An ignition plug — commonly called a spark plug — is a threaded ceramic-and-metal device that screws into the cylinder head and delivers a high-voltage arc across a precise air gap to ignite the compressed air-fuel charge. Unlike the hot-tube and trembler-coil ignitors it replaced around 1902, the modern plug fires on demand at a controlled crank angle. It converts 15-40 kV from the coil into a 0.5-1.0 mJ plasma kernel inside the combustion chamber. That single spark is what turns a charged cylinder into useful torque on every power stroke.
Ignition Plug Interactive Calculator
Vary coil voltage, breakdown threshold, and spark gap to see whether the plug fires and how much voltage margin is available.
Equation Used
The plug fires when available coil voltage exceeds the gas breakdown voltage across the spark gap. Voltage margin shows reserve before misfire, while Vcoil divided by gap estimates the average electric field in the gap.
- Breakdown voltage is treated as a known threshold for the current gas pressure and mixture.
- Coil voltage is the available secondary voltage before the gap conducts.
- Gap electric field is estimated as average field across a uniform air gap.
How the Ignition Plug Actually Works
A spark plug is an air-gap switch that only conducts when the voltage across it exceeds the dielectric breakdown of the gas in the gap. The ignition coil ramps secondary voltage until the gap ionises — typically 8-15 kV in a clean engine at idle, climbing to 25-35 kV under boost or with a worn gap. Once it breaks down, current flows through the plasma channel, dumps roughly 30-100 mJ of stored coil energy into the gas, and lights a flame kernel about 2 mm across. That kernel grows into the full burn that drives the piston down.
The geometry matters more than people think. The gap between centre electrode and ground electrode is usually 0.6-1.1 mm depending on the engine. Open it 0.2 mm too wide and required voltage jumps past what the coil can deliver — you get misfires under load, especially in 4th gear at part throttle where cylinder pressure is high but coil dwell is short. Close it 0.2 mm too tight and you get a weaker kernel, slow burn, and rough idle. The ceramic insulator around the centre electrode has to hold off that 30 kV without tracking down the outside, which is why you wipe the porcelain clean and never run a cracked plug.
Heat range is the other axis. The plug's firing tip has to stay between roughly 500°C and 850°C in service. Below 500°C, unburned fuel and oil deposit on the insulator nose and the plug fouls — black, wet, dead. Above 850°C, the tip becomes a glow-ignition source and the engine starts pre-igniting before the spark even fires, which is how you melt a piston crown. Heat range is set by the length of the insulator nose: a long nose runs hot, a short nose pulls heat into the shell quickly and runs cold. Turbo and high-compression engines need cold plugs. Lawnmowers and idle-heavy engines need hot plugs.
Key Components
- Centre electrode: The high-voltage terminal that protrudes into the combustion chamber. Made from nickel alloy, platinum, or iridium — iridium tips run as fine as 0.4 mm diameter to lower required firing voltage by 3-5 kV versus a 2.5 mm nickel tip. Erodes about 0.01-0.02 mm per 1000 km on a nickel plug, which is why service intervals exist.
- Ground electrode (side electrode): The L-shaped strap welded to the shell that forms the other side of the spark gap. Gap to centre electrode is set to 0.6-1.1 mm typical, with a tolerance of ±0.05 mm — measure with a wire feeler, never a flat blade, because a worn centre electrode has a concave face that fools a flat gauge.
- Ceramic insulator: Aluminium-oxide porcelain that isolates the centre electrode from the steel shell. Has to hold off 30-40 kV without surface tracking and survive 850°C tip temperatures. A hairline crack you cannot see with the naked eye will track to ground and kill the spark — replace any plug you have dropped on a concrete floor.
- Steel shell with thread: Carries the ground path and seals the combustion chamber. Common threads are 14 mm × 1.25 (M14, automotive) and 10 mm or 12 mm for modern compact heads. Torque to spec — 25-30 Nm on a flat-seat 14 mm plug with a fresh gasket, less on a tapered seat. Over-torque crushes the gasket and changes heat transfer.
- Resistor (in resistor plugs): A 4-7 kΩ carbon or ceramic resistor in the centre wire that suppresses RFI from the spark event. Required on any engine with an ECU — without it, the radiated noise will trip the ECU's knock sensor input or scramble CAN-bus traffic on the same harness.
- Terminal nut: The threaded stud or solid post on top that the plug wire or coil-on-plug boot connects to. Must be tight to the centre electrode — a loose terminal arcs internally and burns the resistor open.
Real-World Applications of the Ignition Plug
Spark plugs live in every Otto-cycle engine ever built, from a 25 cc string-trimmer to a 5000 hp marine V12. The variant choice — heat range, reach, gap, electrode metal, projection — comes from the engine's compression ratio, RPM range, fuel chemistry, and how long you want to go between services. A turbocharged 2.0 L sees a different plug than a naturally-aspirated lawn engine even though both use the same basic mechanism.
- Automotive — passenger cars: NGK ILZKAR7B11 iridium plugs in the Toyota GR Corolla G16E-GTS turbo three-cylinder, gapped at 0.7 mm to survive 25+ psi of boost without blow-out misfire.
- Motorcycle: Champion RA8HC plugs in the Harley-Davidson Milwaukee-Eight 114, two per cylinder for faster flame travel across the wide combustion chamber.
- Small engines / outdoor power equipment: Champion RC12YC in the Briggs & Stratton Vanguard 810 V-twin used on commercial zero-turn mowers, gapped at 0.76 mm.
- Marine: NGK BPZ8H-N-10 surface-gap plugs in Mercury OptiMax 2.5 L direct-injected two-strokes — surface gap resists fouling on oil-rich startups.
- Aviation (piston aircraft): Champion REM38S massive-electrode plugs in the Lycoming O-360 powering Cessna 172s, designed for 100LL leaded avgas and 2700 RPM continuous.
- Industrial gas engines: Denso GE3-5 plugs in Caterpillar G3516 stationary natural-gas gensets running at 1500 RPM for 8000-hour service intervals.
- Vintage / restoration: Champion W18 plugs in restored Ford Model T flatheads — large 18 mm thread, hot heat range to suit the 4.5:1 compression and low-octane fuel.
The Formula Behind the Ignition Plug
The required firing voltage is what determines whether a given coil can actually light a given plug under a given cylinder pressure. Practitioners need this because at the low end of the operating range — idle, warm engine, fresh plugs — required voltage sits around 8-12 kV and any coil works. At the high end — full boost, worn plugs, lean mixture — it climbs past 30 kV and a marginal coil starts dropping sparks. The sweet spot for a healthy ignition system is staying below 70% of the coil's maximum output across the worst-case operating point, which leaves headroom for a worn gap at end of service life.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Vreq | Required breakdown voltage across the spark gap | V | V |
| K | Paschen-law constant for the gas mixture (≈ 4.4 × 106 for air-fuel near stoich) | V·K0.5 / (Pa0.5·m) | V·°R0.5 / (psi0.5·in) |
| p | Cylinder pressure at the moment of spark | Pa | psi |
| g | Spark gap distance | m | in |
| T | Gas temperature in the gap at firing | K | °R |
Worked Example: Ignition Plug in an Iveco Cursor 13 NG natural-gas city bus engine
You are commissioning the spark ignition system on an Iveco Cursor 13 NG 12.9 L stoichiometric natural-gas inline-six used in a transit-bus retrofit. The OEM calls for a 0.3 mm gap on Denso GI3-5A iridium plugs, 12.5:1 compression, and you need to confirm the 40 kV peak coil output has enough margin across the operating window from light cruise (8 bar BMEP) to full power up a hill (22 bar BMEP at full WOT, manifold-pressure-corrected cylinder pressure at spark ~35 bar).
Given
- g = 0.30 mm = 3.0 × 10⁻⁴ m
- T = 650 K (compressed charge at spark)
- K = 4.4 × 10⁶ V·K^0.5 / (Pa^0.5·m)
- p (low cruise) = 1.5 × 10⁶ Pa (15 bar)
- p (nominal cruise) = 2.5 × 10⁶ Pa (25 bar)
- p (full load) = 3.5 × 10⁶ Pa (35 bar)
Solution
Step 1 — at nominal 25 bar cruise pressure, compute required voltage with the Paschen-style relation:
Round to 16-17 kV at cruise. The 40 kV coil sits at 41% of rated output here — comfortable margin, and this is exactly where the bus spends 80% of its duty cycle.
Step 2 — at the low end of the operating range, 15 bar light-cruise pressure:
At 12.7 kV the system is barely working the coil. The plug fires cleanly even with deposits, and you would not see misfire even on a coil that has lost 30% of its peak output to age. This is the easy case.
Step 3 — at the high end, 35 bar full-load pressure pulling up a 6% grade:
That sits at 49% of the 40 kV coil. Now factor in end-of-service-life gap erosion — the 0.30 mm gap typically opens to 0.40 mm by 80,000 km on natural gas. Recompute Vhigh with g = 0.40 mm: V ≈ 19.4 × (0.40 / 0.30) ≈ 25.9 kV, or 65% of coil capacity. Still inside the 70% headroom rule, so the design survives a full service interval without misfire on the worst hill.
Result
The nominal required voltage at 25 bar cruise is roughly 16. 4 kV — well inside the 40 kV coil's capability. Low-cruise sits at 12.7 kV (effortless), nominal at 16.4 kV (the design sweet spot), and full-load worst-case end-of-life climbs to 25.9 kV (65% coil utilisation, still safe). If you measure higher firing voltages on a scope than these numbers predict — say 30 kV at cruise instead of 16 — check three things in order: (1) actual measured gap with a wire gauge, because a plug that came out of the box at 0.45 mm instead of the spec 0.30 mm will pull voltage hard from day one; (2) coil secondary resistance, because an aged coil with a partially shorted secondary winding shows the right peak voltage but wrong rise time and will misfire under boost; (3) plug-wire boot seating, since a 1 mm air gap between the boot and the plug terminal adds another 8-10 kV of series demand and looks identical to a wide gap on a scope.
Choosing the Ignition Plug: Pros and Cons
The spark plug is one option for igniting a charge in a piston engine. Compare it against the systems it replaced and the ones competing with it today on the dimensions that actually matter for engine designers and rebuilders.
| Property | Spark plug (modern iridium) | Hot-tube ignitor (pre-1905) | Glow plug (diesel / model engine) |
|---|---|---|---|
| Ignition timing precision | ±0.5° crank, fully controlled by ECU | ±10-20° crank, drifts with tube temperature | Compression-ignition timing in diesel; fixed in glow model engines |
| Required voltage | 10-35 kV pulsed secondary | None — uses external flame | 1.5-12 V DC continuous (glow), no spark in diesel |
| Service interval | 30,000-160,000 km depending on electrode metal | Replace tube every 50-200 hours, reseason daily | 100,000+ km for diesel glow plugs; minutes for nitro model glow |
| Cost per cylinder | $3 (copper) to $25 (iridium) | $15-30 per tube + burner fuel | $8-20 (diesel glow plug) |
| Max practical RPM | 20,000+ RPM with capacitor discharge | ~1000 RPM before tube cannot reheat between cycles | 5000 RPM diesel; 30,000+ RPM model glow |
| Reliability under boost | Excellent with cold-range plug and tight gap | Unusable — pressure blows flame out | Excellent (diesel) |
| Cold-start capability | Instant down to -40°C | Requires 5-15 minutes preheat with blowtorch | Requires 10-30 s preheat (diesel) |
Frequently Asked Questions About Ignition Plug
The iridium tip is finer (often 0.4 mm versus 2.5 mm on copper), which lowers required voltage at idle but does not magically lower it at 25 psi of boost. If you transferred the copper plug's gap directly — say 0.9 mm — onto an iridium plug, you are running too wide for a boosted application. Iridium plugs in turbo engines almost always want 0.6-0.7 mm.
Close the gap 0.2 mm and the misfire usually disappears. The mistake is treating the electrode metal as the only variable when gap is the bigger lever under high cylinder pressure.
Pull the plug after a hard run — not after idling back to the pits — and look at the insulator nose. Light tan or biscuit colour means the heat range is right. Pure white with tiny aluminium specks means too hot and you are on the edge of pre-ignition; go one step colder. Black, sooty, or wet means too cold and the plug is fouling between firing events; go one step hotter or check for a rich-running condition first.
The ground strap colour tells you ignition timing. A blue heat-tint band that ends partway up the strap means timing is in the ballpark. No tint at all means retarded; tint running all the way to the weld means advanced and you are putting heat into the wrong place.
Projected tip puts the spark deeper into the combustion chamber, which gives faster flame travel and a broader heat range — the airflow over the projected tip cools it at high RPM and the combustion gases keep it hot at low RPM. That is why most modern automotive plugs are projected.
Recess the tip (or use a non-projected plug) when piston-to-plug clearance is marginal — high-compression race motors with domed pistons, or any engine where you have shaved the head. Mock up with clay on the piston crown and turn the engine through two full revolutions before committing. A projected plug that contacts the piston destroys both parts on the first crank.
This is almost always insulator tracking. A hairline crack in the ceramic, or a film of oil/silicone on the outside of the porcelain, behaves like an insulator when cold but becomes conductive once it warms up. The 25 kV secondary then jumps down the outside of the plug to ground instead of crossing the gap.
Diagnostic: spray the plug exterior with a fine water mist on a running engine in the dark. If you see the spark walking down the porcelain, replace the plug. Also check that you did not get silicone dielectric grease inside the boot — it migrates onto the insulator and causes the same symptom.
Short term yes, long term no. Bumping one heat range hotter will burn off carbon deposits at idle and cure the immediate fouling, which is a legitimate fix on a cold-running utility engine that spends its life at low load. On a turbo or high-compression engine it is dangerous — the same hotter plug that clears deposits at idle becomes a glow-ignition source at WOT and can detonate the engine within minutes.
If the engine is fouling because of a stuck injector, a leaking valve seal, or a too-rich tune, fix that first. The plug is a symptom-reader, not the disease.
Detonation hammers the ground electrode toward the centre electrode. Normal erosion opens the gap by a few hundredths per 10,000 km; detonation closes it because the shock wave physically deforms the strap. If you pull plugs at 20,000 km and find the gap has dropped from 0.7 mm to 0.5 mm, you have been detonating — usually at a specific load point you are not noticing because the knock sensor is pulling timing fast enough to mask it.
Look for tiny aluminium speckles welded to the plug nose at the same time. That is piston-crown material being deposited on the porcelain and confirms the diagnosis. Pull boost or add octane before you put new plugs in.
The spec assumes a fresh sealing washer and clean threads in a known head material. A used washer is already partially crushed, so re-torquing to full new-washer spec over-compresses it and changes heat transfer between the shell and the head — the plug runs colder than intended and fouls. Aluminium heads with old anti-seize on the threads also throw torque readings off by 20-30% because the friction coefficient is wrong.
Rule of thumb: new flat-seat 14 mm plug with fresh washer gets the full 25-30 Nm. Re-installed plug with already-crushed washer gets a quarter turn past hand-tight and no more. Tapered-seat plugs get half that torque — about 12-15 Nm — because there is no gasket to crush.
References & Further Reading
- Wikipedia contributors. Spark plug. Wikipedia
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