CHAPTER 6
Engine Components
Learning Objectives
• Identify the basic components of a small engine
and describe the function of each component.
• Describe engine block variations.
• Describe the construction and operation of the
crankshaft.
• Explain piston design considerations and
differentiate between types of piston rings.
• Describe connecting rod and bearing variations.
• Identify common valve train configurations.
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Engine Block
• Keeps engine parts in alignment
• Cast iron or aluminum alloy
• Consists of two sections:
– Cylinder block
– Crankcase
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Cylinder Block
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Contains cylinder bore
Cylinder may be bored directly in block
Aluminum cylinder blocks cast around steel sleeve
Cylinder head bolts to block or is cast as integral
part
• Cooling fins on block and head of air-cooled
engines
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Crankcase
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Contains crankshaft
Must withstand rotational forces of the crankshaft
May contain oil for lubrication
May be cast with engine block
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Crankshaft
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Major rotating part of engine
Converts reciprocating motion into rotary motion
Transmits engine torque
Drives camshaft
Supports flywheel
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Crankshaft
• Cast or drop forged steel
• Tapered end receives flywheel
– Flywheel keyed to crankshaft
• Crankshaft throw
– Offset portion of shaft
• Counterweights offset unbalance
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Crankshaft Main Bearings
• Supports crankshaft in block
• Three types:
– Sleeve or bushing
– Roller bearing
– Ball bearing
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Crankshaft Main Bearings
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Crankshaft Main Bearings
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Crankcase Seals
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Prevent leakage
Shell makes fixed contact with crankcase
Knife edge rubs crankshaft
Coil spring keeps sealing lip in contact with shaft
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Crankcase Seal
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Pistons
• Straight line driving member
• Subjected to heat of combustion
– Adequate clearance
• Seal between combustion chamber and crankcase
– Piston rings exert tension on cylinder wall
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Piston Construction
• Steel or aluminum
– Aluminum most popular
• Grooves accept piston rings
– Lands are full-diameter ridges between grooves
– Holes in bottom groove allow oil to flow back to
sump
• Hole for piston pin (wrist pin)
– Pin boss surrounds the piston pin hole
• Piston skirt keeps piston from tipping
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Piston Construction
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Cam Ground Piston
• Oval shape
• Thrust surfaces fit more closely
• Helps prevent piston slap
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Piston Head
• Runs hotter and expands more than skirt
– Smaller in diameter than skirt
• Shape depends on application
– Four-stroke engines
• Flat
• Domed
• Wedge
– Two-stroke
• Flat with loop-scavenged engine
• Raised baffle or deflector with cross-scavenged
engine
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Piston Rings
• Allow piston to compress fuel charge
• Prevent burning gases from leaking
• Ride on cylinder walls
– Separated by thin film or oil
• Rub sides of piston grooves
– Grooves hold rings squarely to bore
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Piston Rings
• Four-stroke pistons use
three rings
– Two upper rings are
compression rings
– Lower ring is oil
control ring
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Compression Rings
• Provide strong seal
– Keeps compressed air-fuel mixture and burning
gases above piston
– Prevents passage between piston and cylinder wall
• Twists in groove during intake stroke
• In tipped position during compression and exhaust
strokes
• Flat against cylinder wall on power stroke
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Compression Rings
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Oil Control Rings
• Remove surplus oil
from cylinder walls
– Light scraping action
• Slotted or perforated
– Oil passes through
holes in ring and
groove
– Oil flows to inside of
piston and into
crankcase
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Piston Ring Construction
• Cast iron or steel
– Plated with long-wearing material
• Outside diameter slightly larger than cylinder bore
– Creates ring tension
• Ring gap
– Opening between ends of ring
– Allows rings to expand and contract
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Piston Ring Movement
• Free to move inward or outward in piston grooves
• Floating rings
– Rotate around grooves
– Installed with end gaps staggered
• Pinned rings
– Held in position by pin in ring groove
– Pin prevents rotation of ring around grooves
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Piston Pins
• Secure piston to connecting rod
• Case-hardened steel
– Solid or hollow
• Full-floating pin
– Retained by snap rings
• Press-fit pin
– Fits tightly in connecting rod
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Connecting Rod
• Attaches piston to
crankshaft
– Upper end accepts
piston pin
– Lower end contains
bearings and fits
around crankshaft
journal
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Friction Bearings
• Used when lower end of connecting rod is split
• Types
– Rod metal
– Bearing bronze
– Removable precision inserts
• Locating tab
– Fits into slot in rod
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Antifriction Bearings
• Rollers or balls reduce friction
• Elements held together by roller cage or can be
free
• Hardened and ground to exact size
• Must fit accurately and have clearance for
expansion
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Intake and Exhaust Ports
• Provide means for admitting air-fuel mixture and
exhausting waste products
• Two-stroke engines
– Porting of cylinder walls
– Alternately covered and exposed by piston
• Four-stroke engines
– Poppet valves open and close port opening
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Reed Valves and Rotary Valves
• Control fuel flow directly into crankcase of twostroke engine
• Reed valve
– Opens during compression stroke and closes
before the power stroke
• Rotary valve
– Attached to end of crankshaft
– Air-fuel charge enters crankcase when holes in
valve and wear plate align
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Reed Valve Action
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Rotary Valve Action
(Evinrude Motors)
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Poppet Valves
• Installed in valve ports of four-stroke engines
• Control flow of air-fuel mixture into the cylinder
and exhaust gases out of the cylinder
• Angled valve face closes tightly against seat
• Valve guide aligns valve and ensures accurate
raising and lowering
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Poppet Valves
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Valve Spring
• Holds valve firmly against seat
• Connect to valve stem with retainer and keeper
• Allows valve to be opened and closes it when
pressure is released from valve stem
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Camshaft
• Used in four-stroke engines
– Opens valves the right amount at the right time
– Holds valve open for a specific period and allows
them to close at correct instant
• Shaft has lobe for each valve
– Lobe lifts valve from seat when shaft rotates
• Rotates at half crankshaft speed
– Camshaft gear meshes with and is driven by
crankshaft gear
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Valve Lifter
• Located between cam
lobe and valve stem
• Rises and falls as
camshaft rotates,
opening and closing
valves
• May provide means to
adjust clearance
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Valve Train
• Transforms crankshaft rotation into opening and
closing of valves
• Valve train configurations
– Valve-in-block
– Overhead valve
– Overhead cam
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Valve-in-Block
• Camshaft located in
crankcase
• Valves located in
cylinder block,
directly above
camshaft lobes
• Lifters act directly on
valve stems
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Overhead Valve (OHV)
• Camshaft installed in
crankcase
• Valves installed in
cylinder head
• Pushrods transfer
motion from valve
lifters to rocker arms
• Rocker arm pushes
down on valve stem
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Overhead Cam (OHC)
• Both camshaft and
valves in cylinder
head
• Camshaft positioned
directly above valves
or offset
– Rocker arms used if
camshaft is offset
• Camshaft driven by
chain or belt
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Starter Assembly
• Rewind starter assembly
– Mounted above flywheel
– When rope is pulled, pawls engage flywheel clutch
and assembly turns crankshaft
– When rope is released, pawls retract and assembly
disengages
• Electric starters
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Automatic Compression Release
• Makes hand cranking
easier
• Lifts exhaust valve
during cranking
– Releases part of
compression pressure
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Flywheel
• Fastened to one end of crankshaft
• Keeps crankshaft spinning during nonpower
strokes
• Metal fins on flywheel force air over cylinder to
cool engine
• Contains magnets that produce electrical current
for ignition system
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Review
The two sections of the engine block are the cylinder
block and the _____.
crankcase
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Review
The _____ converts the reciprocating motion of the
piston into rotary motion
crankshaft
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Review
The shape of a cam-ground piston allows the _____
surfaces to fit more closely.
thrust
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Review
The _____ rings keep the air-fuel mixture and burning
gases above the piston.
compression
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Review
Name the three types of friction bearings used in the
big end of the connecting rod.
Rod metal, bearing bronze, and removable precision
inserts
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Review
_____ bearings use rollers or balls to reduce friction
between moving parts.
Antifriction
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Review
In a(n) _____ valve train, the valves are in the
cylinder block and the camshaft is in the crankcase.
valve-in-block
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Review
In an overhead valve arrangement, _____ transfer
motion from the valve lifters to the rocker arms.
pushrods
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Review
In an overhead cam valve train, both the camshaft
and the valve assemblies are installed in the _____.
cylinder head
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