How Many Spark Plugs Does a Diesel Engine Have?

Although the question “Do diesel engines have spark plugs?” might come off as a diesel spark plugs joke, many folks may not be sure of the answer. Neither do they know the “why”. To get to the root of the question, let’s start with the spark plug’s function. A spark plug is a component that transmits electric current from the ignition system to the combustion chamber of an engine with a spark-ignition design. It sparks the compressed fuel/air mixture via an electric spark, thus starting the fuel’s combustion, which ultimately moves the vehicle.

Spark plugs are essential components in a gasoline engine because it is required to get the car moving. How many spark plugs does a car have if it’s that important? The answer is 4 for the 4cylider engine, 6 for V6, and 8 for V8 it goes on that sequence. Now that we know how essential spark plugs are to a gasoline engine, does the same apply to diesel engines? Well, find out why in the rest of the article.

Why Does a Diesel Engine Have No Spark Plugs?

To understand why the diesel engine does not have spark plugs, let’s take a quick look at the differences in the way both engines work. The diesel engine is similar to the gasoline engine in that both are internal combustion engines. However, as their names suggest, both engines use different types of fuel. But there is more.

Diesel engines work based on a compression-ignited injection system that works differently from gasoline engines’ spark-ignited system. The difference in the combustion systems is down to the nature of the fuel. While gasoline can ignite in whatever state it is, diesel must first become atomized.

Due to the compression-ignited system, diesel gets injected into the engine’s combustion chamber, where it gets ignited at high temperatures. The high temperature is generated by the compression of gas by the engine piston. Given the difference in combustion systems, there is no need for a spark plug to spark up the engine. So what do diesel have instead of spark plugs?

Diesel engines use a glow plug instead. A glow plug is an electrical heating device that is in each of the engine’s cylinders. It has a heating element at the tip, which emits light when heated (hence its name). When diesel is sprayed on the heated tip, it mixes with the compressed air, evaporates, and starts the combustion process.  Here is why the glow plug replaces the spark plug.

 Less Volatility

Diesel engines have high compression ratios when the infusion of air and fuel occurs differently. Gasoline cannot generate the same compression ratio because it is more likely to auto-ignite. Diesel is less volatile, so it has a low self-ignition temperature, allowing it to burn when sprayed on heated air.

When the pressure stroke sucks air, the air is compressed by a piston resulting in the air getting heated. The diesel’s high compression ratio makes it possible for the diesel to be sprayed as droplets over the heated air. Given its low volatility, the diesel no longer needs a high temperature to auto-ignite.

how many spark plugs does a diesel have

Auto-Ignition Temperature

Auto-ignition temperature refers to the exact temperature at which fuel starts to burn. Unlike gasoline, diesel can easily ignite since its auto-ignition temperature is low. That is why the temperature that is generated when air is compressed in the cylinder is enough to ignite the diesel that has been atomized.

Conversely, the risk of engine knocks and the explosion of the air-fuel mixture is high with the gasoline engine. The reason is that gasoline does not work the same way diesel does at high temperatures ratios, so the gasoline will pre-ignite easily, leading to engine knocks. Since diesel is less volatile and has a low auto-ignition temperature, it burns easily, eliminating the need for a spark plug. So do diesel engines have carburetors since most of them do not have sparkplugs? The answer is no.

Generally, It is believed that diesel engines do not have spark plugs, but some of them do. Some agricultural diesel tractors use a variety of fuel types. A good example is Allis-Chalmers’ heavy-duty engine. This heavy-duty engine uses gasoline, diesel fuel, or liquefied propane gas. 

Another example of a diesel engine that uses spark is the Farmall Super M-D diesel tractor. Farmall F-20 and Super M-D diesel tractor is a four-cylinder engine with four spark plugs. The engine is designed to start with gasoline and is equipped with two levers where the operator can switch it to diesel. You can find more about the Farmall F-20 here.

How Many Spark Plugs Does A Diesel Have?

The diesel engine injects atomized fuel into the cylinder, where it is ignited by the compression of the engine, heating the air-fuel mixture. Since the spark plug plays no role in the process, there are no spark plugs in most diesel engines. So how many spark plugs does diesel have? The answer is none.


 What Happens If a Spark Plug Is Used In Diesel Engine?

Spark plugs are designed to function on gasoline engines, which implies spark plugs will not work on diesel engines unless it is designed to run on a variety of fuel. The reason is since most diesel engines are not designed to work with a spark plug, it is not built to accommodate a spark plug. So, currently using a spark plug on a diesel engine that doesn’t run a variety of fuel is impossible.

Will A Diesel Start Without Glow Plugs

If your diesel engine is designed to start with diesel alone, it won’t start. This is because there would be a lack of compression. A certain compression ratio is required to generate the heat that atomizes the diesel. In addition, glow plugs are essentially starting aids so without them, diesel will not start. However, it is possible to start a diesel engine with bad glow plugs but it will take a longer time.

What Do Diesel Engines Use Instead Of Spark Plugs

Since the combustion of fuel in diesel engines does not require spark plugs, some other component replaces it in the engine. This component is called a glow plug. A glow plug is an electrical heating device that is situated in each of the engine’s cylinders.

It is a piece of metal that is structured so that there is a heating element at the tip that emits light when heated. When diesel is sprayed on the glow plug, the diesel mixes with the compressed air, evaporates, and starts the combustion process. How many glow plugs does diesel have? A diesel engine can have four to ten glow plugs, depending on how many cylinders the engine has. One glow plug for each cylinder.

Why Is The Spark Plug In A Diesel Engine Unnecessary

A spark plug is unnecessary in a diesel engine because it is not required to induce the diesel’s combustion. With diesel engines, the air is sucked into the engine’s cylinder, where it is compressed by high pressure. This high pressure increases the temperature to the point where it can ignite the diesel when it is supplied at the end of the compression stroke. Once the diesel is ignited, the gases expand, and the power stroke is gotten. As such, the spark plug plays no role and is redundant to the process of fuel combustion in diesel engines.

Final Words

If you read the article up to this point, you already know the answer to the question, “Do diesel engines have spark plugs”. Just in case you skipped through the article, the answer depends on if your diesel engine uses only diesel or a variety of fuel. And that is mostly because the diesel system has a combustion system that eliminates the need for a spark plug. Instead, the diesel engine uses a different plug called the glow plug, which is different in shape and function from the spark plug.

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Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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