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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. . Small Gasoline Engines:. Principles of Operation 4-stroke and 2-stroke

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how many small engines do you have at home

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.

small gasoline engines
Small Gasoline Engines:
  • Principles of Operation

4-stroke

and

2-stroke

Zach Olinger

Spring 2002

performance objectives
Performance objectives:
  • To explain the principles of operation of a 4-stroke engine
  • To explain the principles of operation of a 2-stroke engine
enabling objectives
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
slide5
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
name some uses of small engines
chainsaws

lawn mowers

weed eaters

four wheelers

wood splitters

garden tillers

Name some uses of small engines.
slide7
There are many things that we use on a regular basis that are powered by small engines.

Small engines make our lives easier.

the four cycle engine operates on a series of four strokes or piston movements

The four-cycle engine operates on a series of four “strokes” or piston movements

slide12
The piston is connected to a crankshaft that converts the up and down motion to a rotary motion, which powers the implement.

crankshaft:

slide13

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
the cylinder is capped with a thick plate called a head which seals the cylinder
The cylinder is capped with a thick plate called a Head, which seals the cylinder.
  • Cylinder Head
  • Head Bolts
engine block valves
Engine Block & Valves
  • Within the block are two valves:Intake and Exhaust

valves

piston

slide16
The Intake Valve lets the fuel mixture into the combustion chamber.
  • The Exhaust valve lets exhaust fumes out.
the valves operate by means of a camshaft which is powered by a gear on the crankshaft
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

the carburetor provides the fuel air mixture to the engine
The Carburetorprovides 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.

in the engine the mixture is ignited and the burning gases provide the force to drive the piston
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.
the spark at the spark plug comes from a process known as electrical induction
The spark at the spark plug comes from a process known as electrical induction.
  • This begins with the flywheel.

Flywheel

magnet

slide21

Armature

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.

Flywheel

in a four stroke engine moving parts are lubricated by motor oil
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

strokes of a four cycle engine
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

intake stroke
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

intake stroke25
Intake Stroke:

KEY:A = Intake passageB = Intake valveC = Spark plugD = Exhaust valveE = Exhaust passageF = PistonG = Piston pinH = Connecting rodI = CrankshaftJ = CrankcaseK = Combustion chamber

compression stroke
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

compression stroke27
CompressionStroke:

KEY:A = Intake passageB = Intake valveC = Spark plugD = Exhaust valveE = Exhaust passageF = PistonG = Piston pinH = Connecting rodI = CrankshaftJ = CrankcaseK = Combustion chamber

ignition
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).
power stroke
Power Stroke
  • The burning and expanding gases drive the piston downward.
  • Both valves remain tightly closed

Both Valves Closed

Piston: Moving downward

power stroke30
PowerStroke:

KEY:A = Intake passageB = Intake valveC = Spark plugD = Exhaust valveE = Exhaust passageF = PistonG = Piston pinH = Connecting rodI = CrankshaftJ = CrankcaseK = Combustion chamber

exhaust stroke
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

exhaust stroke32
ExhaustStroke:

KEY:A = Intake passageB = Intake valveC = Spark plugD = Exhaust valveE = Exhaust passageF = PistonG = Piston pinH = Connecting rodI = CrankshaftJ = CrankcaseK = Combustion chamber

slide33
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.
slide35
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
slide36
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.
slide40
Two cycle engines do not have an oil sump.

Critical parts of the engine are lubricated by oil that is mixed with the fuel.

slide41
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.
summary
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
slide43
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.
slide44
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.
slide45
The 2-cycle engines undergo the same events of intake, compression, power, and exhaust, but only takes 2 piston strokes to complete the cycle.