This is a very basic engine rig that i created. I used constraints such as prent, geometry and point to create one of the piston "sets", then duplicated them, rotating each one by 90° each time.
 

I then parented the main discs to a nurb circle, which allowed me to rotate all of them by hand. By keyframing the rotation of the nurb circle, I was able to produce the animation you see above.

Below is an example of me rotating the nurb circle to illustrate how the rig works. by applying the same rigging principles to models with higher detail, I could create a realisic looking engine.