Ignition Systems Ignition Systems

Page 2 of handoutPage 2 of handout

As we have stated, the ignition As we have stated, the ignition system is split into two circuits, system is split into two circuits, the primary and the secondary. the primary and the secondary.

Describe the operation of the Describe the operation of the two systems.two systems.

• The primary circuit is the low voltage side of the coil. The primary circuit is switched to allow the coil to charge and discharge.

• The discharge of the coil induces a high voltage into the secondary circuit, this ultimately causes the spark at the spark plug.

To make a spark jump across an To make a spark jump across an air gap of 0.6mm under normal air gap of 0.6mm under normal atmospheric conditions requires atmospheric conditions requires

………… to ………… volts. ………… to ………… volts. 2000 3000

To make a spark jump across a To make a spark jump across a spark plug of similar air gap spark plug of similar air gap

size inside an engine cylinder size inside an engine cylinder requires over …………. Volts. requires over …………. Volts. 8000

For higher compression engines For higher compression engines with a lean fuel mixture, the with a lean fuel mixture, the

voltage required can be as high voltage required can be as high as 20 000 volts. as 20 000 volts.

How is the voltage on a motor How is the voltage on a motor vehicle ‘stepped up’ from 12 volts, vehicle ‘stepped up’ from 12 volts, to the voltage required to jump the to the voltage required to jump the

spark plug?spark plug?

• Work in pairs to come to an answer, you have 2 minutes – try to be as in depth as possible.

Here’s my answer Here’s my answer

• Mutual induction takes place inside the coil. This is where a magnetic field is built up in the primary windings, then collapsed rapidly. This rapid collapse causes a high voltage to be induced into the secondary windings. This is discharged at the spark plug via the HT leads.

Label the sectioned coilLabel the sectioned coil

Primary Winding

Laminated sheet surrounding coils

Ceramic base

Laminated core

Secondary Windings

Questions Questions

1. Why is the ceramic base at the bottom of the coil?

2. Which of the two windings inside of the coil has the most windings – primary or secondary?

3. Why is this?4. Why is there a core of metal running

down the centre of the coil?5. How can the secondary voltage be made

higher?

Answers Answers

1. To insulate the coil body, to absorb heat.

2. The secondary has more windings.

3. This is to allow the voltage to be made greater, if it had less windings the voltage would be reduced.

4. To concentrate the magnetic field.

5. Make the primary voltage higher, or have more turns on the secondary winding.