Lesson 4 of 8 · Parts academy

Ignition coil

3 min20 XP3 questions

A battery supplies low voltage. How does an ignition system obtain the pulse a spark plug needs?

Converts low-voltage electrical energy into the high voltage needed for a spark.

The idea

The coil uses a changing magnetic field to produce a high-voltage pulse. The spark plug uses that pulse at its electrode gap.

Many BMW petrol engines use a coil directly above each spark plug, in the cylinder-head plug wells.

A magnetic field builds as current flows in the primary winding. Interrupting that current generates a high-voltage pulse in the secondary winding.

A weak coil can cause misfires, especially under load. Wiring, plugs and other engine faults can produce similar symptoms.

Think it through

In the lesson diagram, current first builds a magnetic field and is then interrupted. Which component makes the high-voltage pulse, and which component creates the spark gap?

Show the reasoning

The coil generates the pulse; the plug contains the gap. Keeping those jobs separate also explains why a misfire alone cannot tell you whether a coil, plug or another engine system is responsible.

Take away
  • Follow electrical energy from supply to spark.
  • Keep coil function separate from plug function.
Sources