Summary The most common air cleaner is called a dry-element type. It has a replaceable dry-element of pleated paper or cellulose fiber that doesn’t need to be wet or use oil to act as a filter. The paper is pleated because that gives a much larger surface area in the space. The element is designed to be fine enough to trap impurities, but porous enough to let air through easily. Most heavy-duty air-cleaners incorporate a cyclone type pre-cleaner. It collects larger particles from the air before it enters a dry-element cleaner. It can be mounted directly onto the air-cleaner unit, or the cyclone principle can be incorporated into the main air-cleaner unit. This dry-element cleaner is fitted close to the engine and it’s linked to the cyclone system by a duct. The name cyclone comes from angled vanes that give the incoming air a swirling motion. Centrifugal force throws the heavier dirt particles outwards. They can collect in a separate bowl at the bottom of the cleaner. In the single unit cyclone element, it collects at the end of the air-cleaner case where it can be emptied out manually. The air then continues into the dry-element air-cleaner. An efficient cyclone pre-cleaner can remove up to 90% of particles before they reach the main element. Some older vehicles may use an oil-bath type air cleaner. The sudden change in direction of the air as it passes over the oil leaves the heavier dust particles to be caught in the oil.