Chapter 4 Compression System

1. Chapter 4Compression System Compression • Valves • Valve Guides • Valve Seats • Pistons • Cylinder Bore • Crankcase Breather System • Compression Release System • Valve Resurfacing System • Valve Resurfacing Service Procedures

2. Competencies • Describe the compression process in a small engine. • List and describe compression problems. • Describe important characteristics of valves. • Describe valve head design and valve dynamics. • Explain the function of valve guides. • Explain the function of valve seats.

3. Competencies • Explain piston design. • Describe the function and types of piston rings. • Explain piston ring dynamics. • List and describe common piston ring materials. • Describe cylinder block and bore design, and compression release systems.

4. Compression Process • an adiabatic process • heat is derived from the process • work applied by the piston during compression produces heat • as compression continues, temperature of the charge increases • as temperature increases, additional volatile vapor is produced

5. Compression Problems • from inadequate or excessive compression • compression leaks • detonation • Pre-ignition • excessive carbon build up

6. Valves • one-piece valves • made from single piece of metal • projection-tip welded valves • small piece of metal welded to stem • two-piece-stem welded valves • head welded onto stem • hardfacing • Heat treated coating added on valve face for better wear • valve stem surface treatments

7. Valve Head Design and Valve Dynamics • 45° exhaust valve face angle • 30° intake valve face angle • valve spring retainer

8. Valve Guides • foundation of the valve system • provide consistent paths for valve stems • valve guide length to valve stem diameter ratio is typically 7:1 • no standard clearance specifications • aluminum, brass, or sintered iron

9. Valve Seats • mate with valve face • seals combustion chamber • removes heat away from valves • used for exhaust valves

10. Piston Design • made from aluminum alloy • cast and machined at the factory into a cam ground shape • designed with a taper or with a barrel-shaped piston skirt • piston windows • machined channel near the piston pin

11. Piston Rings • seal the combustion chamber, transferring heat to cylinder wall • control oil consumption • taper-faced or barrel-faced • wiper ring • oil ring

12. Piston Ring Dynamics • combination of inertia, radial pressure, and conformability ensure required seal • piston ring twist • alignment of the piston ring end gaps

13. Piston Ring Materials • Chromium plating • break-in is accomplished at any speed above idle • piston ring expander prevents improper stressing or bending of the piston ring

14. Cylinder Block and Bore Design • the number and size of cooling fins on a cylinder block are determined by the thermal characteristics of the engine • compression release system

15. Chapter 4 Review What types of valves are commonly used in small engines? • Valves commonly used in small engines include one-piece, projection-tip welded, or two-piece-stem welded valves. What is the function of a piston ring? • A piston ring seals the combustion chamber.