Small Engines

The identification and operating process of four-cycle and two-cycle small engines

Mr. Alan Ford

Performance objectives:

Identification of four-cycle engines

Identification of two-cycle engines

Describe the appropriate operations of these engines

Students will be able to:Discuss the common uses of small

enginesDistinguish between two-cycle and four-

cycle engines Identify the major components of small

enginesExplain the function of the major

components of small engines

Students will be able to:Explain the position of the piston and

valves during each strokeExplain what is taking place during each

stroke of four- and two- cycle engines

What are some uses for Small Engines???

ChainsawsLawnmowersWeed EatersGo-CartsCompressorsGarden Tillers

There are two different types of small engines

Four-cycle Engines

Two-cycle Engines

We will now discuss each of the engines

The Four-cycle EngineLawn Tillers

Go-Carts

Lawn mowers

Compressors

Ect…..

Does anyone know why it is called a Four-cycle Engine??

A four-cycle engine operates in a series of four (cycles or strokes) of a piston moving up and down within the cylinder of a motor.

Here are pictures of the piston as well as the cylinder within which the piston moves up and down inside

The piston is connected to the crankshaft, which converts the linear motion of the piston into the rotary motion, that powers the implement.

Crankshaft

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

crankcase

The crankcase also houses the camshaft which connects to and

rotates along with the crankshaft.

The camshaft has a pair of lobes on it that are responsible for lifting a set of valves that controls the intake and exhaust operations of the engine.

The valves which the camshaft lobes lift are called the Intake and Exhaustvalves. They are located inside the engine block which also surrounds the cylinder IntakeIntake ExhaustExhaustblockblock

cylindcylinderer

The Intake Valve is used to allow air-fuel mixture to enter into the cylinder area called the Combustion Chamber.

The purpose of the Exhaust Valve is to allow the used up air-fuel mixture (exhaust) to escape out of the Combustion Chamber.

Combustion Chamber

The block is capped off by a thick plate of metal called the Head, which is used to seal off the the cylinder.

The Head is secured by numerous head bolts.

Cylinder HeadCylinder Head

What provides the engine with an Air-Fuel mixture?

Gas from the fuel tank enters into the Carburetor, where it is then vaporized and mixed with right amount of air needed for combustion.

What causes the combustion? Ignition of the air-

fuel mixture occurs from an electrical spark coming from the Spark Plug. The result of the burning gas generates the force that moves the piston.

The spark from the spark plug comes from a process called electrical induction. This process starts

at the Flywheel, which is a circular piece of metal that has a magnetic area located on part of its circumference.

Electrical Induction continues as the magnetic area passes over the Armature. The armature captures

the low voltage current produced by the passing magnet in its primary coil. When the circuit is opened it ups the voltage as the current passes through a secondary coil and onto the spark plug.

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

The oil is located in the bottom of the crankcase in an area called the oil sump, where it is distributed through the motor usually from an oil slinger or a dipper that splashes the oil around while the crankshaft rotates

The Strokes of a Four-Cycle Engine

It takes four strokes (movement of the piston either up or down inside the cylinder) to complete one full cycle of the engine

1. Intake 1. Intake StrokeStroke

2. 2. Compression Compression StrokeStroke

3. Power 3. Power StrokeStroke

4. Exhaust 4. Exhaust StrokeStroke

Intake Stroke The piston moves

downward in the cylinder creating a vacuum or area of low pressure.

The rotating camshaft lifts the intake valve allowing air-fuel mixture to enter into the combustion chamber.

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 The piston moves

upward in the cylinder, compressing the air-fuel mixture between the top of the piston and the cylinder head. That is the area called the combustion chamber.

Compression Stroke: KEY:

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

Power Stroke The spark plug fires

and ignites the compressed air-fuel mixture

The expanding gas formed forces the piston back downward

Both valves continue to remain tightly closed

Power Stroke: KEY:

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

Exhaust Stroke The piston begins

moving back upward The exhaust valve is

lifted open by the exhaust lobe on the camshaft

The exhaust fumes are forced out of the cylinder past the exhaust valve and out of the engine through the muffler

Exhaust Stroke: KEY:

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

The Start of a New Cycle

Once the piston finishes the Exhaust Stroke and reaches the top of the cylinder, the Intake Valve begins to open as the piston moves downward, starting the Intake Stroke and beginning the whole process once again.

Two-Cycle Engines

Two-Cycle EnginesThey are generally used for smaller,

hand held equipment like chainsaws and weed eaters because than can be smaller and lighter weight.

They are also unique because, unlike four-cycles, they can be used while on their sides and up-side down.

Two-Cycle Engines

These engines complete all the cycles of intake, compression, power, and exhaust in only two strokes of the piston.

The crankshaft only has to make one full rotation to complete a two-stroke cycle before beginning again.

The Operation of a Two-Cycle

The Operation of a Two-Cycle

The Operation of a Two-Cycle

Unique Features of a Two-Cycle Two-Cycle engines

do not have valves like the four-cycles.

They have an Exhaust Port for the emissions and a Reed Valve for the air-fuel mixture to enter in past.

Unique Features of a Two-Cycle

These engines do not have an oil sump.

The oil needed for lubricating moving parts comes from the oil-gas mixture it is required to use.

This is why the motor does not have to remain upright for operations.

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 magnet on the flywheel is picked up by the armature and is sent to the spark plug which ignites the mixture.

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

The piston is connected to a crankshaft which converts the up and down motion of the piston to a rotary motion which drives the implement.

Summary

The crankshaft is responsible for turning the camshaft, the flywheel, and the lubrication system.

The Lobes on the camshaft are responsible for lifting the Intake and the Exhaust valves at the appropriate times.

As the piston moves back upward the exhaust valve opens and lets the fumes escape the combustion chamber.

Summary

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

Two-cycle engines have the oil needed for lubrication of moving parts mixed in with the fuel. There is not a need for an oil sump and can be used up-side down