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Engine Operation

Engine Operation. AF115 M. Westerman. Events within the four stroke cycle. Intake Compression Ignition Power Exhaust Ignition and power are part of the same stroke but are two different events. The Four Stroke Cycle. Intake Piston begins at top dead center (TDC) Intake valve is open

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Engine Operation

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  1. Engine Operation AF115 M. Westerman

  2. Events within the four stroke cycle • Intake • Compression • Ignition • Power • Exhaust • Ignition and power are part of the same stroke but are two different events.

  3. The Four Stroke Cycle • Intake • Piston begins at top dead center (TDC) • Intake valve is open • Piston is pulled down via the crankshaft • Reduced pressure in the cylinder creates a vacuum that draws air through entire induction system and into cylinder • The amount of fuel/air allowed into cylinder is dependant on throttle position

  4. Compression • Starting at bottom dead center (BDC) piston starts to rise again • Intake Valve remains open momentarily, to take advantage of momentum (valve overlap) • As piston approaches TDC two spark plugs ignite mixture • Spark happens before TDC to optimize the burn of the mixture

  5. Power • Starting from TDC again with both valves closed • Piston is pushed down by the rapidly expanding fuel/air mix • Temp of gases =3000° F • Pushes cylinder down with a force in excess of 15 tons • Exhaust valve opens before BDC

  6. Exhaust • Returning from BDC to TDC • Exhaust valve already open • Exhaust gases leaving cylinder creates an area of low pressure assisting in the intake stroke again • Intake valve opens before TDC to maximize intake of mixture

  7. Valve lead and lag • Lead • Valve opens before cycle begins • Lag • Valve closes after cycle has completed

  8. Factors Effecting the Power of an Engine • Detonation: the uncontrolled, explosive ignition of the fuel/air mixture

  9. Causes • Fuel grade lower than recommended mixed with engine overheating • Ignition timing wrong • Heavy engine load at low RPM • Fuel/air mix too lean

  10. Detonation Cont’d… • Effects • High CHT, and pressure • Leads to rough running engine, overheating, and power loss • Cause a hammering or scrubbing action on cylinder and/or piston, which can burn a hole through either in seconds. • Can detect detonation as a knock in the engine • Rough engine procedure? What can you do once you detect a knocking?

  11. Preignition • When fuel/air mixture ignites too soon • Causes • Hot spots within the cylinder • Hot spots: carbon particles, overheated valve edges, red hot sparkplug electrode, deposits on sparkplug • Causes • Poor engine cooling • Dirty intake filters • Shutting down engine at high rpm

  12. Preignition Cont’d… • Symptoms are similar to detonation • Overheating • Engine roughness • Knock knock • Can happen simultaneously with detonation

  13. Backfiring • Excessively lean mix passes into cylinder • Burns extremely slow • Still burning during new intake cycle • Fire spreads throughout induction system

  14. Afterfiring • Excessively rich mixture • Raw fuel ends up throughout exhaust system • If hot enough will ignite fuel

  15. Other Stuff • Compression Ratio • Ignition timing • Engine speed • More speed= more power • Speed is limited to propeller tip speed, and stress on parts • To operate at high RPM props need to be shortened or geared down

  16. Other stuff cont’d… • Altitude • Density altitude up= air density down= engine power down • To overcome high density alt turbo or superchargers are used • Fuel/air ratio • Perfect ratio- 15:1 • To maintain this ratio as altitude increases, mixture controls are used

  17. Rich mix runs cool • Lean mix runs hot • Too lean temps drop again

  18. Your engine and you • Engine warm-up 800-1000 • Valves need to seat • Oil needs to spread • Need to maintain engine stability throughout warm up- how well does engine run at low RPM? • Idling for long periods can cause? • Temperature check before T/O- Green?

  19. Engine won’t start • Prime more? • Flooded start?

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