To keep combustion gases above the piston, the space between the piston and the cylinder wall must be sealed. And for the piston to move smoothly through the cylinder, a thin oil layer is vital. Both sealing and regulating the lubrication is done with piston rings. That sounds complicated, but is it?
At first glance, piston rings look like fairly simple rings, yet they are essential for the functioning of an internal combustion engine. Regardless of whether it is a diesel or gasoline engine, the internal combustion engine is all about pushing the piston down because pressure is created on top of that piston as a result of combustion. For the greatest effect, it is important that no combustion gases escape between the cylinder and piston walls. Now you could say, “Make the piston so big that no gases can escape.” But unfortunately that is not possible: partly because the piston and the cylinder wall do not expand equally at high temperatures, there must be sufficient space between the two, otherwise things will get stuck and / or too much friction will arise.
With most engines we see three springs per piston. The top one is called the compression spring and is primarily intended not to let gas through. The bottom of the three is the oil scraper spring, which ensures that there is just enough oil on the cylinder wall to be able to slide past it and that too much oil is drained. The latter is important not only for oil consumption, but also for the composition of the exhaust gas. Burned oil creates harmful substances in the exhaust gases. The middle spring contributes to both sealing and draining too much oil.
The springs each have their own shape for their respective tasks. In order to seal as well as possible, the outside of the compression spring is smooth and straight, so that there is the largest possible contact area with which it surfs, as it were, over the thin oil film. This should ensure an optimal seal. In addition, the relatively large contact surface also ensures heat dissipation from the piston towards the cylinder wall. The outer profile of the oil scraper spring is rather pointed (often this spring is built up from several loose parts): here the contact surface is deliberately kept as small as possible so that the spring can easily push through any excessively thick oil film and the oil that is too much can scrape away. The shape of the middle spring is often a combination of a straight side with a sharp bottom so that both sealing and scraping can be done. Oh, and not unimportant: the shape of the springs is also important to ensure that the springs themselves do not get stuck due to unexpected carbon deposits, especially on diesels. In addition, the choice of material of the springs also plays a role, especially for the frictional resistance and the service life.