Igniting Gas and Air Mixture
Electrical energy is transmitted through a spark plug. When sufficient voltage is supplied to this component, a spark jumps the cap in the firing end to ignite a mixture of air and fuel in the combustion chamber. This is the short answer to the question why do spark plugs need to be gapped? An ignition coil or magneto connects spark plugs to a source of high voltage. The ignition process starts as a voltage difference develops between the center and side electrodes, but no current flows because the fuel and air present in the gap act as an insulator.
Rising voltage causes the structure of gases between the electrodes to change until the level exceeds the dielectric strength of the gases. Ionized gas becomes a conductor and enables electrons to surge across the gap. High heat in the spark channel causes the ionized gas to expand quickly and the resulting heat and pressure cause a small ball of fire to form in the spark gap before the gases begin to burn independently.
Transforming Potential Energy Into Kinetic Energy
The combustion process is ignited by spark plugs. These parts start the engine cycle that turns fuel from a source of potential energy into kinetic energy. Valves fill the engine cylinder with a mixture of air and fuel and a piston moves upward to compress this mixture into a small space. A spark ignites the mixture at the peak of compression and the resulting explosion forces the piston downward, turning the crankshaft in the engine and creating power to propel a vehicle.
Removing Heat From the Combustion Chamber
Spark plugs do not create heat, but transfer thermal energy away from the combustion chamber to the engine cooling system. The temperature of the firing end of a spark plug must remain low enough to prevent pre-ignition and high enough to avoid fouling. Over time, constant exposure to high heat, voltage and pressure wears these parts down and may widen the gap, which can result in hard starts, timing problems and reduced fuel economy.
Spark plugs are present in spark-ignition systems that run on gasoline. Compression-ignition systems that run on diesel fuel heat and compress air and inject diesel fuel that either autoignites or relies on glow plugs to achieve a more reliable cold start. Standard spark plugs should be replaced after approximately 30,000 miles, while extended-life spark plugs installed by manufacturers may be rated up to 100,000 miles.