basic principles of 4- cycle & two cycle engines bulletin 8793a

BASIC PRINCIPLES OF 4-BASIC PRINCIPLES OF 4-CYCLE & TWO CYCLE CYCLE & TWO CYCLE ENGINESENGINES

BULLETIN 8793A

BRIGGS & STRATTONBRIGGS & STRATTON

PURPOSEPURPOSE

Small internal combustion engine is an important power source for agricultural operations.

In US alone, some 70,000,000 or more single-cylinder engines are in operation today.

SMALL ENGINE USESSMALL ENGINE USES

Mowers Chainsaw Post hole diggers Feed grinders Air compressors Brush cutters Rotary tillers sprayers

Irrigation pumps Concrete equipment Generators Outboard boats Etc. Etc.

InternalInternal vs vs externalexternal combustion combustion engineengineInternal combustion engine obtained its

power by burning fuel inside an engine.

External combustion engine uses power from fuel burned outside the engine. Example: steam engine

EXTERNAL COMBUSTION EXTERNAL COMBUSTION ENGINEENGINE

EXTERNAL COMBUSTION EXTERNAL COMBUSTION ENGINEENGINE

ENGINESENGINES

www.keveney.com/Engines.html

InternalInternal vs vs externalexternal combustion combustion engineengineBoth engines require air, fuel, and ignition

to produce power in order to move a piston.

– For external engine, steam is the moving force, whereas in,

– Internal engine, an explosion occurs inside an enclosed area that exerts great pressure against a piston, forcing it to move.

Two types of Small Gasoline Two types of Small Gasoline EnginesEngines

1. Four-stroke Cycle Engine

2. Two-stroke Cycle Engine

Four-stroke Gasoline EngineFour-stroke Gasoline Engine

Crankshaft makes two complete revolutions for each cycle of events.

Complete cycle is:1. Intake stroke

2. Compression stroke

3. Power stroke

4. Exhaust stroke

Intake StrokeIntake Stroke

Piston moves downward Downward motion draws in a mixture of

air/fuel into the cylinder.

Intake valve is openExhaust valve is closed

Compression StrokeCompression Stroke

Upward movement of the piston compresses the air/fuel mixture.

Intake valve is closed.Exhaust valve is closed.

Power strokePower stroke

The air/fuel mixture is ignited by an electrical spark creating the explosion to push the piston downward.

Intake valve is closedExhaust valve is closed

Exhaust strokeExhaust stroke

Piston moves upward, forcing burned gases out through the exhaust port.

Exhaust valve is open.Intake valve is closed.

Note: The exhaust valve closes at the end of the stroke, than the intake valve opens as the next cycle begins.

Two-stroke Cycle EngineTwo-stroke Cycle Engine

To complete a cycle, the piston makes two strokes and the crankshaft turns one revolution.

Stroke I – power strokeStroke II – Compression stroke

Exhaust & intake occurs between strokes

Stroke IStroke I

Power Stroke – Piston moves downward from explosion– Reed valve in crankcase is closed.– Intake port is closed– Exhaust port is closed

Two-cycle ExhaustTwo-cycle Exhaust

Piston continues to move downward under pressure.

Exhaust port opensIntake port is still closed

Two-cycle IntakeTwo-cycle Intake

Piston continues to move downward.Intake port opens allowing fuel/oil mixture

to enter into the cylinder

Stroke II - CompressionStroke II - Compression

Piston moves upward.Intake port is closed.Exhaust port is closed.Reed valve in crankcase opens allowing

fuel/oil mixture to enter the crankcase.Fuel mixture is compressed in the cylinder.

Major DifferencesMajor Differences

Four Stroke– Regular intake and

exhaust valves

– Oil is used as a lubricant only

– Takes two revolutions of the crankshaft to complete a cycle.

– Crankcase contains only the lubricant(oil)

Two Stroke– Reed valve serves as a

valve

– Oil is mixed with gasoline to serve as a fuel and lubricant.

– Takes only one revolution of the crankshaft to complete cycle

– Crankcase contains oil and fuel mixture

Major DifferencesMajor Differences

Four stroke– Four strokes of the piston

complete one cycle.

– Has valve lifters, valve springs, and camshaft.

– Difficult to handle on very small equipment.

– Starts easier.

– Less power produced per unit of weight

Two-stroke– Two strokes of the piston

complete one cycle.

– Has no valve lifters, valve springs, and camshaft.

– Less difficult to handle on small equipment

– Difficult to start.

– More power produced per unit of weight

Major DifferencesMajor Differences

Four stroke– Easy to lubricate

– Fuel and oil are separate from each other.

– Limitation – engine cannot be tilted due to oil in the sump.

Two-Stroke– Lubricating oil is

mixed with gasoline—less efficient in lubrication.

– No limitation – engine can be tilted due to oil and fuel mixture.

Methods Used to Distinguish Methods Used to Distinguish between a 4-cycle & a 2-cyclebetween a 4-cycle & a 2-cycle Check operator’s manual for crankcase capacity

or the kind of oil needed. Check to see if engine has an oil sump (place to

add oil or place to drain the oil) Check to see if engine has an exhaust muffler.

(4-stroke is at the end of the cylinder – 2-stroke muffler is located in the center of the cylinder.

Check compression – 4-stroke every other revolution, whereas, 2-stroke is on every revolution.

Primary Functions of Engine Primary Functions of Engine OperationOperation There are 3 primary function for engine

to operate.

1. Compression

2. Carburetion

3. ignition

Secondary FunctionsSecondary Functions

There are two basic secondary functions needed for engine operation.

1. Cooling

2. lubrication

What has to occur during What has to occur during compression?compression?1. No leakage can appear between the piston and

cylinder walls.2. First compression ring is fitted in a groove near

the top of the piston so pressures can expand the ring on the cylinder wall.

3. Second compression ring is considered as a back-up, but also serves as a scraper ring. It scrapes the excess oil from the cylinder, preventing oil to accumulate in the cylinder

What has to occur during What has to occur during compression?compression?4. Perforated Oil ring, located below the

compression rings, lubricates the cylinder wall on each stroke.

Please note: All rings must meet engine specifications for engine performance.

How important are valves to an How important are valves to an engine?engine? Valves are one of the most important parts of

engine. Primary function is the intake to allow fuel-air

mixture and exhaust the burned residue. Engine operating at 3000rpm – two valves are

opening and closing 1/50 of a second. Valves must seal well enough to withstand

pressures up to 500 psi (pounds per square inch)

How important are valves to an How important are valves to an engine?engine?Exhaust valve are damaged by heat than

any other part on the engine. Exhaust valve is subject to high

temperatures as much as 2500 degrees on the head of a valve.

Parts of a valve – head, margin, face, stemPlease note: discard any valve that are

burned, dished, necked

What is displacement?What is displacement?

Displacement is the amount of air/fuel mixture to be compressed in a cylinder.

Depends on the size of bore and the length of stroke.

Piston displacement formulaPiston displacement formula

Piston displacement = (bore)² X 3.1416 X stroke 4Example of a 2” bore and 1-3/4” stroke

2 x 2 X 3.1416 X 1.75 = 5.5 cubic inch 4

Piston displacement formulaPiston displacement formula

Most small engines vary from 5:1 to 6:1 compression ratios.

Generally, engines with high compression ratio is more efficient than one with a low compression ratio.

CarburetionCarburetion

The major functions of the carburetor are:

1. Atomize the fuel (reduce to a fine spray)

2. Mix air with the fuel

3. Vaporize the fuel

4. Deliver the mixture to the combustion chamber.

Best Operating Fuel MixtureBest Operating Fuel Mixture

15 to 17 parts of air by weight to 1 part of gasoline.

What is the venturi?What is the venturi?

The venturi is the narrow part of the pipe in the carburetor.

Accelerated air moves through the narrow part causing fuel to be drawn through the nozzle and atomized as a result of low pressure.

Types of fuel systemsTypes of fuel systems

Two types of fuel systems.

1. Gravity

2. suction

Gravity Fuel systemGravity Fuel system

Fuel tank located above the carburetor.

Fuel flows by gravitation to a carburetor bowl.

A small air vent must be open on the tank cap, allowing air to move into the tank as fuel flows out.

Suction Fuel SystemSuction Fuel System

Fuel is drawn from the tank through a tube that is located below the carburetor.

Suction created by the venturi pulls gasoline from the fuel tank and a foot valve keeps it from flowing back into the tank.

Types of GovernorsTypes of Governors

1. Air-vane governor – hand throttle increases or decreases the tension on the governor spring.The spring moves the flap which is attached to a hinge in the air current created by the fins of the flywheel.

2. Centrifugal governor controls the speed by a set of weights which moves outward for high speed and moves inward for low speed. This action causes the throttle to open and close.

Air CleanersAir Cleaners

An engine running 3600 rpm, it is estimated that it takes in 390 cubic feet of air per hour. Regularly scheduled maintenance is needed for engine performance.

Three type of air cleaners1. Oil bath

2. Oil saturated polyurethane

3. Dry-type air filter element

Ignition Ignition

Ignition system furnishes a spark at the proper time and strong enough to ignite the fuel-air mixture in the combustion chamber.

Engine performs the best when the spark occurs just before the piston reaches TDC on the compression stroke.

Types of Ignition SystemTypes of Ignition System

1. Battery2. Magneto3. Solid state

Battery SystemBattery System

Battery system must include the following parts:– Battery

– Ignition coil

– Spark plug

– Battery cables

– Timer

– Condenser

– Cam

– Ignition wires

– Ignition switch

– Breaker points

– Starters

– lights

Magneto SystemsMagneto Systems

Use on small engines where no electrical load is necessary, except to ignite the fuel.

Most economical to operate.Low maintenance costs.

Magneto SystemsMagneto Systems

Most commonly used magnetos– 1. flywheel– 2. external

Magneto SystemsMagneto Systems

Flywheel magnetos– Found on small engines– Magnets are imbedded in the flywheel– Magnets is made of a special alloy called

“alnico”.– Poles of the armature are separated from the

magneto by a small air gap.

Magneto SystemsMagneto Systems

Armature coil– Primary windings (few turns of large wire)– Secondary windings (many turns of fine wire)

Magneto SystemsMagneto Systems

Primary windings– Connected to the breaker points – other end

connected to the frame of the engine.– Current flows through the primary coil as the

moving magnets in the flywheel reach the poles.

– Breaker points open-magnetic field collapses-sending about 100 volts to the coil.

Magneto SystemsMagneto Systems

Secondary winding– Collapsing magnetic field cuts through the

secondary coil increasing the current to several thousand volts.

SMALL ENGINE SPECIALITY SMALL ENGINE SPECIALITY TOOLSTOOLS

STARTER CLUTCH WRENCH

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FLYWHEEL PULLER

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FLYWHEEL WRENCH

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FLYWHEEL HOLDER

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TELESCOPIC GAUGES

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FEELER GAUGE

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SPARK PLUG GAP GAUGE

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COMPRESSION TESTER

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VALVE SPRING COMPRESSOR

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RING COMPRESSOR

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PISTON RING EXPANDER

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TORQUE WRENCH

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CYLINDER HONE

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GEAR PULLER

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VALVE GRINDING COMPOUND

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RING COMPRESSOR/WRENCH

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SNAP RING PLIERS

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VALVE SPRING COMPRESSOR

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BLADE BALANCER

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VALVE SEATING TOOL

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IGNITION TESTER

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PISTON GROOVE CLEANER