2. 5
Principles of Engine Operation, Two- and Four-Stroke Engines

3. Learning Objectives Explain simple engine operation.
Explain why gasoline is atomized in the small engine.
Describe four-stroke engine operation and explain the purpose of each stroke.
Explain the concept of valve timing.
Compare the lubrication system in a four-cycle engine to the system in a two-stroke engine.

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

5. Principles of Engine Operation
Converts chemical energy into mechanical energy
Gasoline engine is an internal combustion engine
Gasoline must ignite easily and burn quickly
Energy produced by burning gasoline must be controlled

6. Gasoline Must Be Atomized
The more surface area exposed to air, the more vapor will be given off
More vapor leads to faster burning
Gasoline must be atomized
Atomization
Increased burning area
Explosive release of heat energy

7. Elementary Engine

8. Two- and Four-Stroke Engines
Engines identified by number of piston strokes required to complete one operating cycle
Each stroke is either toward the rotating crankshaft or away from it
Bottom dead center
Top dead center
Strokes identified by job they perform

9. Four-Stroke Engine Four strokes needed to complete operating cycle
Intake stroke
Compression stroke
Power stroke
Exhaust stroke
Two strokes occur during each crankshaft revolution
Two crankshaft revolutions complete one operating cycle

10. Four-Stroke Engine

11. Intake Stroke Piston travels downward
Volume of space above piston increases
Creates partial vacuum
Intake valve open and exhaust valve closed

12. Intake Stroke
Atmospheric pressure forces air through carburetor, through intake valve port, and into cylinder
Intake valve must open and close at the correct time
Incoming air-fuel mixture cools valve during engine operation

13. Compression Stroke Piston moves upward Both valves closed
Mixture is compressed
Force of combustion is increased

14. Power Stroke Both valves closed Air-fuel mixture ignited
Burning action forces piston downward

15. Exhaust Stroke Intake valve closed and exhaust valve open
Rising piston pushes exhaust gases from engine

16. Exhaust Stroke Exhaust valve Allow a streamlined flow of exhaust gases
Heat must be controlled

17. Four-Stroke Engines Valve Timing Lubrication
Measured in degrees of crankshaft rotation
Varies with different engines
Valve overlap
Lubrication
Provided by oil in the crankcase
Splash and pump systems
Oil must be drained and replaced periodically

18. Two-Stroke Engine
Two strokes occur during each revolution of crankshaft
Advantages over four-stroke engine
Simpler in design
Smaller
Lighter
Adequate lubrication at extreme angles

19. Two-Stroke Engine
(Kohler Co.)

20. Two-Stroke Engine Operation
(Rupp Industries, Inc.)

21. Intake into Crankcase Piston moving upward Crankcase pressure drops
Intake port exposed
Intake air through carburetor pulls fuel and oil into crankcase

22. Ignition-Power Piston moving upward Compresses air-fuel charge
Spark ignites air-fuel mixture
Piston driven downward
Creates crankshaft motion

23. Exhaust Piston moving downward Exhaust port exposed
Exhaust gases expelled
Complete exhausting occurs when transfer ports are opened
New charge rushes in

24. Fuel Transfer Piston moving downward
Air-fuel charge in crankcase compressed
Transfer port opened
Compressed charge rushes through port

25. Scavenge Loss
Occurs when a significant portion of the air-fuel charge flows out through the open exhaust port
Scavenge loss results in increased hydrocarbon emissions and reduced fuel economy

26. Stratified Scavenge Engine
Two-cycle engine that eliminates scavenge loss
Extra intake port and a specially shaped piston introduce fresh air into the transfer port ahead of the air fuel charge
Fresh air flows into the cylinder before the air-fuel charge, pushing the spent exhaust gases from the cylinder and preventing the air-fuel charge from escaping through the exhaust port

27. Four-Stroke Engine vs. Two-Stroke Engine