiston pin and the oil control ring, thus restricting the expansionof the skirt .Autothermic piston:This type of piston contains low thermal expansion steel insert at the piston pin bosses. These inserts are so moulded that theirends are anchored in the piston skirt as shown.
20Bi-metal piston: Aeconoguide piston: In this type of piston skirt is formed by steel and the aluminium alloy cast inside it forms piston head and piston pin bosses.As the coefficient of thermal expansion for steel is quite small, the piston will not expand much and hence smaller cold clearances can be maintained.Aeconoguide piston:This is a method to reduce skirt friction. The skirt contact area is reduced by about 75% compared to conventional pistons.And consist of raised pads which are specially shaped to assist the hydrodynamic lubrication.
22Damaged piston pin boss Worn out Circlip groove Piston FailuresScoringBurnt PistonScuffingDamaged Ring landDamaged piston pin bossWorn out Circlip groove
23Piston Failures Scuffing : Scoring: Damage to Ring land: This occurs due to excessive heat, the piston expands and becomes tight in the cylinder. As a result lubricant is squeezed out from the cylinder walls and metal to metal contact takes place.Scoring:Piston is scored as a result of carbon build-up. Accumulation of carbon and other deposits on the piston skirt. Particles of carbon breaking away from the exhaust ports, lodging between the piston skirt and cylinder results in scoring the piston.Damage to Ring land:This occurs mainly because excessive ring groove clearance andattempting to remove the piston without first removing the cylinder ridge.Damage to piston pin boss and circlip groove:This occurs rocking motion of the connecting rod due to bent connecting rod or tapered crankpins or loosely installed circlip
24PISTON RINGSThese are circular rings and made of special steel alloys which retain elastic properties even at high temp.These are housed in the circumferential grooves of piston outer surfaceMain function of the piston ring is to impart the necessary radial pressure to maintain the seal between piston and the cylinder bore.
25It prevents gas leakage into the crank case It prevents gas leakage into the crank case. It has contacts with cylinder walls evenly and tightly fit into the grooves.It is made by fine grained cast iron. Modern rings are made by steel.Types of piston rings:Compression rings or Pressure ringsOil control rings or Oil scraper ringsThe compression rings are located at the top portion of the piston.Two or three compression rings are fitted in order to increase the compression ratio and one or two oil rings to remove the excess oil from the cylinder walls.
26Functions of compression rings To seal between the piston and the cylinder liner.To transfer the heatTo absorb piston fluctuations due to side thrust.Any number of piston rings.The heat transfer is better from rings to the liner.It is better to use a number of narrow rings than a few wide shallow rings.
28Functions of oil control rings To scrape the lubricating oil from the surface of the liner.During the upward stroke these should allow sufficient oil to go upwards for the proper lubrication of the liner.
29Types of piston ring Ends Butt cutAngle cutSquare step cut joint or LapRound step cut joint
30CONNECTING ROD Manufactured by drop forging Should have adequate strength, stiffness with minimum weightMaterialShape of connecting rod:RectangularCircularTubularI-sectionH-section
31Connecting Rod There are 2 types of small end and big end bearings Split at right angle to its lengthSplit at an angle
36The Front end of the crankshaft carries three devices- gear or sprocket that drives the camshaft, vibration damper to control torsional vibration and the fan belt pulley.Rear end of the crankshaft carries fly wheel.Crank shaft have drilled oil passages through which oil can flow from the main bearings to the connecting rod bearing
38Valve is a device to admit the Air-Fuel mixture and expel gases from cylinder. Inlet valve is made by Nickel Chromium alloy steel and Exhaust valve is made by Silichrome steel.The face of the valve should be ground to make an angle 450 to 300 in order to match the angle of valve seat in the head or block.Valves used in modern vehicles are treated as mushroom valves.There are two types of valve mechanisms namely“Straight mushroom valve mechanism” and“Overhead mushroom valve mechanism”
40As crank shaft rotates, cam shaft rotates will also rotates there by rotating cams mounted on it. Cams lift the push rod by valve lifter.Push rod actuates the rocker arm and opens the valve.Valves are mounted in the cylinder head and valve clearance is in between rocker arm and valve stem.
42When the crank shaft rotates cam shaft rotates, valve lifter slides up and down due to rotation of cams.When the valve lifter moves upwards valve stem moves upward which operates the valves.Valve mechanism is provided in the engine block.Here valve clearance is between valve stem and valve lifter
44To see how valve-timing works in a 4-stroke engine cycle, let’s show piston motion as a circle. In this simple cycle, each stroke is shown as a semi-circle.The intake valve opens at top dead center, and closes at bottom dead center. The blue line shows that period and it matches the intake stroke.The exhaust valve opens at bottom dead center, then closes at top dead center before the new air-fuel mixture enters the cylinder.In practice, the intake valve usually opens earlier than top dead center, and stays open a little past bottom dead center.The exhaust valve opens a little before bottom dead center, and stays open a little past top dead center.When the valves actually open and close, can be measured by angles. To make these angles easier to read, let’s use a spiral instead of a circle.
45This intake valve opens 12° before the piston reaches top dead center. And it closes 40° after bottom dead center.The exhaust valve opens 47° before bottom dead center - and stays open - until 21° past top dead center. This gives exhaust gases more time to leave.By the time the piston is at 47° before bottom dead center on the power stroke, combustion pressures have dropped considerably and little power is lost by letting the exhaust gases have more time to exit.When an intake valve opens before top dead center and the exhaust valve opens before bottom dead center, it is called lead.When an intake valve closes after bottom dead center, and the exhaust valve closes after top dead center, it is called lag.On the exhaust stroke, the intake and exhaust valve are open at the same time for a few degrees around top dead center. This is called valve overlap. On this engine, it is 33°.Different engines use different timings. Manufacturer specifications contain the exact information.
47Exhaust Port opens before piston reaches BDC and after sometime inlet valve opens. During next stroke of the piston, first inlet valve closes then exhaust valve closes. After exhaust port closes compression of fuel-air mixture takes place