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 flux is equal to the time rate of change of the magnetic flux through 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 Windings S N

Magneto Operation Soft Iron Core Engine Driven Rotor and Magnets 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 the high voltage pulse is generated. Primary Windings Secondary Windings

Magneto Operation The 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 CondenserDistribution 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 and distribution components. Spark Plugs

Ignition Switch Magneto Operation Magneto Unit Coil Magneto Power Generation Spark Generation Contact Breaker Rotor Arm CondenserDistribution 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 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, 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 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 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 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. 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 Plugs Alternator Timing Sensor Timing Disc Engine Speed Sensing Unit Electronic Control 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 Plugs Alternator Timing Sensor Timing Disc Engine Speed Sensing Unit Electronic Control 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 SecuringThread Inner Electrode Seal These surfaces must be kept clean 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 Outer Electrode

Check of Understanding A pulse of electricity is called an EMF what 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 magneto as 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 of the 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, each spark plug has its own coil pack, producing what amount of voltage? Check of Understanding

PISTON ENGINES End of Presentation