Small Engine Valves Prepared by Brian D Alexander

Small Engine Valves Prepared by: Brian D. Alexander Virginia Tech Edited by CTAE Resource Network February 2011

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: 1. Describe the valve parts of a small engines. 2. Explain the function of valves in an engine.

Valves and Compression • Enabling Objectives: 1. 2. 3. 4. 5. 6. 7. Identify three types of valve systems. Describe the function valves serve. Identify the components of a poppet valve train. Differentiate between intake and exhaust valves. List the advantages of Over Head Valve systems. List and identify six different valve problems. 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 spring • Retainer(s)

Components of a Complete Valve Train • • Valve guide Spring Cam

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

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.

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.

Valve Operating Conditions • 1200 - 2500 °F • High pressure - 500 pounds/sq. inch • Open and close - 1/50 of a second (at 3000 RPM)

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° angle, although 30° seat angles are used in a few engines. • The value seat width is important for effective valve system operation. • Valve seat widths range from. 030 -. 060 in. (1/32 - 1/16 in. )

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.

Valves Affect 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 -. 003 in.

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

Valve Failures

Summary Engine valves are essential • Control intake and exhaust • Affect compression • Three major systems § Poppet § OHV § Reed