How Jet Fuel Is Routed Within Aircraft Engines

Sep 03, 2023

Jet fuel passes through various subsystems before being ignited for combustion.

The fuel routing subsystem within jet engines aims to deliver high-pressure, high-temperature clean fuel for combustion. The engine's fuel efficiency depends on the optimal pressure and temperature of fuel entering the combustor.

The aircraft-supplied fuel (from wing tanks) enters the low-pressure stage of the fuel pump. The low-pressure (sometimes impeller) system pressurizes the fuel to approximately 150 psi to prevent high-pressure pump cavitation. The fuel passes through a strainer before entering the high-pressure gear system, where the fuel is further pressurized to nearly 1,800 psi.

A pressure transmitter installed at the fuel pump inter-stage (between low-pressure and high-pressure systems) sends a fuel pressure reading to the cockpit control. Depending on the stage of flight, fuel pressure is optimized for efficient combustion.

The high-pressure fuel enters the Fuel Oil Heat Exchanger (FOHE), where the fuel is heated while the engine oil is cooled. The FOHE comprises metal tubes through which the pressurized fuel pass. Hot engine oil runs around the metal tubes, heating the fuel from the outside.

It is noteworthy that the metal tubes within the FOHE must be routinely checked to prevent fuel from leaking and mixing with the oil, which could result in smoke or fire in the engine.

The fuel enters the fuel filter, where the contaminants are removed using a filter element. Fuel filters are generally designed such that the incoming fuel enters the filter element from the outside, thus leaving any contaminants and debris on the filter face for investigation.

Some fuel system configurations also consist of a second filter (or a wash screen) for additional filtering. A pressure switch installed on the fuel filter transmits a differential pressure reading to the cockpit control, which is used for fuel filter clog warning indication.

A relatively small amount of fuel is routed to the servo systems that use pressurized and heated fuel for actuation mechanisms. Some servo systems include fuel metering valves, variable bleed valves, and variable vanes valves. Other valves that direct the bypass air for shrouds and case cooling may also use fuel for actuation.

It is noteworthy that some engine configurations may require the servo fuel to be further heated, through a designated servo fuel heater, before routing to servo systems. The additional heating prevents tiny ice crystals (that may be present in the fuel) from entering sensitive servo systems.

At this stage, the fuel flow transmitter measures the total mass flow of fuel supplied for combustion. Moreover, the engines with an Integrated Drive Generator (IDG) system installed require the IDG oil to be cooled with fuel. In such cases, the fuel passes through the IDG FOHE, where the fuel is further heated while cooling the IDG oil.

The high-pressure, high-temperature metered fuel is ready for combustion and is routed to the fuel nozzles. Dual flow nozzles consist of primary and secondary spray patterns for different flight conditions. For example, the primary flow is used for low-power settings such as engine startup, taxing, and idling. For high-power settings such as takeoff, the secondary fuel flow complements the primary flow.

Fuel nozzles also prevent engine flame-out during deceleration by holding the flame at low power settings. The atomized fuel dispersed from the fuel nozzles homogenizes with the pressurized air from the compressor before the fuel-oil mixture is ignited for combustion.

What do you think about the fuel routing within a jet engine? Tell us in the comments section.

Writer - Omar is an aviation enthusiast who holds a Ph.D. in Aerospace Engineering. With numerous years of technical and research experience under his belt, Omar aims to focus on research-based aviation practices. Apart from work, Omar has a passion for traveling, visiting aviation sites, and plane spotting. Based in Vancouver, Canada