Spark plug working and its construction

Spark Plug in Automobiles – Working, Functions, Types, and Parts Explained

A spark plug is one of the most critical components of internal combustion (IC) engines. Whether it’s a two-wheeler, three-wheeler, four-wheeler, or even larger automobile engines, spark plugs are indispensable in gasoline-powered vehicles.

They are mounted on the cylinder head and generate an electric spark to ignite the compressed air-fuel mixture, enabling combustion and engine power generation. While diesel engines rely on compression ignition, they may use glow plugs for cold starts, whereas petrol engines cannot function without spark plugs.

---

🔧 Functions of a Spark Plug

A spark plug performs two primary functions:

1. Ignition of Air-Fuel Mixture

• The ignition coil or magneto provides 20 kV – 100 kV high voltage.

• This creates a potential difference between the central and ground electrodes.

• Once the voltage exceeds the dielectric strength of the gap, the air-fuel mixture gets ionized.

• The ionized gas becomes a conductor, allowing a spark to jump across the gap.

• The spark temperature can reach ~60,000 K, igniting the mixture and producing a small explosion for power.

2. Heat Dissipation from Combustion Chamber

• Contrary to the misconception, spark plugs do not create heat; instead, they act as a heat exchanger.

• They transfer unwanted heat from the firing end to the engine’s cooling system.

• The firing end temperature must remain between 900°C – 1,450°C:

  • Below 900°C → risk of carbon fouling.

  • Above 1,500°C → risk of overheating and pre-ignition.

• The ability of a spark plug to remove heat is termed its heat range.

---

⚡ Types of Spark Plugs

1. Cold Spark Plug

• Short heat path → faster heat transfer.

• Smaller insulator nose → less surface area for heat absorption.

• Best suited for high-performance engines that run at high speeds and loads.

2. Hot Spark Plug

• Longer insulator nose → slower heat dissipation.

• Maintains higher tip temperatures → helps prevent carbon fouling.

• Ideal for low-speed, city-driving engines.

---

🛠️ Parts of a Spark Plug and Their Functions

A modern spark plug consists of 11 major parts:

1. Insulator – Made of aluminum oxide ceramic, ensures dielectric strength and thermal conductivity.

2. Hexagonal Section – Provides grip for tightening/removal using a wrench.

3. Shell & Plating – Made from extruded steel, plated for corrosion resistance.

4. Gasket (optional) – Ensures proper sealing; some plugs use a tapered seat instead.

5. Threads – Machined by rolling, allow plug to be screwed into the cylinder head.

6. Ground Electrode – Nickel alloy, resists spark erosion and high-temperature corrosion.

7. Central Electrode – Similar alloy, conducts high-voltage current.

8. Electrode Gap – Distance between central and ground electrodes where the spark jumps.

9. Insulator Nose (Tip) – Designed to resist carbon, oil, and fuel deposit buildup.

10. Terminal Nut – Connection point for ignition coil wire; may be threaded or detachable.

11. Resistance – Built-in resistance (1kΩ – 7kΩ) limits excessive current. Most common is 5kΩ for stability.

---

🔍 Spark Plug Operation (Step-by-Step)

1. Ignition coil generates 20,000–100,000 volts.

2. Voltage reaches the spark plug’s central electrode.

3. Once the dielectric strength of air gap is exceeded, the mixture ionizes.

4. Spark forms across the electrode gap.

5. The spark’s extreme temperature ignites the compressed air-fuel mixture.

6. Controlled explosion produces power, while spark plug dissipates heat into the cooling system.

---

✅ Key Takeaways

• Spark plugs are essential for spark-ignition engines, while diesel engines use glow plugs only for starting.

• Their two main roles are:

  1. Igniting the air-fuel mixture.

  2. Dissipating combustion heat.

• Proper heat range selection is critical to avoid fouling or overheating.

• Cold plugs suit high-speed engines, hot plugs suit low-speed city use.

• Spark plugs consist of 11 precision-engineered parts, each ensuring reliability under extreme temperature and pressure.

---

⚠️ Disclaimer: This article is for educational and informational purposes only. Always refer to the manufacturer’s guidelines for spark plug selection, installation, and maintenance in vehicles.

---