Daily Lesson

Small Engine

Valve Types, Valve Trains,

and Compression

Prepared by: Brian D. Alexander

Virginia TechThis presentation has not been edited by the Georgia Curriculum

Office.

What do a Heart and an Engine have in common?

They both contain Valves!

This unit will cover valves, valve trains and the effect they have on

compression.

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

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

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

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.

Valve Systems

• Poppet valves

• Overhead valves (OHV)

• Reed Valves

Poppet Valve Assembly

• Valve• Valve spring• Retainer(s)

Components of a Complete Valve Train

• Valve• Valve guide• Spring• Cam

Overhead Valve System

Overhead Valve System

• Improves volumetric efficiency

• Eliminates combustion chamber hot-spots

• Increases fuel efficiency by as much as 25%

• Improves engine service life

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.

Reed Valve System

• The reed valve system can be found in 2-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

Valve Part Names

• Sec 14 p. 5 Fig 11 Briggs book

Valve Operating Conditions

• 1200 - 2500 degrees F • High pressure - 500

pounds/sq. inch• Open and close - 1/50

of a second (at 3000 RPM)

• section 14 p. 3 fig 8 (Briggs book)

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

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 16-37 p.281 yel

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 16-36 p.281 yellow

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 16-30 p.279 yellow

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.

Valves Effect Compression

• There are two types of valve guides– Bored in block– Pressed in block

Valve Failures

Engine Valves are Essential

• Control intake and exhaust

• Affect compression

• Three major systems Poppet, OHV and Reed