How spark plugs work: how a spark ignites fuel in an engine

Introduction: why spark plugs are the “heart” of a gasoline engine

Spark plugs are those “little giants” that directly determine whether your car will start in the morning and how it will “breathe” on the highway. Without them, a gasoline internal combustion engine turns into a useless set of metal: no spark, no explosion of the fuel mixture, no energy for the crankshaft. But how exactly do these compact parts, hidden deep in the cylinder head, control the combustion process?

At first glance, a candle is a simple design: a metal body, a ceramic insulator and two electrodes. However, behind this apparent simplicity lies complex physics. This combines high voltage (up to 40,000 volts), extreme temperatures (up to 900°C in the combustion chamber) and chemical reactions. And all this must work synchronously with the engine strokes, producing a spark at a strictly defined moment - with an accuracy of milliseconds.

In this article we will look not only physical principle of sparking, but also how the design of a candle affects its effectiveness, why “cold” and “hot” candles behave differently, and what signs indicate that it’s time to change them. And you will also find out why iridium or platinum Candles last longer than regular candles - and is it worth overpaying for them?

Spark plug design: what does a “spark generator” consist of?

To understand how a candle works, you must first understand its design. Despite the miniature dimensions (usually the thread diameter is 14 mm, length - 50–70 mm), it contains several critically important elements, each of which plays its own role.

Main components:

• 🔧 Housing - a metal shell with a thread that is screwed into the cylinder head. Provides tightness and heat dissipation.
• ⚡ Central electrode - a rod made of heat-resistant metal (copper, nickel, iridium or platinum), through which a high voltage current passes.
• 🛡️ Ceramic insulator — protects the central electrode from breakdown and overheating. Often has a ribbed surface for better heat dissipation.
• 🔌 Side electrode (“ground”) is a curved metal “tongue”, between which a spark jumps and the central electrode.
• 🔥 O-rings — prevent gas leakage from the combustion chamber.

Particular attention should be paid electrode material. In budget candles they use Nickel alloys, which wear out quickly. In premium models (for example, Denso Iridium or NGK Platinum) the central electrode is made of iridium or platinum — these metals withstand higher temperatures and maintain a gap between the electrodes longer.

Why do electrodes “burn out” over time?

With each spark discharge, a microscopic amount of metal evaporates from the surface of the electrodes. Over time, this leads to an increase in the gap between the electrodes, which requires more voltage for breakdown. As a result, the engine begins to “triple” and fuel consumption increases.

The physics of sparking: how 40,000 volts “pierce” the air

The spark in the spark plug is not just a “little lightning bolt”. It's controlled electrical breakdown gas mixture between the electrodes. To understand the process, let's break it down step by step:

1. Energy storage. The ignition coil converts the low voltage of the on-board network (12 V) to high (20,000–40,000 V).
2. Dielectric breakdown. When the voltage exceeds breakdown voltage air-fuel mixture (depending on pressure, temperature and gap between the electrodes), a spark discharge occurs.
3. Gas ionization. The electrons are accelerated in the electric field and collide with air and fuel molecules, creating a plasma channel.
4. Ignition of the mixture. The temperature in the spark channel reaches 6000°C - this is enough to ignite the fuel.

The key parameter here is gap between electrodes. It usually amounts to 0.7–1.1 mm (depending on the spark plug and engine model). Too large a gap requires a higher voltage for breakdown, which can lead to misfires at high speeds. Too small - it gives a weak spark that will not be able to ignite the lean mixture.

Interestingly, the color of the spark depends on the composition of the fuel mixture:

- Blue spark — optimal mixture (14.7:1).

- Red or orange — over-enriched mixture (may indicate a malfunction of the oxygen sensor).

- White discharge - mixture is too lean (risk of detonation).

“Cold” vs “hot” candles: what does the heat rating mean?

If you've ever heard phrases like "put the plugs cooler" or "those plugs are too hot for my engine," we're talking about heat number - a parameter that determines thermal regime candle work. This is not the temperature of the spark, but the ability of the spark plug to remove heat from the electrodes.

The heat rating shows how quickly the spark plug heats up and cools down:

- "Cold" candles (high heat number, for example, 20–26) - quickly remove heat, used in forced and turbocharged engines.

- "Hot" candles (low heat number, e.g. 8–14) - retain heat longer, suitable for naturally aspirated engines with low loads.

Heat number	Candle type	Application	Risks of making the wrong choice
8–11	Hot	Old naturally aspirated engines, low compression engines	Glow ignition (uncontrolled ignition from hot electrodes)
14–17	Average	Most modern gasoline engines	—
20–26	Cold	Turbocharged, sports and forced engines	Oily electrodes (the spark plug does not self-clean)

⚠️ Attention: If you install spark plugs that are too hot in a turbocharged engine, the electrodes can become hot enough to cause the fuel mixture to ignite. on your own, no spark. It's called glow ignition and leads to detonation - one of the most destructive phenomena for the engine.

How spark plugs affect power, fuel consumption and the environment

It may seem that candles are a trifle, but their condition directly affects:

• 🚀 Engine power. Worn spark plugs produce a weak spark, which leads to incomplete combustion of fuel and a drop in efficiency by 5–15%.
• ⛽ Fuel consumption. Misfires cause the ECU to increase the amount of gasoline supplied to “compensate” for inefficient combustion.
• 🌱 Environmental friendliness. Unburned fuel enters the exhaust, increasing the content CO and CH.
• 🔧 Catalyst life. Unburnt gasoline burns out in the outlet, overheating the honeycombs of the catalytic converter.

Research shows that replacing spark plugs every 30,000 km (for conventional ones) or 100,000 km (for iridium ones) allows you to save up to 4% of fuel and reduce emissions CO on 15–20%. At the same time, acceleration dynamics are improved due to more uniform combustion of the mixture in all cylinders.

⚠️ Attention: If, after replacing the spark plugs with platinum or iridium ones, you notice that the engine has become “harder” (vibration has appeared at idle), this may indicate that the new spark plugs have different heat number. Check that the settings comply with the manufacturer's recommendations.

Signs of faulty spark plugs: when is it time to change them?

Spark plugs don't break suddenly - they degrade gradually. Here are the key symptoms that indicate the need for inspection or replacement:

• 🔥 Misfires (“triple” of the engine, especially when cold).
• ⚡ Deterioration in dynamics (the car “does not pull” when accelerating).
• ⛽ Increased fuel consumption on 10–20% for no apparent reason.
• 🔊 Unstable idle speed (the engine is “tuning up”).
• 🚨 Check Engine with errors P0300–P0308 (cylinder misfires).

A visual inspection of the spark plugs can tell a lot about the condition of the engine:

Candle condition	Possible reason	Consequences
Black dry soot	Over-enriched mixture, lambda probe malfunction	Misfires, increased fuel consumption
White or gray coating	Overheating, lean mixture, early ignition	Risk of electrode melting, glow ignition
Oil soot	Worn oil rings or valve guides	Oily electrodes, misfires
Melted electrodes	Detonation, incorrect heat rating, overheating	Damage to pistons and valves

How to choose spark plugs: what to look for when buying

When choosing spark plugs, you cannot focus only on price or brand. Here are the key parameters to consider:

1. Engine compatibility. Check heat number, thread size (M14×1.25 or M12×1.25), thread length and gap between electrodes (indicated in the car's operating manual).
2. Electrode material:

   - Copper/Nickel - budget option, resource 30,000 km.

   - Platinum — middle price segment, resource 60,000–80,000 km.

   - Iridium — premium, resource up to 100,000–120,000 km.

3. Construction:

   - Resistor (with noise suppression resistor) - reduce interference to electronics.

   - Multielectrode — increase the resource by distributing the spark between several side electrodes.

4. Brand. among trusted manufacturers: NGK, Denso, Bosch, Champion. Beware of fakes - they may have an inappropriate heat rating or low-quality ceramics.

⚠️ Attention: If you install spark plugs with non-standard gap (e.g. increased to improve ignition), ensure that the ignition coils are capable of generating sufficient voltage for breakdown. Otherwise, you will get the opposite effect - misfires.

FAQ: Frequently asked questions about spark plugs

Is it possible to clean spark plugs from carbon deposits?

Mechanical cleaning (with sandpaper or brush) is possible, but not recommended for modern candles. The abrasive damages the protective coating of the electrodes, accelerating their corrosion. Better to use ultrasonic cleaning or special chemicals (for example, Cleaner for Spark Plugs from Liqui Moly). However, if the spark plugs have already worked for more than 20,000 km, they are easier to replace.

What happens if you drive on worn-out spark plugs?

The consequences range from worsening dynamics up to serious damage:

• Increased fuel consumption by 10–25%.
• Damage to the catalytic converter due to incomplete combustion of fuel.
• Detonation leading to destruction of pistons or valves.
• Overloading the ignition coils, which can lead to their failure.

Do I need to “break in” new spark plugs?

No, modern spark plugs do not require break-in. However, at first 100–200 km after replacement it is recommended to avoid extreme loads (for example, long-term operation at maximum speed) so that the electrodes “break in” to the conditions of a particular engine.

Is it possible to use candles with a different heat rating?

Slight deviation is allowed (±1–2 units), but sudden change (for example, with 14 on 22) will lead to:

• Overheating of spark plugs (if you put “hot” ones in a turbocharged engine).
• Oiling (if you put “cold” ones in a naturally aspirated engine).

Always follow the vehicle manufacturer's recommendations.

Is it true that iridium spark plugs improve dynamics?

Yes, but the effect is noticeable only when compared with very worn ordinary candles. Iridium spark plugs provide:

• More stable spark (especially at high speeds).
• Better ignition of lean mixtures (relevant for modern eco-engines).
• Reduced misfires.

However, the power increase rarely exceeds 2–3% - the main advantage in durability.