piston engines part 5 piston engine operations (fuel & ignition)

PISTON ENGINES

Part 5

Piston Engine Operations(Fuel & Ignition)

The CarburettorRemember 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.

The Carburettor

FLOAT CHAMBERRESERVOIR

FLOAT

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 fuelfrom the float chamber into the main jet.

FUEL FEED

AIR VENT

FUEL NOZZLE

FUEL LEVEL

VENTURI

LOWER AIR PRESSURE

FUEL FLOW

The Carburettor


FLOAT

AIR/FUEL MIXTURE

FLOWS TO ENGINE

FUEL FEED

AIR VENT

FUEL NOZZLE

FUEL LEVEL

VENTURI

LOWER AIR PRESSURE

FUEL FLOW

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.

NEEDLE VALVE

THIS ONLY HAPPENSWHEN ONE PISTON

IS ON THEINDUCTION STROKE

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 shapethat closely fits the tube in the carburettor.

The Throttle Valve


FLOAT

AIR/FUEL MIXTURE

FLOWS TO ENGINE

LINK TO THROTTLE

LEVER/PEDAL

VALVE SPINDLE

VALVE PLATE

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.

The Throttle Valve


FLOAT

AIR/FUEL MIXTURE

FLOWS TO ENGINE

So an alternative, more effective,Venturi is needed.

OPEN THROTTLELOW THROTTLE

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.

The Throttle Valve


FLOAT

AIR/FUEL MIXTURE

FLOWS TO ENGINE

OPEN THROTTLE

At Full Throttle the main Venturi

comes into operation.

LOW THROTTLE

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 pressureat Low Speed,and preventing too much Fuel mixing with the Air.

Balanced Air Pressure


FLOAT

AIR/FUEL MIXTURE

FLOWS TO ENGINE

ANDHere

VACUUM

Here

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 obtainedby fitting a

Diffuser Tubeonto the main jet.

Mixing Air and Fuel


FLOAT

AIR/FUEL MIXTURE

FLOWS TO ENGINE

So let’s take a look here.

Mixing Air and FuelThe Diffuser Tube

Fuel level in theFloat Chamber

DIFFUSER TUBE

MAIN JET

Fuel feed fromthe Float Chamber

AIR INLET

Air mixes with Fuel

Increased flow of

mixed Air/Fuel

THROTTLEVALVE

IDLE SETTING the diffuser is full of fuel

but very little air,fuel supplied via idle jet

OPEN THROTTLEidle jet stops working,main Venturi comes

into operation

CRUISE SETTINGair bubbles through the diffuser holes,

improving atomization

HIGH-POWER SETTING

The throttle valve is fully open

and maximum airflowis passing throughthe main venturi.

The Pressure Capsule

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


A method of adjusting the fuel and air flow is requiredto 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 chambermade from concertinaed thin metal.

The inside is a partial vacuum,

the outside senses the air pressure,

(termed as‘ambient’ pressure).

FLOAT


ANEROIDCAPSULE

FLOAT



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



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 isthe air/fuel ratio is maintained at

15:1at all atmospheric

conditions.

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.

Fuel Injection SystemsHydro-Mechanical Injection System

Warm Up Regulator

Air Inletfrom air filter

Throttle Valve

Cold Start Valve

Auxiliary Air Device

Idle Speed Adjuster

Mixture Control Unit (Fuel Distribution Valve )

Air Sensor Plate

Fuel injector

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.

Pump

Air Filter

ThrottleValve

Fuel Injection SystemsElectronic Injection System

Injection Control

Unit

EngineECU

Pressure Regulator

Common ‘Rail’

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.

The Injection Control Unit (ICU)controls the timing

of the injector valve opening,

in conjunction withthe Engine

Control Unit (ECU).

Fuel Injectors

Check of UnderstandingIn the carburettor

when is air sucked into the Ventri?

On the Induction down stroke

On the Compression up stroke

On the Compression down stroke

On the Induction up stroke

What shape are Throttle Valves?

Never Triangular

Any Shape

Always Square or Oblong

Always Circular

Check of Understanding

When does the Venturi work at maximum efficiency?

When the engine is at low power

When the engine is idle

When the engine is cruising

When the engine isat full power


What happens in a float chamber carburettor if an air filter causes restrictions to the air flow?

A vortex is formed

The pressure drops rapidly

The pressure rises rapidly

A vacuum is formed


Z

W

Y

X

Check of UnderstandingIn the diagram below,

which arrow points to the main jet?

At what throttle setting is a diffuser tubefull of fuel but with little air?

Ticking Over

Never

Cruising

Full Power


In a float chamber carburettor, what is the function of the device

marked with a ‘X’?

To prevent the fuel/air ratio from being too rich when the aircraft

climbs

To atomize the fuel

To supply extra fuel for acceleration

To prevent the fuel/air ratio from being too rich when the throttle

valve is opened


In a fuel injection system,which of the following does not apply?

A throttle valve is not needed

Fuel can be injected into the inlet manifold

Fuel can be injected into the cylinder

The fuel is not atomized


What is the normal fuel to air ratioin a piston engine?

15 : 1

1 : 5

5 : 1

1 : 15


PISTON ENGINES

End of Presentation

piston engines part 5 piston engine operations (fuel & ignition)

Documents

fuel reservoir

fuel vapour

atomized fuel

atomizing fuel

fuel pump

float chamber vent

efficient mix of fuel

float drops