Thebasicsoffour-Strokeengines

TheBasicsofFour-StrokeEngines

Description

Students willbe introducedtobasicengineparts,theoryand terminology.Understandinghowanengine worksandknowing somekeyrelatedpartsand terminologyis important forworkingon anyvehicle. Theinformation is broken down into three majorsections: “BasicEngine Parts,”“Basic EngineTerminology” and“BasicFour-StrokeCycleEngine Theory.“

LessonOutcomes

The student willbeableto:

• Identify and explain thefunction ofbasic engine parts

• Identify and explain basic engine terminology

• Identify the four piston strokes of a four-stroke cycle engine

• Describe the actionand functionofeach piston stroke

Assumptions

• The studentswill havelittleor noprior knowledge of how engineswork, terminology orparts.

• Theteacher isfamiliar withthe informationbeing taught.

Note: Thisinformationisgiven as aguide tothe minimumamountof material tobe coveredfor a basicunderstanding of the engine and howitworks. Muchmore canbe addedastheinstructorsees fit.

Terminology

Valvetrain: all theparts that are usedtoopen andclose valves.This may include parts suchasvalve springs, keepers, lifters,cam followers, shims,rockers and pushrods.

Any otherterminology used willbeexplained asrequiredduring theactivities.

Estimatedtime

90–120 minutes(including a question and answersession)

Recommendednumberofstudents

20, basedontheBCTechnology Educators’Best PracticeGuide

Facilities

A classroom,computerlab or workshopwithtablesandchairs sufficient for20students.

Materials

•Basic engine parts to showasexamples,or images ofthem. A widevarietywouldbe best(1,4, 6 and 8cylinders)

•Enginecutaway

Resources

FourStrokeEngine

Animationoffour-stroke engineinmotion.Otherdepictedenginescanbefoundonthehomepage.

AutomobileEngine:Introduction

HowCarEnginesWork

World’sLargestandMostPowerfulDieselEngine

Generalinterest—descriptionofthelargesttwo-strokediesel engine,designedforuseincontainerships.

PrintResources

Stockel,Martin.AutoMechanicsFundamentals.Goodheart-WilcoxCo., 1990.

McGraw-HillEducation (author). AutomotiveExcellence,Volume1(3rd Edition).Glencoe/McGraw-Hill,2006.

Activity

This informationcanbepresented in manydifferent ways andformats:straight theory lessons,worksheets, computer researchassignments or textbookassignments. This informationismeant onlytobe theessential basicstobuild afoundation on howan enginefunctions.

1.BasicEngineParts

EngineBlock

Theengineblock is the foundationandcentre of the engine. Blocks come in manydifferentsizes, shapes and styles.

Theengineblock holds the cylinder, the crankshaft, connecting rods (“con rods”) and pistons.Thelarge round holes in the block, called cylinders,arewhere the pistons slide up and down.

Figure1—Cylinder block with head attached

Piston

A piston slides up anddown acylinderandpushesthecrankshaftaround. The pistonconnectstothecrankshaft via theconnectingrods.

PistonRings

Eachpiston hastwocompression rings and one oilcontrolring tohelp seal thepiston in thecylinder.

Firstcompressionring groove

Secondcompressionring groove

Oil controlring groove

Skirt

Figure4—Apistonwith twocompressionringsandone oil controlring

Figure5—Pistonrings(compressionringsand oil controlring,left toright)

ConnectingRods

Connectingrods connect thepistons to thecrank. Theyutilize anI-beamconstruction forstrength.

Figure6—Connectingrod

Crankshaft

Thecrankshaft isboltedto thebottom oftheengine block through bearings soit canrotatefreely. It is sometimesreferredtoas thecrank. Thecrankshaft converts theverticalmotionofthepistons andconnecting rods torotational motion. Oneend of thecrankshaftis connectedto a flywheel thattransferstheenginepowertothewheelsthrough a transmission(manual orautomatic).

CylinderHead

Thecylinderheadisboltedto the topofthecylinderblock. It serves as acover forthecylindersand pistons. The cylinder headhelpstocreatethe toppartof thecombustion chamber.Anengine“breathes”throughthe cylinder head.It letsanair/gasmixture into theengine andexhaust outof the engine.The valvesand valve traincontrol thebreathing of the enginebyopeningand closingthe valves attheappropriate time.

Figure9—Topviewof cylinderhead with camshaftsattached

Figure10—Bottom viewofcylinderhead showingwherethevalvessit

Note:This has fourvalves per cylinder(two intakeand two exhaust).

Figure 11—Cylinder head being attached toengine block

Valves

Valvescanbe divided intotwo groups:

•Intakevalves control theflowoftheair/gasmixtureintotheengine.

•Exhaustvalves control theflowofexhaustoutoftheengine.

Camshaft

Thecamshaft controlstheopening andclosing of the valves. Thereisonelobeonthe camshaftfor each valve inthe engine. Camshaft lobedesign dictates three things:

•Howfarthe valve opens

•Howfastthe valve opens

•Howlong the valve opens

Dependingon theengine type, thecamshaft canbelocated eitherinthe engine blockor overthehead(OHC)or double OHC(DOHC).

Figure14—Cam lobe profileshowingthe openingand closingangles

Figure15—Camshaft

Figure16—In-the-blockcamshaft

Figure17—Camshaft located over thehead

Lifters(Tapetts)

Lifters are the linkbetweenthe cam and valves.They are sonamed because theyactuallylift asthecamlobe rotates and openthe valves.Thereare twobasic types of lifters:

•Thehydrauliclifter

•Thesolidlifter

TimingChain/Belt/Gears

Thesepartsareusedindifferentcombinationsandconfigurationsto connect thecrankshafttothecamshaft. Theykeep thevalves’ openingandclosing timedwith thepiston movement.Timingbeltsshould be replacedevery100,000 km or everyfiveyears.

ValveTrain

The valve trainincludesallthe partsthat are usedto open andclosevalves. Thismayincludepartslikevalve springs, keepers,lifters, cam followers, shims,rockers and pushrods.

Figure18—Diagram ofan enginewith overheadcamshafts, demonstrating valvetraincomponents

Flywheel

The flywheel attaches tothecrankshaft,andusesitsmomentumtopowertheenginethroughthethreenon-power strokes(intake, compression andexhaust).Becausean 8-cylindercar hasmorepowerstrokesthan a 4-cylinder car,the flywheel will be smallerandlighterasthereislessneedfor the momentum carry capabilities.

•Flywheels areused instandardtransmissions.

•Flexplates areusedin automatic transmissions.Flywheels andflexplateshave a ring gearforthestarter.

2.BasicEngineTerminology

Bore:thedistance across thecylinder(or the diameter).

Bottomdeadcentre(BDC):the lowest point inthecylinder that thepistonreaches.

Combustionchamber:thespaceleft at thetop of thecylinder whenthepistonisat topdeadcentre (TDC). Thisalsoincludesany space inthe cylinder head.

Compression:the squishing orsqueezing of theair/fuel mixturefromBDCtoTDC. Themorethemixture is compressed,themore powerit can produce.

Compression ratio: thedifferenceasexpressedthrougharatio of thespaceleft in thecylinderwhenthepiston is at TDC versusBDC. Forexample,8:1 means thatthespace whenthe pistonis at BDC is 8 times biggerthanwhen thepiston is at TDC.

Cubicinchdisplacement(CID):theenginesize.Forexample, Chevy 350(cubicinches),Mustang5.0 (cubiclitres) or Honda1800 cc(cubiccentimetres).Eventhough 5.0L and 1800ccaremetric measurements,they are oftenreferred toas the CIDofan engine.

CIDis amathematical calculationthattakesinto account the boreand stroke of the cylindertimes the numberofcylinders in the engine.It basicallymeasureshow much volumeor air acylinder candisplace or push out fromBDC toTDC.

Enginetypes:engines canbeclassified in manydifferent ways, butthree basic engine typeslikely to beencountered in an automotive shopare:

•Four-strokecycle engine—takes four stokes ofthepiston tocompletea cycle

•Two-stroke cycle engine—takes two strokes ofthepiston tocompletea cycle

•Diesel—(twoor four stroke)usesheat ofcompression rather than a spark plug to ignite thefuel that isdirectly injected into thecylinder

Eachoftheseengines can come inseveraldifferentconfigurations.

Four-stroke cycle: four movements of thepistonequals one cycle.

Stroke:thedistancethepiston travelsfromTDCtoBDCorfromBDCtoTDC.

Topdeadcentre (TDC): thehighestpoint inthecylinder thatthepistonreaches.

3.BasicFour-StrokeEngineTheory

Regardless ofits design,an engineneedsfour thingsin order to deliver a substantialamount ofusefulenergy or work:

1.Air

2.Fuel toburn

3.Ignitionsourceto ignite thefuel

4.Compressionof the air/fuel mixtureto maximizethe powerpotential ofthefuel

Takeawayanyof these itemsandan engine willnotrun.Thereforeall engine designs are basedon allowingthesekeyfactorsto work inharmonyfor a smooth, powerful and efficiently runningengine.

Example: You could pourout some gas on a smallplate andlightit on fire. Althoughitwouldproducesomelightandheat,itwouldnotbe agreatsource of power.However,takingthatsameplate ofgasand compressingthe airaround it byplacing a bowloveritand ignitingthe gaswould produce enough powerto blowthe bowloffthe plate. Thisisthebasicsof how an engineworks.

TheFour-StrokeCycle

NickolausOttoiscreditedwithbuildingthefirst four-stroke cycleenginein1867,consideredthebasisofourmodernengines.InhishonouritisoftencalledtheOttocycle engine.

1.Intakestroke

•Thepiston movesfromTDC toBDC(down).

•The intake valve is open.

•The exhaustvalve is closed.

•Thepiston creates a suction (vacuum) and air and fuelare suckedinto the cylinder.

Intakevalveopen

Exhaustvalveclosed

Topdeadcentre(TDC)

Stroke

Bottomdeadcentre(BDC)

Figure19—Intakestroke

2.Compressionstroke

•Thepiston movesfromBDC toTDC(up).

•Both valves areclosed.

•Thepistoncompressestheair and fuelmixture.

Intakevalveclosed

Exhaustvalveclosed

Topdeadcentre(TDC)

Stroke

Bottomdeadcentre(BDC)

Figure20—Compressionstroke

3.Powerstroke

•Thepiston movesfromTDC toBDC(down).

•Both valves areclosed.

•Thesparkplugfires.

•Thefuel mixtureburnsrapidly. This expanding heatedmixtureforces thepiston down.

Intakevalveclosed

Exhaustvalveclosed

Topdeadcentre(TDC)

Stroke

Bottomdeadcentre(BDC)

Figure21—Powerstroke

4.Exhauststroke

•Piston movesfromBDCtoTDC(up).

•The intake valve is closed.

•The exhaustvalve is open.

•Thepistonpushes the exhaust out.

Intakevalveclosed

Exhaustvalveopen

Topdeadcentre(TDC)

Stroke

Bottomdeadcentre(TDC)

Figure22—Exhauststroke

Thecyclerepeats itself.

The four-stroke cycle ispresented inchart formbelow. Note the following:

•Thepistondirectionhas adistinct pattern.

•The intakevalve isonly open duringthe intake stroke.

•The exhaustvalve isonly open duringthe exhauststroke.

RememberICPE:intake, compression, power,exhaust.Thecyclerepeats itself. This order cannot change!