The "Dippy Bird" is a classic example of a heat engine. The bird repeats this motion as long as the temperature at the top of the bird is different from the temperature in the bottom of the bird. The bird motion keeps repeating. The repetitions are called cycles.
Fluid in the bird undergoes several changes. If you watch the video you can even see the fluid go up and down inside of the bird. The fluid in the bird follows a series of processes. Below is the collection of processes the bird follows.
Any collection of processes is called a thermodynamic cycle. By the time these four processes have completed themselves the bird is back to its starting position and the bird readies itself to "drink" again.
A cycle can be made from any combination or number of processes. The net work in the cycle is the area of the enclosed shape.
If the process at the "top" of the cycle points to increasing volume then the net work is positive. If the "top" process points towards dimenishing volume, then the net work is negative. The process above has a positive net work.
by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)