The induction system is a passage and controlling element for the air supplied to the engine. In small aircraft engines there are two main types of induction systems: Carburetor System and Fuel Injection System. The basic induction system of an aircraft reciprocating engine consists of ducting that transfers the air to the inlet filter and an air scoop used to collect the inlet air. The Boeing 737 induction system includes updraft and downdraft options and incorporates alternate air from inside the engine cowling.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is an induction system in aviation?
An aircraft induction system brings air from the outside into the powerplant and delivers the fuel/air mixture to the cylinder. An induction system consists of an air scoop, mixes air and fuel, and forms the mixture necessary for combustion to occur. This fuel/air mixture is drawn through the intake manifold, and its flow to the combustion chambers is regulated by the throttle valve.
The air induction system for each engine and auxiliary power unit must deliver the flow required by that engine and auxiliary power unit under each operating condition for which certification is requested. Induction system valves can stand in any setting, but the system still supplies the air for proper fuel metering and mixture distribution. Air enters through a circular opening in the center of the compressor casing where fast-moving impeller blades sling it outward to a diffuser. Outside air first flows through the air filter at the intake in the front part of the cowling. Ducting then transfers the air to the inlet filter and that filtered air arrives at the mixer. The intake manifold finally carries the mixture to each combustion chamber where combustion creates thrust or power.
What are the types of induction systems in aviation?
The types of induction systems in aviation are listed below.
- Updraft Induction System
- Nonsupercharged Induction System
- Carburetor Induction System
- Fuel Injection Induction System
Induction systems for reciprocating engines are broadly classified as supercharged or nonsupercharged. Within nonsupercharged designs, two main types of fuel induction systems are carburetors and fuel injectors. A carburetor system mixes fuel and air in the carburetor before the mixture enters the intake manifold. Carburetors are further classified as either float type or pressure type. A float-type carburetor acquires its name from a float that meters fuel at low pressure, while a pressure-type carburetor uses a fuel pump to discharge fuel into the airstream well above atmospheric pressure, resulting in better vaporization and practically eliminating fuel-vaporization icing.
Small aircraft engines typically use either a carburetor system or fuel injection system, the latter providing greater power and more accurate fuel distribution, though it is more complicated and expensive to maintain. Physical arrangements of the induction inlet create types known as updraft and downdraft induction systems. Supercharged induction systems include a supercharger located between the fuel-metering device and the intake manifold; this true supercharger boosts manifold pressure above ambient and can be internally driven from the engine crankshaft or externally driven by an exhaust-powered turbosupercharger. Single-stage single-speed superchargers are found on many high-powered radial engines, while larger installations employ multi-stage systems.
What is induction system icing in aviation?
Induction system icing is ice that forms in the air intake and air passages of an engine, blocking or restricting the airflow required for combustion. It appears in two kinds: carburetor icing, which is common in piston-engine aircraft, and air-intake blockage that affects both carbureted and fuel-injected engines, including many jet installations. The cause is usually a sudden temperature drop inside the venturi or throttle valve, produced by fuel vaporization, adiabatic expansion or a local pressure drop. If enough ice accumulates it robs the engine of air, reduces intake flow to below that necessary for operation, and always lowers engine performance; in extreme cases the engine ceases to operate. Because induction ice occurs under a wide range of meteorological conditions, regulations require every reciprocating-engine air-induction system to carry means to prevent and eliminate icing, while certification guidance provides the same protection for turbine installations. Carburetor heat, alternate air doors and heated inlet screens are the common anti-icing measures.
