Engine Cycles

Engine

Cycles

Engine

Cycles

Engine

Cycles

Engine

Cycles

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Introduction to Engine Cycles

•For an engine to operate,

a series of events must occur in a “specific sequence”

This “Series of Events” is as follows:

1.Fill the cylinder with a combustible mixture. This is referred to as “INTAKE”.

2.Compress this mixture into a smaller space. This is referred to as “COMPRESSION”

3.Producing Power, the mixture ignites which causes it to expand. “POWER”

4.Exhaust the burnt gases from the cylinder. “EXHAUST”

The result of these events is power.

• To produce sustained power, the engine must repeat this sequence over and over again.

•One complete series of these events in an engine is called a cycle.

Engine Cycles

• Most engines have one of the two types of cycles:

–Four-Stroke Cycle

–Two-Stroke Cycle

Four-Stroke Cycle

What are those 4 parts to the cycle again?

Four-Stroke Cycle

• In the four-stroke cycle engine, there are four strokes of the piston, up & down then up & down again during each cycle.

• Then it starts over again on another cycle of the same four strokes.

• This cycle occurs during two revolutions of the crankshaft.

• Most engines today operate on the four-stroke cycle.

Four Stroke Cycles

4-Stroke Cycle Engine in Action

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Intake Stroke• The intake stroke starts with the piston

near the top and ends shortly after the bottom of its stroke.

• The intake valve is opened, allowing the cylinder to receive the fuel-air mixture as the piston moves down.

• As the piston moves down, it creates a vaccuum

• The valve is then closed, sealing the cylinder.

Compression Stroke

• The compression stroke begins with the piston at the bottom of the cylinder and rises up to compress the air-fuel mixture.

• Compressed air-fuel is easier to ignite!

• Since the intake and exhaust valves are closed, there is no escape for the fuel-air mixture and it is compressed to a fraction of its original volume.

Power Stroke• The power stroke begins when the

piston almost reaches the top of its stroke and the fuel-air mixture is ignited.

• As the mixture burns and expands, it forces the piston down.

• The piston being pushed down creates power!

• The valves remain closed so that all the force is exerted on the piston.

Exhaust Stroke

• The exhaust stroke begins when the piston nears the end of its power stroke.

• The exhaust valve is opened and the piston rises, pushing out the burned gases.

• When the piston reaches the top, the exhaust valve is closed and the piston is ready for a new four-stroke cycle.

Advantages of 4-Stroke

• good fuel economy

• quiet

Dis-advantages of 4-Stroke

• more moving parts

• expensive to make

Examples of Four-Stroke Cycle Engines

• Cars

• Trucks

• Tractors

SCOOTERS!!!

So, that was the theory. Let’s see

these engines work

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Camshaft & OHV

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OHV with pushrods.

No Overhead Cam

Note: Electrical

Points, Condenser, Coil,

and Battery QuickTime™ and a

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Engine Cam

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Any Questions