Download Piston Prop. Pt 5

Piston Engines
Ignition, Cooling
and Thrust
Lecture Five
Introduction
Faraday Law
Every time a magnetic field
passes through the wires of a circuit,
a pulse of electricity is generated in those wires.
S
N
Coil
Windings
Volt
Meter
Each time the magnetic field ‘washes’ through the wire
an electrical current is produced called
EMF – Electro Motive Force
Magneto Operation
Magnetic fields of force flow from the magnets’ north pole
through the soft iron core to the south pole.
The coil and ignition circuit consist of two sub-circuits:
the primary windings, which carries low voltage;
and the secondary windings, where high voltage is generated.
The magnetic poles continuously change
by mechanical
Secondary drive from the engine which rotates the magnets.
Windings
For each rotation of the magnet rotor,
the magnetic field in the soft iron core
Primary
will
build and collapse four times.
Windings
NO
Magnetic
Magnetic Field
Field
EMP
No EMF
When the core is magnetised
an EMF is generated in the
primary windings.
Soft Iron
Core
This
generates
an EMF in the
Secondary windings which is used
Engine Driven
to
the spark in the spark plugs.
Rotorprovide
and Magnets
Magneto Operation
The essential
components
ofsystem
a magneto
are: unit,
The Magneto
is a complete
ignition
in a single
(except for
the sparktoplugs
and ignition
switch). field.
The Magneto
System,
provide
the magnetic
The Contact
Breaker,
topower
break generation,
the primary circuit
It consists
of the
when the current
is at its maximum.
spark flow
generation
The Condenser, to reduce burning/pitting
of the contact points.
and
components.
The Rotor Arm to distribution
direct the spark
surge to each spark plug,
in the correct sequence for the combustion stroke.
Magneto
Unit
Power
Generation
Spark Generation
Rotor Arm
Condenser
Coil
Magneto
Contact
Breaker
Distribution
Ignition
Switch
Spark Plugs
Dynamo/Alternator System
In the Dynamo and Alternator systems
the battery powers the primary windings in the coil unit.
The battery is continuously re-charged
by the dynamo / alternator,
and these systems generally feature an engine driven distributor,
containing the condenser, points and rotor arm,
Coil Unit
Magneto
Unit
Spark Generation
Rotor Arm
Distributor
Condenser
Ignition
Switch
Power Secondary
GenerationWindings
Coil Primary
Windings
Magneto
+ _
Battery
Contact
Breaker
Distribution
Ignition
Switch
Dynamo/
Alternator
Spark Plugs
The Distributor
The Distributor combines the main distribution elements,
directing the high tension current to each sparking plug.
The Vacuum Advance Mechanism
consists of a plate where the condenser,
contact breaker and rotor arm are mounted.
The Centrifugal Advance Mechanism
consists of weights thrown outwards by centrifugal force,
altering the position of the cam relative to the Rotor
input
Armdrive shaft.
Distributor
This can alter the ignition
advance by up to 30 degrees.
Condenser
Vacuum Advance
Adjustable
Plate
Centrifugal
Weights
Rotor
Arm
Vacuum
Chamber
Contact Breaker
Contact
Breaker
Centrifugal
Advance
Electronic Systems
Mechanically timed ignition systems suffer limitations : The contact breaker points deteriorate.
Contact breaker current is limited;
limiting the spark at the plugs.
At higher revs the points can bounce on the cam when they close;
this can weaken the spark.
At higher revs the points are close for a very short time
(called the ‘dwell’ angle)
this produces a weaker spark.
Despite these problems,
mechanical systems were used until the 1970s
when electronic systems started to appear.
Electronic Systems
Initially Electronic Systems continued to use
the mechanical contact breaker points,
but have now replaced them with a ‘solid’ state timing sensor,
and an Electronic Control Unit (ECU).
The timing disc and sensor
indicate the position of the crankshaft
to the ignition control unit.
Coil Unit
Rotor Arm
Engine Speed Sensing Unit
Ignition
Ignition
Switch
Switch
Timing
Disc
Secondary
Windings
Condenser
Timing
Sensor
Electronic
Control Unit
Primary
Windings
Contact
Breaker
+ _
Battery
Dynamo/
Alternator
Alternator
Spark Plugs
Electronic Systems
Engine Management Systems (EMS)
With the development of fuel injection systems,
it became logical to combine fuel and ignition in one system.
On most systems each spark plug now has its own coil pack,
producing a current of some 30,000 volts at the plugs.
Engine Speed Sensing Unit
Ignition
Switch
Timing
Disc
Timing
Sensor
Electronic
Control Unit
+ _
Battery
Alternator
Spark Plugs
Coil Packs
Spark Plugs
The first commercially available Spark Plug
was invented in 1902,
and the basic construction has been the same since.
When supplied with a high voltage pulse,
a spark jumps across from the inner to the outer electrode.
The gap needs to be set correctly;
too wide and the spark won’t jump the gap,
too close and the spark may not be
big enough to initiate combustion.
The gap can increase due to erosion and
could be bridged by contaminants
as a result of the combustion process.
Regular cleaning and gap adjustment
is necessary.
Gap
Cap Connector
Ceramic Body
Seal
Outer Casing
Inner Electrode
Copper Sealing
Gasket
Securing
Thread
These surfaces must be kept clean
Outer Electrode
Introduction
Of the total heat produced in a piston engine,
about 50% is used as energy, 40% passes into the atmosphere,
and the remaining 10% is absorbed by
the pistons, cylinder heads, valves etc.
and by the lubricating oil that splashes against them.
Unless these components are cooled by some means,
they become over-heated,
causing distortion, increased wear, and a reduction in strength.
Excessive temperatures can also result in engine seizure.
Two methods of cooling are in common use;
Air Cooling and Liquid Cooling.
Air Cooling
The advantages of Air Cooling include :-
Simple and less costly to produce.
Lighter engines.
But there are disadvantages :Difficult to cool all cylinders evenly, especially inline engines.
Difficult to get engine to run at most efficient temperature
on ground and at altitude.
Except for small amount of ‘prop-wash’,
early systems only effective when aircraft or vehicle
is moving forward.
Air cooled engines tend to be noisier than liquid cooled engines.
Not so easy to streamline.
Air Cooling
Air Cooling Flows
In air-cooled engines the outer surfaces of the cylinder head
is extensively finned
and a flow of air is directed over the exposed surfaces.
The fins are kept thin and close together to obtain
a maximum surface in contact with the air
Cylinders
to allow the air to take the heat away.
Air Intake
Air Outlet
Cross section of
typical light aircraft
A typical cooling airflow goes
through the forward facing intakes,
one each side of the spinner,
directly inline with the cylinders.
An outlet at the bottom is provided
to allow the cooling flow to escape.
Liquid Cooling
The advantages to Liquid Cooling include :-
A steady engine operating temperature
is more easily maintained
and can be kept within a narrower range,
making the engine more efficient.
The cooling ‘jacket’ makes the engine quieter.
The engine can be more easily streamlined
and thus produce less drag.
But there are also disadvantages :Tend to be heavier.
More complex, more weight, costlier to produce.
The system needs more maintenance compared to air cooling.
Liquid Cooling
The Thermostatic Valve
prevents cooling in the radiator
until the engine has warmed up.
It is usually housed in an easy to access bell housing
on the engine block,
generally where the top hose is connected to the engine.
Thermostatic
Valve
Consisting of
Typical
a temperature sensitive waxThermostatic
pellet,
within a rubber diaphragm,Valve
it is no more than a heat switch.
Rubber
Diaphragm
Wax Pellet
Liquid Cooling
The Thermostatic Valve
With a cold engine,
the valve is closed by spring force;
isolating the radiator flow,
and forcing coolant around the engine block only.
As the coolant heats up, it heats the wax pellet,
pushing the valve open.
CLOSED
OPEN
No Flow
with
Flow
To Radiator
‘Jiggle’ pin
Rubber
Diaphragm
Wax Pellet
When the valve opens,
coolant is directed to flow
through the radiator.
A ‘Jiggle-pin’ in the valve plate
allows air to escape,
and a small coolant flow to
the radiator so it starts to heat up
before the hot coolant
is allowed full flow.
Liquid Cooling
Coolant / Engine COLD:HOT:Pressure
Relief Valve
Thermostatic Valve
CLOSED
OPEN
Cooling Fan
Radiator
Coolant
Coolant
circulates
circulates
around
around
engine
engine
block block
and radiator
only
Thrust Augmentation
What is it?
A method of extracting more power
from Internal Combustion Engines.
It’s a form of ‘Power Boosting’
There are two types of booster: -
Supercharging and Turbocharging
both working on air compressors
The Air Compressor
Works with a rotor using centrifugal force,
and is driven by the engine crankshaft.
The rotor spins the air between the rotor vanes,
forcing it from the centre out to the rim,
then pushing the air through the stator vanes,
and finally to the engine via the outlet ducts.
OUTLET
DUCTS
STATOR
VANES
CROSS
SECTION
ROTOR
VANES
TAKE A CROSS SECTION
The Super Charger
Supercharging is an air compressor driven by the engine
via a gear train or belt drive.
Its main function is to increase the amount of air and fuel
delivered to the cylinders
and increase the pressure in the induction manifold
It can increase overall engine power up to 50% or more.
GEAR OR BELT DRIVE
COMPRESSED
AIR TO ENGINE
The Turbo Charger
Turbocharging is driven by the engine exhaust gases,
passing through a turbine.
Turbo’s operate best at high engine speeds
taking very little power from the engine to drive them,
therefore the net gain in power is relatively greater,
although they can suffer from what is called ‘turbo-lag’.
COMPRESSED
AIR TO ENGINE
THROUGH
EXHAUST PIPE
TURBINE
BLADES
Other Cooling Methods
Intercooling
Intercoolers are fitted in the induction system
between the booster and fuel controller
and work exactly the same as engine cooling radiators.
Water Injection
Water injection systems were introduced primarily
to cool the boosted air to the engine.
Adding fluid reduces the heat of the inlet air to the engine.
This protects against detonation
and allows more fuel to be added.
More air and fuel means more power is developed,
and more thrust produced !
Check of Understanding
A pulse of electricity is called an EMF
what does EMF stand for?
Electro Magnetic Force
Energy Motion Force
Electro Motive Force
Energy Magnet Force
Check of Understanding
Which of the following describes a magneto
as used in a piston engine?
A dynamo to produce a spark.
A dynamo to turn the engine
A fuel pumping system
A fuel metering system
Check of Understanding
In a magneto, one purpose of
the condenser (capacitor) is to Make the secondary current
flow evenly
Reduce pitting of
the contact breaker points
Make the primary current
flow evenly
Reduce erosion at
the spark plug points
Check of Understanding
How many times will the magnetic field
build and collapse with each rotation
of the magnet rotor?
Twelve
Eight
Six
Four
Check of Understanding
In a magneto system,
where is the power for the spark generated?
In the contact breaker points
In the coil secondary windings
In the condenser
In the coil primary windings
Check of Understanding
In a piston engine, the purpose of
the distributor is to distribute -
The correct mix of fuel and air to the cylinders
Power to the primary windings in the coil
Electrical impulses to the cylinders
Power to the secondary windings in the coil
Check of Understanding
In a distributor, where are the condenser,
contact breaker and rotor arm mounted?
The Vacuum Advance Mechanism
The Electronic Control Unit
The Centrifugal Advance Mechanism
The Spark Plug Coil Packs
Check of Understanding
In an electronic ignition system,
what creates the spark and ensures
the correct firing sequence?
The spark plug coil packs
The electronic control unit
The solid state timing sensor
The input drive shaft
Check of Understanding
On most electronic systems,
each spark plug has its own coil pack,
producing what amount of voltage?
300 volts
3,000 volts
30,000 volts
300,000 volts
Check of Understanding
The distributor includes two mechanisms,
centrifugal weights and a pressure diaphragm,
to advance the spark timing.
What are these mechanisms for respectively?
To advance the timing for
engine temperature and air pressure
To advance the timing for
engine speed and air pressure
To advance the timing for engine pressure
and exhaust temperature
To advance the timing for engine speed
and engine intake manifold pressure
Check of Understanding
In a breakerless electronic ignition system,
what components indicate the position of
the crankshaft to the ignition control unit?
The coil packs
The distributor timing disc
and sensor
The altenator
The valve timing belt
Check of Understanding
The 2 main methods of cooling
piston aero engines are
liquid cooling and air cooling.
Which of the following is an advantage
that air cooling has over liquid cooling?
Can hold the engine
at a more even temperature
Much cheaper to make
Engine can be more easily streamlined
Promotes longer life
between engine overhauls
Check of Understanding
What is the purpose of the fins
arranged about the cylinder and cylinder head
of an air-cooled engine?
To reduce the engine weight
To allow air to move freely through
the engine compartment
To allow air to take the heat away
To prevent the engine coolant
from boiling
Check of Understanding
What is the main benefit of using liquid,
as opposed to air, to cool an engine?
The engine will be lighter
A steady temperature
is easier to maintain
The engine will be simpler
The engine will be less costly
to maintain
Check of Understanding
In a liquid cooled engine,
what is the purpose of the thermostatic valve?
It prevents cooling in the radiator
until the engine has warmed up
It prevents cooling in the radiator
until the driver/pilot selects it open
It prevents heating in the vehicle interior
until the engine has warmed up
It allows the coolant to be cooled
until the engine is hot, then it closes
Check of Understanding
How is the thermostatic valve operated?
Manually closed,
opened by a spring
Closed by heat,
opened by a spring
Opened by heat,
closed manually
Opened by heat,
closed by a spring
Check of Understanding
How is the air compressor driven
in a piston engine supercharger?
By air pressure
By the exhaust gases
By an electric motor
By the engine crankshaft
Check of Understanding
What is the main function of a supercharger
when fitted to a piston engine?
It increases the amount of air and fuel
delivered to the cylinders
It delivers a stronger spark
to the sparking plugs
It increases the amount of lubricating oil
delivered to the bearings
It ensures that the batteries
are always fully charged
Check of Understanding
Which of these statements,
about an exhaust-driven turbocharger,
is true?
It operates best
at low engine speeds
It operates
as soon as the throttle is opened
It operates best
at high engine speeds
It is more effective than
an engine-driven supercharger
at increasing the power output
Check of Understanding
What is the primary function
of a supercharger on a piston engine?
To ensure that the batteries
are always fully charged
To make use of unburnt fuel
in the exhaust gases
To speed up the extraction
of exhaust gases
To increase the pressure
in the induction manifold
Check of Understanding
Water injection can be used with,
or instead of, intercoolers.
How does water injection help to
increases engine power?
It cools the air flow
allowing more fuel to be added
It makes the combustion
burn slower and longer
It heats the air flow
allowing more fuel to be added
It allows less fuel to be used
Piston Engines
End of Presentation