1. Small Engine Operation

2. Objectives Identify the main engine components of a 4 cycle engine

3. Engine Block Main Structure of an engine
Supports and maintains alignment of internal and external components
Can be produced as one or two piece unit
Consists of two pieces
Cylinder block
Crankcase

4. Cylinder Block Consists of the following components: Cylinder bore
Cooling fins
Valve train components

5. Cylinder block

6. Cylinder bore
Hole in the engine block that houses and aligns the piston in the engine block
Bore- is the diameter of the cylinder bore
Stroke- is the linear distance that a piston travels from TDC to BDC in the cylinder bore

7. Displacement
Volume that a piston displaces when traveling from TDC to BDC
When bore and stroke are know displacement fro single-cylinder engine is found by
D=0.7854*Bsquared*S

8. Displacement D=0.7854*B squared*S D=displacement 0.7854= constant
B squared= bore squared in inches
S= stroke in inches

9. Practice
What is the displacement of a single cylinder engine that has a 2.5” bore and a 2”stroke?
D=0.7854* B squared* S

10. Answer
D= * (2.5*2.5)*2
D=0.7854*6.25*2
D=9.82 cu in.

11. Multiple Cylinder Displacement
To calculate the displacement of multiple cylinder engine;
Find the displacement of a single cylinder and multiply by the total number of cylinders
The larger the displacement the more power produced

12. Cooling Fins Used on an air cooled engine
Thin cast strip on block that dissipates heat away from engine cylinder
Increase the surface area of cylinder block to which ambient air has contact
Also cast into the flywheel
Act as fan blades providing air circulation

13. Crankcase Houses and supports the crankshaft
In a four-stroke engine the crankcase also acts as oil reservoir for lubrication of engine components

14. Crankcase components Sump Horizontal shaft engines do not have a sump
Removable part of engine crankcase that serves as an oil reservoir and provides access to internal parts
Horizontal shaft engines do not have a sump
The crankcase and engine block serve as an oil resivoir

15. Crankcase Componenets
Crankcase cover
Provides access to internal parts
Supports the crankshaft
Crankcase breather functions as a check valve releasing pressure created by the cylinder moving towards BDC

16. Cylinder block construction
Cylinder blocks are commonly constructed from:
cast aluminum alloy
cast iron
or cast aluminum alloy with cast iron cylinder sleeves

17. Cast Aluminum Alloy Pro’s Con’s Cheap to produce Lightweight
Dissipate heat more rapidly than cast iron
Con’s
More prone to heat distortion
Less resistant to wear

18. Cast Iron Pro’s Con’s Last longer more resistant to wear
Can handle higher temps
Con’s
Heavier
More expensive to produce

19. Cylinder Head
Component that is fastened to the end of the cylinder block farthest from the crankshaft
Head gasket is placed between cylinder head and cylinder block
Usually constructed of soft metals and graphite layered together
Used to completely seal the combustion chamber

20. Cylinder Head
In some two-stroke engines the cylinder head and block are made as one piece otherwise know as a jug.
This single cast unit provides the maximum strength
Eliminates possible leaks in combustion chamber

21. Cylinder Head Design OHV- overhead valve L-head engine
Valves and related components are housed in the cylinder head
L-head engine
Valves and related components located in cylinder block
DOV-direct overhead valves

22. Crankshaft
Converts the up and down motion of the piston into a rotary motion
Orientation of the crankshaft in small engines is either
Horizontal or vertical shaft

23. Crankshaft Components
Crankpin journal
Throw- measurement of distance between crankshaft and crankpin journal
Throw is equal to one half the stroke
Bearing journal
Counterweights
Crank gear
and power take off (PTO)

24. Bearing journal
Crankpin journal
Crank gear
Bearing journal
Throw
PTO
counterweight

25. Piston and Piston Rings
Component that is fitted into the cylinder bore
Pistons are usually built out of cast aluminum allow
Proper tolerance must be maintained to compensate for the expansion of metals under extreme heat

26. To little and an engine might seize
To much and proper compression will not be met

27. Piston componenets Piston head Piston pin Skirt Ring grooves
Ring lands
Piston rings

28. Piston Head The shape of a piston head is either Flat Contoured
Irregular
Domed
Dished

29. Piston rings
Usually composed of cast iron to hold their structural integrity
The most common types of rings found in small engines include
Compression ring
Wiper ring
Oil ring

30. Piston rings Compression Wiper Oil
Located closest to the top of the piston
Seals the combustion chamber
Wiper
Ring with a tapered face
Further seals combustion process/wipes cylinder wall clear of oil
Oil
Located closest to crankshaft
Wipe oil off cylinder walls return it to oil reservoir
2 stroke engines do not need an oil ring

31. Piston ring components

32. Connecting Rod Transfers movement from piston to the crankshaft
Includes the following pieces
Piston pin (wrist pin) w/pin clips
Connecting rod
Rod cap w/rod cap screws

34. Flywheel Helps maintain engine power and speed between power strokes
Cooling fins also provide air circulation during operation

35. Valving system
Includes valve components as well as the cam shaft

36. L-Head

37. OHV