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
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.
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