By Ryan Saucier Introduction to Engine Cycles For

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 fourstroke 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 fourstroke cycle occurs in each cylinder.