Study Notes on Engine Fuel and Fuel Metering Systems

Expert: Engr. Vindell Nino Singco
Institution: Indiana Aerospace University, 1992
Subject: Engine Fuel and Fuel Metering Systems, focusing on carburetion principles, venturi principles, and various carburetor systems.

Definition: The carburetor measures airflow to regulate fuel discharge into the airstream.
Air Measuring Unit: The venturi is used as the air measuring unit.
- Fundamental Principle: As the velocity of a gas or liquid increases, the pressure decreases (Bernoulli's principle).
Structure of the Venturi:
- Composed of a tube with a narrow section called the throat.
- As air velocity increases to pass through the throat, pressure decreases:
- Pressure in the throat < Pressure in other parts of the venturi.
- Proportional Relationship: The pressure drop is proportional to velocity, serving as a measure of airflow.
Operating Principle of Carburetors: Depends on the differential pressure between the inlet and the venturi throat to measure and regulate fuel discharge.

Airflow Dynamics:
- As air passes through the venturi's throat, velocity increases and pressure drops (
  VELOCITY ↑ and PRESSURE ↓).
Throttle Valve Functionality:
- Located between the venturi and the engine, mechanically linked to a cockpit lever.
- Regulates airflow to the cylinders, controlling engine power output by manipulating the throttle valve position.
- In wide-open throttle position, the air passage is minimally obstructed.

Components:
- Fuel-Air Mixture: The blend is routed to combustion chambers for burning.
- Throttle Valve: Controls flow of fuel-air mixture; cannot permit fuel discharge when closed.
- Venturi: Creates low-pressure area.
- Discharge Nozzle: Fuel is forced through this nozzle into the venturi due to atmospheric pressure in the float chamber.
- Float Chamber: Maintains fuel level regulated by a float mechanism.
- Mixture Needle: Controls the fuel to the discharge nozzle; its position can be adjusted using the mixture control.
Air Bleed: Allows air to mix with fuel, reducing density to enhance vaporization.

Purpose: To ensure engine operation under varying loads and speeds.
Types of Systems (Total of 6):
1. Main Metering System
2. Idling System
3. Accelerating System
4. Mixture Control System
5. Idle Cutoff System
6. Power Enrichment (Economizer)

Supplies fuel at all speeds above idling, with fuel flow determined by pressure drop in the venturi throat.

A separate system for low engine speeds due to low airflow at closed throttle positions.
Provides fuel via an idling jet situated in the low-pressure area near the throttle valve.

Supplies additional fuel during rapid increases in engine power to counteract momentary leanness of fuel/air mixture due to fast throttle opening.
Includes an accelerating pump that pushes fuel into the venturi for immediate enrichment.

Adjusts the fuel/air ratio to counteract variations in air density, especially at higher altitudes.
Manual Control Types:
- Needle-type: Valve opens or closes based on cockpit control.
- Back-Suction Type: Uses venturi-created low pressure to manage fuel proportion in float chamber successfully.

Shuts off fuel to stop the engine, integrated into the manual mixture control lever that stops discharge when in idle cutoff position.

Enriches fuel during high power operations to cool combustion chambers, operates at throttle settings above 60–70% rated power.

Float-Type Carburetors: Most common but has distinct disadvantages including icing and incomplete vaporization issues. Consists of six subsystems:
- Float chamber mechanism
- Main metering system
- Idling system
- Mixture control system
- Accelerating system
- Economizer system

Serves as a reservoir maintaining steady fuel level for the main metering system.
Must maintain fuel slightly below outlet holes to prevent leakage when the engine is off.

Functions of Venturi in Main Metering System:
1. Proportions fuel/air mixture.
2. Reduces pressure at the discharge nozzle.
3. Limits airflow at full throttle.

The low air velocity at idle can stop fuel discharge through the main nozzle, so a separate discharge system (the idling jet) is utilized.
Allows the engine to idle by feeding fuel into low-pressure area near the throttle.

At 18,000 feet, air density halves, limiting oxygen intake per cylinder.
Venturi action draws the same fuel volume through the discharge nozzle regardless of altitude, leading to a richer fuel mixture as altitude increases.

Objective: To ensure the mix is richer for maximum power at full throttle to cool the engine and prevent detonation.
Mechanism: Needle valve opens at near full throttle settings to allow more fuel flow, supplementing fuel from the main jet.

Understanding carburetor functionality and its components is critical for effective engine operation in varying conditions and maintaining optimal performance within aircraft systems.