How Many Small Engines do you have at home

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How Many Small Engines do you have at home?

• Do you understand how they work and what makes
  them run?
• This presentation is from Virginia Tech and has
  not been edited by the Georgia Curriculum Office.

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Small Gasoline Engines

• Principles of Operation
• 4-stroke
• and
• 2-stroke
• Zach Olinger
• Spring 2002

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Performance objectives

• To explain the principles of operation of a
  4-stroke engine
• To explain the principles of operation of a
  2-stroke engine

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Enabling Objectives

• Following the instruction in this lesson, the
  student will be able to
• Discuss the common uses of small engines
• Distinguish between two- and four- cycle engines
• Identify the major components of small engines

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• Explain the function of the major components of
  small engines
• Explain the position of the piston and valves
  during each stroke
• Explain what is taking place during each stroke
  of four- and two- cycle engines

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Name some uses of small engines.

• chainsaws
• lawn mowers
• weed eaters

• four wheelers
• wood splitters
• garden tillers

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• There are many things that we use on a regular
  basis that are powered by small engines.
• Small engines make our lives easier.

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There are two types of small gasoline engines
that are common today

• Four- cycle engines

• Two- cycle engines

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The Four-Cycle Engine
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The four-cycle engine operates on a series of
four strokes or piston movements
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The piston operates in an up and down, or back
and forth motion within a cylinder.

• Piston

• Cylinder

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The piston is connected to a crankshaft that
converts the up and down motion to a rotary
motion, which powers the implement.

• crankshaft

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The crankshaft extends through the crankcase and
is attached to a flywheel at one end and a blade,
gear, transmission or pulley at the other end.

• Crankcase

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The cylinder is capped with a thick plate called
a Head, which seals the cylinder.

• Cylinder Head
• Head Bolts

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Engine Block Valves

• Within the block are two valves Intake
  and Exhaust

valves
piston
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• The Intake Valve lets the fuel mixture into the
  combustion chamber.
• The Exhaust valve lets exhaust fumes out.

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The valves operate by means of a Camshaft. Which
is powered by a gear on the crankshaft

• As the camshaft turns, the lobes push open the
  valves with precise timing so that intake and
  exhaust occur at the right instant.

Lobes
Camshaft
Camgear
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The Carburetor provides the fuel/air mixture to
the engine.

• Gas from the fuel
• tank enters the
• carburetor, where
• it is vaporized and
• mixed with the
• correct amount
• of air.

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In the engine, the mixture is ignited, and the
burning gases provide the force to drive the
piston.

• Ignition in the combustion chamber occurs in the
  form of an electrical spark from the spark plug.

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The spark at the spark plug comes from a process
known as electrical induction.

• This begins with the flywheel.

Flywheel
magnet
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• As the flywheel magnet passes the armature
• coil, a low-voltage current is induced in a
  primary circuit.
• When the circuit is
• opened, the decaying
• magnetic field sends
• a high voltage charge
• through a secondary
• circuit to the spark
• plug.

Armature
Flywheel
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In a four-stroke engine, moving parts are
lubricated by motor oil.

• In most small engines, the oil is located in the
  bottom of the crankcase in the oil sump, and is
  distributed throughout the engine to vital parts
  by means of a oil slinger or oil dipper depending
  upon the crankshaft position.

oil sump
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Strokes of a Four-Cycle Engine

• It takes four strokes (piston moving within the
  cylinder) to complete one cycle. These are
• 1. Intake Stroke
• 2. Compression Stroke
• 3. Power Stroke
• 4. Exhaust Stroke

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Intake Stroke

• The piston moves downward in the cylinder,
  creating a vacuum or area of low pressure.
• The intake valve opens allowing a mixture of air
  and atomized (vaporized) fuel to flow into the
  combustion chamber.

intake valve opening
exhaust valve closed
piston moving downward
cylinder
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Intake Stroke
KEYA Intake passageB Intake valveC
Spark plugD Exhaust valveE Exhaust
passageF PistonG Piston pinH Connecting
rodI CrankshaftJ CrankcaseK Combustion
chamber
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Compression Stroke

• Piston moves upward in the cylinder, compressing
  the air/fuel mixture into the space between the
  top of the piston and the cylinder head. This
  area is called the combustion chamber.

Piston moving upward
Both valves Closed
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CompressionStroke
KEYA Intake passageB Intake valveC
Spark plugD Exhaust valveE Exhaust
passageF PistonG Piston pinH Connecting
rodI CrankshaftJ CrankcaseK Combustion
chamber
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Ignition

• For the engine to operate properly, ignition of
  the compressed fuel/air mixture must occur with
  precise timing, slightly before the piston
  reaches TDC (Top Dead Center).

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Power Stroke

• The burning and expanding gases drive the piston
  downward.
• Both valves remain tightly closed

Both Valves Closed
Piston Moving downward
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PowerStroke
KEYA Intake passageB Intake valveC
Spark plugD Exhaust valveE Exhaust
passageF PistonG Piston pinH Connecting
rodI CrankshaftJ CrankcaseK Combustion
chamber
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Exhaust Stroke

• The piston moves upward, the exhaust valve opens
  and forces the exhaust fumes out of the cylinder.

Piston moving upward
Intake Valve Closed
Exhaust Valve Open
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ExhaustStroke
KEYA Intake passageB Intake valveC
Spark plugD Exhaust valveE Exhaust
passageF PistonG Piston pinH Connecting
rodI CrankshaftJ CrankcaseK Combustion
chamber
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• At the end of the exhaust stroke, when the piston
  reaches TDC, the intake valve begins to open and
  the piston begins moving downward, beginning a
  new cycle.

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Two Cycle Engines
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• Two- cycle engines are typically used in smaller,
  hand-held equipment like chainsaws and string
  trimmers because they can be smaller and lighter
  than four-cycles.
• Also
• Used in these types of equipment because unlike
  four-cycle engines, two cycle engines can operate
  on their side or even up-side down

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• The two cycle engine completes its cycle of
  intake, compression, power, and exhaust with only
  two strokes of the piston.
• It takes only one revolution of the shaft to
  complete the 2-stroke cycle.

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• Two cycle engines do not have an oil sump.
• Critical parts of the engine are lubricated by
  oil that is mixed with the fuel.

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• Two-stroke engines do not have valves like
  four-strokes.
• They have an exhaust port that is opened and
  closed by the moving piston, and a Reed valve to
  let air/fuel mixture in.

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Summary

• There are two major classifications of small
  engines
• 2-cycle
• 4-cycle
• The four strokes in a four cycle engine are
• intake
• compression
• power
• exhaust

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• Major parts and functions summary
• With the piston moving down, air/fuel mixture
  from the carburetor enters the combustion chamber
  through the intake valve.
• An electrical charge induced from the magneto is
  sent to the spark plug, and ignites the mixture.
• The burning gases drive the piston downward while
  both valves are closed.
• The piston is connected to a crankshaft which
  converts the up and down motion of the piston to
  a rotary motion.

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• The crankshaft is responsible for turning the
  camshaft, which raises and lowers the valves, the
  flywheel, and the lubrication system.
• The momentum of the counterweights on the
  crankshaft carry the piston back up the cylinder,
  while the exhaust valve opens to expel exhaust
  fumes.

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• The 2-cycle engines undergo the same events of
  intake, compression, power, and exhaust, but
  only takes 2 piston strokes to complete the cycle.