Four Stroke Engine

Four Stroke Engine OperationM.M.Palitha Mahinda MunasingheMechanical InstructorFour stroke cycleIntake stroke

Piston moving downIntake valve openExhaust valve closed

2009/10/052mmpmm2Compression stroke

Piston moving upIntake valve closedExhaust valve closed

Four stroke cycle2009/10/053mmpmm3Power stroke

Piston moving downIntake valve closedExhaust valve closed

Four stroke cycle2009/10/054mmpmm4Exhaust stroke

Piston moving upIntake valve closedExhaust valve open

Four stroke cycle2009/10/055mmpmm5 Each stroke takes 180 of crankshaft rotation to complete

All cylinders fire in 720 of crankshaft rotation

720 divided by number of cylinders = firing interval

Odd fire V-6 engine (90 block with 120 rod journals)

Four stroke cycle(Petrol)2009/10/056mmpmm62009/10/05mmpmm7Four stroke cycle(Diesel)

2009/10/05mmpmm8

Fuel Injection System2009/10/05mmpmm9

Fuel Pump(Diesel)Rotary Type PumpInline Type Pump2009/10/05mmpmm10

Fuel Feed Pump

Draw fuel from tank and feed to injection pump2009/10/05mmpmm11

Engine Cooling SystemEngine heat is transferred through walls of the combustion chambers and through the walls of cylindersPiston Dwell TimePiston travel is at a minimum TDC and BDCCrank moves horizontally

Piston velocityMaximum when rod is 90 to crank

AccelerationMaximum 30 earlier

Best VE is obtained by synchronizing valve opening with piston speeds2009/10/0512mmpmm12Other Valve PositionOverlapBoth valves are openEnd of exhaust & start of intakeLow pressure in exhaust port

Blow downExhaust valve opens before BDCTo help evacuate cylinder before piston reversesPumping losses at end of exhaust stroke 2009/10/0513mmpmm13

Valve Mechanism Intake valve opening BTDC Low pressure in cylinder Intake valve closing Intake valve closing ABDC Cylinder pressure is effected by timing Exhaust valve opening BBDC Residual pressure helps blow down Exhaust valve closing ATDC Low pressure in exhaust port draws air in2009/10/0514mmpmm14Effects On Valve TimingIntake valve openingLate Reduced VEEarly Dilution of intake with exhaustIntake valve closingLate Reduces cylinder pressureEarly Increases cylinder pressureExhaust valve openingLate Pumping lossesEarly Power reductionExhaust valve closingLate Reduces vacuumEarly Reduces VE2009/10/0515mmpmm15COMBUSTIONSpark ignition

Maximum cylinder pressure 15 ATDCTumble and swirlMotion reduces misfiresExcess motion inhibits flowAFR 14.7:1 at part throttle, 12.5:1 under load

Compression ignition

18:1 direct injection23:1 pre-chambers for better startingCompression heats to 800-1200 F2009/10/0516mmpmm16Valve MechanismOHV (overhead valve)

Pushrod configurationMany reciprocating partsHigher valve spring pressure requiredCompact engine size compared to OHC

2009/10/0517mmpmm17

OHC (overhead cam)

Fewer reciprocating partsReduced valve spring pressure requiredHigher RPM capabilityCylinder head assemblies are taller2009/10/0518mmpmmValve Mechanism18Cam-in-head

No pushrodsUse rocker arms

2009/10/0519mmpmmValve Mechanism19Valve lash compensatorsSolid lifters

No internal partsPeriodic adjustment

2009/10/0520mmpmm20

Hydraulic lifters

To maintain zero lashQuieter No periodic adjustmentAnti-scuff additives are required in oilsValve lash compensators2009/10/0521mmpmm21

Hydraulic lifter operationValve closed

Oil flows through lifter bore & past check valvePlunger return spring maintains zero lash2009/10/0522mmpmm22

Valve open

Check valve seats and limits the slippageNow operates as a solid lifterHydraulic lifter operation2009/10/0523mmpmm23Hydraulic lifter operationReturn to valve closed

New oil enters the lifter bodyThis oil replaces oil that has leaked between plunger and body (predetermined leakage)

2009/10/0524mmpmm24Metering DeviceMetering valve meters the oil flow to the pushrod

2009/10/0525mmpmm25Gear sets

Cam and crank rotate in opposite directions Noisy if not free of burrs Helical and spur cut gears2009/10/0526mmpmm

Timing Gear Wheel sets26Timing chains

Single and double rollerTension Pulley2009/10/0527mmpmmTiming Belt sets

27Timing belts

Require maintenanceSilent operation2009/10/0528mmpmmTiming Chain Drive

282009/10/05mmpmm29

Engine Lubricating SystemLubrication with oil through pressure to every moving partsSplash and spray

2009/10/0530mmpmmLubricating System30Oil pan bafflesTo keep oil in sump during braking, accelerating, and cornering

2009/10/0531mmpmmLubricating System31Oil pan windage tray

To prevent oil aeration in the sump

2009/10/0532mmpmmLubricating System32Oil pumpsDriven by distributors, gear on camshaft, or crankshaft

2009/10/0533mmpmmLubricating System33Oil pumps with pressure relief valvesGear type pump Rotor type pump

2009/10/0534mmpmmLubricating System34

Full flow oil filtering system

Oil pump output flows through filter first

Bypass circuit for restricted filters will allow oil to flow to engine 2009/10/0535mmpmmLubricating System35Diesel EngineAdvantages

Higher engine torque Better fuel economy Long engine lifeDisadvantages

Engine noise Exhaust smell Hard start2009/10/0536mmpmm36