Simple Carburettor Fuel System for
a Piston Engine
And how it works
Inlet
Tank
PISTON ENGINE – Carburettor Fuel System
Exhaust
COCKPIT
FUEL
GAUGE
Filler
Cap
FUEL
1/2
E
F
Filler
Neck
FUEL
LEVEL
SENSOR
Tank
Cavity
PISTON ENGINE – Carburettor Fuel System - Tanks
FUEL
1/2
E
F
PISTON ENGINE – Carburettor Fuel System - Tanks
WHERE ARE THE FUEL TANKS IN AIRCRAFT?
IN THE OTHERWISE EMPTY WING SECTIONS
CI30
HERCULES
PISTON ENGINE – Carburettor Fuel System - Aircraft Tanks
Cockpit
Gauge
Inlet
Tank
Pump
PISTON ENGINE – Carburettor Fuel System
Exhaust
One-way
Inlet Valve
One-way
Outlet Valve
Diaphragm
Driving Cam
PISTON ENGINE – Mechanical Diaphragm Pump
One-way
Inlet Valve
One-way
Outlet Valve
Diaphragm
Magnetic
Plate
Solenoid
Windings
Switch
Mechanism
PISTON ENGINE – Solenoid Diaphragm Pump
PUMP FLOW AND RESTRICTION TO FLOW IN
CONTROLLER CAUSES PRESSURE TO INCREASE
‘SPUR’
GEARS
FLOW
Flow
OUT
Controller
PISTON ENGINE – Gear Type Pump
Air
Filter
Carburettor
Cockpit
Gauge
Inlet
Tank
Pump
PISTON ENGINE – Carburettor Fuel System
Exhaust
Filter element – corrugated breathable paper
Dirt trapped
Air
and
Filter element
Dirt
by
PISTON
ENGINE
– Air Filter
Basic Air
Filter Operation
Clean
Air
Air
Filter
Carburettor
Cockpit
Gauge
Inlet
Tank
Pump
PISTON ENGINE – Carburettor Fuel System
Exhaust
We are now going to look at how the carburettor controls the flow to the engine
This is done by the: -
THE FLOAT CHAMBER
FUEL
NOZZLE
VENTURI
NEEDLE VALVE
AIR VENT
FUEL
LEVEL
FLOAT
FLOAT CHAMBER
FUEL
FEED
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
FUEL
FLOW
AIR FLOW
AIR FLOW
AIR/FUEL
MIXTURE
FLOWS TO
ENGINE
AIR IS SUCKED THROUGH VENTURI BY…..
A PISTON MOVING DOWN ON INDUCTION STROKE
THE LOWER AIR PRESSURE PULLS FUEL THROUGH THE JET
FUEL LEVEL DROPS
MOVING AIR HAS LOWER PRESSURE
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
FUEL LEVEL DROPPING LOWERS FLOAT
PULLING AIR INTO FLOAT CHAMBER
FUEL LEVEL DROPS
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
FUEL LEVEL DROPPING LOWERS FLOAT
FUEL LEVEL RISING FORCES THE NEEDLE VALVE CLOSED
AND ALLOWS FUEL TO ENTER FLOAT CHAMBER FROM PUMP
FUEL LEVEL DROPS
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
FUEL LEVEL RISING FORCES THE NEEDLE VALVE CLOSED
FUEL IS PULLED OUT OF THE FLOAT CHAMBER
ONLY WHEN ONE OF THE PISTONS IS ON THE INDUCTION STROKE
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
We now need to look at controlling the air/fuel mixture flowing into the engine
Controlling the air/fuel mixture means controlling the engine
The carburettor part which controls the flow is….
THE THROTTLE
THROTTLE
VALVE
VALVE
DISC
VALVE
SPINDLE
LINK TO
THROTTLE
LEVER/PEDAL
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
THROTTLE VALVE
THIS WOULD BE A HIGH THROTTLE SETTING OR ‘FULL –POWER’
OPEN THROTTLE ALLOWS VENTURI TO WORK AT MAXIMUM EFFICIENCY
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
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 SYSTEM
LOW THROTTLE SETTING - CALLED ‘IDLE’ OR ‘TICK-OVER’
EDGE GAPS BECOME THE VENTURI FOR THE LOW AIR FLOW
AT IDLE - SLOW RUNNING JET
T CONTROLS FUEL FLOW
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
NEXT IS….
BALANCED AIR PRESSURE AT THE MAIN JET
THIS WOULD CREATE A PRESSURE
DIFFERENCE BETWEEN:-
HERE AND HERE
FLOAT
CHAMBER VENT
THE AIR FILTER CAN ALSO CAUSE RESTRICTION TO AIR FLOW
AND THEREFORE A SLIGHT VACUUM BEFORE CARB
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
AND THE FLOAT CHAMBER IS CONNECTED TO THE CARB AIR INTAKE
EQUALISING THE PRESSURE AT LOW SPEED
SO THE VENT TO THE FLOAT CHAMBER IS CLOSED OFF
THEREFORE PREVENTING TOO MUCH FUEL MIXING WITH THE AIR
FLOAT
CHAMBER VENT
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
NEXT….
MORE EFFICIENT MIXING OF AIR AND FUEL
MORE EFFICIENT MIXING OF AIR AND FUEL
IS ACHIEVED BY FITTING A DIFFUSER TUBE
LET’S TAKE A CLOSER LOOK AT THIS AREA
HERE, ABOVE THE MAIN JET
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
INCREASED
FLOW OF MIXED
AIR/FUEL
AIR/FUEL MIXING IN
DIFFUSER TUBE
AIR INLET
FUEL LEVEL IN
FLOAT CHAMBER
AIR MIXES
WITH FUEL
DIFFUSER TUBE
IDLE
SETTING
FUEL
CRUISE
SETTING
HIGH
POWER
OPEN
THROTTLE
SUPPLIED
VIA IDLE
JET
SETTING
IDLE
JET
STOPS
WORKING
MAIN JET
FUEL FEED FROM
FLOAT CHAMBER
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
NEXT..….
AUTOMATIC MIXTURE CONTROL
This is required to take 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.
CAPSULE EXPANDS IN LOWER
PRESSURES AND CONTRACTS IN
HIGHER PRESSURES
FIXED END
PRESSURE CAPSULE
MOVING END
NEEDLE
VALVE
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
AT HIGH
LOW ALTITUDE
ALTITUDE –– HIGHER
LOWER ATMOSPHERIC PRESSURE
CAPSULE EXPANDS
CONTRACTED
HIGH FUEL FLOW
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
AT HIGH ALTITUDE – LOWER ATMOSPHERIC PRESSURE
CAPSULE EXPANDS
E AND REDUCING FUEL FLOW
CLOSING FUEL ORIFICE
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
NEXT..….
AUTOMATIC MIXTURE CONTROL
This is required to take 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.
Auxiliary Air
Device
Mixture Control
Unit (Fuel
Distributor Valve)
Warm Up
Regulator
Fuel
Injector
Air Sensor
Plate
Primary
Pressure
Valve
Throttle
Valve
Idle Speed
Adjuster
Air (from Air
Inlet Filter)
Fuel
Accumulator
Cold Start
Valve
Engine Temp
Sensor
Safety
Relay
Ignition
Switch
Filter
Fuel Tank
Pump
Battery
Legend
High Pressure Fuel
Low Pressure Fuel
Supply Pressure Fuel
Servo Pressure Fuel
Electrical Signal
PISTON ENGINE – Hydro-Mechanical Fuel Injection System
Injection
Control
Unit
Engine
ECU
‘Common Rail’
Engine Temp
Engine RPM/TDC
Exhaust Temp
Throttle Position
Forward Speed
Air Temps – Inlet & Ambient
Fuel
Injectors
Air Filter
Throttle
Cockpit
Gauge
Pressure
Regulator
Tank
Pump
PISTON ENGINE – Electronic Fuel Injection System
Any Questions?