Spark plugs deliver electrical current to the engine's combustion chamber to ignite the fuel, which burns in the air mixed with it to create pressure in the cylinder and push the piston to create movement and force. For the gasoline engine, the lowly and simple spark plug is at the core of the entire combustion process.
Modern Spark Plugs
Today's spark plugs are a part of the ever more sophisticated ignition and timing systems used to control the engine's operation. While the lowly spark plug has seen little change for decades, the components around it and controlling it have been revolutionized several times.
A spark plug is comprised of five basic parts, none of which move and a few of which will fail over time. Many spark plugs can have a lifespan nearing half or more of the car's expected lifespan. The only thing affecting a plug outside of physical damage is erosion of the electrode through exposure to carbon buildup or misapplied power input.
A spark plug consists of:
- A central electrode
- A resistor
- Surrounded by a porcelain shell
The shell insulates it from its metallic base through which an electrode and ground from the base nearly meet. The space between them is called the plug's 'gap' and will vary according to the needs of the specific engine the plug is made for. It's in this gap that the high-intensity spark the plug creates will appear.
A wire from the ignition system, usually a coil or distributor, will pulse electricity to the top of the plug's central electrode. This will fire down the electrode, possibly meeting resistance depending on the plug's design characteristics, and coming out the end that is inside the combustion chamber to arc to the ground hook. This spark ignites the fuel which burns inside the air injected into the cylinder.
Spark Plugs Over Time
Over time, carbon buildup from the fuel and air burning, erosion of the electrode and/or ground from the effects of high-temperature spark burn, or fluctuation of power input due to worn or loose spark plug cables will take their toll on the plug until it loses efficiency and must be replaced. Most vehicles have a tune-up interval for this, ranging from 10,000 miles to over 50,000, depending on the application.
Spark plugs can be purchased with premium features such as multiple grounding prongs, coated metal grounds for improved conductivity, and lower or higher resistance than recommended by the manufacturer to tune the ignition timing. Whether or not these improve performance is often a matter of debate.