1. Valve Types, Valve Trains,
Daily Lesson
Small Engine
Valve Types, Valve Trains,
and Compression
Prepared by: Brian D. Alexander
Virginia Tech
This presentation has not been edited by the Georgia Curriculum Office.

2. What do a Heart and an Engine have in common?

3. They both contain Valves!
This unit will cover valves, valve trains and the effect they have on compression.

4. Valves and Compression
Performance Objectives:
SWBAT (Students Will be Able To):
1. Describe the valve parts of a small engines
2. Explain the function of valves in an engine

5. Valves and Compression
Enabling Objectives
SWBAT:
1. Identify three types of valve systems
2. Describe the function valves serve
3. Identify the components of a poppet valve train
4. Differentiate between intake and exhaust valves

6. Valves and Compression
Enabling Objectives (continued)
SWBAT:
5. List the advantages of Over Head Valve systems
6. List and identify six different valve problems
7. List ways valves affect compression

7. Purpose of Engine Valves
Intake valve - a valve that permits a fluid or gas to enter a chamber and seals against exit.
Exhaust valve - a valve that permits burned and unburned gases to escape, and seals against exit.

8. Valve Systems
Poppet valves
Overhead valves (OHV)
Reed Valves

9. Poppet Valve Assembly
Valve
Valve spring
Retainer(s)

10. Components of a Complete Valve Train
Valve guide
Spring
Cam

11. Overhead Valve System

12. Overhead Valve System Improves volumetric efficiency
Eliminates combustion chamber hot-spots
Increases fuel efficiency by as much as 25%
Improves engine service life

13. Valves Effect Compression
It is essential that valve springs meet manufacturer specifications.
Through overheating and extensive use, valve springs can lose their elasticity and become distorted.

14. Reed Valve System
The reed valve system can be found in cycle engines
Reed valve - a flat , flexible plate that permits air or liquid to pass in one direction but seals when the flow reverses.
Picture from Ag Mec book

15. Valve Part Names
Sec 14 p. 5 Fig 11 Briggs book

16. Valve Operating Conditions
degrees F
High pressure pounds/sq. inch
Open and close - 1/50 of a second (at 3000 RPM)
section 14 p. 3 fig 8 (Briggs book)

17. Inspecting Valves & Seats
1) Eroded, cracked, or pitted valve faces, heads, or stems
2) Warped head
3) Worn or improperly ground valve stems
4) Bent valve stems
5) Margin less than 1/64 inches
6) Partial seating

18. Valves Affect Compression
The correct valve seat angle is necessary for proper valve seating.
Valve seats are generally cut to a 45 degree angle, although 30 degree seat angles are used in a few engines.
Fig p.281 yel

19. Valves Affect Compression
The value seat width is important for effective valve system operation.
Valve seat widths range from .030 to .060 in. (1/32 - 1/16 in.)
Fig p.281 yellow

20. Valves Affect Compression
It is essential that valve springs meet manufacturer specifications.
Through overheating and extensive use, valve springs can lose their elasticity and become distorted.
Fig p.279 yellow

21. Valves Effect Compression
Valve guides align and “steer” the valves so that they can open fully and close completely.
Valve-to-guide clearance should run about .002 to .003 in.

22. Valves Effect Compression
There are two types of valve guides
Bored in block
Pressed in block

23. Valve Failures

24. Engine Valves are Essential
Control intake and exhaust
Affect compression
Three major systems
Poppet, OHV and Reed