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Understanding Two and Four Cycle Engines: Principles and Operation

This chapter delves into the fundamentals of two and four stroke cycle engines, exploring their operation, including the intake, compression, power, and exhaust strokes. Learn the distinctions between rotary and hybrid designs, valve timing, and how proper lubrication affects engine performance. Understand the principles of scavenging and how different types of valves contribute to fuel transfer efficiency. This overview provides a comprehensive resource for identifying small engines and understanding their mechanics.

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Understanding Two and Four Cycle Engines: Principles and Operation

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  1. Chapter 5 Two & Four Cycle Engines page 77 Questions on pg 90

  2. Small Engine Identification • # of strokes for a cycle • 2 • 4 • rotary

  3. Four Stroke Cycle Engine • Intake • Compression • Power • Exhaust

  4. Intake • Intake open • Exhaust closed • Piston moves downward • Vacuum

  5. Compression • Both valves closed • Piston moves up • Compression 9:1

  6. Power • Both valves closed • Spark • Ignites air/fuel mixture • 14.7:1 • Piston forced down

  7. Exhaust • Exhaust valve opens • Intake valve closed • Piston moves up • Pushes exhaust out

  8. Valve Timing • Valves must open and close at correct time • Align marks • Backfire • Lack power

  9. Lubrication • Splash • Pressure • Hybrid

  10. Two Stroke Cycle Engine • See: • http://science.howstuffworks.com/two-stroke2.htm

  11. Variation in Design • Cross-scavenged • Contoured piston head • Deflect air upward • Reed valves/rotary valves • Loop-scavenged • Flat or domed piston • Intake offset for exhaust

  12. Principles of Operation • Location of parts essential

  13. Intake into Crankcase • Piston moves up • Causes vacuum • Reed/rotary valve opens

  14. Fuel Transfer • Piston moves down • Pressurizes crankcase • Transfer port • Intake port uncovered • Air/fuel rush in

  15. Ignition Power • Air/fuel compressed • 8:1 compression • TDC-spark • BTDC-advance (warm)

  16. Exhaust • Exhaust port exposed • Intake pressure in • forces exhaust out

  17. Scavenging & Tuning • Proper design • Full scavenging

  18. Rotary Disc Valve Engine • Intake in crankcase • No reed valve • better fuel transfer

  19. Reed Valve Engine • Intake directly into crankcase • Flexible spring steel • Reed stop • Limits valve opening • Reed opened by atmospheric pressure • Closed seals crankcase

  20. 4 Cycle VS 2 Cycle • 4 cycle • More power • More fuel efficient • No-premix • 2 cycle • Simple design • Smaller lighter • Runs at any angle • High speeds

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