Principles of Engine Operation, Two- and Four-Stroke Engines

C H A P T E R 5

Principles of Engine Operation, Two- and Four-Stroke Engines

Permission granted to reproduce for educational use only.© Goodheart-Willcox Co., Inc.

• Explain simple engine operation.• Explain why gasoline is atomized in the small

engine. • Describe four-stroke engine operation and explain

the purpose of each stroke.• Explain the concept of valve timing.• Compare the lubrication system in a four-cycle

engine to the system in a two-stroke engine.

Learning Objectives


• Describe two-stroke engine operation and explain the principles of two-cycle operation.

• List the advantages and disadvantages of two-stroke and four-stroke engines.

Learning Objectives


Principles of Engine Operation

• Converts chemical energy into mechanical energy• Gasoline engine is an internal combustion engine• Gasoline must ignite easily and burn quickly• Energy produced by burning gasoline must be

controlled


Gasoline Must Be Atomized

• The more surface area exposed to air, the more vapor will be given off

• More vapor leads to faster burning• Gasoline must be atomized• Atomization

– Increased burning area– Explosive release of heat energy


Elementary Engine


Two- and Four-Stroke Engines

• Engines identified by number of piston strokes required to complete one operating cycle

• Each stroke is either toward the rotating crankshaft or away from it– Bottom dead center– Top dead center

• Strokes identified by job they perform


Four-Stroke Engine

• Four strokes needed to complete operating cycle– Intake stroke– Compression stroke– Power stroke– Exhaust stroke

• Two strokes occur during each crankshaft revolution

• Two crankshaft revolutions complete one operating cycle


Four-Stroke Engine


• Piston travels downward

• Volume of space above piston increases– Creates partial

vacuum

• Intake valve open and exhaust valve closed

Intake Stroke


Intake Stroke

• Atmospheric pressure forces air through carburetor, through intake valve port, and into cylinder

• Intake valve must open and close at the correct time– Incoming air-fuel mixture cools valve during engine

operation


• Piston moves upward• Both valves closed• Mixture is

compressed– Force of combustion is

increased

Compression Stroke


• Both valves closed• Air-fuel mixture

ignited• Burning action forces

piston downward

Power Stroke


• Intake valve closed and exhaust valve open

• Rising piston pushes exhaust gases from engine

Exhaust Stroke


• Exhaust valve– Allow a streamlined

flow of exhaust gases– Heat must be

controlled

Exhaust Stroke


Four-Stroke Engines

• Valve Timing– Measured in degrees of crankshaft rotation– Varies with different engines– Valve overlap

• Lubrication– Provided by oil in the crankcase

• Splash and pump systems

– Oil must be drained and replaced periodically


Two-Stroke Engine

• Two strokes occur during each revolution of crankshaft

• Advantages over four-stroke engine– Simpler in design– Smaller– Lighter– Adequate lubrication at extreme angles


Two-Stroke Engine

• Cross-scavenged– Contoured piston head prevents air-fuel charge

from going out exhaust port– Use reed valves or rotary valve

• Loop-scavenged– Flat or slightly domed piston head– Transfer ports cause incoming air to swirl


Two-Stroke Engine

(Kohler Co.)


Two-Stroke Engine Operation

(Rupp Industries, Inc.)


• Piston moving upward• Crankcase pressure

drops• Intake port exposed• Intake air through

carburetor pulls fuel and oil into crankcase

Intake into Crankcase


• Piston moving upward• Compresses air-fuel

charge• Spark ignites air-fuel

mixture• Piston driven

downward– Creates crankshaft

motion

Ignition-Power


• Piston moving downward

• Exhaust port exposed• Exhaust gases

expelled• Complete exhausting

occurs when transfer ports are opened– New charge rushes in

Exhaust


• Piston moving downward

• Air-fuel charge in crankcase compressed

• Transfer port opened– Compressed charge

rushes through port

Fuel Transfer


Four-Stoke Engine vs. Two-Stroke Engine


Review

Why is a gasoline engine considered an internal combustion engine?

Because gasoline is combined with air and burned inside the engine.


Review

What is atomization and why is gasoline atomized?

Atomization involves breaking gasoline up into tiny droplets and mixing it with air. Gasoline is atomized to produce the rapid burning required in an engine.


Review

What are the four-strokes in a four-stroke cycle?

Intake stroke

Power stroke

Compression stroke

Exhaust stroke


Review

What is valve overlap?

Valve overlap is a condition in which both valves are open at the same time.


Review

Why do two-stroke engines receive adequate lubrication even when operated at extreme angles?

Because it receives its lubrication as fuel mixed with oil is passed through the engine.


Review

How many crankshaft revolutions are required to complete a two-stroke cycle?

One


Review

What advantages does a two-stroke engine have over a four-stroke engine?

A two-stroke engine:

…is simpler in design than a four-stroke engine

…is smaller and lighter than a four-stroke engine of equivalent horsepower

…will receive adequate lubrication even when operated at extreme angles

Documents

Principles of Engine Operation, Two- and Four-Stroke Engines