1. How Many Small Engines do you have at home?
Do you understand how they work and what makes them run?
This presentation is from Virginia Tech and has not been edited by the Georgia Curriculum Office.

2. Small Gasoline Engines:
Principles of Operation
4-stroke
and
2-stroke
Zach Olinger
Spring 2002

3. Performance objectives:
To explain the principles of operation of a 4-stroke engine
To explain the principles of operation of a 2-stroke engine

4. Enabling Objectives
Following the instruction in this lesson, the student will be able to:
Discuss the common uses of small engines
Distinguish between two- and four- cycle engines
Identify the major components of small engines

5. Explain the function of the major components of small engines
Explain the position of the piston and valves during each stroke
Explain what is taking place during each stroke of four- and two- cycle engines

6. Name some uses of small engines.
chainsaws
lawn mowers
weed eaters
four wheelers
wood splitters
garden tillers

7. There are many things that we use on a regular basis that are powered by small engines.
Small engines make our lives easier.

8. There are two types of small gasoline engines that are common today:
Four- cycle engines
Two- cycle engines

9. The Four-Cycle Engine

10. The four-cycle engine operates on a series of four “strokes” or piston movements

11. The piston operates in an up and down, or back and forth motion within a cylinder.

12. The piston is connected to a crankshaft that converts the up and down motion to a rotary motion, which powers the implement.
crankshaft:

13. The crankshaft extends through the crankcase and is attached to a flywheel at one end and a blade, gear, transmission or pulley at the other end.
Crankcase

14. The cylinder is capped with a thick plate called a Head, which seals the cylinder.
Cylinder Head
Head Bolts

15. Engine Block & Valves
Within the block are two valves: Intake and Exhaust
valves
piston

16. The Intake Valve lets the fuel mixture into the combustion chamber.
The Exhaust valve lets exhaust fumes out.

17. The valves operate by means of a Camshaft
The valves operate by means of a Camshaft. Which is powered by a gear on the crankshaft
As the camshaft turns, the lobes push open the valves with precise timing so that intake and exhaust occur at the right instant.
Lobes
Camshaft
Camgear

18. The Carburetor provides the fuel/air mixture to the engine.
Gas from the fuel
tank enters the
carburetor, where
it is vaporized and
mixed with the
correct amount
of air.

19. In the engine, the mixture is ignited, and the burning gases provide the force to drive the piston.
Ignition in the combustion chamber occurs in the form of an electrical spark from the spark plug.

20. The spark at the spark plug comes from a process known as electrical induction.
This begins with the flywheel.
Flywheel
magnet

21. As the flywheel magnet passes the armature
coil, a low-voltage current is induced in a primary circuit.
When the circuit is
opened, the decaying
magnetic field sends
a high voltage charge
through a secondary
circuit to the spark
plug.
Armature
Flywheel

22. In a four-stroke engine, moving parts are lubricated by motor oil.
In most small engines, the oil is located in the bottom of the crankcase in the oil sump, and is distributed throughout the engine to vital parts by means of a oil slinger or oil dipper depending upon the crankshaft position.
oil sump

23. Strokes of a Four-Cycle Engine
It takes four strokes (piston moving within the cylinder) to complete one cycle. These are:
1. Intake Stroke
2. Compression Stroke
3. Power Stroke
4. Exhaust Stroke

24. piston: moving downward
Intake Stroke:
The piston moves downward in the cylinder, creating a vacuum or area of low pressure.
The intake valve opens allowing a mixture of air and atomized (vaporized) fuel to flow into the combustion chamber.
intake valve: opening
exhaust valve: closed
piston: moving downward
cylinder

25. Intake Stroke:
KEY: A = Intake passage B = Intake valve C = Spark plug D = Exhaust valve E = Exhaust passage F = Piston G = Piston pin H = Connecting rod I = Crankshaft J = Crankcase K = Combustion chamber

26. Compression Stroke:*
Piston moves upward in the cylinder, compressing the air/fuel mixture into the space between the top of the piston and the cylinder head. This area is called the combustion chamber.
Piston: moving upward
Both valves: Closed

27. Compression Stroke:
KEY: A = Intake passage B = Intake valve C = Spark plug D = Exhaust valve E = Exhaust passage F = Piston G = Piston pin H = Connecting rod I = Crankshaft J = Crankcase K = Combustion chamber

28. Ignition
For the engine to operate properly, ignition of the compressed fuel/air mixture must occur with precise timing, slightly before the piston reaches TDC (Top Dead Center).

29. Piston: Moving downward
Power Stroke
The burning and expanding gases drive the piston downward.
Both valves remain tightly closed
Both Valves Closed
Piston: Moving downward

30. Power Stroke:
KEY: A = Intake passage B = Intake valve C = Spark plug D = Exhaust valve E = Exhaust passage F = Piston G = Piston pin H = Connecting rod I = Crankshaft J = Crankcase K = Combustion chamber

31. Exhaust Stroke
The piston moves upward, the exhaust valve opens and forces the exhaust fumes out of the cylinder.
Piston: moving upward
Intake Valve: Closed
Exhaust Valve: Open

32. Exhaust Stroke:
KEY: A = Intake passage B = Intake valve C = Spark plug D = Exhaust valve E = Exhaust passage F = Piston G = Piston pin H = Connecting rod I = Crankshaft J = Crankcase K = Combustion chamber

33. At the end of the exhaust stroke, when the piston reaches TDC, the intake valve begins to open and the piston begins moving downward, beginning a new cycle.

34. Two Cycle Engines

35. Two- cycle engines are typically used in smaller, hand-held equipment like chainsaws and string trimmers because they can be smaller and lighter than four-cycles.
Also:
Used in these types of equipment because unlike four-cycle engines, two cycle engines can operate on their side or even up-side down

36. The two cycle engine completes its cycle of intake, compression, power, and exhaust with only two strokes of the piston.
It takes only one revolution of the shaft to complete the 2-stroke cycle.

40. Two cycle engines do not have an oil sump.
Critical parts of the engine are lubricated by oil that is mixed with the fuel.

41. Two-stroke engines do not have valves like four-strokes.
They have an exhaust port that is opened and closed by the moving piston, and a Reed valve to let air/fuel mixture in.

42. Summary There are two major classifications of small engines
2-cycle
4-cycle
The four strokes in a four cycle engine are
intake
compression
power
exhaust

43. Major parts and functions summary:
With the piston moving down, air/fuel mixture from the carburetor enters the combustion chamber through the intake valve.
An electrical charge induced from the magneto is sent to the spark plug, and ignites the mixture.
The burning gases drive the piston downward while both valves are closed.
The piston is connected to a crankshaft which converts the up and down motion of the piston to a rotary motion.

44. The crankshaft is responsible for turning the camshaft, which raises and lowers the valves, the flywheel, and the lubrication system.
The momentum of the counterweights on the crankshaft carry the piston back up the cylinder, while the exhaust valve opens to expel exhaust fumes.

45. The 2-cycle engines undergo the same events of intake, compression, power, and exhaust, but only takes 2 piston strokes to complete the cycle.