How Many Small Engines How Many Small Engines do you have at home?do you have at home?

Do you understand how they Do you understand how they work and what makes them work and what makes them run?run?

This presentation is from Virginia Tech and has not This presentation is from Virginia Tech and has not been edited by the Arizona Curriculum Office. been edited by the Arizona Curriculum Office.

Small Gasoline Engines:Small Gasoline Engines:

Principles of Principles of OperationOperation

4-stroke4-stroke

andand

2-stroke2-stroke

Zach OlingerZach Olinger

Spring 2002Spring 2002

Performance objectives:Performance objectives:

To explain the principles of To explain the principles of operation of a 4-stroke engineoperation of a 4-stroke engine

To explain the principles of To explain the principles of operation of a 2-stroke engineoperation of a 2-stroke engine

Enabling ObjectivesEnabling Objectives Following the instruction in this Following the instruction in this

lesson, the student will be able to:lesson, the student will be able to:

• Discuss the common uses of small Discuss the common uses of small enginesengines

• Distinguish between two- and four- Distinguish between two- and four- cycle enginescycle engines

• Identify the major components of Identify the major components of small enginessmall engines

• Explain the function of the major Explain the function of the major components of small enginescomponents of small engines

• Explain the position of the piston and Explain the position of the piston and valves during each strokevalves during each stroke

• Explain what is taking place during Explain what is taking place during each stroke of four- and two- cycle each stroke of four- and two- cycle enginesengines

Name some uses of small Name some uses of small engines.engines.

chainsawschainsaws lawn mowerslawn mowers weed eatersweed eaters

four wheelersfour wheelers wood splitterswood splitters garden tillersgarden tillers

There are many things There are many things that we use on a regular that we use on a regular basis that are powered basis that are powered by small engines. by small engines.

Small engines make our Small engines make our lives easier.lives easier.

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

Four- cycle Four- cycle enginesengines

Two- cycle Two- cycle enginesengines

The Four-Cycle EngineThe Four-Cycle Engine

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

The The piston piston operates in operates in an up and down, or back an up and down, or back and forth motion within a and forth motion within a cylinder.cylinder.

PistonPiston CylinderCylinder

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

crankshaft:crankshaft:

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

CrankcaseCrankcase

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

Cylinder HeadCylinder Head

Head BoltsHead Bolts

Engine Block & ValvesEngine Block & Valves

Within the Within the blockblock are two valves: are two valves: IntakeIntake

and and ExhaustExhaust

The Intake Valve lets the fuel The Intake Valve lets the fuel mixture into the combustion mixture into the combustion chamber.chamber.

The Exhaust valve lets exhaust The Exhaust valve lets exhaust fumes out.fumes out.

The valves operate by The valves operate by means of a means of a CamshaftCamshaft. . Which is powered by a Which is powered by a gear on the crankshaftgear on the crankshaftAs the camshaft turns, the lobes As the camshaft turns, the lobes

push open the valves with precise push open the valves with precise timing so that intake and exhaust timing so that intake and exhaust occur at the right instant.occur at the right instant.

The The CarburetorCarburetor provides provides the fuel/air mixture to the the fuel/air mixture to the engine.engine.

Gas from the fuelGas from the fuel

tank enters the tank enters the

carburetor, where carburetor, where

it is vaporized andit is vaporized and

mixed with the mixed with the

correct amountcorrect amount

of air.of air.

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

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

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

This begins with the This begins with the flywheel.flywheel.

As the flywheel magnet passes the As the flywheel magnet passes the armaturearmature

coil, a low-voltage current is induced coil, a low-voltage current is induced in a primary circuit. in a primary circuit.

When the circuit is When the circuit is

opened, the decayingopened, the decaying

magnetic field sendsmagnetic field sends

a high voltage chargea high voltage charge

through a secondarythrough a secondary

circuit to the sparkcircuit to the spark

plug.plug.

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

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

Strokes of a Four-Cycle Strokes of a Four-Cycle EngineEngine

It takes four strokes (piston It takes four strokes (piston moving within the cylinder) to moving within the cylinder) to complete one cycle. These are:complete one cycle. These are:

1. Intake Stroke1. Intake Stroke

2. Compression Stroke2. Compression Stroke

3. Power Stroke3. Power Stroke

4. Exhaust Stroke4. Exhaust Stroke

Intake Stroke:Intake Stroke: The piston moves downward in the The piston moves downward in the

cylinder, creating a vacuum or cylinder, creating a vacuum or area of low pressure.area of low pressure.

The intake valve opens allowing a The intake valve opens allowing a mixture of air and atomized mixture of air and atomized (vaporized) fuel to flow into the (vaporized) fuel to flow into the combustion chamber.combustion chamber.

Intake Stroke: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, Piston moves upward in the cylinder,

compressing the air/fuel mixture into compressing the air/fuel mixture into the space between the top of the the space between the top of the piston and the cylinder head. This area piston and the cylinder head. This area is called the is called the combustion chambercombustion chamber. .

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

IgnitionIgnition

For the engine to operate properly, For the engine to operate properly, ignition of the compressed fuel/air ignition of the compressed fuel/air mixture must occur with precise mixture must occur with precise timing, slightly before the piston timing, slightly before the piston reaches reaches TDCTDC ( (TTop op DDead ead CCenter).enter).

Power StrokePower Stroke

The burning and expanding gases The burning and expanding gases drive the piston downward. drive the piston downward.

Both valves remain tightly closedBoth valves remain tightly closed

PowerPowerStroke:Stroke:

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

Exhaust StrokeExhaust Stroke

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

ExhaustExhaustStroke:Stroke:

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

At the end of the exhaust stroke, At the end of the exhaust stroke, when the piston reaches TDC, the when the piston reaches TDC, the intake valve begins to open and intake valve begins to open and the piston begins moving the piston begins moving downward, beginning a new cycle.downward, beginning a new cycle.

Two Cycle EnginesTwo Cycle Engines

Two- cycle engines are typically used in Two- cycle engines are typically used in smaller, hand-held equipment like smaller, hand-held equipment like chainsaws and string trimmers because chainsaws and string trimmers because they can be smaller and lighter than four-they can be smaller and lighter than four-cycles.cycles.

Also:Also: Used in these types of equipment because Used in these types of equipment because

unlike four-cycle engines, two cycle unlike four-cycle engines, two cycle engines can operate on their side or even engines can operate on their side or even up-side down up-side down

The two cycle engine The two cycle engine completes its cycle of intake, completes its cycle of intake, compression, power, and compression, power, and exhaust with only two strokes exhaust with only two strokes of the piston.of the piston.

It takes only one revolution of It takes only one revolution of the shaft to complete the 2-the shaft to complete the 2-stroke cycle. stroke cycle.

Two cycle engines do not have an oil Two cycle engines do not have an oil sump. sump.

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

Two-stroke engines do not have Two-stroke engines do not have valves like four-strokes.valves like four-strokes.

They have an They have an exhaust portexhaust port that is that is opened and closed by the moving opened and closed by the moving piston, and a piston, and a Reed valveReed valve to let to let air/fuel mixture in.air/fuel mixture in.

SummarySummary

There are two major classifications of There are two major classifications of small enginessmall engines

– 2-cycle 2-cycle – 4-cycle4-cycle

The four strokes in a four cycle engine areThe four strokes in a four cycle engine are• intakeintake• compressioncompression• powerpower• exhaustexhaust

Major parts and functions summary:Major parts and functions summary:• With the With the pistonpiston moving down, air/fuel moving down, air/fuel

mixture from the mixture from the carburetorcarburetor enters the enters the combustion chambercombustion chamber through the through the intake intake valvevalve..

• An electrical charge induced from the An electrical charge induced from the magnetomagneto is sent to the is sent to the spark plugspark plug, and , and ignites the mixture.ignites the mixture.

• The burning gases drive the piston The burning gases drive the piston downward while both valves are closed.downward while both valves are closed.

• The piston is connected to a The piston is connected to a crankshaftcrankshaft which converts the up and down motion of which converts the up and down motion of the piston to a rotary motion.the piston to a rotary motion.

The crankshaft is responsible for The crankshaft is responsible for turning the turning the camshaftcamshaft, which , which raises and lowers the valves, the raises and lowers the valves, the flywheel, and the lubrication flywheel, and the lubrication system. system.

The momentum of the The momentum of the counterweights on the crankshaft counterweights on the crankshaft carry the piston back up the carry the piston back up the cylinder, while the exhaust valve cylinder, while the exhaust valve opens to expel exhaust fumes.opens to expel exhaust fumes.

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