2. 6
Engine Components

3. Learning Objectives
Identify the basic components of a small engine and describe the function of each component.
Describe engine block variations.
Describe the construction and operation of the crankshaft.
Explain piston design considerations and differentiate between types of piston rings.
Describe connecting rod and bearing variations.
Identify common valve train configurations.

4. Engine Block Keeps engine parts in alignment
Cast iron or aluminum alloy
Consists of two sections:
Cylinder block
Crankcase

5. Cylinder Block Contains cylinder bore
Cylinder may be bored directly in block
Aluminum cylinder blocks cast around steel sleeve
Cylinder head bolts to block or is cast as integral part
Cooling fins on block and head of air-cooled engines

6. Crankcase Contains crankshaft
Must withstand rotational forces of the crankshaft
May contain oil for lubrication
May be cast with engine block

7. Crankshaft Major rotating part of engine
Converts reciprocating motion into rotary motion
Transmits engine torque
Drives camshaft
Supports flywheel

8. Crankshaft Cast or drop forged steel Tapered end receives flywheel
Flywheel keyed to crankshaft
Crankshaft throw
Offset portion of shaft
Counterweights offset unbalance

9. Crankshaft Main Bearings
Supports crankshaft in block
Three types:
Sleeve or bushing
Roller bearing
Ball bearing

10. Crankshaft Main Bearings

11. Crankshaft Main Bearings

12. Crankcase Seals Prevent leakage
Shell makes fixed contact with crankcase
Knife edge rubs crankshaft
Coil spring keeps sealing lip in contact with shaft

13. Crankcase Seal

14. Pistons Straight line driving member Subjected to heat of combustion
Adequate clearance
Seal between combustion chamber and crankcase
Piston rings exert tension on cylinder wall

15. Piston Construction Steel or aluminum Grooves accept piston rings
Aluminum most popular
Grooves accept piston rings
Lands are full-diameter ridges between grooves
Holes in bottom groove allow oil to flow back to sump
Hole for piston pin (wrist pin)
Pin boss surrounds the piston pin hole
Piston skirt keeps piston from tipping

16. Piston Construction

17. Cam Ground Piston Oval shape Thrust surfaces fit more closely
Helps prevent piston slap

18. Piston Head Runs hotter and expands more than skirt
Smaller in diameter than skirt
Shape depends on application
Four-stroke engines
Flat
Domed
Wedge
Two-stroke
Flat with loop-scavenged engine
Raised baffle or deflector with cross-scavenged engine

19. Piston Rings Allow piston to compress fuel charge
Prevent burning gases from leaking
Ride on cylinder walls
Separated by thin film or oil
Rub sides of piston grooves
Grooves hold rings squarely to bore

20. Piston Rings Four-stroke pistons use three rings
Two upper rings are compression rings
Lower ring is oil control ring

21. Compression Rings Provide strong seal
Keeps compressed air-fuel mixture and burning gases above piston
Prevents passage between piston and cylinder wall
Twists in groove during intake stroke
In tipped position during compression and exhaust strokes
Flat against cylinder wall on power stroke

22. Compression Rings

23. Oil Control Rings Remove surplus oil from cylinder walls
Light scraping action
Slotted or perforated
Oil passes through holes in ring and groove
Oil flows to inside of piston and into crankcase

24. Piston Ring Construction
Cast iron or steel
Plated with long-wearing material
Outside diameter slightly larger than cylinder bore
Creates ring tension
Ring gap
Opening between ends of ring
Allows rings to expand and contract

25. Piston Ring Movement Free to move inward or outward in piston grooves
Floating rings
Rotate around grooves
Installed with end gaps staggered
Pinned rings
Held in position by pin in ring groove
Pin prevents rotation of ring around grooves

26. Piston Pins Secure piston to connecting rod Case-hardened steel
Solid or hollow
Full-floating pin
Retained by snap rings
Press-fit pin
Fits tightly in connecting rod

27. Connecting Rod Attaches piston to crankshaft
Upper end accepts piston pin
Lower end contains bearings and fits around crankshaft journal

28. Friction Bearings Used when lower end of connecting rod is split Types
Rod metal
Bearing bronze
Removable precision inserts
Locating tab
Fits into slot in rod

29. Antifriction Bearings
Rollers or balls reduce friction
Elements held together by roller cage or can be free
Hardened and ground to exact size
Must fit accurately and have clearance for expansion

30. Intake and Exhaust Ports
Provide means for admitting air-fuel mixture and exhausting waste products
Two-stroke engines
Porting of cylinder walls
Alternately covered and exposed by piston
Four-stroke engines
Poppet valves open and close port opening

31. Reed Valves and Rotary Valves
Control fuel flow directly into crankcase of two-stroke engine
Reed valve
Opens during compression stroke and closes before the power stroke
Rotary valve
Attached to end of crankshaft
Air-fuel charge enters crankcase when holes in valve and wear plate align

32. Reed Valve Action

33. Rotary Valve Action
(Evinrude Motors)

34. Poppet Valves Installed in valve ports of four-stroke engines
Control flow of air-fuel mixture into the cylinder and exhaust gases out of the cylinder
Angled valve face closes tightly against seat
Valve guide aligns valve and ensures accurate raising and lowering

35. Poppet Valves

36. Valve Spring Holds valve firmly against seat
Connect to valve stem with retainer and keeper
Allows valve to be opened and closes it when pressure is released from valve stem

37. Camshaft Used in four-stroke engines Shaft has lobe for each valve
Opens valves the right amount at the right time
Holds valve open for a specific period and allows them to close at correct instant
Shaft has lobe for each valve
Lobe lifts valve from seat when shaft rotates
Rotates at half crankshaft speed
Camshaft gear meshes with and is driven by crankshaft gear

38. Valve Lifter Located between cam lobe and valve stem
Rises and falls as camshaft rotates, opening and closing valves
May provide means to adjust clearance

39. Valve Train
Transforms crankshaft rotation into opening and closing of valves
Valve train configurations
Valve-in-block
Overhead valve
Overhead cam

40. Valve-in-Block Camshaft located in crankcase
Valves located in cylinder block, directly above camshaft lobes
Lifters act directly on valve stems

41. Overhead Valve (OHV) Camshaft installed in crankcase
Valves installed in cylinder head
Pushrods transfer motion from valve lifters to rocker arms
Rocker arm pushes down on valve stem

42. Overhead Cam (OHC) Both camshaft and valves in cylinder head
Camshaft positioned directly above valves or offset
Rocker arms used if camshaft is offset
Camshaft driven by chain or belt

43. Starter Assembly Rewind starter assembly Electric starters
Mounted above flywheel
When rope is pulled, pawls engage flywheel clutch and assembly turns crankshaft
When rope is released, pawls retract and assembly disengages
Electric starters

44. Automatic Compression Release
Makes hand cranking easier
Lifts exhaust valve during cranking
Releases part of compression pressure

45. Flywheel Fastened to one end of crankshaft
Keeps crankshaft spinning during nonpower strokes
Metal fins on flywheel force air over cylinder to cool engine
Contains magnets that produce electrical current for ignition system

46. The two sections of the engine block are the cylinder block and the _____. crankcase

47. The _____ converts the reciprocating motion of the piston into rotary motion crankshaft

48. The shape of a cam-ground piston allows the _____ surfaces to fit more closely. thrust

49. The _____ rings keep the air-fuel mixture and burning gases above the piston. compression

50. Name the three types of friction bearings used in the big end of the connecting rod. Rod metal, bearing bronze, and removable precision inserts

51. _____ bearings use rollers or balls to reduce friction between moving parts. Antifriction

52. In a(n) _____ valve train, the valves are in the cylinder block and the camshaft is in the crankcase. valve-in-block

53. In an overhead valve arrangement, _____ transfer motion from the valve lifters to the rocker arms. pushrods

54. In an overhead cam valve train, both the camshaft and the valve assemblies are installed in the _____. cylinder head

55. Glossary Antifriction bearing Automatic compression release Camshaft
A bearing constructed with balls or rollers between journal and bearing surfaces to provide rolling instead of sliding friction.
Automatic compression release
Mechanism on the camshaft designed to lift the exhaust valve slightly during cranking to release part of the compression pressure.
Camshaft
Shaft containing lobes or cams that operate engine valves.

56. Glossary Compression rings Connecting rod Cooling fins
Piston rings designed to provide a strong seal, keeping the compressed air-fuel mixture and the burning gases above the piston by preventing passage between the piston and the cylinder wall.
Connecting rod
Link that attaches the piston to the crankshaft.
Cooling fins
Thin metal projections on the cylinder block that increase the surface area around the outside of the cylinder on air-cooled engines.

57. Glossary Crankcase Crankcase seals Crankshaft
The housing for the crankshaft and other related internal parts.
Crankcase seals
Seals that prevent leakage of oil from the areas where the crankshaft and crankcase come together.
Crankshaft
The major rotating part of the engine. It converts the reciprocating (back-and-forth) motion of the piston into rotary (circular) motion.

58. Glossary Crankshaft throw Cylinder block Engine block
The offset portion of the crankshaft measured from the centerline of the main bearing bore to the centerline of the connecting rod journal.
Cylinder block
The portion of the engine block that contains the cylinder bore.
Engine block
Casting of iron or an aluminum alloy designed to keep all engine parts in alignment.

59. Glossary Floating rings Flywheel
Piston rings installed with the ring end gaps staggered to avoid gap alignment and possible oil flow through the series of gaps to the combustion chamber.
Flywheel
A heavy wheel fastened to one end of the crankshaft to improve the running quality of the engine. The inertia of the flywheel keeps the crankshaft spinning and smoothes engine operation.

60. Glossary Friction bearings Lands Oil control rings
Bearing that use smooth, sliding surfaces to reduce friction between moving parts.
Lands
The full-diameter ridges between the piston grooves.
Oil control rings
Piston rings designed to remove surplus oil from the cylinder walls.

61. Glossary Overhead cam (OHC) design Overhead valve (OHV) arrangment
Valve train arrangement in which both the camshaft and the valve assemblies are located in the cylinder head.
Overhead valve (OHV) arrangment
Valve train arrangement in which the camshaft is installed in the crankcase and the valves are installed in the cylinder head.
Pin boss
The section of the piston surrounding the piston pin hole. It is thick and often reinforced with cast-in webs.

62. Glossary Pinned rings Piston
Piston rings that are held in position by a short pin that fits into the piston ring groove. The pin prevents rotation of the ring around the groove.
Piston
A part that moves up and down in the cylinder in order to draw air-fuel mixture into the cylinder during the intake stroke, compress the mixture during the compression stroke, transfer force to the crankshaft during the combustion stroke, and force exhaust out of the cylinder during the exhaust stroke.

63. Glossary Piston rings Piston skirt Poppet valve
Circular rings that fit the piston grooves and exert tensions on the cylinder wall. Piston rings prevent blowby of exhaust gases into the crankcase and leakage of oil into the combustion chamber.
Piston skirt
The part of the piston below the rings and the bosses.
Poppet valve
A valve structure consisting of a circular head with an elongated stem attached in the center.

64. Glossary Porting Pushrods
System that consists of two holes (ports) in the cylinder wall of some two-stroke engines. One port admits the air-fuel mixture and the other port allows exhaust gases to escape.
Pushrods
A connecting link in an operating mechanism, such as the rod between the valve lifter and rocker arm on an overhead valve engine.

65. Glossary Reed valve Rewind starter assembly Ring tension
A flat, springy valve covering the ports between the carburetor and the crank chamber in a two-cycle engine.
Rewind starter assembly
Mechanism mounted above the flywheel on many small engines that spins, or cranks, the engine during starting.
Ring tension
Force that the piston ring exerts on the cylinder wall.

66. Glossary Rocker arms Rotary valves Slap
Device used in overhead valve system to transfer the upward motion of the pushrod to a downward force on the valve.
Rotary valves
A valve construction in which ported holes move in and out of register with each other to allow fluids or gases to enter and exit.
Slap
Side-to-side movement of the piston in the cylinder caused by too much clearance.

67. Glossary Snap rings Sump Thrust surfaces
Fasteners that are compressed and placed in grooves in the piston pin bosses. They prevent the pin from rubbing on the cylinder surface.
Sump
The part of the block in a small four-stroke engine that holds and collects the lubricating oil.
Thrust surfaces
The sides of the piston forced against the cylinder wall. These surfaces are at right angles (90°) to the centerline of the crankshaft and piston pin.

68. Glossary Valve guide Valve lifter
Hole machined in the engine block to align the valve and ensure accurate raising and lowering in relation to the seat.
Valve lifter
A rod or plunger that transfers motion from the cam and the other valve train components.

69. Glossary Valve spring Valve train
A spring attached to a valve to return it to the seat after it has been released from the lifting or opening operation.
Valve train
All the components that work together to transform the rotation of the crankshaft into the opening and closing of the valves.

70. Glossary Valve-in-block arrangement Wrist pin
Valve train arrangement in which the camshaft is located in the crankcase and the valves are located in the cylinder block, directly above the camshaft lobes.
Wrist pin
The journal for the bearing in the small end of an engine connecting rod. The piston pin also passes through the piston walls, holding the piston to the connecting rod.