What are Spark Plugs and What Do They Do?

The spark plug is a seemingly simple device, although it is tasked with a couple of different but critical jobs. First and foremost, it creates (literally) an artificial bolt of lightning within the combustion chamber (cylinder head) of the engine.

The electrical energy (voltage) it transmits is extremely high in order to create a spark and to “light the fire” within the controlled chaos of the combustion chamber. Here, the voltage at the spark plug can be anywhere from 20,000 to more than 100,000 voltage.

What is Spark Plug?

A spark plug is an electrical device used in an internal combustion engine to produce a spark which ignites the air-fuel mixture in the combustion chamber.

As part of the engine’s ignition system, the spark plug receives high-voltage electricity, generated by an ignition coil in modern engines and transmitted via a spark plug wire, which it uses to generate a spark in the small gap between the positive and negative electrodes.

The timing of the spark is a key factor in the engine’s behavior, and the spark plug usually operates shortly before the combustion stroke commences.

The spark plug was invented in 1860, however its use only became widespread after the invention of the ignition magneto in 1902. Diesel engines use compression ignition instead of spark ignition, therefore they do not normally use spark plugs.

What Do Spark Plugs Do?

Your engine is a pretty remarkable machine, designed to turn an energy source (gasoline) into actual movement. But how does it do this? The answer is the principle known as internal combustion. In order to turn the fuel in your car from a source of potential energy into a source of kinetic energy, your engine needs to find a way to release it, and it does so through the combustion process.

The engine cycle is what makes this process happen. In your engine cycle, your valves fill your cylinder with a mixture of air and fuel, which are highly-explosive when combined. As the piston in your engine moves upward, it compresses this mixture until it’s in an extremely small space, creating even more potential energy.

At the peak of this compression, your engine ignites this mixture with a small spark, creating an explosion that forces the piston back downward, turning the crankshaft in your engine and creating the power which makes your car move forward.

Your spark plugs are what supply the spark that ignites the air/fuel mixture, creating the explosion which makes your engine produce power. These small but simple plugs create an arc of electricity across two leads which are not touching, but close enough together that electricity can jump the gap between them.

Your spark plugs, along with the electrical and timing equipment which powers them, are part of what’s known as your ignition system.

Generally, your spark plugs are made from extremely durable material, and are capable of withstanding millions and millions of explosions before wearing out or needing to be replaced.

But it’s true that over time, the explosions and corrosion lead to smaller or weaker sparks, which leads to reduced efficiency in your engine, and could lead to other issues including misfiring, or failure to fire.

How Do Spark Plugs Work?

Think of spark plugs as the tiniest bolt of lightning. Small but mighty, they spark electricity that ignites an air-fuel mixture deep within your car’s engine. In turn, combustion creates the energy required to power your car’s pistons, and ultimately, get you to your destination.

These components also play an important role in helping to dissipate heat from the combustion chamber to the engine’s cooling system. Here’s the step-by-step:

Step 1: You turn the key in the ignition.

When you start your car by pushing the “ignition button” or inserting and turning your key, the starter motor engages, which cranks your engine.

Step 2: Energy from the battery ignites the plugs.

Electricity from the battery travels to an induction coil on your car’s combustion engine, which transforms the battery’s 12 volts to as much as 45,000 volts before supplying it to the spark plugs.

Step 3: Sparks fly.

As the induction coil’s voltage increases and is transferred to the spark plugs, the plugs ignite the air-fuel mixture inside the combustion chamber — generating a small and controlled explosion in the gap between the plug’s electrodes.

Step 4: Combustion moves your car.

Once the air-fuel mixture ignites, a chemical reaction occurs, turning the mixture into an expanded gas or exhaust. The pressure generated by this sudden expansion within the combustion chamber moves your car’s pistons, ultimately turning chemical energy into the kinetic energy required to power the engine.

Step 5: The cycle continues.

As you continue your journey, the spark plugs power through this cycle repeatedly, helping your car run smoothly till you reach your destination.

Spark Plug Diagram

The diagram of the spark plug hasn’t changed much since the inception of the combustion engine in the late 1800s. A central electrode that carries the current runs the entire length of the spark plug. The spark plug wires (if applicable – older technology) or the ignition coil connected to the top of the spark plug electrode. A ceramic insulator separates the electrode from the rest of the spark plug.

diagram of Spark plugs

The plug is screwed into the cylinder head and protrudes into the combustion chamber with a threaded metal section, called the ground point of the spark plug. At the bottom of the spark plug is a small metal piece that looks like a “J” sitting sideways – this is the side (or ground) electrode. There is a gap between the center electrode and the side (ground) electrode, which is where the spark jumps the gap.

Spark Plug Construction

A spark plug comprises three main parts: the housing, insulator, and electrodes.

Spark Plug Diagram
  1. Terminal. Connects to the ignition system and conducts the high voltage to the central electrode.
  2. Insulator. Insulates the terminal, center shaft and center electrode against high voltage and provides flashover protection. Keeps high voltage from escaping before it gets to the electrode tip.
  3. Resistor. Suppresses ignition noise that is generated during sparking. Helps prevent electrical interference with other electrical components in the vehicle and disruption to radio reception.
  4. Central Electrode. Connects to the terminal by an internal wire. Tip is made of copper, nickel, chromium or other precious metals. The metal carries the high voltage through the spark plug so it can spark when it goes across the small gap between the central electrode and the side electrode.
  5. Ground or Side Electrode. Located on the side of the metal, it extends into the combustion chamber to create the spark that ignites the fuel.
  6. Hex Head. Steel shell where a socket wrench fits for tightening and loosening the plug. Provides electrical ground to cylinder head and transfers heat to the head to help cool the plug.
  7. Gasket. Located between the insulator and the housing, it helps keep combustion gases from escaping.
  8. Ribs. Helps stop voltage from jumping to the hex head.

Different Types Of Spark Plugs

Spark Plugs can be put into two different primary classifications, based on their operating temperatures, and based on their construction.

Based On Operating Temperatures

Once the combustion process is completed in the combustion cycle, the heat generated needs to dissipate. The heat escapes through the exhaust gases, the cylinder wall of the engine, and the spark plug surface. Based on the operating temperature and level of heat dissipation, spark plugs can be classified into two types:

  • Hot Spark Plug: A hot spark plug operates in a higher temperature range. It has a lesser ceramic area which is used to insulate the heat. A hot spark plug dissipates lesser combustion heat and allows the tip and electrode to stay hotter. This ensures that any deposit accumulation is burned off and isn’t allowed to stay for long.
  • Cold Spark Plug: For high-performance engines that run hot by default, using a hot spark plug will cause pre-ignition. In extreme cases, it can also lead to the tip melting off. In such cases, a cold spark plug is used. Here the ceramic insulation area is higher, which will dissipate more heat. But on the flip side, it is prone to greater deposit accumulation. Be sure to follow your instruction manual and use the correct type of plug recommended for your engine for optimum performance.

Based On The Material Used

Spark Plugs are further classified based on the material used on the ends of the electrodes. They are of 4 types:

#1. Copper Spark Plugs.

Copper plugs have been around for over a century and have a solid copper core with a nickel alloy electrode. Nickel is softer than platinum or iridium, and thus has a much shorter life.

These plugs are usually best for older, low-voltage systems. Because of copper’s high conductivity, some high-performance vehicles require them.

#2. Platinum Spark Plugs.

Platinum plugs became popular in the late-80’s, when ignition systems began to migrate from distributor-based systems, to coil pack or DIS (wasted spark) systems, which demanded a plug that could withstand the higher-energy, more demanding system.

The harder, platinum material lasted longer, as manufacturers made a push to develop spark plugs and antifreeze that lasted upwards of 80-100K miles.

Platinum plugs use a platinum disc that is welded to the end of the center electrode, which allows the plug to last longer than a copper plug, retaining its edge better, which means better conductivity. Platinum spark plugs tend to run hotter, preventing fouling and buildup of deposits.

#3. Double Platinum Spark Plugs.

In the early 1990’s, DIS “wasted spark” ignition systems were created, which essentially means that one coil pack controls the firing of two spark plugs in the system.

In doing this, both spark plugs that attach to that single coil pack spark at the exact same time – the difference is that one is firing on the compression stroke, and one is firing on the exhaust stroke of the sister-cylinder, which is referred to as a “wasted spark”.

To make this even more complicated, one spark plug attached to each coil is positive polarity, while one is reverse polarity. This means that the spark on one plug travels from the center electrode to the side electrode.

On the other plug, the spark travels from the side electrode to the center electrode. What this means is that instead of the center electrode wearing as it does in traditional ignition systems, now, the side electrode wears on the plug that is reversed-polarity!

This created the need for a double-platinum plug that has a platinum disc on both the center electrode, and the side electrode. This created a plug that is very durable in DIS systems, lasting upwards of 80-100K miles.

You don’t want to install single platinum plugs if your owner’s manual recommends a double platinum spark plug. Granted, the plug will work, but will wear prematurely and not perform as well.

#4. Iridium Spark Plugs.

Harder than platinum, Iridium plugs came on the market in the mid-2000’s as coil-on-plug systems began to get more popular. Iridium plugs usually feature the smallest diameter center, which can increase efficiency by reducing voltage needed to create a spark.

Iridium is also famous for continuing to fire under extreme conditions. If your manual specifies iridium spark plugs, best practice is to absorb the extra cost associated with this rarer metal for best performance.

#5. Double Iridium Spark Plugs.

Like double platinum spark plugs, double Iridium spark plugs feature two discs of rare metal for increased performance and durability.

Double Iridium Spark Plugs are designed to deliver the firepower necessary under any driving conditions, especially extreme heat or continual high-speeds, and can provide up to 4 times the normal life of service compared to a standard copper spark plug.

What Causes Spark Plugs To Wear Out?

To ensure that your vehicle is running on all cylinders, your engine’s spark plugs need to be clean, with no damage to the electrodes. Failing spark plugs can have an adverse effect on the performance of your vehicle, and driving with bad or fouled spark plugs can cause many problems for your engine. You could experience:

  • Reduced Gas Mileage
  • Poor Acceleration
  • Hard Starts
  • Engine Misfires
  • Rough Idling

If you recognize any of these symptoms of failing spark plugs, consult your mechanic who will be able to determine if a bad spark plug is the source of your problem.

Let’s Find out what can cause your spark plugs to fail.

#1. Oil in Combustion Chamber.

A leading cause of spark plug problems is a flow of engine oil into the combustion chamber. If oil leaks into the combustion chamber, it can cause the tip of the spark plug to get oily and dirty leading to premature failure.

Especially a problem in older vehicles, take notice if your vehicle starts to burn oil, as it can be an indicator that your spark plugs may be damaged, and therefore have a shorter lifespan.

#2. Overheating.

Repeated overheating of the spark plug tip can cause the plug to prematurely fail. Overheating can be caused by many things like pre-ignition and a malfunctioning cooling system. Pre-ignition can lead to heat building up in the combustion chamber causing the spark plugs to fail.

In addition, if the cooling system isn’t functioning correctly, it can cause the engine and spark plugs to overheat. This overheating can lead to the spark plug’s electrode wearing out faster.

#3. Carbon Buildup.

Black, dry soot on the electrodes and insulator tip indicates a carbon-fouled spark plug. This carbon buildup decreases the lifespan of a spark plug and can lead to hard starts, decreased acceleration, engine misfires and the check engine light coming on.

Causes of a carbon-fouled spark plug include a dirty air filter, excessive driving at low speeds, too rich of a fuel/air mixture, dirty fuel injectors or idling your vehicle for too long.

#4. Improper Spark Plug Gap.

The gap between the spark plug’s center and side electrodes needs to be calibrated perfectly to ensure optimal engine performance. Having the right gap ensures that the arcing occurs at the proper voltage to ignite the fuel and generate the combustion that makes the engine run.

If the gap isn’t set correctly, extra stress could be placed on the spark plug tip which could cause it to erode and wear out prematurely.

How often do you need to change spark plugs?

If none of the previous issues are forcing you to replace your spark plugs, you can typically change them every 30,000 to 100,000 miles depending on the make and model of your car. In order to determine which spark plug replacement schedule is the best for your vehicle, refer to your vehicle’s owner’s manual or contact a dealership for your vehicle’s manufacturer.

Whether you can change the spark plugs yourself depends on how convenient they are to reach. With older vehicles, the job is relatively easy and needs only some wrenches and a new set of spark plugs. You can easily find instructions in your owner’s manual or online.

You may find the procedure impossible to do in more modern vehicles. With additional electronics, sensors, and computers, current engines have expanded to fill nearly every open space in their compartments. The spark plugs are generally difficult to reach because they’re blocked by other components.