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Under the Bonnet – Spark Plug Explained

By Hanna

Spark Plug


Although a small piece of equipment, spark plugs are a critical component of combustion engines that pack a big punch. The plugs bridge electricity across a gap, producing a lightning bolt-type current used to ignite combustion engines and ultimately power your vehicle.


There is typically only one spark plug per cylinder, so failing spark plugs can result in non-combusted fuel, a dangerous thing to have freely pumping through your internal engine components. Therefore, it’s very important that you understand how a spark plug functions, and know when and how to service them.


Here’s a guide of everything you need to know about your combustion engine’s best pal – the engine spark plug!





In 1860, Belgian engineer Jean Étienne Lenoir used an electric spark plug in his combustion engine – the first commercially successful implementation of spark plugs to date. Various patents followed as the spark plug gained popularity, including the notable innovation by Bosch of combining a high-voltage magneto with a spark plug, earning a patent and becoming the world’s leading spark plug manufacturer. This past year, Bosch manufactured over 350 million spark plugs, and show no signs of slowing down.



How Do They Function?


Spark plugs are located at the top of each cylinder head. The piston begins by moving down the cylinder, filling up with a mixture of fuel and air. Next, the piston is compressed towards the spark plug. Right when the piston is near the top dead center (TDC), the spark plug emits an electric current, igniting the mixture and forcing the piston down back down the cylinder. This action of forcing the piston via fuel combustion creates the energy to power your vehicle. The exhaust is then cleared, and the process begins all over again.



Secondary Responsibilities – Heat Dissipation


Besides providing the spark to ignite the fuel, engine spark plugs also function as heat dissipaters to maintain ideal engine temperature. The center electrode and insulator are made of highly conductive materials, like copper, nickel, or porcelain. Excess heat from combustion flows through these materials directly to the car’s cooling system to regulate temperature. High-end spark plugs use precious metals like platinum or iridium, which transfer heat more efficiently than standard materials.



 Cold Plugs vs. Hot Plugs


When purchasing new spark plugs, you may come across both “cold” and “hot” plug options. The key difference is their level of ceramic insulation. Cold plugs have less insulation, allowing more heat to be transferred out of the engine. This is the safer route, because if the engine is too hot, it can encounter pre-ignition, knocking, or permanent engine damage.


Alternatively, “hot” plugs have more insulation and are found in many standard engines. The extra insulation helps the plug burn off carbon deposits, extending the plug’s lifetime.  If you’re unsure which type to buy for your car, a cold plug is a safer option.



Do My engine Spark Plugs Need to be Replaced?


Spark plugs should be replaced every 30,000 miles due to general use. However, there are a few telltale signs of spark plug failure to keep in mind:


  • Engine Misfires, evidenced by your engine halting momentarily, before returning to usual function


  • High Fuel Consumption – incomplete combustion from a broken plug can cause up to 30% inefficiencies in fuel consumption


  • Poor Acceleration – taking longer than usual to get up to speed is another side effect of incomplete combustion


  • Rough Idle – rough, jittery sounds and vibrations throughout the car during idling


If you experience any of these abnormalities, it’s likely due to a spark plug deterioration or malfunction. Take your car in for service, or order the replacement yourself!


engine spark plugs


Spark plug replacement – DIY


Replacing a spark plug is totally manageable, even for a car-repair novice. You can order your manufacturer’s recommended spark plug replacement online, then install them yourself with limited tools in a few quick steps.


  1. Clean the area around the spark plug. Each one is connected to a rubber-coated wire, one per cylinder.
  2. Break the O-ring seal by twisting a quarter turn, then gently lift the wires out of the spark plug container
  3. Unscrew the plug using a swivel spark plug socket (an extension may be required here) and remove
  4. Install the new socket in the reverse order, making sure to use the manufacturer’s torque recommendation
  5. Lube the rubber boot with dielectric grease and reattach wire to the new plug


Check out this video for an easy walkthrough!


We hope this post got you a big more comfortable with the infamous spark plug. A basic understanding and regular simple maintenance of the plugs can have a great impact on your fuel economy – better for the environment and your wallet!


Until next time,


The Engie Team