The Spark Plug: Firing a Spark 50 Times a Second

From 12 volts to 40,000: the ignition coil

Air is an excellent insulator, and a car battery supplies only about 12-14 volts — nowhere near enough to force current across even a one-millimeter gap. The device that closes that gap is the ignition coil, a step-up transformer with a primary winding of a few hundred turns and a secondary of roughly 20,000 turns, a ratio near 100:1. But a transformer only converts changing current, so the coil first behaves as an inductor: a low-voltage switch (once mechanical breaker points, now a transistor or IGBT) closes and lets primary current ramp up to about 5-8 A over the dwell period, storing magnetic energy in the core.

The stored energy is E = ½LI². With a primary inductance of a few millihenries and several amps, the coil banks on the order of 100 mJ. When the switch opens, that field collapses in microseconds; the enormous dΦ/dt induces a few hundred volts across the primary and, multiplied by the turns ratio, 20,000-40,000 V across the secondary. This is the inductive (Kettering) system that Charles Kettering and DELCO commercialized around 1910-1912, itself building on the high-tension magneto that Robert Bosch and engineer Gottlob Honold made practical in 1902. High-revving and racing engines often use capacitor-discharge ignition (CDI), which dumps a pre-charged ~400 V capacitor into the primary for a faster voltage rise when dwell time runs short. Modern engines put one coil directly on each plug (coil-on-plug), abolishing the distributor and its rotating high-voltage contacts.

Breaking down the air: Paschen's law and the spark

As the secondary voltage climbs, an electric field builds across the electrode gap. At atmospheric pressure air breaks down at roughly 3 kV per millimeter (~30 kV/cm), but the plug fires late in the compression stroke when the charge is squeezed to 8-20 bar. Paschen's law (Friedrich Paschen, 1889) says breakdown voltage scales with the product of pressure and gap on the operating branch of the curve, so the dense, compressed air demands far more — tens of kilovolts — which is exactly why the coil is sized for 20-40 kV of reserve.

When the field finally strips electrons from air molecules, a Townsend avalanche ionizes a thin channel into conductive plasma and current jumps. The spark then unfolds in three phases: a breakdown phase lasting nanoseconds, in which the channel momentarily reaches tens of thousands of kelvin at enormous power but tiny energy; a brief arc phase (~100 V, ~1 µs); and a long glow discharge (a few hundred volts, ~1-2 ms) that delivers the bulk of the total 30-100 mJ into the gas. Fine, sharp electrodes concentrate the field, lowering the voltage needed to fire — the core reason iridium and platinum tips exist.

The flame kernel and the combustion that follows

The spark's job is not to burn the whole cylinder — it cannot — but to create a flame kernel: a millimeter-scale ball of burning gas hot enough to sustain itself. The minimum ignition energy of a stoichiometric gasoline–air mixture is only about 0.2 mJ, so the 30-100 mJ a plug delivers is a large safety margin, needed because real mixtures are turbulent, sometimes lean, diluted with recirculated exhaust (EGR), and never perfectly uniform.

Once lit, the kernel grows into a self-propagating flame front. The intrinsic laminar flame speed of gasoline–air is only ~0.4 m/s, which alone would take far too long. In-cylinder turbulence — deliberately generated by intake tumble and swirl and by squish near top dead center — wrinkles and stretches the flame so its effective turbulent flame speed reaches 10-25 m/s. That lets the flame cover the ~40-50 mm from a centrally placed plug to the wall of an 80-100 mm bore in roughly 1-3 ms: turbulence intensity climbs with engine speed, so the burn occupies a roughly constant 30-40 crank-angle degrees — about 2 ms at 3,000 rpm and nearer 1 ms at 6,000 rpm. As the charge burns, heat release drives peak gas temperatures near 2,500 °C and peak cylinder pressures of 50-100+ bar (naturally aspirated engines toward the low end, turbocharged and racing engines well past 100 bar).

Timing is everything: spark advance and MBT

Because flame development takes a roughly fixed couple of milliseconds regardless of engine speed, the spark must fire before top dead center (BTDC) so that combustion finishes just after the piston passes over the top. The goal is MBT — maximum brake torque timing — which places peak cylinder pressure about 10-15 degrees after top dead center, where the crank geometry converts pressure into torque most effectively. Typical spark advance runs from ~10° BTDC at idle to ~35-40° BTDC at high rpm and light load, precisely because a fixed burn time occupies more crank degrees as the engine spins faster.

The timing window is bounded on both sides. Fire too early and the compressed, heating end-gas can auto-ignite ahead of the flame — knock — which hammers the piston and can wreck it; the engine's piezoelectric knock sensor listens for this and retards timing. Fire too late and combustion trails off during expansion, wasting energy and dumping heat into the exhaust. As for the headline number: a four-stroke fires each plug once every two crank revolutions, so the firing frequency is f = rpm / 120. At 6,000 rpm that is exactly 50 Hz per cylinder; at a 800 rpm idle it is under 7 Hz; and a V8 at 6,000 rpm demands 400 sparks per second from the ignition system as a whole.

Surviving the fire: insulator, electrodes, and heat range

The plug's white body is alumina (aluminum oxide, typically 96-99% Al₂O₃), chosen for high dielectric strength, mechanical toughness, and enough thermal conductivity to carry combustion heat away. The exposed ribs are anti-flashover corrugations that lengthen the surface path so the 30 kV cannot arc down the outside instead of across the gap. The exposed center electrode faces the worst of the fire, so its metal is tuned: base nickel alloys with a copper core for heat conduction; platinum (melting point 1,768 °C) for erosion resistance; and iridium (melting point 2,446 °C), which survives as a fine 0.4-0.6 mm tip that lowers the required firing voltage and lasts past 100,000 miles. Denso and NGK brought iridium plugs to market in the 1990s.

Just as important is heat range — how fast the insulator nose sheds heat, set by the length of its heat path. The firing tip must sit in a narrow thermal window of roughly 450-870 °C: below ~450 °C the tip cannot burn off carbon and it fouls; above ~870-950 °C it becomes a glowing hot spot that causes pre-ignition. A hot plug has a long insulator nose that retains heat, suited to low-output or low-speed engines; a cold plug has a short nose that dumps heat fast, suited to high-output, turbocharged, and racing engines. Threads are standardized (M14×1.25 is common, with M12, M10, and M8 on compact heads), sealed by either a flat gasket or a conical taper seat, and most plugs carry a ~5 kΩ resistor to suppress the radio-frequency interference the spark would otherwise broadcast.

How it fails: erosion, fouling, misfire, and the catalyst

Every spark ablates a few atoms of electrode. Over tens of thousands of miles the gap slowly widens — roughly 0.01 mm per 1,000 miles for plain nickel — and a wider gap needs a higher firing voltage. When the demand voltage finally exceeds the coil's reserve, the plug misfires: no spark, no combustion. Fouling does the opposite, coating the insulator with conductive carbon, oil, or ash that lets the charge leak away before it can build to breakdown. A wrong heat range or lean, over-advanced running invites pre-ignition and, in the worst case, runaway auto-ignition.

Misfires are not merely a stumble. A cylinder that fails to burn pumps raw hydrocarbons into the exhaust, wasting fuel and running rough. Worse, that unburned fuel oxidizes exothermically on the catalytic converter, which can spike past 900 °C and even melt or sinter the catalyst substrate — a repair far costlier than a plug. That is why OBD-II systems watch for misfire directly, detecting the tiny crankshaft angular deceleration a dead cylinder produces and logging codes P0300-P0308. Beyond simple plugs, engineers are chasing better ignition for lean, high-EGR combustion: pre-chamber turbulent jet ignition (used in the Maserati Nettuno V6, Formula 1, and Mahle's designs), corona and barrier-discharge systems (Tenneco/Federal-Mogul's Advanced Corona Ignition), and laser ignition in the lab. All aim past what a single point of plasma, fired 50 times a second, can reliably light.

Spark-plug electrode materials and what the metal choice buys you

Center electrode	Melting point	Tip diameter	Typical service life
Nickel alloy (copper core)	~1,455 °C (Ni)	~2.0-2.5 mm	~30,000 mi / 50,000 km
Single platinum	1,768 °C	~0.6-0.8 mm	~60,000 mi / 100,000 km
Double platinum	1,768 °C	~0.6-0.8 mm	~60,000-100,000 mi
Iridium	2,446 °C	~0.4-0.6 mm	~100,000+ mi / 160,000 km