The cylinders found on the side of dump trucks and 18-wheelers are called air breathers or air cleaners. They filter the air that goes into the engine of the truck.
Long-haul trucks that travel over dusty terrain face the risk of engine damage caused by dirt particles entering with air. The air breathers help to protect the engine of the truck from dust and other impurities to which the truck's engine may be exposed. They contain air filters that prevent dust and other contaminants from entering the engine of the truck through the air intake inlet. The breathers are cylindrical in shape and made of painted steel or stainless steel. They generally have a capacity for storing dirt that is prevented from entering the engine. The size of the breather is dependent on its capacity for storing the dirt.