How Many Spark Plugs Does A Diesel Have? [The Answer]

How many spark plugs does a diesel have? The answer to this seemingly straightforward question might surprise you.

Unlike gas-powered internal combustion engines, diesel cars and trucks don’t use spark plugs to ignite their fuel. Rather, diesel engines use compression to raise the air temperature inside the combustion chamber to a point where the diesel spontaneously combusts.

Do diesels have spark plugs? We gave you a simple answer, but you’ll need to keep reading to learn how diesel engines are able to operate without them.

How Many Spark Plugs Does A Diesel Have? A Deep Dive

Having established that diesel engines don’t use spark plugs, let’s look at the reason why that’s the case and what device, if any, they use as a substitute.

If you have a gas car and need new spark plugs, we have a detailed guide that will help you find one.

Why Don’t Diesel Engines Have Spark Plugs?

Diesel engine vehicles do not have spark plugs because they don’t need spark plugs. The biggest reason is that diesel has a much different property than gasoline.

More specifically, gasoline is more volatile than diesel, and it doesn’t take much for it to ignite. A simple spark is all that’s needed.

Diesel, on the other hand, is much less flammable and requires a lot more agitation to ignite. In fact, it won’t catch fire in the combustion chamber without first being atomized.

Simply put, an electrical spark from a spark plug isn’t going to cut it. A more robust system is required.

Fun Fact: If you throw a lit match into a bucket of diesel, the fuel will extinguish the match rather than catch fire. It takes an intense amount of pressure or sustained flame to ignite diesel fuel.

How Do Diesel Engines Work Without Spark Plugs?

Because diesel requires more effort to ignite than gasoline, diesel engines use an entirely different ignition system. Rather than the compression-ignited injection system found in gas engines, they use a compression-ignited injection system.

A compression-ignited injection system uses glow plugs to help heat up a charge of compressed air in the compression chamber to such an extent that the diesel ignites spontaneously once injected into the chamber.

So, instead of spark plugs, diesel engines typically employ glow plugs to aid ignition. The question, therefore, isn’t “how many spark plugs does a diesel have?” but rather “how many glow plugs does a diesel have?”

How Do Glow Plugs In A Diesel Work?

Glow plugs are essential for the operation of modern diesel engines, so it’s worth exploring in more detail what exactly they are and how they work.

A glow plug is for all intents and purposes a heating coil contained in a metal tube. More specifically, it’s a pencil-shaped metal piece with a heating element at the tip.

When electrified, the heating element glows much like the heating element in a toaster does, heating the air in the compression chamber. The fuel injector then sprays the diesel near the glowing hot tip, where the high heat and compression of the chamber causes it to ignite.

Glow plugs can get as hot as 1,000°C (1,832 °F) in just a few seconds and are primarily intended to ignite the fuel when the engine is too cold for normal operation. They also help heat the entire engine so that it runs optimally in colder temperatures. 

Ceramic glow plugs are more durable and efficient than metal plugs, heating up faster and providing more stability, better performance, and superior fuel efficiency. However, they are more expensive.

Unlike spark plugs, which work continuously to keep a vehicle’s engine running, a glow plug is only necessary for the initial ignition. 

Can A Diesel Engine Run Without A Glow Plug?

You might even be surprised to know that it’s possible for a diesel engine to run without glow plugs, especially in warm weather. In fact, many diesel engines DO NOT HAVE glow plugs and operate just fine without them.

That’s because, while useful, glow plugs are not an integral part of a diesel engine’s operating cycle like spark plugs are to a gasoline unit. They are merely a starting aid found in some but not all engines.

In a sense, a diesel engine that has glow plugs can start even if the plugs become faulty, but you probably wouldn’t want to take any chances.

Will a diesel engine still start in cold weather without a glow plug? Yes, but it will undergo more cranking than usual to run properly.

However, a diesel engine that is designed to start with glow plugs will usually not start without them.

Reasons Why Glow Plugs Fail

How Many Spark Plugs Does A Diesel Have - faulty plugs

Glow plugs are typically designed to last a long time; however, like every other part of your vehicle, circumstances can cause them to fail prematurely.

Excessive heat and high voltages are the two main causes of premature failure. If your engine excessively overheats due to poor timing, a leaking fuel injector, carbon buildup in the pre chambers, or other factors, the tips of the glow plugs can burn out.

High voltage resulting from a defective controller can cause the glow plugs to receive power constantly, making them burn out prematurely since they are not designed to be on all the time.

Additionally, boost-starting an engine with a high-amperage battery charger can pump too much voltage into the glow plugs, causing their tips to explode.

Having the wrong glow plugs for your engine is perhaps the most obvious reason why glow plugs fail. If you have the wrong make or model for your engine, the heat or voltage produced by your engine could be too much for them to handle and they may strain to work properly, if at all.

Finally, an incorrect spray pattern of your fuel injectors can also be problematic. If they are squirting large streams of raw fuel, this can cause the tips of the glow plugs to cool down rapidly, stressing and possibly even burning them out.

Symptoms Of Bad Glow Plugs In A Diesel

While it’s possible for a diesel engine to start and operate without glow plugs, having them makes the operating cycle smoother, more efficient, and less taxing on the engine. For optimal engine performance, you want to make sure they always work properly.

Your diesel vehicle might exhibit one or more of the following symptoms if it has a faulty glow plug or several. 

1. Hard Start 

The main purpose of glow plugs is to heat the air and diesel fuel in the combustion chamber just enough to facilitate ignition. If you notice that your car or truck is taking longer to start than usual, then you might be dealing with problematic glow plugs.

A faulty glow plug may not be able to achieve a hot enough temperature to aid the combustion process, resulting in hard starts, especially in colder weather.

It’s worth pointing out that hard starts can also be caused by issues with the battery or fuel system. But regardless, they can be so rough that it’s not uncommon for the vehicle to shake or vibrate.

2. Engine Doesn’t Start

As with gasoline engines, diesel engines require more heat to start in cold weather. If some of the glow plugs are malfunctioning, they may not start at all.

Hearing the engine turn over without starting is usually a good sign that faulty glow plugs are at fault. If the battery is the problem, you would hear clicking sounds and nothing else, indicating that the starter motor isn’t receiving enough power.

3. Poorer Fuel Efficiency

Malfunctioning glow plugs can cause your engine to continuously inject diesel into the combustion chamber without igniting it, essentially wasting fuel. 

Not only that but the strain placed on its various parts can lead to unnecessary wear and tear, causing the cylinders to lose some of their capacity to compress air and rendering the engine less efficient than before.

4. White Exhaust Smoke

If the diesel does not ignite and burns in the combustion chamber because of faulty glow plugs, it can cause your engine to release thick black or white smoke from the exhaust.

Then again, an abnormal amount of exhaust smoke can also be caused by a variety of other issues such as metal shavings in your oil, so you should have the engine properly inspected to diagnose the underlying problem.

5. Engine Misfire

Engine misfires are a common symptom of faulty glow plugs. Glow plugs that don’t work properly won’t be able to provide the additional heat needed to aid diesel combustion, potentially causing the engine to misfire. 

Engine misfires, in turn, can lead to a drop in power and fuel efficiency.

6. Check Engine Light

Cars and trucks have many moving parts, and there are many reasons why the check engine light would come on. In the case of diesel vehicles, faulty glow plugs are one of those reasons.

Granted your vehicle will still run even when the ‘check engine’ or ‘service engine soon’ warning light is on, you’d be wise to have the underlying problem diagnosed and fixed sooner than later. 

The engine is the single most expensive part of a gas or diesel-powered vehicle, so any damage to it can be particularly expensive to repair.

Do Diesels Have Spark Plugs? FAQs

Question mark in road sign

Questions, questions — you’ve got some, and we’ve got answers. 

How Many Glow Plugs Are in a Diesel Engine?

The number of glow plugs in a diesel engine usually depends on the size of the engine and the number of cylinders it has. In other words, four-cylinder engines typically have four glow plugs, while an eight-cylinder unit will have eight plugs.

Some large diesel engines can have upwards of ten glow plugs.

How Often Do You Need to Replace Glow Plugs?

Glow plugs can last 100,000 miles (160,934 km) before they start deteriorating; however, you may need to replace them earlier if they fail or become defective prematurely.

Using quality glow plugs that are compatible with your diesel engine, ensuring the engine doesn’t overheat, and preventing high voltage spikes are critical for keeping your glow plugs in optimal working condition.

In addition to being long-lasting, diesel engine glow plugs are also quite affordable, costing just a little more than your average spark plug. Replacing a whole set won’t cost an arm and a leg.

What Happens When Glow Plugs Fail?

Faulty or failed glow plugs can disrupt the diesel combustion process, possibly resulting in poor engine performance. The engine might be slow to start or not start at all, experience misfires, and produce white or black smoke.

You might even notice a drop in power, acceleration, and fuel efficiency.

Faulty glow plugs can place so much stress on your engine that other parts of the engine may wear out more quickly or fail outright.

Can We Use Spark Plugs In A Diesel Engine?

Spark plugs are designed specifically for use by gasoline engines. So, unless they have a design that allows them to be used on a variety of fuels, which is very rare, they will not work on a diesel engine.

It’s also worth pointing out that there is nowhere in a diesel engine where spark plugs can be installed.

So, no, it’s often impossible to use spark plugs in a diesel engine.

Final Thoughts

Do diesels have spark plugs? No! Unlike their gasoline counterparts, diesel engines do not use spark plugs.

Rather than a spark-ignited system, they employ a compression-ignited system partly aided by glow plugs, a novel device that helps kick start the combustion process.

As highlighted in the article, glow plugs work in a fundamentally different way than spark plugs. They don’t ignite the fuel but rather help heat up the combustion chamber to encourage efficient combustion.

Glow plugs are used only for the initial combustion and don’t continuously operate as spark plugs do. However, like spark plugs, they can fail prematurely and undermine the performance of your engine if not properly maintained.

So, how many spark plugs does a diesel have? In case you somehow missed the answer, none. If you found this article useful, you might also want to know how much your car or truck weighs, how to dispose of old gas properly, and why your turn signals sometimes act up.