Spark plugs have been an integral part of the internal combustion engine for more than 120 years. The job of a spark plug is to create a spark that ignites the air/fuel mixture in the engine’s combustion chamber. When the air/fuel mixture is ignited, the energy created expands and forces the piston downward. The timing has to be exact so the spark occurs when the air/fuel mixture is at its maximum compression. In older cars, this timing was controlled by a mechanical distributor, but in modern cars it is controlled by software. Despite improvements in the technology, today’s spark plugs are remarkably similar to the very first ones made in the late 19th century.
A spark plug consists of the these compoments:
- Electrode tip at the bottom of the spark plug that extends inside the combustion chamber that creates the spark for fuel ignition
- Ceramic insulator that keeps the high voltage current from escaping before it gets to the electrode tip, and provides thermal conductivity to keep the spark plug operating at an ideal temperature
- Terminal on top that connects the spark plug to the engine ignition source to generate the high voltage current for the spark
- Center stem shaft that connects the terminal to the electrode
- Housing or outer shell that supports the insulator, the grounding electrode, and the threads that mount the spark plug in the engine
While all spark plugs generally have the same parts, the devil is in the details. There are nearly as many versions of spark plugs as there are cars.
During the design of a modern engine, spark plug characteristics are finely tuned to provide an optimum spark for all sorts of conditions, engine loads, fuel quality, compressions, and fuel delivery methods. The differences are in spark plugs materials, the type of electrode gap, and the efficiency of the spark plugs insulation.
Manufacturers make spark plug outer shells and insulators from proprietary material combinations that are designed to reduce wear from corrosion, heat, chemicals, and moisture. The center stems are usually a combination of copper and glass powder that is fused to the insulator and outer shell. This center stem material is designed to provide optimum electrical current conductivity to the spark gap.
The insulator shape determines how the spark plug keeps itself at the right temperature for operation—not so hot that it causes uncontrolled ignition of the fuel mixture, and not too cool that unburned fuel accumulates as carbon deposits around the spark plug gap and prevents it from firing.
Types of Spark Plugs
Spark plug types are generally characterized by the type of electrode tip.
Copper spark plugs
Copper spark plugs, also called normal or standard, have a copper electrode. They are the most inexpensive plugs, and have the shortest life span. They are best for older cars and engines, producing a good spark under adverse conditions. Because of the fast erosion of the copper electrode, these plugs typically only last between about 20,000 and 40,000 miles.
Platinum-tipped spark plugs
Platinum-tipped spark plugs replace the copper tip with one made of platinum, which can extend the life to as much as 100,000 miles. Because platinum is more rare than copper, they cost more—but they pay for themselves because they last longer. Modern engines usually specify platinum-tipped plug because they perform to specification much longer, thereby reducing engine maintenance.
Iridium tipped spark plugs are the newest type of spark plug, with the tip made of the element iridium. These plugs are designed to produce the highest spark in the combustion process, and are best for the highest performance engines like direct fuel injection and turbocharged high-compression engines. The longevity of iridium plugs is somewhere between copper and platinum plugs, but it’s the most expensive spark plug. Life expectancy for iridium plugs is about 50,000 to 60,000 miles.
Iridium-tipped spark plugs
In most cases, deciding which plug is best for your vehicle comes down to what the vehicle manufacturer recommends. If copper plugs are recommended, then these plugs will do fine, but if your budget allows, you can upgrade to platinum plugs and extend your maintenance interval. If your car is equipped with platinum plugs, it’s best to stay with those plugs, and not save a few bucks by switching to copper plugs. Since iridium plugs are the most expensive, they are not recommended except for modern high-compression high performance engines when the manufacturer specifies that type of plug.
Is It Time to Replace Your Spark Plugs?
The performance of all spark plugs gradually degrades over time, and is not usually obvious. If your vehicle’s performance is not what it used to be, and it’s relatively close to its recommended spark plug maintenance interval, then consider changing the plugs early. If your vehicle is starting to misfire or becoming hard to start, then it also might be time to change the plugs.
When you change the plugs, look at the tip that’s inside the engine. It should have a light grey or brown color. It should be free of oil and foreign substances—and should not be wet with fuel. Dirty or damaged spark plugs indicate more serious issues lurking in your engine.
