piston engines: fuel

Piston Engine Propulsion

Fuel

The complete fuel system of an aircraft can be divided into two principle sections:

• the aircraft section and

• the engine section.

Aircraft fuel System• The aircraft fuel system consists of the fuel

tanks, the fuel boost pump, the fuel strainer and filters, fuel lines (pipes/hoses), selector valves, fuel flow and pressure gauges, fuel drain valves and a fuel control.

• Fuel systems for different aircraft will vary in complexity and may or may not include all of the above components.

Engine Section

• The engine fuel system where the fuel is delivered to the engine driven pump and includes all of the fuel controlling units fitted to the engine itself

Carburettors

• Carburetion, as applied to the internal combustion engine, is the process by which air and fuel vapour are mixed in suitable proportions, and the supply of this mixture regulated according to the requirements of any given operating condition.

• The mechanical means by which this mixture is achieved is by the use of a carburettor

• The purpose of a carburettor is to supply a well atomized and correctly proportioned mixture of fuel and air to the engine, and to provide a method of limiting the power output by limiting the flow of the mixture.

• Liquid fuels will not burn unless they are mixed with air, or more precisely, with oxygen.

• For the mixture to burn efficiently in an engine cylinder the fuel/air ratio must be kept within a certain range.

• The basic principle of operation is that during the induction stroke of a cylinder the pressure reduces in the inlet manifold and thereby causes the air to flow though the choke tube from the carburettor air inlet.

• Projecting into the choke tube is a fuel nozzle from which petrol is sprayed into the air as droplets of various sizes.

• All carburettors depend on the differential pressure created at the venturi throat to measure the amount of air delivered to an engine and meter the proper amount of fuel.

• When air flows though a venturi, the speed of the air flowing through the venturi throat increases. At the same time, the pressure and temperature of the air in the venturi throat decreases.

Tank

PISTON ENGINE – Carburettor Fuel System

ExhaustInlet

Tank

Cavity

Filler

Cap

Filler

Neck

E F1/2

FUEL

FUEL

LEVEL

SENSOR

COCKPIT

FUEL

GAUGE

PISTON ENGINE – Carburettor Fuel System - Tanks

PISTON ENGINE – Carburettor Fuel System - Tanks

E F1/2

FUEL

E F1/2

FUEL

WHERE ARE THE FUEL TANKS IN AIRCRAFT?

IN THE OTHERWISE EMPTY WING SECTIONS

CI30

HERCULES

PISTON ENGINE – Carburettor Fuel System - Aircraft Tanks

Tank


Cockpit

Gauge

Pump

ExhaustInlet

Air

Filter

Tank


Cockpit

Gauge

Pump

Carburettor

ExhaustInlet

Air and

DirtClean

Air

Filter element – corrugated breathable paper

Dirt trapped

by

Filter element

Basic Air Filter OperationPISTON ENGINE – Air Filter

Air

Filter

Tank


Cockpit

Gauge

Pump

Carburettor

ExhaustInlet

• The carburettor has to control the flow to the engine.

• This is done by the float chamber

Float type carburettor

• This carburettor uses a float to regulate the amount of fuel that enters a carburettor.

• The amount of fuel allowed to flow into a float chamber is controlled by a float operated needle valve installed in the fuel inlet.

SIMPLIFIED PISTON ENGINE FUEL SYSTEM

FLOAT CHAMBER

NEEDLE VALVE AIR VENTVENTURI

FUEL

FEED

FUEL

NOZZLE


FLOAT

FUEL

LEVEL

AIR IS SUCKED THROUGH VENTURI BY…..

A PISTON MOVING DOWN ON INDUCTION STROKE

SIMPLIFIED PISTON ENGINE FUEL SYSTEMSIMPLIFIED PISTON ENGINE FUEL SYSTEMSIMPLIFIED PISTON ENGINE FUEL SYSTEM

MOVING AIR HAS LOWER PRESSURE

FUEL

FLOWAIR

FL

OW

AIR

FL

OW

THE LOWER AIR PRESSURE PULLS FUEL THROUGH THE JET

AIR/FUEL

MIXTURE

FLOWS TO

ENGINE

FUEL LEVEL DROPS

PULLING AIR INTO FLOAT CHAMBER


FUEL LEVEL DROPS

FUEL LEVEL DROPPING LOWERS FLOAT

AND ALLOWS FUEL TO ENTER FLOAT CHAMBER FROM PUMP


FUEL LEVEL DROPS

FUEL LEVEL RISING FORCES THE NEEDLE VALVE CLOSED

FUEL LEVEL DROPPING LOWERS FLOAT


FUEL IS PULLED OUT OF THE FLOAT CHAMBER

ONLY WHEN ONE OF THE PISTONS IS ON THE INDUCTION STROKE

FUEL LEVEL RISING FORCES THE NEEDLE VALVE CLOSED

• To provide for proper engine operation under various engine loads, speeds, and air densities, most carburettors include at least the following systems.

• Main Metering

• Idling

• Mixture Control

• Accelerating

• Power Enrichment or Economizer

Main Metering System

• The purpose of the main metering system is to supply the correct amount of fuel to the engine at all speeds above idle.

• The main metering system is comprised of one or more venturi tubes, a main metering jet and discharge nozzle, and a throttle valve.

• Fuel metering begins with the venturi. As air flows through the venturi, its pressure decreases.

• It is this drop in pressure that the metering system relies on to meter the appropriate amount of fuel.

• In some carburettors, a single venturi is unable to create the pressure drop necessary to meter fuel. In this case, a second boost venturi is installed inside the primary venturi.

• The discharge nozzle delivers fuel to the intake air and in installed between the float chamber and the venturi.

• When an engine is at rest, the fuel level in the discharge nozzle matches that in the float chamber. In most cases, the float maintains a fuel level just below the opening in the discharge nozzle.

• This distance is referred to as the fuel metering head. The purpose of the fuel metering head is to prevent fuel leaking out of the discharge nozzle when the engine is shut down.

• To control the volume of air that passes through a venturi, all carburettors are equipped with a throttle valve.

• The throttle valve consists of a flat, circular piece of metal that is always installed between the venturi and the engine.

• When the throttle valve is positioned parallel with the airflow, the maximum volume of air and fuel enter the engine and the engine develops its maximum power.

• The only thing that limits the volume of air entering the engine is the venturi.

• However, as the throttle valve is moved so that it is perpendicular to the airflow, less air is admitted into the engine and engine power output decreases.


LINK TO

THROTTLE

LEVER/PEDAL

THROTTLE

VALVE

VALVE

SPINDLE

VALVE

DISC


THROTTLE VALVE

THIS WOULD BE A HIGH THROTTLE SETTING OR ‘FULL –POWER’

OPEN THROTTLE ALLOWS VENTURI TO WORK AT MAXIMUM EFFICIENCY

LOW THROTTLE SETTING - CALLED ‘IDLE’ OR ‘TICK-OVER’

ALMOST CLOSED THROTTLE MEANS THE VENTURI DOES NOT WORK VERY

WELL

LOW AIR FLOW MEANS VERY LITTLE OR NO FUEL/AIR MIXING IN THE

VENTURI

SO AN ALTERNATIVE AND EFFECTIVE VENTURI NEEDS TO BE FOUND

SIMPLIFIED PISTON ENGINE FUEL SYSTEMSIMPLIFIED PISTON ENGINE FUEL SYSTEM

T CONTROLS FUEL FLOW

SIMPLIFIED PISTON ENGINE FUEL SYSTEMSIMPLIFIED PISTON ENGINE FUEL SYSTEM

LOW THROTTLE SETTING - CALLED ‘IDLE’ OR ‘TICK-OVER’

EDGE GAPS BECOME THE VENTURI FOR THE LOW AIR FLOW

AT IDLE - SLOW RUNNING JET

• The mixing of air and fuel needs too be done is an efficient way.

• This is done by fitting a diffuser tube.

MORE EFFICIENT MIXING OF AIR AND FUEL

IS ACHIEVED BY FITTING A DIFFUSER TUBE


HERE, ABOVE THE MAIN JET

LET’S TAKE A CLOSER LOOK AT THIS AREA

IDLE SETTING FUEL

SUPPLIED VIA IDLE JET

OPEN THROTTLE

IDLE JET STOPS WORKING

CRUISE SETTINGHIGH POWER

SETTING

AIR INLET

DIFFUSER TUBE

INCREASED

FLOW OF MIXED

AIR/FUEL

FUEL FEED FROM

FLOAT CHAMBER

MAIN JET

AIR MIXES

WITH FUEL

AIR/FUEL MIXING IN

DIFFUSER TUBE

FUEL LEVEL IN

FLOAT CHAMBER



FIXED END

MOVING END

NEEDLE

VALVE

CAPSULE EXPANDS IN LOWER

PRESSURES AND CONTRACTS IN

HIGHER PRESSURES

PRESSURE CAPSULE


AT LOW ALTITUDE – HIGHER ATMOSPHERIC PRESSURE

CAPSULE CONTRACTED

HIGH FUEL FLOW

AT HIGH ALTITUDE – LOWER ATMOSPHERIC PRESSURE

CAPSULE EXPANDS


AT HIGH ALTITUDE – LOWER ATMOSPHERIC PRESSURE

CAPSULE EXPANDS

CLOSING FUEL ORIFICEE AND REDUCING FUEL FLOW

BatteryFuel Tank

Pump

Warm Up

Regulator

Fuel

Accumulator

Ignition

Switch

Filter

Primary

Pressure

Valve

Air Sensor

Plate

Mixture Control

Unit (Fuel

Distributor Valve)

Auxiliary Air

Device

Cold Start

ValveIdle Speed

Adjuster

Fuel

Injector

Throttle

Valve

Engine Temp

Sensor

Safety

Relay

Air

Inlet

High Pressure Fuel

Low Pressure Fuel

Supply Pressure Fuel

Servo Pressure Fuel

Electrical Signal

Legend

PISTON ENGINE – Hydro-Mechanical Fuel Injection System

(from Air

Filter)

Air Filter

PISTON ENGINE – Electronic Fuel Injection System

Engine

ECU

TankPump

Cockpit

Gauge

Pressure

Regulator

Injection

Control

Unit

Engine Temp

Engine RPM/TDC

Exhaust Temp

Throttle Position

Forward Speed

Air Temps – Inlet & Ambient

Fuel

Injectors

‘Common Rail’

Throttle

• An automatic mixture control is required to totake into account that at higher altitude there is less air, so therefore less fuel is required.

• This change in fuel flow is controlled by a device that can sense the change in air pressure.

• It is called a pressure bellows or pressure capsule.

BatteryFuel Tank

Pump

Warm Up

Regulator

Fuel

Accumulator

Ignition

Switch

Filter

Primary

Pressure

Valve

Air Sensor

Plate

Mixture Control

Unit (Fuel

Distributor Valve)

Auxiliary Air

Device

Cold Start

ValveIdle Speed

Adjuster

Fuel

Injector

Throttle

Valve

Engine Temp

Sensor

Safety

Relay

Air

Inlet

High Pressure Fuel

Low Pressure Fuel

Supply Pressure Fuel

Servo Pressure Fuel

Electrical Signal

Legend

PISTON ENGINE – Hydro-Mechanical Fuel Injection System

(from Air

Filter)

Air Filter

PISTON ENGINE – Electronic Fuel Injection System

Engine

ECU

TankPump

Cockpit

Gauge

Pressure

Regulator

Injection

Control

Unit

Engine Temp

Engine RPM/TDC

Exhaust Temp

Throttle Position

Forward Speed

Air Temps – Inlet & Ambient

Fuel

Injectors

‘Common Rail’

Throttle

piston engines: fuel

Engineering

fuel flow

fuel tanks

fuel control

fuel strainer

fuel vapour

fuel nozzle

fuel systems

proportioned mixture