By Ryan Saucier

Introduction to Engine Cycles

• For an engine to operate, a series of events must occur in sequence:

Series of Events

1. Fill the cylinder with a combustible mixture.

2. Compress this mixture into a smaller space.

Series of Events

3. Ignite the mixture and cause it to expand, producing power.

4. Remove the burned gases from the cylinder.

Series of Events

• 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:

–Two-Stroke Cycle

–Four-Stroke Cycle

Two-Stroke Cycle

• In the two-stroke cycle engine, there are two strokes of the piston, one up and one down, during each cycle.

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

• The entire cycle occurs during one revolution of the crankshaft.

Two-Stroke Cycle Engines: Intake

and Compression

Two-Stroke Cycle Engines:

Power and Exhaust

http://www.amsoil.com/graphs/2_cycle_schem_640px.jpg

Two-Stroke Cycle

Engine in Action

Examples of Two-Stroke Engines

• Weed eaters

• Lawnmowers

• Chainsaws

Four-Stroke Cycle

• In the four-stroke cycle engine, there are four strokes of the piston, two up and two down, 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

Intake Stroke

• The intake strokes 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.

• 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 fuel-air mixture.

• 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 on its power stroke.

• 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.

Examples of Four-Stroke Cycle Engines

• Cars

• Trucks

• Tractors

Multiple Cylinder Engines

• Used for a more continuous flow of power.

• Modern engines use four, six, eight or more cylinders.

• The same four-stroke cycle occurs in each cylinder.

Examples of Various Multi-Cylinder Engines

Opposing Four Cylinder Engine or Boxer Engine

Inline Four Cylinder Engine

V- 6 Engine

Rotary Engine

Rotary Engine

Examples of Uses of Rotary Engines

Datsun 280Z

Mazda RX-8

Any Others?

Any Questions