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
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