Welcome to a deep dive into the fascinating world of Aircraft Fuel and Fuel Metering Systems! Understanding how aircraft are fueled and how that fuel is precisely delivered to the engine is crucial for safe and efficient flight. This guide will break down the complexities, from fuel types to advanced metering systems, making it accessible for students.
Understanding Aircraft Fuel: Types and Handling
Aircraft utilize different types of fuel depending on their engine design. Reciprocating engines and turbine engines have distinct fuel requirements and handling procedures.
Turbine Engine Fuel Types
Turbine engines, commonly known as jet engines, typically use kerosene-based or gasoline-based fuels. The two basic types are:
- Jet A and A-1: These are special types of kerosene-based fuel, similar to military JP-5. Jet A-1 has a lower freezing point than Jet A.
- Jet B: This is a gasoline-based fuel, comparable to military JP-4. It offers better cold-weather performance but is more volatile.
What if the Wrong Fuel is Used?
Operating a reciprocating engine on turbine engine fuel can be extremely damaging. If this occurs, a series of critical steps must be taken:
- The entire fuel system should be drained and flushed with the proper fuel.
- The engine must undergo a compression check.
- All cylinders require a borescope inspection.
- The oil must be drained, and all filters and strainers checked for contamination.
- After proper refueling, the engine must be given a thorough run-up.
Essential Fuel System Components and Their Functions
Aircraft fuel systems incorporate several specialized components to ensure fuel safety, proper delivery, and optimal engine performance.
Fuel Heaters and Anti-Freezing Agents
Some turbine engine fuel systems include a fuel heater. Its purpose is to keep the fuel warm enough to prevent any water that precipitates out from freezing on the filters, which could block fuel flow.
Prist is a crucial additive for turbojet aircraft fuel for two main reasons:
- It acts as a biocidal agent, killing scum-forming bacteria in the fuel tank.
- It serves as an antifreeze agent, lowering the freezing point of entrained water released from the fuel.
Fuel Filters and Boost Pumps
Aircraft fuel filters sometimes have a built-in relief valve. This valve opens to allow unfiltered fuel to flow to the fuel control device if the filter becomes plugged with ice or other contaminants, preventing engine starvation.
Boost pumps are vital components in most aircraft fuel systems, typically electrically operated centrifugal pumps. They serve three primary purposes:
- To provide fuel pressure for starting the engine.
- To pressurize the fuel lines, preventing vapor lock.
- To transfer fuel from one tank to another.
Boost pumps in the fuel tank pressurize the fuel in the lines, forcing it into the fuel metering system, which helps prevent vapor lock by keeping the fuel under pressure.
Engine-Driven Fuel Pumps and Valves
An engine-driven fuel pump often includes a bypass valve. This valve allows fuel to flow around the pump during engine starting or in an emergency if the engine-driven pump fails.
Excess fuel from an engine-driven fuel pump's pressure relief valve is directed back to the inlet side of the pump. Some pumps feature a compensated relief valve, which uses a diaphragm acted upon by atmospheric pressure. This system varies the pump discharge pressure to maintain a consistent differential above the air pressure entering the carburetor.
The engine fuel shutoff valve is critically located on the side of the firewall away from the engine, ensuring accessibility in an emergency.
Turbine Engine Specific Valves
In a turbine engine, the pressurizing and dump valve plays a dual role. During normal operation, it acts as a flow divider, directing fuel into the main or pilot manifold for discharge from the correct orifice in the duplex fuel nozzle. When the engine shuts down, its dump function expels all fuel from the manifold.
Aircraft Fuel Metering Systems: Ensuring Precise Delivery
Fuel metering systems are responsible for accurately measuring airflow and dispensing the appropriate amount of fuel for all engine operating conditions.
Reciprocating Engine Fuel Metering
Float Carburetors
A float carburetor measures airflow through the engine's induction system and dispenses the correct amount of gasoline, aiming for the most vaporized state possible by ignition.
When setting idling conditions on a float carburetor, two things are adjusted:
- Idling RPM, by adjusting the throttle stop.
- Idling mixture, by adjusting the idle needle valve.
If the mixture is not adjusted, it becomes richer as the aircraft gains altitude. Applying carburetor heat also makes the mixture richer. Conversely, if the main air bleed of a float carburetor becomes plugged, the mixture would become excessively rich.
The economizer (or power enrichment) system allows the engine to run with an economically lean mixture for conditions other than full power. When the throttle opens to full power, it enriches the mixture to dissipate heat.
The acceleration system operates when the throttle is suddenly opened, providing an immediate surge of fuel.
Pressure Carburetors
Pressure carburetors employ an automatic mixture control to maintain a constant air-fuel mixture as altitude changes. It progressively bleeds air between the two sides of the air diaphragm, decreasing the air metering force, which leans the mixture as the aircraft ascends.
Before flight, the diaphragms of a pressure carburetor must be soaked to restore their original flexibility for accurate calibration.
For starting an engine with a pressure carburetor, the mixture control is placed in the IDLE CUTOFF position, and the engine is started using the primer system. The carburetor heat control should be in the COLD position when starting any aircraft engine.
In engines with an antidetonation injection (ADI) system, a de-richment valve in the pressure carburetor automatically closes to lean the air-fuel mixture when ADI fluid flows. It reopens and the mixture returns to rich when ADI flow stops.
Fuel Injection Systems
In a continuous-flow fuel injection system, fuel is discharged through injector nozzles screwed into the cylinder heads near the intake valves.
For the Teledyne-Continental fuel injection system:
- The mixture control in FULL RICH sends all fuel to the manifold valve. In IDLE CUTOFF, all fuel returns to the pump inlet. Intermediate positions vary the fuel amount to the engine.
- The adjustment of the orifice in the fuel pump controls the high unmetered fuel pressure.
- Letters stamped on the flats of a fuel injection nozzle indicate the relative size of its orifice. A lower letter in the alphabet means a smaller orifice and less fuel flow.
- The manifold valve (or flow divider) in horizontally opposed aircraft engines serves two purposes: it provides a constant discharge fuel pressure for idling and a positive shutoff for fuel when the engine is shut down.
Turbine Engine Fuel Metering
The fuel metering system of a turbine engine discharges its fuel through spray nozzles directly into the combustion chambers.
Duplex nozzles are a feature in turbine engine fuel metering systems, designed to offer a desirable spray pattern for combustion across a wide range of operating pressures.
Turbine Engine Fuel Control Unit
Turbine engines utilize fuel control units which are either hydropneumatic or electro-hydromechanical. These units sense several engine parameters:
- Engine RPM
- Inlet air pressure
- Compressor discharge pressure
- Burner can pressure
- Inlet air temperature
When the fuel control unit of a turbine engine is trimmed, the idle speed and the maximum-thrust speed are adjusted.
When trimming a jet aircraft, its position relative to the wind is important:
- If wind velocity is less than 10 MPH, it can face any direction.
- If wind velocity is between 10 and 25 MPH, it should face into the wind.
- Trimming should not be done if wind velocity exceeds 25 MPH.
Troubleshooting and Best Practices
Vapor Lock Prevention
Vapor lock occurs when fuel becomes hot enough to boil, releasing vapors that block fuel lines and prevent liquid fuel flow to the engine. In most aircraft fuel systems, vapor lock is prevented by using boost pumps in the fuel tank to pressurize the fuel in the lines and force it into the fuel metering system.
Engine Shutdown Procedures
An aircraft reciprocating engine is typically killed by placing the mixture control in the Cutoff position, rather than using the magneto switch. Shutting off fuel to the cylinders leaves combustion chambers full of air with no fuel. This reduces the likelihood of the engine firing if the propeller is accidentally pulled through with the ignition switch on.
Exhaust Gas Temperature (EGT) and Mixture
Exhaust Gas Temperature (EGT) is a key indicator for reciprocating engine air-fuel mixture. A stoichiometric mixture (where all constituents of the air-fuel mixture are burned) produces the highest EGT. The mixture is adjusted to achieve the highest EGT, then enriched slightly to place it on the rich side of stoichiometric for optimal operation.
Frequently Asked Questions about Aircraft Fuel and Fuel Metering Systems
What is the primary function of a float carburetor?
The primary function of a float carburetor is to measure airflow through the engine induction system and dispense the appropriate amount of gasoline into the airflow for all engine operating parameters and conditions, ensuring the fuel is as vaporized as possible by the time ignition occurs.
Why is Prist added to turbine engine fuel?
Prist is added to turbine engine fuel because it acts as a biocidal agent to kill scum-forming bacteria in the fuel tank and as an antifreeze agent to lower the freezing point of water entrained in the fuel, preventing ice formation on filters.
What are the main parameters sensed by a turbine engine's fuel control unit?
The fuel control unit of a turbine engine senses engine RPM, inlet air pressure, compressor discharge pressure, burner can pressure, and inlet air temperature to precisely manage fuel flow.
How does an engine-driven fuel pump's bypass valve contribute to safety?
The bypass valve in an engine-driven fuel pump allows fuel to flow around the pump for engine starting and provides an essential emergency backup operation if the engine-driven pump should fail, ensuring continuous fuel supply to the engine.
What causes vapor lock in an aircraft fuel system and how is it prevented?
Vapor lock is caused by the fuel becoming hot enough to boil, releasing vapors that obstruct fuel lines. It is prevented in most aircraft fuel systems by using electrically operated centrifugal boost pumps in the fuel tank to pressurize the fuel lines and force fuel into the metering system, keeping it from vaporizing.