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 Cross-scavenged Loop-scavenged
Contoured piston head prevents air-fuel charge from going out exhaust port
Use reed valves or rotary valve
Loop-scavenged
Flat or slightly domed piston head
Transfer ports cause incoming air to swirl

20. Two-Stroke Engine
(Kohler Co.)

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

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

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

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

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

26. Four-Stoke Engine vs. Two-Stroke Engine

27. Why is a gasoline engine considered an internal combustion engine
Why is a gasoline engine considered an internal combustion engine? Because gasoline is combined with air and burned inside the engine.

28. What is atomization and why is gasoline atomized
What is atomization and why is gasoline atomized? Atomization involves breaking gasoline up into tiny droplets and mixing it with air. Gasoline is atomized to produce the rapid burning required in an engine.

29. What are the four-strokes in a four-stroke cycle
What are the four-strokes in a four-stroke cycle? Intake stroke Power stroke Compression stroke Exhaust stroke

30. What is valve overlap? Valve overlap is a condition in which both valves are open at the same time.

31. Why do two-stroke engines receive adequate lubrication even when operated at extreme angles? Because it receives its lubrication as fuel mixed with oil is passed through the engine.

32. How many crankshaft revolutions are required to complete a two-stroke cycle? One

33. What advantages does a two-stroke engine have over a four-stroke engine? A two-stroke engine: …is simpler in design than a four-stroke engine …is smaller and lighter than a four-stroke engine of equivalent horsepower …will receive adequate lubrication even when operated at extreme angles

34. Glossary Atomization Bottom dead center Compression ratio
Process of breaking gasoline into tiny droplets and mixing it with air to produce the rapid burning required in an engine.
Bottom dead center
The lowest point of piston travel. Abbreviation is BDC.
Compression ratio
Volume of combustion chamber at end of compression stroke as compared to volume of cylinder and chamber with piston on bottom center.

35. Glossary Compression stroke Cross-scavenged Exhaust stroke
Occurs as the piston moves upward in the cylinder with both valves closed. During this stroke, the air-fuel mixture is compressed into a smaller space.
Cross-scavenged
An engine with a special contour on the piston head, which acts as a baffle to deflect the air-fuel charge upward in the cylinder.
Exhaust stroke
Piston stroke during which the exhaust valve opens and the rising piston pushes the exhaust gases from the cylinder.

36. Glossary Four-stroke engine Intake stroke
Also known as Otto cycle. A combustion cycle that consists of an intake, a compression, a power; and an exhaust stroke. Also called four-stroke engine.
Intake stroke
Piston stroke in which the piston travels downward in the cylinder with the intake valve open. This increases the volume above the piston and creates a partial vacuum that draws the air-fuel mixture into the cylinder.

37. Glossary Internal combustion Loop-scavenged engine Power stroke
The burning of a fuel within an enclosed space.
Loop-scavenged engine
Engine in which the fuel transfer ports are shaped and located so that the incoming air-fuel mixture swirls.
Power stroke
Piston stroke that occurs when both valves are in the closed position and the force of combustion drives the piston downward.

38. Glossary Stroke Top dead center Two-stroke engine
Movement of a piston in the cylinder from one end of its travel to the other.
Top dead center
The point at which the piston is at its upper most position on the compression stroke.
Two-stroke engine
An engine design that produces one power stroke for each revolution of the crankshaft.

39. Glossary Valve overlap
An interval expressed in degrees where both valves of an engine cylinder are open at the same time.