How a Spark Plug Works

A spark plug ignites the air-fuel mixture in a gasoline engine by creating a high-voltage spark across a tiny gap at precisely timed moments, forming a flame kernel that grows into full combustion. It does this by channeling tens of thousands of volts from the ignition coil to jump the gap between its center and ground electrodes, ionizing the compressed mixture and lighting it reliably under harsh, high-pressure conditions.

The Core Physics of Ignition

In each cycle, an ignition coil generates high voltage—commonly 20,000 to 45,000 volts in modern systems—that the spark plug uses to push electrons across a 0.6 to 1.1 mm gap. When the electric field is strong enough to break down the gas (a process governed by pressure, gap size, and mixture composition), a conductive channel forms and the spark arcs. The discharge has a very fast breakdown phase (nanoseconds), followed by an arc lasting roughly 0.5 to 2 milliseconds in inductive systems, during which 30 to 60 millijoules—or more in some systems—are deposited. That energy ignites a small flame kernel, which then grows rapidly, driven by turbulence in the combustion chamber. Higher cylinder pressure (from turbocharging or high compression) and lean or diluted mixtures demand more voltage and often a tighter gap to maintain reliable ignition. The plug’s job is to repeat this precisely, thousands of times per minute, without overheating, fouling, or eroding prematurely.

What’s Inside a Spark Plug

Several engineered parts work together to deliver the spark reliably while withstanding heat, pressure, vibration, and chemical attack. Here are the key components and what they do.

• Terminal/post: Connects to the ignition lead or coil-on-plug boot to receive high voltage.
• Internal resistor: Typically 3–7 kΩ to suppress electromagnetic interference that can affect sensors and infotainment electronics.
• Center electrode: Conducts high voltage to the tip; may use a copper core for conductivity and a fine-wire iridium or platinum tip to reduce required voltage and wear.
• Ceramic insulator: Electrically isolates the center electrode and sheds heat; its nose length helps set the plug’s “heat range.”
• Metal shell with threads: Anchors the plug to the cylinder head and conducts heat to the head for cooling.
• Gasket or taper seat: Seals combustion gases and sets proper crush/preload.
• Ground (side) electrode(s): Provides the counter-electrode to form the spark gap with the center electrode.
• Tip materials/coatings: Iridium/platinum fine-wire tips resist erosion and maintain a stable gap over long service intervals.

Together, these elements ensure the spark occurs at the right place and temperature, remains consistent over time, and does not create radio interference or leak heat too slowly or too quickly.

Step-by-Step: From Coil to Combustion

The ignition event is a tightly timed sequence orchestrated by the engine control unit (ECU). The following steps outline how the spark plug turns coil energy into a flame kernel.

1. Dwell/charging: The ECU energizes the coil’s primary winding; magnetic energy builds.
2. Trigger/collapse: At the commanded ignition timing (degrees before top dead center), the ECU cuts current; the magnetic field collapses, inducing high voltage in the coil’s secondary.
3. Voltage rise: Secondary voltage climbs rapidly; if it exceeds the gap’s breakdown threshold (which increases with cylinder pressure), the gas ionizes.
4. Breakdown and arc: Electrons avalanche across the gap; voltage drops to sustaining levels (~1–2 kV), and arc current heats the mixture.
5. Flame kernel formation: In roughly a millisecond, a stable kernel forms and expands, then the bulk flame front takes over combustion.
6. Spark strategies: Inductive systems provide a longer-duration spark; capacitive discharge (CDI) systems deliver high energy in a shorter burst and may use multiple sparks at low rpm to improve idle stability.
7. Timing control: The ECU adjusts spark timing cycle-by-cycle using inputs like knock sensors, airflow, temperature, and crank/cam position.

The exact timing and energy delivery are adapted continuously to load, rpm, and fuel quality to maximize torque, efficiency, and knock resistance while minimizing misfires.

Key Specs That Determine Performance

Choosing and setting up spark plugs correctly is as important as having a strong coil. The following specifications govern ignition reliability, durability, and emissions.

• Gap: Typical 0.6–1.1 mm; larger gaps ease ignition in clean, low-pressure conditions but demand more voltage; boosted/high-pressure engines often run 0.6–0.8 mm to avoid blowout.
• Heat range: A “hotter” plug (longer insulator nose) runs warmer, resisting fouling; a “colder” plug runs cooler to prevent pre-ignition. Self-cleaning typically begins around 450°C insulator temperature; sustained tip temperatures above ~850–900°C risk pre-ignition.
• Tip material: Fine-wire iridium/platinum reduces required voltage and erosion, enabling long service intervals (often 60,000–120,000+ miles) and stable gaps.
• Resistor value: 3–7 kΩ is common; required for modern ECUs to prevent radio-frequency noise errors.
• Thread size/reach: Must match the head (e.g., M14×1.25 or M12×1.25; specific “reach” ensures the electrode sits correctly in the chamber).
• Seat type and torque: Gasket-seat plugs are typically tightened to manufacturer specs (often ~18–25 N·m for M14 with gasket, ~10–15 N·m for M12), or by angle after gasket crush. Over- or under-tightening can cause leaks or damage.
• Projected tip and ground-electrode design: Projected-tip plugs can improve ignitability in some chambers; multi-ground designs emphasize durability over maximum kernel exposure.
• Indexing: Orienting the open side of the ground electrode can marginally help in certain high-performance engines but is rarely necessary on street engines.

Matching these parameters to the engine’s design and usage ensures the plug runs clean, avoids overheating, and lights the charge reliably across the operating range.

Ignition Systems and Compatibility

Spark plugs must work with the ignition architecture and combustion strategy. The items below outline common systems and their implications for plugs and gaps.

• Distributor vs. coil-on-plug (COP): COP minimizes energy losses and crossfire, boosting available voltage at the plug and enabling finer control.
• Inductive vs. CDI: Inductive systems provide longer-duration sparks that help with lean or EGR-diluted mixtures; CDI produces very high initial voltage and short duration—useful for high rpm or high-pressure conditions, sometimes supplemented by multispark at low rpm.
• Direct injection gasoline (GDI): Higher in-cylinder pressures at ignition can demand tighter gaps and robust coils; tip design and placement are more critical to avoid fuel wetting.
• Boosted engines: Elevated pressures increase breakdown voltage needs; colder heat ranges and reduced gaps are common, especially at high boost.
• Lean-burn/stratified modes: Require strong, consistent sparks; fine-wire iridium helps lower the required voltage threshold and maintain stability.
• Ion-sense systems: Some engines use the spark plug to measure post-ignition ionization currents for knock and misfire detection, necessitating resistor plugs and clean electrical interfaces.

Understanding the system helps select plugs and gaps that balance energy demand, durability, and electromagnetic compatibility.

Reliability, Symptoms, and Maintenance

Even with modern materials, spark plugs are wear items. The following points cover common issues, signs of trouble, and best practices for service.

• Fouling: Short trips, rich mixtures, oil consumption, or weak ignition can produce wet fuel/oil or dry carbon fouling, causing misfire.
• Erosion and gap growth: Electrode wear increases required voltage, raising misfire risk under load; iridium/platinum slows this process.
• Glazing and pre-ignition: Overheating can glaze the insulator or cause pre-ignition, leading to severe engine damage; use proper heat range.
• Misfire symptoms: Rough idle, hesitation, loss of power, poor fuel economy, and diagnostic codes (e.g., P0300–P030X) under load.
  
• Service intervals: Conventional plugs often 20,000–40,000 miles; iridium/platinum 60,000–120,000+ miles—follow the vehicle’s maintenance schedule.
• Gapping and handling: Re-gapping fine-wire iridium/platinum plugs can damage the precious tip; adjust carefully with proper tools or use pre-gapped OEM-specified parts.
• Installation: Start by hand to avoid cross-threading; use specified torque; most nickel-plated shells do not require anti-seize (it can cause over-torque).
• Boots and coils: Inspect for cracks, oil intrusion, or carbon tracking; use dielectric grease on boots if recommended to ease removal and seal out moisture.

Regular inspection and correct installation prevent misfires and protect catalytic converters and turbochargers from unburned-fuel damage.

Myths and Facts

Spark plugs attract marketing claims. These clarifications separate durable truths from garage folklore.

• “Bigger spark = big power”: A strong, reliable spark prevents power loss from misfires; beyond that, gains are small unless the old system was marginal.
• Exotic materials: Iridium/platinum primarily extend life and maintain gap stability; they don’t add power by themselves.
• Multi-electrode plugs: Improve durability and gap stability; they do not inherently produce multiple simultaneous sparks.
• Octane and plugs: Higher octane doesn’t require a “hotter” plug; heat range is about plug temperature, not fuel octane.
• Always open the gap: Larger gaps can improve ignitability but may exceed available voltage under boost, high rpm, or high EGR; follow OEM guidance.

Choosing the right OEM-specified plug and ensuring the ignition system is healthy will usually outperform “universal” performance claims.

Edge Cases: Diesel, Gaseous Fuels, and Hybrids

Not all engines use—or use the same—spark plug strategies. These examples highlight important differences.

• Diesel engines: Use compression ignition and do not use spark plugs; they may use glow plugs for cold-start assistance.
• Natural gas/propane engines: Lean mixtures and higher ignition energy requirements often call for smaller gaps and robust coils.
• Hybrids and stop-start: Frequent restarts and low-load operation can change fouling patterns; long-life iridium plugs are commonly specified.
• Pre-chamber ignition (some high-efficiency engines): A small pre-chamber with a spark ignites jets of burning gas into the main chamber; plug placement and design are specialized.

Application-specific requirements can significantly change the optimal plug type, gap, and service interval.

Summary

A spark plug is a precisely engineered switch that, at the ECU’s command, turns coil energy into a controlled, repeatable spark that lights the compressed air-fuel mixture. Its ability to ionize the gap under varying pressures, form a stable flame kernel, shed heat within a safe window, and resist erosion determines how smoothly, efficiently, and cleanly an engine runs. Correct specification—gap, heat range, materials—and careful installation are the difference between reliable combustion and costly misfires.

How does the spark plug know when to fire?

Computer-controlled ignition systems
The computer will send a signal to the ignition coil at the indicated time in the timing map in order to fire the spark plug. Most computers from original equipment manufacturers (OEM) cannot be modified so changing the timing advance curve is not possible.

How do spark plugs work step by step?

The piston is forced back down to create power for the vehicle. Then pushed back up again to clear out the exhaust. At that point the process starts all over. Again.

How do spark plugs ignite gas?

Ignition by electrical spark occurs because the fuel particles between the electrodes are activated by the spark to discharge, a chemical reaction (oxidation) is triggered, the heat of reaction is generated, and the flame core is formed.

What does a spark plug do in simple terms?

The function of a spark plug is to produce a spark at the required time to ignite the combustible mixture. The plug is connected to the high voltage generated by an ignition coil or magneto. As current flows from the coil, a voltage develops between the central and side electrodes.