11

Chapter 5Chapter 5

Two & Four Cycle EnginesTwo & Four Cycle Engines

page 77page 77

Questions on pg 90Questions on pg 90

22

Small Engine IdentificationSmall Engine Identification

# of strokes for a cycle# of strokes for a cycle– 22– 44– rotaryrotary

33

Four Stroke Cycle EngineFour Stroke Cycle Engine

IntakeIntake CompressionCompression PowerPower ExhaustExhaust

44

IntakeIntake

Intake openIntake open Exhaust closedExhaust closed Piston moves downwardPiston moves downward VacuumVacuum

55

CompressionCompression

Both valves closedBoth valves closed Piston moves upPiston moves up Compression 9:1Compression 9:1

66

PowerPower

Both valves closedBoth valves closed SparkSpark Ignites air/fuel mixtureIgnites air/fuel mixture 14.7:114.7:1 Piston forced downPiston forced down

77

ExhaustExhaust

Exhaust valve opensExhaust valve opens Intake valve closedIntake valve closed Piston moves upPiston moves up Pushes exhaust outPushes exhaust out

88

Valve TimingValve Timing

Valves must open and close at Valves must open and close at correct timecorrect time

Align marksAlign marks BackfireBackfire Lack powerLack power

99

LubricationLubrication

SplashSplash PressurePressure HybridHybrid

1010

Two Stroke Cycle EngineTwo Stroke Cycle Engine

See:See: http://science.howstuffworks.com/twhttp://science.howstuffworks.com/tw

o-stroke2.htmo-stroke2.htm

1111

Variation in DesignVariation in Design

Cross-scavengedCross-scavenged– Contoured piston headContoured piston head– Deflect air upwardDeflect air upward– Reed valves/rotary valvesReed valves/rotary valves

Loop-scavengedLoop-scavenged– Flat or domed pistonFlat or domed piston– Intake offset for exhaustIntake offset for exhaust

1212

Principles of OperationPrinciples of Operation

Location of parts essentialLocation of parts essential

1313

Intake into CrankcaseIntake into Crankcase

Piston moves upPiston moves up Causes vacuumCauses vacuum Reed/rotary valve opensReed/rotary valve opens

1414

Fuel TransferFuel Transfer

Piston moves downPiston moves down Pressurizes crankcasePressurizes crankcase Transfer portTransfer port Intake port uncoveredIntake port uncovered Air/fuel rush inAir/fuel rush in

1515

Ignition PowerIgnition Power

Air/fuel compressedAir/fuel compressed 8:1 compression8:1 compression TDC-sparkTDC-spark BTDC-advance (warm)BTDC-advance (warm)

1616

ExhaustExhaust

Exhaust port exposedExhaust port exposed Intake pressure inIntake pressure in forces exhaust outforces exhaust out

1717

Scavenging & TuningScavenging & Tuning

Proper designProper design Full scavengingFull scavenging

1818

Rotary Disc Valve EngineRotary Disc Valve Engine

Intake in crankcaseIntake in crankcase No reed valveNo reed valve better fuel transferbetter fuel transfer

1919

Reed Valve EngineReed Valve Engine

Intake directly into crankcaseIntake directly into crankcase Flexible spring steelFlexible spring steel Reed stopReed stop Limits valve openingLimits valve opening Reed opened by atmospheric Reed opened by atmospheric

pressurepressure Closed seals crankcaseClosed seals crankcase

2020

4 Cycle VS 2 Cycle4 Cycle VS 2 Cycle 4 cycle4 cycle More powerMore power More fuel efficientMore fuel efficient No-premixNo-premix 2 cycle2 cycle Simple designSimple design Smaller lighterSmaller lighter Runs at any angleRuns at any angle High speedsHigh speeds