Air must pass through a number of passages to reach the combustion spaces within an engine. So far, this discussion has considered the passage of air through components that clean, silence, and compress the intake air. From the blower or turbocharger, however, the air is discharged into a unit or passage that conducts it to the intake valves or ports of the cylinders. The design of such a unit as well as the associated terminology will differ depending on the type of engine.

In 2-stroke cycle engines, the passages that conduct intake air to the cylinders are generally referred to as an AIR BOX. The air box surrounds the cylinders (fig. 6-2) and, in many engines, is built into the block. In 2-stroke cycle, V-type diesel engines, the air box consists of the space (within the block) between the two banks of the V-construction and the open space between the upper and lower deckplates of each bank. (Refer to fig. 6-8.) The scavenging air passages in a Fairbanks-Morse, opposed-piston engine are referred to as the AIR RECEIVER. The air receiver is located at the upper part of the block and surrounds the cylinder liners. In some 2-stroke cycle engines, the passage that serves as a reservoir for intake air from the blower is called an AIR HEADER.

Drains are generally provided in air boxes, receivers, and headers to drain off any liquids that may accumulate. A slight amount of vapor from the air charge may condense and settle in the air box, or a small amount of lubricating oil may be blown into the air box as the piston passes the ports on the downstroke following the power event. On some engines, the drains are vented to the atmosphere. In others, a special drain tank collects the drainage from the air box. The purpose of the tank is to prevent drainage of oil into the engine room.

Drains are of primary importance when the air cooler is installed between the blower (or turbocharger) discharge and air intake manifold or receiver. The drains are usually left open during engine operation so that condensation or water from leaky coolers is prevented from being carried into the engine cylinders with the combustion air.

In 4-stroke cycle diesel engines, the intake air passages from the blower to the cylinders differ, in general, from those in 2-stroke cycle engines in that these passages are not an integral part of the block. Instead, a separate unit is attached to the block to conduct intake air to the engine cylinders. The attached unit is generally called an air intake MANIFOLD.