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How does a spark plug work?

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Quick answer

A spark plug ignites the fuel-air mixture in a petrol engine. An ignition coil raises the 12-volt supply to tens of thousands of volts and sends it to the plug's centre electrode. The voltage jumps a small gap to the ground electrode as a spark, lighting the compressed mixture and driving the piston down.

A spark plug works by creating a high-voltage electric spark across a small gap inside the cylinder, igniting the compressed fuel-air mixture at precisely the right moment. The ignition coil boosts the car's 12-volt supply to tens of thousands of volts so current can jump that gap.

Step by step

  1. The piston rises on its compression stroke, squeezing the fuel-air mixture into the small space above it.
  2. Just before the piston reaches the top, the engine computer triggers the ignition coil to fire.
  3. The coil releases a high-voltage pulse that travels along the plug's centre electrode through the ceramic insulator.
  4. The voltage breaks down the gas in the electrode gap, and a spark jumps across to the ground electrode.
  5. The spark ignites the mixture, and the expanding burning gases force the piston down to turn the crankshaft.

Key things to understand

  1. Each cylinder of a petrol engine has a spark plug screwed into the top of its combustion chamber.
  2. An ignition coil transforms the 12-volt battery supply into a pulse of tens of thousands of volts.
  3. That voltage travels down the plug's central electrode, insulated from the metal body by a ceramic sleeve.
  4. The voltage ionises the gas in the gap to the ground electrode, letting a spark of current jump across.
  5. The spark's heat starts a flame front that spreads through the mixture, pushing the piston down.

Labelled diagram

  1. Terminal

    Connects to the ignition coil and receives the high-voltage pulse.

  2. Ceramic insulator

    Keeps high voltage from leaking to the metal shell and engine block.

  3. Metal shell

    Threads into the cylinder head and conducts heat away from the tip.

  4. Centre electrode

    Carries the voltage down to the firing tip inside the combustion chamber.

  5. Ground electrode

    Bent metal tab across the gap that the spark jumps to.

Parts of a spark plug, top to bottom

Real-world examples

  • Petrol scooters and motorcycles common on Indian roads each use a single spark plug per cylinder.
  • Some motorcycle engines use two spark plugs per cylinder so the mixture burns faster and more completely.
  • Lawnmowers and small garden tools use one spark plug to run their simple single-cylinder engines.
  • CNG cars in Indian cities also use spark plugs, because compressed natural gas ignites by spark like petrol.

Common mistake

✗ Myth: Spark plugs explode the fuel inside the cylinder.

✓ Fact: The spark starts a rapid but controlled burn whose flame front spreads across the chamber in milliseconds. A true explosion, called knock or detonation, is abnormal and can damage the engine.

People also ask

How does a spark plug create a spark?
The ignition coil sends a pulse of tens of thousands of volts to the plug's centre electrode. Air and fuel normally insulate the gap, but this voltage is high enough to ionise the gas, turning it briefly into a conducting plasma. Current then jumps across the gap as a visible spark.
What is the gap on a spark plug?
The gap is the small distance between the centre and ground electrodes, usually around a millimetre for car engines. Too small a gap makes a weak spark; too large needs more voltage and can cause misfires. Manufacturers specify the correct gap for each engine.
What does a bad spark plug look like?
A healthy used plug tip is light grey or tan. Black, sooty deposits suggest a rich mixture or misfiring; oily deposits can indicate oil entering the cylinder; white, blistered tips can point to overheating. Rounded or heavily worn electrodes mean the plug is due for replacement.
How often should spark plugs be changed?
It depends on plug type and the vehicle maker's schedule. Standard copper-nickel plugs wear out sooner, while iridium and platinum plugs last much longer. Following the owner's manual interval is the reliable guide.
Do diesel engines have spark plugs?
No. Diesel engines compress air so strongly that it becomes hot enough to ignite fuel injected into it. Many diesels have glow plugs, which are heaters that help cold starts rather than spark igniters.
What is the heat range of a spark plug?
Heat range describes how quickly the plug's tip sheds heat into the cylinder head. The tip must stay hot enough to burn off carbon deposits but cool enough to avoid igniting the mixture early, so manufacturers specify a heat range for each engine.
What happens when a spark plug fails?
A worn or fouled plug can misfire, leaving that cylinder's fuel unburned. Symptoms include rough idling, hesitation, harder starting, higher fuel use and sometimes a check-engine light.

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