What do spark plugs do?

One of the smallest automobile parts you’ll find on your vehicle, spark plugs play a critical role in making sure that your vehicle operates exactly as intended. In fact, it’s possible that if more than one of your spark plugs isn’t working properly, your engine might not start at all.

These little modern miracles need to be checked (and replaced) from time to time. You won’t know when to make the change, or how, unless you pay close attention to the inside information we have for you below.

What do spark plugs do?

The actual mechanical function of your spark plugs is ridiculously small and almost invisible, but the chain reaction that the proper firing of your spark plugs is responsible for is the entire backbone of the modern day internal combustion engine.

You see, the only way that your vehicle is going to be able to move, or run at all, for that matter is if the fuel and the air that is brought into the combustion chamber of your vehicle is able to ignite, causing a cascade of reactions from there that shoot you down the road at top speed.

The combustion, or burning, of the gasoline is what powers the engine. The fuel is ignited by the spark from the spark plug, and that’s what keeps your engine running.

How important of a role do spark plugs play?

Spark plugs play a very critical role in your engines performance and efficiency. As the pistons in your vehicle come up on the compression stroke, fuel and air is going to be pulled into the compression area of your engine so that it is properly mixed. Just before the compression stroke is finalized, an electrical current is going to shoot through your ignition system.

What does it look like?

The path of spark plug current

The current starts at your ignition coil. It goes from there, up to your distributor and through the spark plug wires. When the current reaches the end of the spark plug wires, that’s when the plugs do their job. The electrical current travels through the center of the spark plug. It is insulated from the engine block by ceramic material. When the current gets to the end of the plug, it has so much energy that the electricity jumps across the gap at the end of the plug to get to ground. The jolt of current that was initially sent from the coil has been trying to get to ground throughout its trip through the ignition system. The conductors that carried the current were insulated from ground through the whole path until the current reached the exposed end of the spark plug. The spark plug is screwed into the combustion chamber so that the spark produced is inside the combustion area of the engine.

What’s really going on at the plug?

The spark plug is a simple device that has a lot going on with it. The gap between the electrodes at the spark end needs to be set just right. If the gap is set too close together, the spark won’t be big enough to properly ignite all of the fuel. If the gap is set too far apart, you run the risk of the electrical current not being strong enough to jump across the gap. If the gap is set too large for the spark to jump it, your engine will not run at all. If the gap is too small, you will have poor power and poor fuel economy.

Setting the spark plug gap

What do spark plugs do when the gap is wrong?

When you purchase new spark plugs, they usually need to have the gap set. Setting the proper plug gap for your particular vehicle is an important thing to remember before you install the plugs. Because a particular plug will fit in many different vehicles, you must look up the recommended plug gap for your vehicle and adjust the new plugs accordingly. The proper gap setting can usually be found in the owner’s manual. If the owner’s manual is not available, it may be printed with the emissions information under the hood. If all else fails, a quick search on the internet will surely give you the info you need.

Spark plug gap is stated in thousandths of an inch. If the gap required is listed as .043 it is said to have a “forty-three thousandths gap”. Similarly, if you are told that your plugs need to be set at thirty thousandths, which means to set them at .030”. All spark plug gaps are set by thousandths of an inch and not with a metric measurement.

The gap of a spark plug is set with a special tool. All auto parts stores carry spark plug gapping tools. There are a few different styles of gapping tool to choose from. The most common type is called a “coin” style gapping tool. It is shaped like a coin and is usually made to go on your key chain. Its outer edge changes in size as you go around its diameter. There are size marks engraved in it at specific points to tell you what the thickness is at a particular point on the coin.

How fast is all of this?

The ignition system fires a spark thousands of times every minute. Each spark is fired with more than 40,000 volts of electricity. Electrical current goes ripping through your system and making that leap every time a compression stroke happens inside of your engine.

What do spark plugs do when you step on the gas pedal to go a little bit faster? The compression strokes speed up. So your spark plugs need to fire a bit faster. Hit the brakes to slow down (or idle in your driveway) and the spark plug firing rate slows down as well.

How do I know my spark plugs aren’t acting the way they should be?

If you’re wondering what spark plugs do because you feel like they aren’t firing quite as effectively or as efficiently as they should be, you’re going to want to pay close attention to how your vehicle acts when you accelerate or when you first turn the engine over.

If your vehicle “fights you” when you turn the ignition, or if you feel like it “bucks or kicks” when you hit the gas for faster acceleration, the odds are pretty good that you are dealing with a spark plug issue.

This doesn’t always mean that you need to replace your spark plugs entirely (though that is pretty cheap and almost effortless to do on your own), but it probably means you need to clean and better maintained your spark plugs.

One way to determine if there is a problem with your plugs is to remove them and compare them to a chart such as this one:

what do spark plugs do when they're burned?

What should I look for in new spark plugs?

Honestly, pretty much any old spark plug will do, provided that it has been engineered to fit your specific make, model, and year. Some spark plugs are designed to last longer than others and may be worth the premium price tag associated with them, but you should be able to get away with inexpensive replacements just as well.

People asking the question “What do spark plugs do?” are often surprised to find out just how important they are. It’s impossible to get good fuel economy or power without your spark plugs functioning properly.

Filed in: Gasoline Engines
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