1. Piston Engine Operations
PISTON ENGINES
Part 5
Piston Engine Operations
(Fuel & Ignition)

2. The Carburettor Remember that the fuel is pumped to the carburettor.
The carburettor is a fuel reservoir
and air delivery tube.
It controls the mixing of air and atomized fuel,
and the flow of the mixture to the engine,
therefore control of the fuel flow,
via the carburettor,
is control of the engine.

3. The Carburettor Fuel is mostly supplied through the main jet nozzle,
AIR/FUEL MIXTURE FLOWS TO ENGINE
Fuel is mostly supplied through the main jet nozzle,
on the same level as fuel in the float chamber.
Air is sucked through the Venturi by air pressure,
as the piston goes DOWN on an Induction Stroke.
This pressure reduction pulls fuel
from the float chamber into the main jet.
VENTURI
FUEL FLOW
AIR VENT
FUEL NOZZLE
FUEL LEVEL
FLOAT
FLOAT CHAMBER
RESERVOIR
LOWER AIR PRESSURE
FUEL FEED

4. The Carburettor As fuel leaves the float chamber,
AIR/FUEL MIXTURE FLOWS TO ENGINE
As fuel leaves the float chamber,
air enters via the vent,
the float drops, opening a needle valve,
which allows fuel to be replaced by the fuel pump.
Fuel entering the chamber forces air out of the vent,
the float rises, closing the needle valve again.
VENTURI
FUEL FLOW
NEEDLE VALVE
AIR VENT
FUEL NOZZLE
THIS ONLY HAPPENS
WHEN ONE PISTON
IS ON THE
INDUCTION STROKE
FUEL LEVEL
FLOAT
FLOAT CHAMBER
RESERVOIR
LOWER AIR PRESSURE
FUEL FEED

5. LINK TO THROTTLE LEVER/PEDAL
The Throttle Valve
AIR/FUEL MIXTURE FLOWS TO ENGINE
The throttle valve consists of a plate,
mounted in a central spindle,
linked to the throttle lever (or foot pedal).
The plate can be any shape
that closely fits the tube in the carburettor.
VALVE PLATE
VALVE SPINDLE
FLOAT
LINK TO THROTTLE LEVER/PEDAL
FLOAT CHAMBER
RESERVOIR

6. The Throttle Valve An Open Throttle (Full Power)
AIR/FUEL MIXTURE FLOWS TO ENGINE
An Open Throttle (Full Power)
allows the Venturi to work at maximum efficiency.
A Low Throttle setting (Idle or Ticking Over)
means the Venturi do not work very well,
with only sufficient Fuel/Air mixture to the engine.
LOW THROTTLE
OPEN THROTTLE
FLOAT
So an alternative,
more effective,
Venturi is needed.
FLOAT CHAMBER
RESERVOIR

7. The Throttle Valve The Edge Gaps now become the Venturi
AIR/FUEL MIXTURE FLOWS TO ENGINE
The Edge Gaps now become the Venturi
for the Low Air Flow.
The Fuel Nozzle is re-designed,
with an extension to the throttle valve.
At Idle – Slow Running,
the extension jet controls the fuel flow.
LOW THROTTLE
OPEN THROTTLE
FLOAT
At Full Throttle
the main Venturi
comes into
operation.
FLOAT CHAMBER
RESERVOIR

8. Balanced Air Pressure
AIR/FUEL MIXTURE FLOWS TO ENGINE
Air Filters can cause restrictions to the Air Flow,
with a slight vacuum before the carburettor.
This creates Pressure Differences between:
So the Float Chamber vent is closed off,
and the Float Chamber is connected to
the Carburettor Air Intake.
Equalising pressure
at Low Speed,
and preventing
too much Fuel
mixing with the Air.
Here
AND
Here
FLOAT
FLOAT CHAMBER
RESERVOIR
VACUUM

9. Mixing Air and Fuel Diffuser Tube
AIR/FUEL MIXTURE FLOWS TO ENGINE
Efficient air/fuel mixing is essential.
So fuel needs to be ‘atomized’
(broken down into the smallest particles possible).
Fuel in liquid form doesn’t burn,
it’s fuel vapour which burns,
so atomizing fuel creates
a greater vapour surface.
A more efficient
mix of fuel & air
can be obtained
by fitting a
Diffuser Tube
onto the main jet.
FLOAT
FLOAT CHAMBER
RESERVOIR
So let’s take a look here.

10. Mixing Air and Fuel The Diffuser Tube HIGH-POWER SETTING IDLE SETTING
Increased
flow of
mixed Air/Fuel
AIR INLET
HIGH-POWER SETTING
The throttle valve
is fully open
and maximum airflow
is passing through
the main venturi.
Fuel level in the
Float Chamber
IDLE SETTING
the diffuser is full of fuel
but very little air,
fuel supplied via idle jet
CRUISE SETTING
air bubbles through
the diffuser holes,
improving atomization
OPEN THROTTLE
idle jet stops working,
main Venturi comes
into operation
Air mixes with Fuel
DIFFUSER TUBE
MAIN JET
Fuel feed from
the Float Chamber
THROTTLE VALVE

11. The Pressure Capsule 15:1 Air at higher altitudes is ‘thinner’,
causing lower pressure.
But the piston travels the same distance
whatever the altitude it is working in.
Therefore at higher altitudes
less air is taken into the engine.
Less airflow means a richer mixture,
(more fuel than air),
which would exceed the ideal ratio of
15:1

12. The Pressure Capsule
A method of adjusting the fuel and air flow is required
to maintain the idea 15:1 ratio.
Since air pressure is easily sensed,
the device used is an Aneroid Capsule.
The aneroid capsule is a sealed chamber
made from concertinaed thin metal.
The inside is
a partial vacuum,
the outside senses
the air pressure,
(termed as
‘ambient’ pressure).
FLOAT
FLOAT CHAMBER
RESERVOIR
ANEROID
CAPSULE

13. The Pressure Capsule At LOW Altitude – Higher Atmospheric Pressure,
the capsule is compressed
by the outside air pressure,
allowing the correct quantity of fuel through,
for the weight of air passing into the engine.
FLOAT
FLOAT CHAMBER
RESERVOIR

14. The Pressure Capsule
At HIGH Altitude – Lower Atmospheric Pressure,
the capsule expands,
pushing the needle into the main jet,
allowing a lower quantity of fuel
for the lower weight of air passing into the engine.
The result is
the air/fuel ratio
is maintained at
15:1
at all atmospheric
conditions.
FLOAT
FLOAT CHAMBER
RESERVOIR

15. Fuel Injection Systems
The injection system differs from the carburettor system
in the way the fuel is delivered.
In carburettors,
fuel is mixed with air before it enters the cylinders.
In injection systems,
fuel is injected directly into the air,
either in the inlet manifold,
or even directly into the cylinders.
Injection systems squirt high pressure fuel
through a small nozzle, or jet, to atomise the fuel.

16. Fuel Injection Systems
Hydro-Mechanical Injection System
The airflow is sensed by an ‘Air Sensor Plate’,
which regulates the fuel flow to the engine in the
Mixture Control Unit/Fuel Distribution Valve.
In this system sufficient fuel is continuously injected
into the inlet manifold.
Warm Up Regulator
Air Inlet
from air filter
Throttle Valve
Cold Start Valve
Auxiliary Air Device
Idle Speed Adjuster
Fuel injector
Mixture Control Unit
(Fuel Distribution Valve )
Air Sensor Plate

17. Fuel Injection Systems
Electronic Injection System
The electronic system is very much simpler,
at least with the number of manufactured parts.
Pressure is limited by the Pressure Regulator,
and the injectors fed from a common ‘Rail’ by the fuel pump.
Injection Control Unit
Pump
Air Filter
Throttle
Valve
Common ‘Rail’
The Injection
Control Unit (ICU)
controls the timing
of the injector
valve opening,
in conjunction with
the Engine
Control Unit (ECU).
Engine
ECU
Fuel Injectors
Pressure Regulator

18. Check of Understanding
In the carburettor
when is air sucked into the Ventri?
On the Compression up stroke
On the Induction up stroke
On the Compression down stroke
On the Induction down stroke

19. Check of Understanding
What shape are Throttle Valves?
Any Shape
Always Circular
Always Square or Oblong
Never Triangular

20. Check of Understanding
When does the Venturi work
at maximum efficiency?
When the engine is idle
When the engine is
at full power
When the engine is cruising
When the engine is
at low power

21. Check of Understanding
What happens in a float chamber carburettor
if an air filter causes restrictions to the air flow?
The pressure drops rapidly
A vacuum is formed
The pressure rises rapidly
A vortex is formed

22. Check of Understanding
In the diagram below,
which arrow points to the main jet? W X Y Z

23. Check of Understanding
At what throttle setting is a diffuser tube
full of fuel but with little air?
Never
Full Power
Cruising
Ticking Over

24. Check of Understanding
In a float chamber carburettor,
what is the function of the device
marked with a ‘X’?
To atomize the fuel
To prevent the fuel/air ratio from being too rich when the aircraft climbs
To supply extra fuel for acceleration
To prevent the fuel/air ratio from being too rich when the throttle valve is opened

25. Check of Understanding
In a fuel injection system,
which of the following does not apply?
Fuel can be injected
into the inlet manifold
The fuel is not atomized
Fuel can be injected
into the cylinder
A throttle valve is not needed

26. Check of Understanding
What is the normal fuel to air ratio
in a piston engine?
1 : 5
1 : 15
5 : 1
15 : 1

27. PISTON ENGINES
End of Presentation