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

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

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

4. 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

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

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

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

8. 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

9. 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.

10. 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.

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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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).

20. 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

21. 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

22. 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

23. 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

24. 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.

25. Two Cycle Engines

26. 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

27. 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.

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

32. 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.

33. 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

34. 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.

35. 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.

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

37. Small Engine Parts Four-Cycle
By Katie Murray, UGA Agriculture Education
Edited by Georgia Agriculture Education Curriculum Office
May 2006
1. Crankcase
Revised May 2007

38. Engine Block
2. Cylinder Block
1. Crankcase
Crankcase Cover

39. Cylinder Block 1. Cylinder Bore 3. Cooling Fins 2. Valves
Head gasket goes here
3. Cooling Fins
2. Valves

40. Spark Plug
Cooling Fins-Cooling fins keep heat away from the cylinder block by increasing the surface area of the cylinder block and contacting the already existing cooler air for cooling efficiency.

41. Crankshaft
Throw-measurement from the center of the crankshaft to the center of the crankpin journal. Determines the stroke of an engine. Throw = ½ stroke.
Crankshaft-converts the linear motion of the piston into rotary motion.
Counterweights-balances the forces of the reciprocating piston and reduces the load on crankshaft bearing journals.
Crankgear-interlocks with the cam gear to turn the crankshaft

42. Crankshaft Magneto Journal PTO Journal Bearing Journal
Crankpin Journal-attaches the connecting rod to the crankshaft
Magneto Journal
PTO Journal
Bearing Journal

43. Piston
Rings
Skirt
Piston Head
Piston Pin Bore

44. Piston Rings
1. Compression Ring-seals the combustion chamber from any leakage during the combustion process.
2. Wiper Ring- used to further seal the combustion chamber and to wipe the cylinder wall clean of excess oil.
3. Oil Ring-Wipes excess oil from the cylinder wall during piston movement. Openings in the ring returns the excess oil to the engine block.

45. Ring Grooves
Ring Lands

46. Crankpin Journal Bearing Surface
Connecting Rod-transfers motion from the piston to the crankshaft and functions as a lever arm
Piston Pin
Piston
Piston pin Bearing Surface
Crankpin Journal Bearing Surface
Rod Cap

47. Valve Tappets
Valve Tappets-Rides on the camshaft and pushes the bottom of the valve stem to open the valve

48. Cam Shaft- includes cam gear and cam lobes; driven by the crankgear
1. Cam Gear-portion of the camshaft that interlocks with the crankgear.
2. Cam Lobes- egg-shaped protrusion on the camshaft that moves a tappet to open a valve; controls the lift and duration of the opening and closing of the valves
Timing Mark

49. Intake Valve- allows the air-fuel mixture to flow into the cylinder
Valve Spring
Valve Retainer
Valve Stem
Valve Head
Intake Valve- allows the air-fuel mixture to flow into the cylinder
Exhaust Valve- allows exhaust valves to flow out of the cylinder

50. Flywheel
Flywheel Nut

51. Magneto
Spark Plug Wire
Armature
Magneto

52. Air Filter

53. Carburetor
Carburetor- provides the proper mixture of air and fuel to the intake valve

54. Intake Manifold

55. Muffler
Muffler Guard

56. Breather Assembly
Breather Cover

57. Shields
Cylinder Shield

58. Dipstick

59. Starter Housing

60. Gas Tank

61. Blower Housing Cover