Spark plugs: how they work in an engine and why they break

Spark plugs are the “heart” of a gasoline engine, without which ignition of the air-fuel mixture is simply impossible. Every driver has heard about them at least once, but not everyone understands exactly how they function in the extreme conditions of the combustion chamber. Temperatures up to 2500°C, pressure up to 50 atmospheres, constant vibrations - and all this throughout 100,000+ km mileage How does a small metal device manage to withstand such loads?

In this article we will analyze not only the physical principle of operation of spark plugs, but also their role in modern ignition systems. You'll find out why a spark between the electrodes must occur strictly at a given moment - accurate to millisecondshow the design of the spark plug affects engine power, and what “symptoms” indicate an imminent replacement. And also why even expensive iridium spark plugs NGK or Denso may fail prematurely if simple rules are not followed.

Spark plug design: what does it consist of?

At first glance, the candle looks simple: a metal body with a thread, a ceramic insulator and two electrodes. But every part performs a critical function. Let's look at the main elements:

• 🔧 Housing — made of alloy steel, withstands high pressure and ensures the tightness of the combustion chamber. It has a thread for screwing into the cylinder head.
• ⚡ Central electrode - usually made of nickel alloy, copper or iridium. It is from this that the spark “jumps” to the side electrode.
• 🧱 Ceramic insulator — protects the central electrode from overheating and prevents current leakage. Often has a ribbed surface for better heat dissipation.
• 🔌 Side electrode - “receiver” of the spark. Modern spark plugs may have several side electrodes for reliability.
• 🔥 O-ring — prevents the breakthrough of gases from the combustion chamber. Usually made of copper or aluminum.

Particular attention should be paid to the material of the electrodes. For example, copper candles dissipate heat well, but wear out quickly, and iridium (for example, NGK Iridium IX or Denso IK20) last 3–4 times longer, but are more expensive. The choice depends on the engine type and driving style.

Fun fact: racing engines sometimes use spark plugs. gold plating on the electrodes - this reduces resistance and improves sparking under extreme loads. However, for production cars such solutions are not economically feasible.

Operating principle: how a spark is formed

The sparking process can be divided into several stages:

1. Supply voltage. When the piston approaches top dead center (TDC), the engine control unit (ECU) sends a signal to the ignition coil. The voltage in the primary winding of the coil reaches 200–400 V.
2. Voltage Transformation. The coil converts low voltage to high voltage - up to 20–45 kV (kilovolt!). This is necessary to “break through” the gap between the electrodes.
3. Gap breakdown. An electrical discharge—a spark—occurs between the central and side electrodes. Her temperature reaches 10 000°C, which is sufficient to ignite the air-fuel mixture.
4. Ignition of the mixture. The spark ignites the mixture of gasoline and air, which leads to a micro-explosion - this is how the piston moves.

It is important to understand that a spark must occur strictly at a certain moment. If the ignition is too early, the mixture will ignite before the piston approaches TDC - this causes detonation (knock of the “fingers”). If later, the fuel will not burn completely, which leads to loss of power and increased consumption.

Why don't diesel engines have spark plugs?

In diesel engines, ignition occurs by compressing air to 30–50 atmospheres, which heats it to 700–900°C. The injected fuel ignites spontaneously upon contact with hot air. However, some diesel engines use glow plugs — they heat the combustion chamber during a cold start.

Modern ignition systems (for example, Coil-on-Plug in cars Toyota or BMW) place an individual coil on each spark plug. This allows you to more accurately control the moment of sparking and increases the reliability of the system.

Thermal regime: why are candles “cold” and “hot”

One of the key characteristics of candles is heat number. It determines how quickly the spark plug transfers heat to the cylinder head. The higher the heat value, the “cooler” the candle (it removes heat better). And vice versa - a low heat value means a “hot” candle.

Candle type	Heat number	Application	Risks of making the wrong choice
Cold	20–26	Highly boosted and turbocharged engines	Overheating when used in “cold” engines → glow ignition
Average	14–19	Most naturally aspirated engines	Minimal risks with proper selection
Hot	8–13	Low speed and old engines	Oiling when used in “hot” engines → misfires

For example, if in a turbocharged engine Subaru WRX install too “hot” spark plugs, then under high loads their temperature will exceed 900°C, and the fuel will begin to ignite not from a spark, but from the hot elements of the spark plug. This phenomenon is called glow ignition and can lead to destruction of the pistons.

Manufacturers often label candles according to the following standards:

- NGK: the higher the number (for example, BP6ES vs BP8ES), the “colder” the candle.

- Bosch: reverse logic - WR7DC colder than WR5DC.

Always check the catalogs!

Signs of faulty spark plugs

Spark plugs wear out gradually, and their malfunction does not appear immediately. Here are the key “symptoms” to look out for:

• ⚠️ Difficulty starting the engine, especially “in the cold”. If the starter turns, but the engine does not “catch”, oily or worn spark plugs may be to blame.
• 🚗 Dips during acceleration - feeling as if the engine is “twitching”. This is a sign of misfire in one or more cylinders.
• ⛽ Increased fuel consumption by 10–15%. Inefficient combustion of the mixture causes ECU enrich it, which leads to overspending.
• 💨 Black or white smoke from the exhaust. Black is a sign of a rich mixture (the spark plugs are oily), white is a sign of a poor mixture (the spark plugs are overheated).
• 🔊 Popping sounds in the exhaust system — unburned fuel burns out already in the outlet, which is accompanied by loud sounds.

If you notice at least 2-3 signs from the list, it's time to check the candles. To do this, you don’t have to go to a service station - just unscrew the spark plug and inspect it:

Carbon deposits on the electrodes (black, white, red) → indicates problems with fuel or oil

Cracks in the insulator → replacement required, risk of breakdown

Electrode erosion (rounded edges) → natural wear, time to replace

Oil marks on the threads → possible problems with valve stem seals

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Particularly dangerous red soot - it indicates the presence of metal additives (for example, ferrocenes) in the fuel. Such deposits conduct current and can cause breakdown of the insulator.

How to extend the life of candles

Average candle life:

- Nickel: 30–50 thousand km.

- Platinum: 60–90 thousand km.

- Iridium: 100–120 thousand km.

But these figures are relevant only if a number of conditions are met. Here's what really affects durability:

1. Fuel quality. Poor gasoline with high sulfur content or additives accelerates the formation of soot. Refuel at trusted gas stations (for example, Lukoil Ecto, Gazpromneft G-Drive).
2. Riding style. Frequent short trips (especially in winter) do not allow the candles to clean themselves - carbon builds up faster. Periodically “drive” the engine at high speeds (for example, on the highway).
3. Engine condition. Worn piston rings or valve stem seals lead to oil entering the combustion chamber, which kills the spark plugs within 5–10 thousand km.
4. Correct tightening torque. An overtightened spark plug can damage the threads in the cylinder head, and an undertightened spark plug can cause overheating. Use a torque wrench (the torque is usually specified in the manual, e.g. 20–25 Nm).

Many car owners ask: do spark plugs need to be cleaned? Mechanical cleaning (for example, with a metal brush) is strictly not recommended - it damages the coating of the electrodes. Acceptable only ultrasonic cleaning or treatment with special means (for example, Liqui Moly Ventil Sauber). However, even after cleaning, the life of the candles is restored by a maximum of 30–40%.

Common mistakes when replacing spark plugs

It would seem, what could go wrong when replacing spark plugs? In practice, even experienced drivers make mistakes that lead to serious consequences. Let's look at the most common ones:

⚠️ Attention! Never use candles with an inappropriate heat rating. For example, installing “hot” candles Bosch WR7DP instead of the recommended “cold” WR9DP in a turbocharged engine Audi 1.8 TFSI can lead to detonation and destruction of pistons after 500–1000 km.

• 🔧 Using the wrong key. The spark plug wrench must have a rubber bushing or magnet to secure the spark plug. Without it, you can drop the candle into the well or damage the ceramic insulator.
• 🧲 Use of excessive force. Overtightening the spark plug leads to deformation of the threads in the aluminum cylinder head (especially important for engines VW and Ford EcoBoost).
• 🔥 Hot replacement. If the engine has not cooled down, when unscrewing the spark plug, the threads can be stripped due to thermal expansion of the metal.
• 🔄 Wire confusion. When replacing high voltage wires (in systems with a distributor), it is important to connect them in the correct order. An error will lead to a violation of the ignition sequence.

Another typical problem is broken candle in the well. This happens if the spark plug is “stuck” to the thread, and they try to unscrew it without first treating it with a penetrating lubricant (for example, WD-40 or Liqui Moly Rostloser). In such cases, you have to drill out the fragment and cut a new thread, which is not cheap.

If you are not confident in your abilities, it is better to entrust the replacement to professionals. For example, in services Bosch Car Service or Mobile Auto use specialized tools and guarantee the correct selection of spark plugs according to VIN-car code.

Myths and misconceptions about spark plugs

There are many myths surrounding spark plugs that interfere with proper operation. Let's look at the most persistent ones:

⚠️ Attention! The common statement that “iridium spark plugs increase engine power” is a marketing ploy. In fact, they only last longer and work more stably under extreme loads. The increase in power (if any) does not exceed 1–2%, which is not noticeable in everyday driving.

• 🔥 “The larger the gap between the electrodes, the more powerful the spark”. In fact, too large a gap (more than 1.3 mm) requires a higher voltage for breakdown, which increases the load on the ignition coil and can lead to its failure.
• 🛢️ “Plugs should only be changed when problems arise”. Even if the engine runs smoothly, worn spark plugs increase fuel consumption and emissions. Manufacturers recommend changing them according to regulations, and not due to breakdown.
• ⚡ “All candles are the same, you can put any suitable size”. In fact, even spark plugs that are identical in size may differ in electrode material, heat rating and resistance. For example, candles for Toyota Corolla and Honda Civic of the same year are often indistinguishable in appearance, but have different characteristics.
• 🔧 “Cleaning spark plugs with sandblasting restores them like new.”. Sandblasting removes carbon deposits, but damages the surface of the electrodes, impairing heat dissipation. Such candles will not last long.

Another popular myth is “The spark plugs do not need to be checked if the engine is running smoothly”. In fact, early misfires can only be detected using a diagnostic scan tool (e.g. Launch X431 or ELM327). Even one “trim” cylinder increases fuel consumption by 5–7%.

FAQ: Frequently asked questions about spark plugs

Is it possible to drive with a faulty spark plug?

Technically possible, but not advisable. The engine will “triple”, lose power and overuse fuel. In addition, unburned fuel will end up in the catalyst, which will shorten its life. In turbocharged engines, driving with misfires can lead to overheating and failure of the turbine.

What gap should be between the electrodes?

The gap depends on the type of ignition system:

• Carburetor engines: 0.7–0.9 mm.
• Injection with distributor: 0.9–1.1 mm.
• Modern systems Coil-on-Plug: 1.0–1.3 mm.
• Engines with gas equipment: 0.7–0.8 mm.

The exact value is indicated in your vehicle's repair manual. The gap is checked with a round feeler gauge, not a flat one!

What happens if high-voltage wires are mixed up?

If you connect the wires in the wrong order, the sequence of operation of the cylinders will be disrupted. The engine will vibrate strongly, stall, or not start at all. In some cases this may lead to collision of pistons with valves (if the timing belt is not synchronized with the ignition). Always check the wiring diagram for your motor!

Why do new spark plugs quickly become covered with soot?

The reasons may be different:

• Malfunction of the crankcase ventilation system (clogged valve PCV).
• Worn oil seals or piston rings (oil enters the combustion chamber).
• Rich air-fuel mixture (lambda probe or injectors are faulty).
• Poor fuel quality (high sulfur content or additives).

If carbon deposits appear 1–2 thousand km after replacement, comprehensive engine diagnostics are required.

Can I use spark plugs from another car?

Only if they completely match:

• Thread (diameter and pitch, e.g. M14×1,25).
• The length of the threaded part.
• Heat number.
• The gap between the electrodes.
• Type of electrodes (for example, it is not recommended to replace iridium spark plugs with nickel spark plugs).

Even if the spark plugs are similar in appearance, differences in thermal conditions can lead to overheating or misfire. Always check manufacturers' catalogs (NGK, Bosch, Denso).