The diagrams show the four primary steps in the production of work in a four-stroke heat engine. An illustration of a internal combustion engine cylinder at four different stages. The first illustration is labeled W with red ejected out of the left valve, the second is labeled X with purple in the cylinder trapped by the piston and valves, the third is labeled Y with the piston going down and the fluid red, the last is labeled Z with purple coming into the cylinder from the right valve.

What is the correct order in which these steps occur?