PISTON ENGINES

Part 6

Piston Engine Operations(Ignition)

Introduction

“The induced electromotive force or EMF in any closed circuit

is equal to the time rate of change of the magnetic fluxthrough the circuit”

Every time a magnetic field (flux)

passes through the wires of a circuit,

a pulse of electricity (EMF)

is generated in those wires.

EMF – Electro Motive Force

Faraday Law

Introduction

Each time the magnetic field ‘washes’ through the wire

an electrical current is produced called

EMF – Electro Motive Force

Faraday Law

Volt Meter

Coil WindingsS N

Magneto Operation

Soft Iron CoreEngine Driven Rotor and Magnets

Magnetic fields of force flow from the magnets’ north polethrough 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 the high voltage pulse is generated.

Primary Windings

Secondary Windings

Magneto OperationThe magnetic pole positions are continuously changed

by mechanical drive from the engine rotating the magnets.

For each rotation of the magnet rotor, the magnetic field in the soft iron core

will build and collapse four times

Magneto Operation

Magnetic Field EMF produced

NO Magnetic Field NO EMF

Each time the core is magnetised and then demagnetised,an EMF is generated in the primary coil windings.

This power generates an EMF in the secondary windings,which is used to provide the spark in the spark plugs.

Ignition Switch

Magneto Operation

Magneto Unit

Coil

Magneto

Power Generation

Spark Generation

Contact Breaker

Rotor Arm

Condenser

Distribution

The Magneto is a complete ignition system in a single unit,(except the spark plugs and ignition switch).

It consists of the power generation, spark generation anddistribution components.

Spark Plugs

Ignition Switch

Magneto Operation

Magneto Unit

Coil

Magneto

Power Generation

Spark Generation

Contact Breaker

Rotor Arm

Condenser

Distribution

The essential components of a magneto are:The Magneto System, to provide the magnetic field.The Contact Breaker, to break the primary circuit

when the current flow is at its maximum.The Condenser, to reduce burning/pitting of the contact points.

The Rotor Arm to direct the spark surge to each spark plug in the correct sequence for the combustion stroke.

Spark Plugs

Magneto Unit

Coil

Magneto

Power Generation

Ignition Switch

Spark Generation

Distribution

Dynamo/Alternator SystemIn 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,

Rotor Arm

Spark Plugs

Contact Breaker

Condenser

+ _

Battery

Secondary Windings

Primary Windings

Ignition Switch

Dynamo/ Alternator

Coil Unit

Distributor

Contact Breaker

Condenser

Rotor Arm

The DistributorThe 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 Mechanismconsists of weights thrown outwards by centrifugal force,

altering the position of the cam relative to the input drive shaft.

This can alter the ignition advance by up to 30 degrees.Distributor

Vacuum Advance

Vacuum Chamber

Adjustable Plate

Contact Breaker

Rotor Arm Centrifugal

Advance

Cam Centrifugal Weights

Electronic SystemsMechanically 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.

Dynamo/ Alternator

Rotor Arm

Ignition Switch

Contact Breaker

Condenser

Secondary Windings

Primary Windings

Coil Unit

Electronic Systems

+ _

Battery Spark PlugsAlternator

Timing SensorTiming

Disc

Engine Speed Sensing Unit

ElectronicControl Unit

Ignition Switch

Initially Electronic Systems continued to use the mechanical contact breaker points,

but now the distributor has all but disappeared, to be replaced by a ‘solid’ state timing sensor,

and an Electronic Control unit (ECU).

The ECU switches a large flow of current through the coils,creating the spark to ensure the correct firing sequence.

Electronic Systems

+ _

Battery Spark PlugsAlternator

Timing SensorTiming

Disc

Engine Speed Sensing Unit

ElectronicControl Unit

Ignition Switch

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.

Coil Packs

Engine Management Systems (EMS)

Spark Plugs

Cap Connector

Outer Casing

Ceramic Body

Copper Sealing Gasket

Securing Thread

Inner Electrode

Seal

These surfaces must be kept clean

The first commercially available Spark Plugwas 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 andcould be bridged by contaminants

as a result of the combustion process.

Regular cleaning and gap adjustmentis necessary.

Gap

Outer Electrode

Check of Understanding

A pulse of electricity is called an EMFwhat does EMF stand for?

Energy Magnet Force

Electro Magnetic Force

Electro Motive Force

Energy Motion Force

A fuel metering system

A dynamo to produce a spark.

A fuel pumping system

A dynamo to turn the engine

Which of the following describes a magnetoas used in a piston engine?

Check of Understanding

Reduce erosion at the spark plug points

Make the secondary current flow evenly

Make the primary current flow evenly

Reduce pitting ofthe contact breaker points

In a magneto, one purpose of the condenser (capacitor) is to -

Check of Understanding

Four

Twelve

Six

Eight

How many times will the magnetic field build and collapse with each rotation

of the magnet rotor?

Check of Understanding

Power to the secondary windings in the coil

The correct mix of fuel and air to the cylinders

Electrical impulses to the cylinders

Power to the primary windings in the coil

In a piston engine, the purpose of the distributor is to distribute -

Check of Understanding

The Spark Plug Coil Packs

The Vacuum Advance Mechanism

The Centrifugal Advance Mechanism

The Electronic Control Unit

In a distributor, where are the condenser, contact breaker and rotor arm mounted?

Check of Understanding

The input drive shaft

The spark plug coil packs

The solid state timing sensor

The electronic control unit

In an electronic ignition system,what creates the spark and ensures

the correct firing sequence?

Check of Understanding

300,000 volts

300 volts

30,000 volts

3,000 volts

On most electronic systems, each spark plug has its own coil pack,

producing what amount of voltage?

Check of Understanding

PISTON ENGINES

End of Presentation