Summary An air cleaner filters air that passes through it to stop harmful particles reaching the engine. The air cleaner on a carbureted engine can be on top of the carburetor, or beside the engine, connected to the carburetor by a hose or duct. Position is usually decided by how much space there is, or bonnet profile. On some electronically fuel injected engines, the air cleaner is on top of the throttle body, similar to a carburetor. Other air cleaners are connected by ducts. Diesel engines often have more than one air cleaner. This may be due to their severe working conditions. They’re usually mounted away from the engine to obtain cleaner, cooler air. A lot of air passes through the intake system into the engine. In a gasoline engine, it’s about 15 times the amount of fuel by weight. By volume that’s 10,000 times more air than fuel. The air-fuel mixture enters the engine so the air needs to be clean. Any abrasives that enter the engine can cause wear and damage. It also has a silencing effect, muffling noise produced by the air entering the engine. It can act as a flame trap. So if a gasoline engine backfires, the air cleaner can contain the flame within the intake manifold or carburetor.