Engine Components

C H A P T E R 6

Engine Components

Permission granted to reproduce for educational use only.© Goodheart-Willcox Co., Inc.

• 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.

Learning Objectives


Engine Block

• Keeps engine parts in alignment• Cast iron or aluminum alloy• Consists of two sections:

– Cylinder block– Crankcase


Cylinder Block

• 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


Crankcase

• Contains crankshaft• Must withstand rotational forces of the crankshaft• May contain oil for lubrication• May be cast with engine block


Crankshaft

• Major rotating part of engine• Converts reciprocating motion into rotary motion• Transmits engine torque• Drives camshaft• Supports flywheel


Crankshaft

• Cast or drop forged steel• Tapered end receives flywheel

– Flywheel keyed to crankshaft

• Crankshaft throw– Offset portion of shaft

• Counterweights offset unbalance


Crankshaft Main Bearings

• Supports crankshaft in block• Three types:

– Sleeve or bushing– Roller bearing– Ball bearing






Crankcase Seals

• Prevent leakage• Shell makes fixed contact with crankcase• Knife edge rubs crankshaft• Coil spring keeps sealing lip in contact with shaft


Crankcase Seal


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


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


Piston Construction


Cam Ground Piston

• Oval shape• Thrust surfaces fit more closely• Helps prevent piston slap


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


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


Piston Rings

• Four-stroke pistons use three rings– Two upper rings are

compression rings– Lower ring is oil

control ring


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


Compression 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

Oil Control Rings


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


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


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


• Attaches piston to crankshaft– Upper end accepts

piston pin– Lower end contains

bearings and fits around crankshaft journal

Connecting Rod


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


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


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


Reed Valves and Rotary Valves

• Control fuel flow directly into crankcase of two-stroke 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


Reed Valve Action


Rotary Valve Action

(Evinrude Motors)


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


Poppet Valves


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


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


• Located between cam lobe and valve stem

• Rises and falls as camshaft rotates, opening and closing valves

• May provide means to adjust clearance

Valve Lifter


Valve Train

• Transforms crankshaft rotation into opening and closing of valves

• Valve train configurations– Valve-in-block– Overhead valve– Overhead cam


• Camshaft located in crankcase

• Valves located in cylinder block, directly above camshaft lobes

• Lifters act directly on valve stems

Valve-in-Block


• 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

Overhead Valve (OHV)


• 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

Overhead Cam (OHC)


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


• Makes hand cranking easier

• Lifts exhaust valve during cranking– Releases part of

compression pressure

Automatic Compression Release


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


Review

The two sections of the engine block are the cylinder block and the _____.

crankcase


Review

The _____ converts the reciprocating motion of the piston into rotary motion

crankshaft


Review

The shape of a cam-ground piston allows the _____ surfaces to fit more closely.

thrust


Review

The _____ rings keep the air-fuel mixture and burning gases above the piston.

compression


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


Review

_____ bearings use rollers or balls to reduce friction between moving parts.

Antifriction


Review

In a(n) _____ valve train, the valves are in the cylinder block and the camshaft is in the crankcase.

valve-in-block


Review

In an overhead valve arrangement, _____ transfer motion from the valve lifters to the rocker arms.

pushrods


Review

In an overhead cam valve train, both the camshaft and the valve assemblies are installed in the _____.

cylinder head

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Engine Components