Did you know that engines suck in as much as 1200 cubic feet of air to properly burn one gallon of fuel? The internal combustion engine is really nothing more than a big air pump. It inhales a tremendous amount of air.

Air filters have a challenging job because they must balance two functions that are at odds with each other. Their first function is to stop dirt particles. Their second is to allow airflow into the combustion chamber. If they stop too much dirt, they can inadvertently restrict airflow. If they allow unrestricted airflow, they'll be guilty of allowing too much dirt to pass. You can see the problem.

Most air filters have a paper media, though there are also cotton gauze and foam filters. There's an oiled foam filter that works sort of like flypaper. A lot of dirt that makes it in is somewhere around fifteen microns. About a sixth the width of a human hair. (note: human hair has a width of 100 microns.)

Gasoline that is squirted into the combustion chamber mixes with the air and airborne particles, including sand, salt and other minerals, which is then compressed by the piston and ignited. The ignition blast sets off a real shockwave effect. Most of the leftover air and debris gets swept out through the exhaust port

The majority of the dirt particles that enter into the combustion chamber pass right through to the exhaust port and out the exhaust pipe. It's a thrill for the dirt particle, but no big deal for the engine.

Dirt that comes into contact with the moving parts within the chamber can get wedged between the piston and the cylinder walls. And in that area, the presence of the dirt will result in wear, both on the piston rings and the cylinder wall itself. Down a little farther, they cause wear on the piston itself. Eventually, dirt is released into the crankcase area of the engine, where the oil reservoir is. It will mix with the oil, and the oil will pick up this contaminant, this abrasive material, and circulate it throughout the engine.


You might be surprised at variety of particles in your engine. First, there's the leftover debris from when the engine was made: casting materials, machining swarf, polishing compounds, even lint. It's a hostile crowd and it brings out the worst in a dirt particle. Then there's all those wear metals, metal from the cylinder wall that was scratched free. Dirt particles bring out the worst in an engine. Finally, there are exhaust gases, soot, acids and moisture. These can enter the engine via combustion blowby and cause corrosion or lubricant breakdown.

Oil filters can be dangerous for dirt particles, once they are trapped inside an oil filter it's all over. Unfortunately, oil filters are not very effective for particles smaller than 20 microns. That is where bypass systems pay for themselves.