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PISTON ENGINES

PISTON ENGINES. Part 2 The Four Stroke Cycle. Introduction. The Operating Cycle. The four stroke piston engine is so called because one ‘Stroke’ is the piston sliding up or down the cylinder; and to complete one operating cycle,

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PISTON ENGINES

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  1. PISTON ENGINES Part 2 The Four Stroke Cycle

  2. Introduction The Operating Cycle The four stroke piston engine is so called because one ‘Stroke’ is the piston sliding up or down the cylinder; and to complete one operating cycle, the piston must travel along the cylinder four times as follows :- 1 Induction Down Suck 2 Compression Up Squeeze 3 Combustion Down Bang 4 Exhaust Up Blow That means that one cycle is completed for every two revolutions of the crankshaft. Down Up Up 1 Induction 4 Exhaust 2 Compression 3 Combustion BUT EXACTLY HOW DOES IT WORK? Down

  3. The Operating Cycle Air/Fuel drawn (Induced/Sucked) Into Cylinder Piston moves DOWN Cylinder Crank Rotates The First Stroke - Induction TDC 1 Induction BDC

  4. Air/Fuel trapped (Compressed/Squeezed) in Cylinder Piston moves UP Cylinder Crank continues to Rotates The Operating Cycle The Second Stroke – Compression or Power TDC BDC

  5. Air/Fuel ignited & burns (Combustion/Bang) in Cylinder Piston FORCED DOWN Cylinder Crank starts Second Rotation The Operating Cycle The Third Stroke - Combustion TDC BDC

  6. Burnt Air/Fuel pushed out of Cylinder (Exhaust/Blow) Piston moved back UP Cylinder again Crank continues Second Rotation The Operating Cycle The Fourth Stroke - Exhaust TDC BDC

  7. The Operating Cycle All Four Strokes It’s the start of the Induction Stroke and the whole process starts again TDC When the end of the Exhaust Stroke is reached . . . and the whole process starts again . . . BDC

  8. The Operating Cycle TDC and BDC Because valves don’t open and close instantly, and the air/fuel mixture doesn’t explode instantly, (it’s a rapid burning process) valve operation and air/fuel ignition isn’t set at the TDC and BDC positions. These events occur at other positions called, Angular Positions of the Crank Shaft.

  9. Angular Positions of the Crank Shaft Start of Induction - Stroke One We’ll start the process at 30o before TDC At which point, the Inlet valve begins to open TDC And the Exhaust valve is almost closed BDC

  10. Angular Positions of the Crank Shaft Start of Induction - Stroke One We’ll start the process at 30o before TDC At this point, Induction begins TDC Inlet valve opens BDC

  11. Angular Positions of the Crank Shaft Start of Compression - Stroke Two When the Inlet valve closes TDC Inlet valve opens Trapping the air/fuel mixture in the cylinder Induction stroke ends at 15o after BDC. BDC

  12. Ignition Angular Positions of the Crank Shaft Start of Combustion - Stroke Three Ignition occurs 30o before TDC. At this point both valves are closed TDC Inlet valve opens Trapping the air/fuel mixture in the cylinder Which is the start of the combustion stroke Inlet valve closes BDC

  13. Ignition Angular Positions of the Crank Shaft Start of Exhaust - Stroke Four 15o before BDC the Exhaust valve opens TDC Inlet valve opens The burning air and fuel mixture reaches maximum expansion Inlet valve closes Exhaust valve opens BDC

  14. Ignition Angular Positions of the Crank Shaft End of Exhaust - Stroke Four As the Exhaust valve starts to close And the beginning of the Induction stroke At which point, the Inlet valve begins to open TDC Inlet valve opens This is the endof the Exhaust stroke Inlet valve closes Exhaust valve opens BDC

  15. Ignition Angular Positions of the Crank Shaft The Overlap - Stroke Four to One This is called ‘Valve Overlap’ Both the inlet and exhaust valves are partially open TDC Exhaust valve closes Inlet valve opens Therefore the Induction stroke starts 45o before the end of the Exhaust stroke Inlet valve closes Exhaust valve opens BDC

  16. Ignition Angular Positions of the Crank Shaft Start of Induction – Stroke One And the Exhaust valve is now closed This is at the end of the Exhaust stroke The Inlet valve is now open TDC Exhaust valve closes Inlet valve opens and is 45o into the Induction stroke Inlet valve closes Exhaust valve opens BDC

  17. Angular Positions of the Crank Shaft Start of Induction – Stroke One ‘Valve Overlap’ is the brief period when the inlet valve opens before TDC, and the exhaust valve does not close until after TDC. ‘Valve Lead’ is the valve operating angle before either TDC or BDC. ‘Valve Lag’ is the valve operating angle after either TDC or BDC. ‘Ineffective Crank Angle’ is an arc of travel at both TDC and BDC where there is very little piston movement.

  18. The Operating Cycle The cycle is happening in every cylinder, in all 4 stroke engines, no matter how many cylinders there are. Lets look at the most typical vehicle engine, the ‘Inline 4’

  19. The Operating Cycle The cylinders are numbered from FRONT to REAR SQUEEZE BANG BLOW SUCK 1 3 4 2

  20. The Operating Cycle And the cycle runs . . . . . FRONT REAR SQUEEZE SQUEEZE SQUEEZE BANG BANG BANG BANG BANG BLOW BLOW BLOW BLOW BLOW SQUEEZE SQUEEZE SUCK SUCK SUCK SUCK SUCK 1 3 4 2

  21. The Operating Cycle The Firing Order So far we have seen the operating cycle as it takes place in one cylinder, but it is necessary to ensure that all cylinders in an engine fire at different times. • What do you think would happen • if all of the cylinders fired at the same time ? • The moving parts would be subjected to 4 times the load. • Fuel and inlet manifolds would need to be bigger • to cope with 4 times the fuel flow. • The exhaust would need to be bigger • to cope with 4 times the flow of the exhaust strokes.

  22. The Operating Cycle So the cycle now runs - 1 - 3 - 4 - 2 SQUEEZE SQUEEZE SQUEEZE SQUEEZE SQUEEZE BANG BANG BANG BANG BANG BLOW BLOW BLOW BLOW BLOW SUCK SUCK SUCK SUCK SUCK 3 1 4 2

  23. The Operating Cycle The Firing Order • This illustration includes the inlets and exhaust, and shows the same firing order and cyclic sequence as before, but looking from above. and so it goes on . . . . 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 3 1 2 4 3 2 1 4

  24. The Operating Cycle The Firing Order • This illustration includes the inlets and exhaust, and shows the same firing order and cyclic sequence as before, but looking from above. 1 3 4 2

  25. Check of Understanding In a 4-stroke cycle, which is the second stroke? Combustion / Power Exhaust Compression Induction

  26. Check of Understanding Valve operation and air/fuel ignition isn’t set at the TDC and BDC positions. These events occur at other positions called what? Angular Positions of the Crank Shaft The Ineffective Crank Angle Valve Overlap Valve Lead and Lag

  27. Check of Understanding In the 4-stroke cycle, where does ignition take place? 15o after TDC on the combustion/power stroke 30o before TDC on the combustion/power stroke At TDC on the compression stroke At TDC on the ignition stroke

  28. Check of Understanding In a piston engine, in which region does ineffective crank angle occur? Only at TDC Only at BDC At both TDC and BDC 90o after both TDC and BDC

  29. Check of Understanding In a 4-stroke cycle, where does the induction stroke end? 15o before BDC 15o after BDC 15o after TDC 15o before TDC

  30. Check of Understanding What is the valve operating angle just before both TDC and BDC called? Valve Lead The Ineffective Valve Angle Valve Lag Valve Overlap

  31. Check of Understanding • What is the firing order • in the 4-stroke cycle? 1 - 2 - 4 - 3 1 - 2 - 3 - 4 1 - 3 - 2 - 4 1 - 3 - 4 - 2

  32. PISTON ENGINES End of Presentation

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