- The modern combustion engine needs a free flow of air to operate properly. Since it draws this essential air in from outside the engine, small particles of dust, dirt, oil and even small obstructions like gravel have a way to get inside the engine. Once inside, these particles can contaminate engine oil and act as an abrasive, causing premature engine wear. To prevent this, air filters are installed on all modern combustion engines. Air is drawn into the engine through a tube, but it must first pass through the filter before it can reach the engine. The filter catches all of these particles and deposits. Because they accumulate over time, filters must be replaced or cleaned regularly.
- Most engine air filters use paper as their primary filtration medium. On newer cars, the filter is rectangular in shape and is filled with several pleated layers of a mesh-like paper that resembles an anti-static dryer sheet. It is usually housed in a plastic compartment that sits on top of the engine block. On older cars, the filter is shaped like a large ring, often about the size of a steering wheel, with accordion-like layers of paper all the way around. It fits in a metal compartment on top of the carburetor. The running engine creates a vacuum which sucks in air through these filters, bringing dust, dirt and other particles along for the ride. These particles get stuck in the many layers of the filters. They should be checked about every three months, and when they are visibly dirty, they should be replaced.
- Although the paper filter is standard on today's engines, there are a few other types of filters that use different filtration media. Some filters are stuffed with oil-soaked cotton instead of paper. These types of filters are more commonly used on high-octane sports cars. Other filters, which are more common on off-road vehicles, use oil-soaked polyurethane foam. Vehicles that predate the spread of the paper filter may also have an oil bath filter installed; in these filters, air is drawn in through a contoured, sponge-like disc that sits on top of a small pool of oil. When the air is sucked through the chambers of this filter, the smaller particles get trapped in the filter medium and the larger particles get trapped in the puddle of oil.
- There is also another type of automotive air filter that is completely separate from the engine air filter. The cabin air filter filters impurities and allergens out of the passenger compartment's outside air intake. It should be subjected to regular visual inspections just like the engine air filter and replaced as needed. Just like engine air filters, the filtration media is typically made of paper.
