Piston Engine Propulsion

Ignition

Ignition in a piston engine requires:

• Spark plugs • Magneto • Condensers • Distributor • Contact breakers electronic systems

Magneto• The Rotating Armature Magneto - The

assembly of the primary and secondary coils on a soft iron core is known as the armature.

• In the rotating armature magneto this is mounted on a shaft driven from the engine and rotated between the poles of a permanent magnet.

• As only two sparks are produced for each revolution of the armature, this type of magneto is normally used only on engines with up to six cylinders.

• The Rotating Magnet Magneto - The most usual type of rotating magnet magneto is the polar inductor magneto where the permanent magnets are actually stationary and soft iron inductors, mounted on a non-magnetic shaft driven from the engine, are used to guide the magnetic flux through the armature.

• Four sparks are produced for each revolution of the inductor shaft, making this type of magneto suitable for use on engines with more than six cylinders.

IGNITION SYSTEMIGNITION SYSTEM

Magneto Operation

Coil

Soft Iron Core

Secondary Windings

Primary Windings

Engine Driven Rotor and Magnets

• The magneto is a completely self-contained ignition generating device.

• Typically, two magnetos are installed on each engine for redundancy.

• When the aircraft engine rotates, gears located in the engine accessory case turn the magneto rotor shaft containing permanent magnets.

• With the rotating shaft, a magnetic field is produced that is transformed into high tension current through primary and secondary coil windings.

• The high tension current is disturbed to the appropriate cylinder through a distributor block assembly and ignition cables.

• One of the primary limitations of magnetos is that, when the magnet inside a magneto turns at a slow speed, the magneto produces relatively little voltage.

• However, as magneto speed increases, the amount of current induced into the primary circuit increases and the magneto produces a higher voltage spark.

• In most cases, the voltage generated at low magneto speeds is insufficient to fire a spark plug.

S N

IGNITION SYSTEM – Faraday Law

S NS NS NS NS NS NS N

Principal -

Each time the magnetic field ‘washes’ through wires

Volt Meter

An electrical current is produced

Called an EMF – Electro Motive Force

Coil Windings

IGNITION SYSTEMIGNITION SYSTEM

Magneto Operation

The magnetic field passes through soft iron core

EMF produced in the coil windings

IGNITION SYSTEM

No magnetic field in soft iron core

No EMF produced in the coil windings

IGNITION SYSTEM

The magnetic field passes through soft iron core again

EMF produced in the coil windings again

IGNITION SYSTEM – Magneto System

Ignition Switch

Distribution

Contact Breaker

Coil

Magneto

Condenser

Power Generation

Spark GenerationMagneto Unit Rotor Arm

IGNITION SYSTEM – Dynamo/Alternator System

Dynamo/ Alternator

Distributor

Contact Breaker

Coil

Ignition Switch

Secondary Windings

Primary Windings

Condenser

Battery

Distributor

• The distributor basically distributes the ignition spark between cylinders.

IGNITION SYSTEM – Distributor

Typical Distributor

Centrifugal Advance

Diaphragm Adjustable Plate

‘Points’

Cam

Cap

Body

Input Shaft Centrifugal

Weights

Adjuster

Vacuum Chamber

Vacuum Advance

Inlet Pressure

Magneto Timing

• There are two different timing requirements on most magnetos, internal timing and timing of the magneto to the engine.

• Internal timing is necessary to make sure that the voltage is delivered to the proper contact in the distributor block at the correct time. Since internal timing procedures vary between different magnetos, you should always follow the manufacturers instructions, but in all cases the magneto has to be stripped down to complete internal timing.

• When timing the two magnetos to an engine, the engine manufacturer can specify either synchronized or staggered ignition timing.

• Different procedures are used for timing flange and base mounted magnetos, but in nearly all cases magnetos are timed to fire between 20 and 25 degrees before top dead centre.

• As well as in the maintenance manuals the timing figures can be found on the engine data plates.

Ignition Switch

Coil Packs

IGNITION SYSTEM – Electronic Systems

Control Unit

Timing SensorTiming

Disc

Engine Speed Sensing Unit

Alternator

Battery

Ignition Switch

• Whenever a magneto is rotated sufficiently to open the contact breaker points, a spark will occur.

• All magnetos are therefore provided with an earthing wire, which is connected to the contact breaker end of the primary coil and through a suitable switch to earth.

• Since this switch is connected in parallel with the contact breaker, with the switch closed the effect of the opening and closing of the contact breaker is by-passed and no spark can occur.

Spark Plugs

• The end result in any ignition system is the spark plug that ignites the fuel/air mixture in a cylinder.

• Spark plugs transmit the short impulses of high voltage current from the ignition harness into the combustion chamber.

• The construction and operation of a spark plug is simple in concept but the demands placed on this part of an ignition system are high.

• Each ignition event begins with a 20,000 volt spark that jumps the air gap between a spark plugs electrodes, and the spark plug must be able to operate in temperatures of 3,000 degrees or higher, with gas pressures as high as 2000psi.

IGNITION SYSTEM – Spark Plug

Cap Connector

Outer Casing

Hexagon

Ceramic Body

Copper Sealing Gasket

Outer Electrode

GapSecuring Thread

IGNITION SYSTEM – Spark Plug

Centre Electrode

These surfaces must be kept clean

Seal

Change Spark Plugs at specified times

Make sure the correct Spark Plug is fitted

Cap Connector

Outer Casing

Hexagon

Ceramic Body

Copper Sealing Gasket

Outer Electrode

Securing Thread