Ignition systems: battery, electronic and distributorless

Why it matters

A spark-ignition engine needs a spark of roughly 10–30 kV at exactly the right crank angle, thousands of times a minute, from a 12 V supply. Ignition timing affects power, fuel economy, knock and emissions, and a weak or mistimed spark causes misfire that can destroy a catalytic converter. The evolution from contact-breaker systems to electronic and distributorless ignition explains most of what you will meet in workshops and two-wheelers today.

Key ideas

What the system must do. Store energy, convert 12 V into a high voltage able to break down the compressed mixture across the plug gap, deliver it to the right cylinder, and fire at the right moment — advanced with speed (the flame needs a roughly fixed time to burn) and with light load (lean, slow-burning mixture), and retarded when knock is detected.

Battery (coil, Kettering) ignition.

• Primary circuit: battery → ignition switch → ballast resistor → coil primary (about 150–300 turns of thick wire) → contact-breaker points → earth.
• Secondary circuit: coil secondary (about 15,000–30,000 turns of fine wire, turns ratio about 100:1) → distributor rotor and cap → HT leads → spark plugs.
• With the points closed, primary current builds up and stores energy in the coil's magnetic field. A cam on the distributor shaft opens the points; the field collapses rapidly and induces a few hundred volts in the primary and, by mutual induction, tens of kilovolts in the secondary.
• The capacitor (condenser), about 0.2 µF, across the points absorbs the primary's self-induced voltage so the points do not arc and the current falls quickly.
• Dwell angle is the cam angle for which the points stay closed; it sets how long the current has to build up.
• Centrifugal advance (flyweights) advances timing with speed; vacuum advance (diaphragm on manifold vacuum) advances it at part load.
• Weaknesses: point wear and burning, timing drift, and falling spark energy at high speed because dwell time shrinks.

Electronic (transistorised) ignition. The points are replaced by a power transistor switched by a non-contact pickup — magnetic (variable reluctance), Hall-effect or optical. The distributor may still route the high voltage. Higher primary current and constant timing give a stronger, more reliable spark; dwell can be controlled electronically. Capacitor-discharge ignition (CDI), common on two-wheelers, charges a capacitor to a few hundred volts and dumps it into the coil primary through a thyristor, giving a very fast-rising spark.

Programmed and distributorless ignition (DIS). The ECU computes timing from a speed–load map using the crankshaft (and camshaft) position sensors, corrected for coolant temperature and knock sensor signals.

• Waste-spark DIS: one double-ended coil fires two cylinders whose pistons reach TDC together (1–4 and 2–3 on a four-cylinder engine). One spark is on compression, the other "wasted" on exhaust, where little voltage is needed.
• Coil-on-plug (COP): one coil per cylinder sitting on the plug — no HT leads, less loss and interference, individual timing and misfire detection.
• No rotating distributor, so no rotor/cap wear or timing drift.

Spark plug. The required breakdown voltage rises with gap, cylinder pressure and electrode wear and falls with temperature. The plug's heat range must keep its tip hot enough to burn off deposits but cool enough to avoid pre-ignition.

Links. Crank-position and knock sensors are covered in the sensors topic; timing maps in the ECU topic.

Formulas

V_s ≈ (N_s / N_p) × V_p

• V_s: secondary voltage (V), V_p: primary induced voltage (V), N_s/N_p: turns ratio. Ideal transformer estimate; the actual peak is limited by plug breakdown.

W = ½ × L_p × I_p²

• W: energy stored in the coil (J), L_p: primary inductance (H), I_p: primary current when the circuit is broken (A).

I_p(t) = (V / R) × (1 − e^(−t/τ)), τ = L_p / R

• V: supply voltage (V), R: total primary resistance including ballast (Ω), t: dwell time (s), τ: time constant (s).

f_spark = n × z / 120 (four-stroke)

• f_spark: sparks per second (1/s), n: engine speed (rpm), z: number of cylinders.

t = θ / (6 × n_shaft)

• t: time (s) for a shaft turning at n_shaft (rpm) to rotate through θ (degrees). For dwell use distributor angle with distributor speed (half engine speed in a four-stroke); for advance use crank angle with engine speed.

Cam angle per cylinder = 360° / z (distributor degrees); dwell percentage = θ_dwell / (360° / z) × 100.

Worked examples

Example 1 (standard). A four-cylinder four-stroke engine runs at 3000 rpm with ignition timed 20° before TDC. The coil turns ratio is 100:1 and the primary develops 300 V when the points open. Find (a) sparks per second, (b) the time between spark and TDC, (c) the ideal secondary voltage.

1. f_spark = n × z / 120 = 3000 × 4 / 120 = 100 sparks/s.
2. t = θ / (6 × n) = 20 / (6 × 3000) = 1.11 × 10⁻³ s = 1.11 ms.
3. V_s = 100 × 300 = 30,000 V = 30 kV.

Answer: 100 sparks/s, 1.11 ms, about 30 kV (ideal).

Example 2 (GATE level). The same engine uses a coil with primary inductance 6 mH and total primary resistance 3 Ω on 12 V. The dwell angle is 54° of distributor rotation. Find the stored energy at 3000 rpm and at 6000 rpm engine speed.

1. Distributor speed at 3000 rpm = 1500 rpm. Dwell time t = 54 / (6 × 1500) = 6.0 ms. (Cam angle per cylinder 90°, so dwell = 60 %.)
2. τ = L / R = 0.006 / 3 = 2 ms; V/R = 4 A.
3. I_p = 4 × (1 − e^(−6/2)) = 4 × 0.9502 = 3.80 A; W = ½ × 0.006 × 3.80² = 43.3 mJ.
4. At 6000 rpm the dwell time halves to 3.0 ms: I_p = 4 × (1 − e^(−1.5)) = 4 × 0.7769 = 3.11 A; W = ½ × 0.006 × 3.11² = 29.0 mJ.

Answer: about 43 mJ at 3000 rpm, falling to about 29 mJ at 6000 rpm — the reason electronic systems control dwell time rather than dwell angle.

Common mistakes

• Using Ohm's law on the coil (V = IR) to "find the spark voltage". The high voltage comes from the rapid collapse of flux, V = L·di/dt and the turns ratio, not from IR drop.
• Using engine speed with distributor degrees (or vice versa); the distributor turns at half crank speed in a four-stroke engine.
• Thinking electronic ignition always removes the distributor. Early electronic systems only replaced the points; DIS removes the distributor.
• Forgetting that both waste-spark plugs fire at once, one on compression and one on exhaust.
• Quoting the breakdown strength of air at atmospheric pressure for a plug inside a compressed cylinder; in-cylinder breakdown voltages are much higher.
• Setting a wider plug gap "for a bigger spark" without checking the coil has enough voltage reserve.

For GATE ME

This topic links to IC engines and basic electrical science. Expect numericals on spark frequency, converting crank angle to time, RL current growth, energy stored in an inductor and transformer turns ratio, plus conceptual questions on advance mechanisms and the reasons for electronic and distributorless systems.

Quick check

1. What is the purpose of the capacitor across the contact-breaker points?
2. Why must ignition timing advance as engine speed increases?
3. In a waste-spark system on a four-cylinder engine, which cylinders share a coil?
4. A coil has L = 5 mH and breaks a current of 8 A. How much energy is stored?

Answers: 1. It absorbs the primary self-induced voltage so the points do not arc, giving a faster collapse and longer point life. 2. Combustion takes a roughly fixed time, which spans more crank angle at higher speed. 3. Cylinders 1 and 4, and 2 and 3. 4. ½ × 0.005 × 64 = 160 mJ.