Many engines are fitted with a fan at the end. They are "self-cooling" because the fan only runs when the engine is running. The purpose of the fan is to cool the engine's cooling fins, so that the engine does not overheat.
If the cooling fins are "blocked" by dirt, then there is virtually no cooling and the engine's lifespan is significantly reduced.
Velum has found a clever solution to that problem!
Watch the video about Velum Engine Filters here:
Danish: https://www.youtube.com/watch?v=uI3ug9dhN5s
NOTE: Once you have purchased a complete set, you can get individual filters for as low as 14 kr./piece. Therefore, it is no longer worthwhile to cut them yourself.