Daily Lesson

1
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.

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What do a Heart and an Engine have in common?
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They both contain Valves!

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

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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

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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

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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

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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.

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Valve Systems

• Poppet valves
• Overhead valves (OHV)
• Reed Valves

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Poppet Valve Assembly

• Valve
• Valve spring
• Retainer(s)

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Components of a Complete Valve Train

• Valve
• Valve guide
• Spring
• Cam

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Overhead Valve System
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Overhead Valve System

• Improves volumetric efficiency
• Eliminates combustion chamber hot-spots
• Increases fuel efficiency by as much as 25
• Improves engine service life

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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.

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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

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Valve Part Names

• Sec 14 p. 5 Fig 11 Briggs book

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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)

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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

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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

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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

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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

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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.

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Valves Effect Compression

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

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Valve Failures
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Engine Valves are Essential

• Control intake and exhaust
• Affect compression
• Three major systems
• Poppet, OHV and Reed