How a Spark Plug Works: How a 5mm Discharge Starts an Engine

The spark plug is a tiny but critical component of a gasoline engine that is responsible for every combustion cycle. Without it, the car simply won’t start, and even the slightest malfunctions lead to loss of power, increased fuel consumption and the risk of serious breakdowns. But how exactly does this small element, hidden deep in the cylinder head, convert an electrical impulse into a spark capable of igniting an air-fuel mixture under a pressure of 20 atmospheres?

In this article we will look at physics of spark formation, the structure of the spark plug (from the central electrode to the insulator), and we will also explain why iridium and platinum candles last longer than classic ones, despite the high price. You will learn how to recognize a malfunction by the color of soot, why “cold” and “hot” spark plugs are not interchangeable, and what happens if you install spark plugs with the wrong heat rating. The material will be useful to both beginners and experienced car owners who want to understand the intricacies of the internal combustion engine.

Physics of the process: how a spark is formed

The spark in a spark plug is not just a “little lightning bolt”, but the result of a complex interaction of high voltage, pressure and the chemistry of the air-fuel mixture. When the piston approaches top dead center (TDC), ignition coil generates a voltage pulse 20–45 kV (100–200 times higher than the on-board network of a car!). This impulse arrives at the central electrode of the spark plug, where an electric field is created between it and the side electrode.

Under normal conditions, air is a dielectric, but at this voltage it occurs electrical breakdown: Gas molecules are ionized, forming a plasma channel. The temperature in this channel reaches 6000–10 000°C - enough to ignite the air-fuel mixture. At the same time, the spark itself lasts only 0.001–0.003 seconds, but this is enough to start a chain reaction of combustion.

• ⚡ Breakdown voltage depends on the gap between the electrodes: the larger it is, the higher the required voltage (but also the more powerful the spark).
• 🔥 Spark temperature exceeds the surface temperature of the Sun, but due to its short duration does not destroy the electrodes.
• ⚖️ Cylinder pressure at the moment of sparking it reaches 15–25 bar, which complicates breakdown (therefore the voltage must be extremely high).

Interestingly, the color of the spark can vary from white to blue depending on the composition of the mixture and the material of the electrodes. For example, iridium spark plugs produce a more stable spark with less voltage, which reduces the load on the ignition coil.

Spark plug device: analysis by components

A spark plug consists of 6 key elements, each of which performs a strictly defined function. Even a minor defect in one of them can lead to misfires or complete engine failure. Let's look at the device using the example of a classic candle with nickel central electrode:

1. Contact pin — the upper part of the spark plug, to which the high-voltage wire from the ignition coil is connected. Typically made of steel and threaded for secure fastening.
2. Isolator - ceramic housing (most often made of aluminum oxide), which prevents current leakage to ground and withstands temperatures up to 1000°C.
3. Central electrode - the heart of the candle through which current passes. The material (nickel, platinum, iridium) determines the service life and sparking efficiency.
4. Side electrode - a “mass” electrode, between which a breakdown occurs and the central one. Modern spark plugs may have 2–4 side electrodes for reliability.
5. O-ring — ensures the tightness of the connection between the spark plug and the cylinder head, preventing gas breakthrough.
6. Threaded part — metal housing with threads for screwing into the cylinder head. Also serves to remove heat from the electrodes.

Component	Material	Function	Typical faults
Central electrode	Nickel, platinum, iridium	Conducts current and forms a spark	Erosion, melting, carbon deposits
Isolator	Aluminum oxide	Isolates current and removes heat	Cracks, ground breakdown
Side electrode	Nickel alloy	Creates a spark gap	Wear, corrosion
O-ring	Copper or steel	Seals the connection	Deformation, gas leakage

Particular attention should be paid gap between electrodes. For most modern engines it is 0.8–1.1 mm, but may vary depending on the type of ignition system. For example, for multi-spark systems (as in some Mazda Skyactiv) a smaller gap is required, since the spark is formed several times per cycle.

Why do some spark plugs have multiple side electrodes?

Multi-electrode spark plugs (for example, Bosch Super Plus or NGK V-Line) have 2–4 side electrodes. This increases the reliability of sparking, since when one electrode wears out, the spark passes to the next. In addition, this design reduces the risk of misfires under high loads or poor fuel quality.

Types of candles: what they are and how they differ

Spark plugs are classified according to several criteria: electrode material, design, heat rating and purpose. Selecting the wrong type may result in detonation, overheating or, conversely, to “flooding” of spark plugs during a cold start. Let's look at the main types:

1. According to the material of the central electrode

• 🔧 Nickel - the cheapest and most common. Service life: 20–30 thousand km. Suitable for older carburetor engines.
• 💎 Platinum — the electrode is coated with platinum, which increases the service life to 50–60 thousand km and improves sparking.
• ⚡ Iridium — central electrode made of iridium alloy (diameter up to 0.4 mm). The most durable (80–100 thousand km) and effective, but expensive.

2. By design

• 🔥 Single electrode - classic design with one side electrode. Cheaper, but less reliable.
• ⚡ Multielectrode — 2–4 side electrodes. They last longer and work better on gas.
• 🌀 flare - the side electrode has a V-shaped cutout, which improves the ignition of lean mixtures (used in some Toyota and Honda).

3. By heat number

The heat number characterizes thermal properties candles - their ability to remove heat. The higher the number, the “colder” the candle (and vice versa). For example:

• 🔥 Hot candles (heat number 11–14) - used in low-power engines with a low compression ratio.
• ❄️ Cold candles (heat number 20–26) - for turbocharged and forced engines.

⚠️ Attention: Installing a “hot” spark plug in a turbocharged engine will lead to glow ignition — uncontrolled ignition of the mixture from hot electrodes, which destroys the pistons and valves.

Find out the recommended heat rating in the instruction manual.

Check compatibility according to the manufacturer's catalog (for example, NGK or Denso)

Determine the type of electrodes (iridium/platinum for modern internal combustion engines)

Make sure the gap is correct (indicated in the manual)

Buy only from official dealers (the risk of counterfeits is high) -->

Heat number: why is it important

The heat rating is not just an abstract number on the packaging, but a key parameter that determines whether the candle will burn self-cleanse from carbon deposits or overheating. It reflects thermal range spark plugs, that is, their ability to dissipate heat into the cylinder head.

If a spark plug is too hot for a given engine, its insulator and electrodes will overheat, resulting in:

• 🔥 Glow ignition - the mixture is ignited not by a spark, but by the hot parts of the spark plug (this destroys the engine!).
• 💥 Detonation - explosive combustion of fuel, which is heard as “ringing of fingers.”
• 🚗 Loss of power due to premature combustion.

On the other hand, a plug that is too “cold” does not heat up to the self-cleaning temperature (400–500°C), and carbon deposits quickly form on its electrodes, leading to misfires. This is especially true for engines running on gas (GBO), where the combustion temperature is higher than with gasoline.

Heat number	Candle type	Application	Risks of making the wrong choice
11–14	hot	Low-power naturally aspirated engines, old carburetor cars	Glow ignition in forced engines
16–19	Average	Most modern gasoline engines	Overheating during aggressive driving
20–26	Cold	Turbocharged, sports engines	Oiling and carbon deposits in “quiet” engines

Manufacturers often indicate the recommended heat rating in instruction manual or on a sticker under the hood. For example, for VAZ 2114 with an 8-valve engine, spark plugs with the number 17 (for example, NGK BPR6ES), and for Subaru WRX STI with turbo engine - 22–24 (for example, Denso Iridium Power IK20).

Signs of faulty spark plugs

Spark plugs wear out gradually, and their failure is often disguised as other engine problems. However there is 5 Key Symptoms, which directly indicate the need for inspection or replacement:

1. Difficulty starting the engine, especially “in the cold”. If the starter turns, but the engine “sneezes” and does not start, flooded or worn spark plugs are to blame.
2. Misfires (triple). It manifests itself as twitching at idle or loss of power during acceleration.
3. Increased fuel consumption by 10–15%. Inefficient combustion of the mixture forces the ECU to enrich it, which leads to overconsumption.
4. Detonation - metallic knocking noise when under load (for example, when going uphill). Indicates that the mixture has not ignited properly.
5. Declining dynamics. The car accelerates poorly and does not pull at high speeds.

The most reliable diagnostic method is visual inspection. To do this, candles are unscrewed and analyzed according to three criteria:

• 🔍 Soot color:
  • ⚪ White coating - mixture is too lean or overheated.
  • ⚫ Black oily soot - rich mixture or oil entering the combustion chamber.
  • 🟤 Brown-yellow soot - normal condition.
• 🔥 Electrode condition:
  • Melting or erosion is a sign of detonation or incorrect heat rating.
  • The gap has increased by more than 20% of the nominal value - it’s time to change the spark plug.
• 💧 Oiling - indicates problems with valve stem seals or piston rings.

⚠️ Attention: If there are vertical black stripes on the spark plug insulator (traces of breakdown), this means that the high-voltage wire or ignition coil is faulty. Replacing spark plugs will not help here - you need to check the entire system.

How to extend the life of candles

The average life of spark plugs is:

• 🔧 Nickel: 20–30 thousand km
• 💎 Platinum: 50–60 thousand km
• ⚡ Iridium: 80–100 thousand km

However, these figures are only relevant if a number of conditions are met. Here 7 rules, which will help delay replacement as much as possible:

1. Use quality fuel. Poor gasoline with high sulfur content or additives accelerates the formation of soot and corrosion of the electrodes.
2. Monitor the condition of the engine. Burnt-out valves, worn piston rings or faulty oil seals lead to oil entering the combustion chamber, which “kills” the spark plugs.
3. Check the electrode gap every 10–15 thousand km. It increases with wear, which impairs sparking.
4. Avoid short trips. If the engine does not warm up to operating temperature, the spark plugs do not self-clean from carbon deposits.
5. Monitor the oil level. Its excess or low quality leads to oiling of the candles.
6. Change the air filter on time. A clogged filter enriches the mixture, which leads to increased carbon formation.
7. Do not rev the engine at high speeds in cold weather. This increases the load on the spark plugs and the risk of detonation.

Particular attention should be paid gas cylinder equipment (GBO). When operating on gas, the combustion temperature is higher than on gasoline, therefore:

• ⛽ Spark plugs need to be changed to 1–2 heat numbers colder (for example, instead of 17 put 19).
• ⚡ Recommended to use iridium or platinum spark plugs with a thin central electrode.
• 🔧 Check the condition of the candles every 10 thousand km (even if they are “iridium”).

Mechanical cleaning (with sandpaper or a brush) is strictly not recommended - it damages the protective coating of the electrodes and accelerates their wear. Acceptable only ultrasonic cleaning in specialized services or processing special chemicals (for example, Liqui Moly Ventil Sauber). However, even after cleaning, the life of the candle is restored by a maximum of 30%.-->

Common mistakes when replacing spark plugs

Replacing spark plugs seems like a simple procedure, but even experienced car owners make mistakes that lead to serious consequences. Here 5 most dangerous mistakes:

1. Incorrect tightening torque. An overtightened spark plug can damage the threads in the cylinder head or deform the sealing ring, while an undertightened spark plug can lead to gas breakthrough and overheating.

   ⚠️ Attention: For most spark plugs, the tightening torque is 20–25 Nm. Use a torque wrench!

2. Ignoring manufacturer's recommendations. Installing spark plugs with an incorrect heat rating or gap will lead to detonation or misfire.
3. Dirt getting into wells. Before unscrewing the spark plugs, it is necessary to blow out the wells with compressed air, otherwise sand or dust will get into the cylinder and damage the piston group.
4. Using incompatible candles. For example, candles for Mitsubishi with a conical seat are not suitable for Volkswagen with a flat fit.
5. Twisting candles by eye. Even if the spark plug is “in place,” without the correct tightening torque it may fly out under pressure or, conversely, “stick” during the next replacement.

Another common mistake is use of anti-stick paste on the thread. For most modern candles (for example, NGK or Denso) it is not required, since the threads already have a special coating. The exception is aluminum cylinder heads (for example, on some Alfa Romeo or Jaguar), where the paste prevents “sticking”.

If after replacing the spark plugs the engine starts to stall or an error appears P0300 (misfire), check:

• Correct connection of high-voltage wires (on engines with a distributor).
• Condition of ignition coils (on engines with individual coils).
• The gap between the electrodes (must comply with the manufacturer's recommendations).

FAQ: Answers to frequently asked questions

Is it possible to drive with a faulty spark plug?

Technically possible, but highly undesirable. one spark plug has failed, this leads to:

• Power loss up to 25% (engine “troits”).
• Increased fuel consumption by 10–15%.
• Increased wear of the catalytic converter (due to unburnt fuel).
• Risk of damage to pistons and valves due to detonation.

On modern cars with Euro-5/Euro-6 Driving with misfires will quickly damage the catalyst (replacement cost is from 50 000 ₽).

What gap should be between the electrodes?

The gap depends on the type of ignition system:

• Carburetor engines: 0.7–0.9 mm.
• Injection (distributed ignition): 1.0–1.1 mm.
• Direct injection (GDI, TSI): 0.8–1.0 mm.
• Turbocharged engines: 0.6–0.8 mm (a smaller gap is more reliable at high pressure).

For exact value, check with repair manual your car. For example, for Toyota Corolla 1.6 (3ZZ-FE) recommended clearance - 1.1 mm, and for Ford Focus 1.5 EcoBoost — 0.8 mm.

How do spark plugs for gas differ from regular spark plugs?

Spark plugs for HBO have three key differences:

1. Smaller gap (0.6–0.7 mm against 0.8–1.1 mm for gasoline ones). This is due to the higher breakdown voltage of the methane-propane mixture.
2. Higher heat rating (1–2 units “colder”), since the combustion temperature of the gas is higher.
3. Electrode material: preferred iridium or platinum, as they better resist erosion at high temperatures.

Examples of specialized candles for HBO: NGK LPG LaserLine 2, Denso IW20, Bosch WR7DPX.

Why do new spark plugs quickly become covered with soot?

If new spark plugs turn black after 1–2 thousand km, the problem is not in them, but in the engine. Main reasons:

• 🛢️ Rich air/fuel mixture — the lambda probe is faulty, the injectors are clogged, or the mass air flow sensor (MAF) gives incorrect readings.
• 💨 Oil entering the combustion chamber — oil seals, valve guides or piston rings are worn out.
• 🔥 Low combustion temperature — short trips in winter do not allow the candles to clean themselves.
• ⚡ Ignition coil malfunction - a weak spark does not burn carbon deposits, and it accumulates.

For diagnostics, check ECU errors (for example, P0172 - rich mixture) and inspect the exhaust: if blue smoke comes from the pipe, the problem is in the oil.

Is it possible to use iridium spark plugs instead of regular spark plugs?

Yes, but with reservations:

• ✅ Pros: better sparking, stable operation at high speeds, extended service life.
• ⚠️ Cons:
  • If the engine is not designed for the thin electrode of iridium plugs, the gap may need to be adjusted.
  • In older carburetor engines, the advantages of iridium are not obvious (there are no high loads).
  • The cost is 3-5 times higher than regular candles (for example, NGK Iridium IX worth it~1 500 ₽ per piece).

Iridium spark plugs are justified in turbocharged, highly boosted or modern engines with direct injection (for example, TSI, Skyactiv). For VAZ 2107 or Daewoo Nexia They will not give a noticeable increase, but they will not cause harm either.