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Basic Engine Construction
• Components and Function – Engine block
– Cylinder head
– Combustion chamber
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Engine Cylinder Block
The cylinder block is the basic frame of a liquid-cooled enginewhether it be in-line, horizontally opposed, or V-type. The
cylinder block is a solid casting made of cast iron or aluminum
that contains the crankcase, the cylinders, the coolant passages,the lubricating passages, and, in the case of flathead engines,
the valves seats. the ports, and the guides.
The cylinder block is a one-piece casting usually made of an iron
alloy that contains nickel and molybdenum. This is the best
overall material for cylinder blocks. It provides excellent wearing
qualities, low material and production cost, and it only changes
dimensions minimally when heated. Another material that is usedfor cylinder blocks, although not extensively, is aluminum.
Aluminum is used whenever weight is a consideration.
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Engine Block
• Contains:
– Cylinder bores – Pistons and rings
– Connecting rods
– Crankshaft – Bearings
– Timing gears
– Oil pan and filter
– Camshaft and lifters (pushrod engines)
– Flywheel or flexplate
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Engine Block
• Cylinder bores:
– Round hole cast in block• Bored to a precise diameter
• Honed to smooth finish
– Guide for piston
– Contains forces generated during 4-stroke
cycle – Some engines use steel sleeves
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It forms the base of the engine. Two types of cylinder
blocks are used in vehicles.
Single piece casting:
In this type cylinder block and crankcase are cast as
one piece.
It gives better rigidity and it is easy to cast, which
reduce the cost of manufacturing.
Two piece casting:
In this type the cylinder block and crankcase are cast
separately.
The crankcase is bolted to cylinder block.It reduces the problem of lifting the cylinder block from
the crankcase, during repairing or overhauling.
This type of casting is used in heavy generating sets.
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ARRANGEMENT IN CYLINDER BLOCK
• The cylinder block is made of cast iron or aluminiumalloy.
• Inside the cylinder block, passages for the coolent andlubracating oil are provided.
• The cylinder head along with the valve assembly is fittedon the top of the cylinder block by nut and bolts.
• The oil sump is bolted to the cylinder block/ crankcasefrom the bottom.
• The crankshaft is supported on the split bearings.
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• One half bearing is fixed on the web which is cast withthe cylinder block, the other half bearing is fixed in the
bearing cap.• The bearing cap is fastened with the web by nuts and
studs.
• This portion where the crankshaft is fixed is known asthe crankcase.
• In the cylinder block space is provided for the camshaftand camshaft bearings, push-rod, tappets etc.
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The cylinder block also provides the foundation for the coolingand lubricating systems. The cylinders of a liquid-cooled
engine are surrounded by interconnecting passages cast in
the block. Collectively, these passages form the water jacketthat allows the circulation of coolant through the cylinder block
and the cylinder head to carry off excessive heat created by
combustion.The water jacket is accessible through holes machined in the
head and block to allow removal of the material used for
casting of the cylinder block. These holes are called core
holes and are sealed by core hole plugs (freeze plugs). These
plugs are of two types: cup and disk.
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Cylinder Block:The cylinder block, also called engine block, is the main bottom end structure. Iron or
aluminum is normally used for its construction. Nickel may be added to the iron to
increase strength and wear.
Aluminum blocks weigh less and are better at dissipating heat. Core plugs (1), alsocalled freeze plugs, seal holes left in the block after casting. The plugs prevent the
leakage of coolant from the water jackets.
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Block Design:
The design of the cylinder block varies among manufacturers.
The block's geometry is determined by the number of cylinders(2), cylinder orientation (opposed, in-line, slant, V-shaped), and
camshaft location (overhead cam or in-block cam). Regardless
of design, the block must withstand the forces exerted by the
pistons and crankshaft, serve as a mounting structure for other
engine components, and be light in weight.
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Main Caps:
The main caps (3) secure the crankshaft to the engine block. When bolted in place,
they form the main bearing bore in which the crankshaft's main journals rotate. Two
or four bolts are used to secure the caps. Some blocks use a main cap girdle which
is a structure in which the caps rest. The caps and girdle are secured to the block.
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The cylinders are bored right into the block. A good cylinder
must be round, not varying in diameter by more than
approximately 0.0005 inch (0.012 mm). The diameter of thecylinder must be uniform throughout its entire length. During
normal engine operation, cylinder walls wear out-of-round, or
they may become cracked and scored if not lubricated orcooled properly. The cylinders on an air-cooled engine are
separate from the crankcase. They are made of forged steel.
This material is most suitable for cylinders because of its
excellent wearing qualities and its ability to withstand hightemperatures that air-cooled cylinders obtain. The cylinders
have rows of deep fins cast into them to dissipate engine
heat. The cylinders are commonly mounted by securing thecylinder head to the crankcase with long studs and
sandwiching the cylinders between the two. Another way of
mounting the cylinders is to bolt them to the crankcase, andthen secure the heads to the cylinders.
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CYLINDER LINERS
In automobile engine two types of liners are used.
• Dry type liner • Wet type liner
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Cylinder Sleeves:
Cylinder sleeves (1) or liners are
removable, cylindrical inserts. Two types
are used. The dry sleeve is separated from
the water jacket (2) by the cylinder wall (3).
The wet sleeve is in direct contact with the
coolant and has thicker walls since nooutside structure provides support. Sleeves
are typically cast iron.
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Dry Sleeves:
Dry sleeves are thinner in construction since structural support
is provided by the engine block cylinder wall. The loads
generated by combustion pressure are absorbed by the engine
block and not the sleeve. Dry sleeves can be pressed into olderengines to repair damaged cylinders.
Wet Sleeves
Wet sleeves are designed to withstand the loads generated by
combustion pressure. The sleeves are in direct contact withengine coolant. A rubber or copper O-ring (yellow circle) is used
at the bottom flange to prevent coolant from leaking into the
crankcase. The cylinder head gasket prevents any coolant
leaking through the top.
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AIR COOLED ENGINE
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• Cylinder Barrel
• ___________________ or nickel-molybdenum
steel• Used to guide and seal piston and to mount
cylinder assembly to head
• Barrel _______ into head to form cylinderassembly
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Cylinder interior wall
Cylinder Walls
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• Cylinder Walls• Inside surface of cylinder barrel is _________
to a controlled amount of roughness
• Rough enough to __________ but smoothenough to minimize friction and wear
• Plain steel cylinder walls are not _______ to
prevent wear or corrosion• Nitrided cylinder walls are _________ to reduce
wear but still rust as easily as plain steel walls.
Nitriding is exposing the cylinder wall toammonia at high temperatures and it hardensthe wall to a thickness of approximately .005”
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• Chrome cylinder wallsuse ______________ toresist wear and provide
a corrosion resistantsurface.
• Cylinders may be
chromed back to __________ insidedimensions if they
become worn• Chrome is too smooth to
hold oil without etching
or channeling during the ____________________
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• Cylinder wall “CHOKE”
• The cylinder wall is
tapered inward
towards the top so that
as the engine warms
up, the hotter top of
the wall expands more
than the bottom,creating a round barrel
at operating
temperature.
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Combustion Chamber
• Area formed by bottom of cylinder head and top of piston
• Air and fuel mixture is compressed and ignited in
chamber and applies force from combustion to piston
• Spark plug protrudes into chamber
• Chamber is sealed by:
– Valves – Piston rings
– Head gasket
• Chamber is exposed to very high pressures and hightemperatures
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Types of Combustion Chambers
Hemispherical or Pentroof - The "Hemi" Combustion Chamber
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A chamber of this design is considered to offer the least
amount of compromise for the efficiency gained.
Effectively, a hemispherical combustion chamber is one half of
a sphere cast into the bottom of the cylinder head. The valves
are placed at the outside of the bore area and at a specificangle from the crankshaft centerline. Optimizing this position
allows for huge airflow gains, since it moves the valve away
from the wall to keep it from creating turbulence.
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This creates a more efficient cross-flow movement of thecharge during valve overlap and limits thermal transfer from
the exhaust valve to the fresh charge. As mentioned
previously, this design offers the best surface-to-volume
ratio and also creates a very short direct exhaust port,essential in limiting heat rejection into the coolant.
Hemispherical chambers generally have a central sparkplug, which offers excellent octane tolerance. An additional
benefit is the distance between the intake and exhaust
valves, which further limits heat transfer.
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Wedge-Shaped Chambers
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Used over the years by almost every manufacturer, this type
of chamber resembles an inclined basin recessed into thedeck of the head. Inline valves are normally tilted to
accommodate the sloping roof of this design. The spark plug
is located on the thick side of the wedge and is usually
positioned midway between the valves.
The relatively steep walls in such chamber design force the
air/fuel flow path and deflect and force it to move in adownward spiral around the cylinder axis. During the
compression stroke, the compressed air/fuel area reduces to
such an extent that the trapped mixture is violently thrust fromthe thin to the thick end of the chamber. This builds up
significant kinetic energy, which when ignited contributes to
overall power.
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Bathtub or Heart-shaped Combustion Chamber
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The bathtub designation is generally reserved for any
chamber that's not a wedge or hemispherical. Most domestic
engines of pushrod design have used it in varying forms.In some instances the shape of the combustion chamber
was almost oval, with later trends being the more efficient
heart shape.
The valves are inline and partially masked by the chamber
wall, being more exposed on the plug side. The area across
from the major squish region is generally tapered and doesnot have the steep wall of a wedge style. Spark plug location
is maximized by biasing toward the exhaust valve and as
central as possible, making the overall design very efficientfor producing power. However, heat transfer from the close
proximity of the valves limits volumetric efficiency and
octane tolerance
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Bowl in Piston
Bowl & Piston with flathead on right.
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• Cylinder heads• Constructed of cast
aluminium
• Provides combustion
chamber, and mounting
areas for spark plug andvalve parts
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• The cylinder head is designed to transfer heatby conduction to the fins and then from the
fins to the air by convection
• The exhaust side of the head has the mostfins as it runs the hottest
• The head also may incorporate a drain linefitting to allow excess oil to return to the
crankcase (inter cylinder drain lines on
radials)
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• The crankcase is that part of the cylinder block below thecylinders. It supports and encloses the crankshaft andprovides a reservoir for lubricating oil. The lower part ofthe crankcase is the oil pan, which is bolted at thebottom. The oil pan is made of cast aluminum or pressedsteel and holds the lubricating oil for the engine. Sincethe oil pan is the lowest part of the engine, it must bestrong enough to withstand blows from flying stones andobstructions sticking up from the road surface.
• The crankcase also has mounting brackets to support theentire engine on the vehicle frame. These brackets areeither an integral part of the crankcase or are bolted to itin such a way that they support the engine at three orfour points. These points are cushioned by rubbermounts that insulate the frame and body of the vehiclefrom engine vibration. This prevents damage to enginesupports and the transmission.
Crankcase
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• The crankcase is the basic foundation of all air-cooledengines. It is made as a one- or two-piece casting that
supports the crankshaft, provides the mounting surfacefor the cylinders and the oil pump, and has thelubrication passages cast into it. It is made of aluminumsince it needs the ability to dissipate large amounts' ofheat. On air-cooled engines, the oil pan usually is madeof cast aluminum, and it is covered with cooling fins. Theoil pan on an air-cooled engine plays a key role in the
removal of waste heat from the engine through itslubricating oil.