An ignition coil is the part of a gasoline engine ignition system that turns a car battery’s low voltage into the very high voltage needed to make a spark. A typical 12 V battery cannot jump the gap at a spark plug by itself, because air is normally an electrical insulator. The ignition coil solves this by storing energy in a magnetic field and then releasing it quickly as a voltage pulse.
That spark starts combustion, which is why coil timing and coil health strongly affect starting, power, fuel economy, and emissions.
Inside the coil are two windings of wire wrapped around an iron core: a primary winding with relatively few turns and a secondary winding with many more turns. When current flows through the primary winding, the iron core becomes magnetized and stores energy in its magnetic field. When the ignition switch or electronic control module suddenly stops the primary current, the magnetic field collapses and induces a large voltage in the secondary winding.
This high voltage travels to the spark plug, where it forces charge across the plug gap and ignites the compressed air fuel mixture.
Key Facts
- An ignition coil is a step-up transformer used in pulses, not a device that creates energy from nothing.
- Typical battery voltage is about 12 V, while spark plug voltage is often about 20,000 V to 40,000 V.
- Turns ratio formula for an ideal transformer: Vs / Vp = Ns / Np.
- Magnetic energy stored in the primary circuit: E = 1/2 L I^2.
- A fast change in magnetic flux creates induced voltage: V = -N ΔΦ / Δt.
- A larger secondary turn count and a faster collapse of the magnetic field both increase the output voltage.
Vocabulary
- Ignition coil
- An electrical device that converts low battery voltage into a brief high-voltage pulse for a spark plug.
- Primary winding
- The low-voltage coil of wire that carries battery current and builds the magnetic field.
- Secondary winding
- The high-voltage coil of wire with many turns that produces the spark voltage.
- Iron core
- A magnetic material inside the windings that strengthens the magnetic field and improves induction.
- Spark plug gap
- The small space between spark plug electrodes where high voltage causes an electric spark to jump.
Common Mistakes to Avoid
- Thinking the coil simply multiplies voltage continuously is wrong because an ignition coil works by building and collapsing a magnetic field in timed pulses.
- Ignoring the turns ratio is wrong because the secondary winding must have many more turns than the primary winding to produce a much higher voltage.
- Assuming voltage alone guarantees a good spark is wrong because the coil must also store enough energy and deliver it at the correct time.
- Forgetting the role of current interruption is wrong because the high voltage is produced mainly when the primary current is switched off quickly.
Practice Questions
- 1 An ignition coil has 150 turns in the primary winding and 30,000 turns in the secondary winding. If 12 V is applied to the primary in an ideal transformer model, what secondary voltage is predicted by Vs / Vp = Ns / Np?
- 2 A coil primary has an inductance of 0.006 H and carries 6 A before the switch opens. How much magnetic energy is stored? Use E = 1/2 L I^2.
- 3 Explain why a faster collapse of the magnetic field in the ignition coil can produce a higher spark voltage at the spark plug.