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 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 N S N S N S N S N S N S 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 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 Generation Magneto 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 Sensor Timing 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 Gap Securing 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