1. Engine Operation
Four-Stroke Cycle

2. Objectives
Describe the four-stroke cycle engine operation and explain the purpose of each stroke
Explain the concept of valve timing

3. Engine Operation
Fuel is burned inside the engine’s combustion chamber to produce heat
Heat causes expansion of gases in the engine
Expansion in the combustion chamber produces pressure
Engine components convert pressure into rotating motion

4. Engine Operation

5. Result of Combustion
Combustion Pressure

6. Conversion of Motion
The crankshaft converts the reciprocating (Up and down) motion and force of the piston into rotational motion and force (Torque).
Combustion Pressure

7. Piston Travel (TDC, BDC)
Top dead center (TDC)
piston is at its highest point in the cylinder
Bottom dead center (BDC)
piston is at its lowest point in the cylinder
Piston stroke
distance the piston slides up or down from TDC to BDC

8. Piston Travel (TDC, BDC)

9. Four-Stroke Cycle Requires four piston strokes to complete one cycle
Every four strokes, the engine produces one power stroke
Two complete crankshaft rotations are required to complete the four-stroke cycle (720o)
One complete camshaft rotation is required as well (360o)

10. Intake Stroke Purpose: Draw fuel and air into the engine
Piston slides down and forms a low pressure area in the cylinder
Atmospheric pressure pushes the air-fuel mixture into the cylinder

11. Intake Stroke Intake valve open Exhaust valve closed
Piston moving down
Crankshaft rotates 180o
Camshaft rotates 90o

12. Compression Stroke
Purpose: Compresses (squeezes) the air-fuel mixture, making it more combustible

13. Compression Stroke Both valves are closed Piston moving up
Crankshaft rotates to 360o
Camshaft rotates to 180o

14. Power Stroke
Purpose: Burns the air-fuel mixture and pushes the piston down with tremendous force (nearly 2 tons!)
Spark plug fires, igniting the air-fuel mixture
Pressure forms on the top of the piston
Piston is forced down, rotating the crankshaft

15. Power Stroke Both valves are closed Piston moving down
Crankshaft rotates to 540o
Camshaft rotates to 270o

16. Exhaust Stroke
Purpose: Removes the burned gases from the cylinder

17. Exhaust Stroke Exhaust valve opens Intake valve closed
Piston moving up
Crankshaft rotates to 720o
Camshaft rotates to 360o

18. Four-Stroke Cycle – All Together

19. Strokes & Valve Timing

20. Check This Out!

21. Can you see it?

23. Engine Operation
Two-Stroke Cycle

24. Objectives
Describe two-stroke cycle engine operation and explain the principles of two-cycle operation
List the advantages and disadvantages of two-stroke and four-stroke engines

25. Intake into the Crankcase
As the piston moves upwards, crankcase pressure drops
Atmospheric pressure causes air-fuel/oil mixture to flow in
While in the crankcase, the oil sticks to parts providing lubrication

26. Fuel Transfer
As the piston moves downward, it compresses the mixture in the crankcase
The pressure forces the mixture up through the transfer port

27. Ignition-Power The piston travels upwards compressing the mixture
The spark plug fires when the piston is just before TDC

28. Exhaust
The piston moves down to expose the exhaust port, exhaust gases rush out
Intake bypass port is exposed and new air-fuel mixture rushes in pushing any remaining exhaust gases out

29. Four-Stroke vs. Two-Stroke
1. # of major moving parts
Nine
Three
2. Power Strokes
One every two revolutions
One every revolution
3. Running temperature
Cooler
Hotter
4. Overall size
Larger
Smaller
5. Weight
Heavier
Lighter
6. Bore size equal Hp
Page 93 Fig. 5-18

30. Four-Stroke vs. Two-Stroke
7. Fuel and oil
No mixture required
Must be pre-mixed
8. Fuel consumption
Fewer gallons per hour
More gallons per hour
9. Oil consumption
Oil re-circulates & stays in engine
Oil is burned with fuel
10. Sound
Generally quiet
Louder
11. Operation
Smoother
More erratic
12. Acceleration
Slower
Very quick
Page 93 Fig. 5-18

31. Four-Stroke vs. Two-Stroke
13. General maintenance
Greater
Less
14. Initial cost
15. Versatility of operation
Limited slope operation
Unaffected at any angle of operation
16. Efficiency - hp/wt. ratio
17. Pull starting
Two crankshaft revolutions required
One crankshaft revolution required
18. Flywheel
Needs heavier flywheel to carry engine through non-power strokes
Lighter flywheel
Page 93 Fig. 5-18