How Many Spark Plugs Does a Diesel Have?

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Even though diesel and gas-powered engines share many parts that function the same, it doesn’t mean there aren’t parts that differ.

In this article, we will delve deeper into the key differences and answer a burning question, how many spark plugs does a diesel have? (do they even have any at all?)

How many spark plugs does a diesel have?

Zero. Since the very first diesel engine, all the way up to modern diesel engines, no diesel engine has ever had spark plugs.

Diesel engines don’t follow the same combustion methods as gas-powered vehicles. Gas-powered vehicles use spark plugs to ignite the fuel in the engine to create energy while diesel engines use immense pressure and heat to ignite the diesel in the engine. 

How diesel engines work

As we know by now a diesel engine needs pressure and heat to combust the diesel itself. To start a diesel engine the engine needs heat inside the combustion chamber, in modern diesel engines especially the smaller displacement ones, this is done by glow plugs. (Modern big capacity diesel engines don’t need glow plugs since they can draw in so much air that temperature doesn’t affect their cold-start capabilities.)

Glow plugs are a pencil-shaped part with a heating element at the tip, and this tip can get so hot that it visibly changes color to a bright red/pink color, hence the name ‘glow’ plug.

There are two different styles of glow plugs plus a very strange interesting engine heating method:

How Many Spark Plugs Does a Diesel Have
A common-style glowplug.

In-Cylinder glow plug

This is the most modern glow plug style when installed the tip of the glow plug is inside the combustion chamber of every cylinder of the engine. 

This is the most common glow plug in direct injection diesel engines (direct injection being when the diesel is directly sprayed into the combustion chamber).

In indirect injection, the glow plug tip is inside a pre-combustion chamber, this is a small chamber next to the piston where the fuel is injected. This is an old method of diesel injection and you are very unlikely to find a modern diesel-powered vehicle using this injection method.

Thermostart/In-Manifold glow plug

This is the oldest glow plug style and most likely you will never see one or have to work on one.

There is only one manifold glow plug for the entire engine and it heats up the intake manifold air before the engine starts.

This is very inefficient and in extremely cold weather, it will take numerous re-glowing to start the engine.

The Pony Engine

This is a really interesting invention, the pony engine was mainly used on tractors. 

It was a small gasoline-powered engine that heated up the big diesel engine. 

Since gasoline engines start easily in cold weather, this pony engine was used as a big glow plug and as a starter in some tractors. 

The pony engine will be started and run for a few minutes heating up the water/block of the diesel engine and then in some cases spin up the flywheel of the diesel engine and the operator would then engage the flywheel and start the diesel engine. 

As you can see this is a bit of a hassle since now you need two different fuels just to operate one vehicle.

Glow plug vs spark plug

We now know that a glow plugs job is to heat up the combustion chamber, only at startup and while the engine is running it is inactive.

Compared to a spark plug that works the whole time while the engine is running. While a glow plug is designed to heat up the combustion chamber a spark plug is designed to ignite the fuel in the combustion chamber with a spark, thus the name ‘spark’ plug.

To make a glow plug start glowing it just needs an electrical connection, this can either be 12 volts or 24 volts depending on the vehicle/truck/semi to heat up the element. 

A spark plug needs a coil to increase the voltage so the spark plug can generate a spark strong enough to ignite the fuel and keep sparking with pressure changes inside the combustion chamber. 

The spark plug also needs a distributor or electronic controlled ignition system to control when the spark plugs should spark. 

If there wasn’t any control over the spark, the engine would run really poorly and damage itself by backfiring (combusting the fuel in the intake manifold) or combusting the fuel too early in the combustion stroke resulting in pinging/detonation (this can put unnecessary force on the internals of the engine resulting in engine damage)

Frequently Asked Questions

How many glow plugs does a Diesel have?

This can range from one, like the Thermostart, a one-cylinder diesel engine, up to twelve depending on how many cylinders it has. 

The most modern bigger capacity diesel engines are so advanced that they don’t need glow plugs anymore. 

The smaller-engined diesels still use them still to this day though.

How many spark plugs does a gas engine have?

It ranges from 1 up to 24 on production vehicles. But what is strange is that it’s not in some cases relevant to how many cylinders the engine has. 

Some cars were equipped with twin spark technology, Alfa Romeo and Mercedes-Benz were fond of using it in their engines.

The older 6 liters and 5.5 liters V12 Mercedes-Benz engines used this twin spark technology and had a whopping 24 spark plugs!

What is a pony engine?

This was a gasoline engine that heated up old diesel engines to help them start in colder conditions, they were also used as a starter by spinning up the flywheel on the big diesel engines.

What is a glow plug?

In short, it is a heating device/component that helps in the startup of diesel engines. 

It has a shape of a pencil and at the tip an electric heating element that heats up the combustion chamber or the intake manifold, depending on what style glow plug it is.

Final thoughts on how many spark plugs a diesel has

A diesel has no spark plugs, instead, they use glow plugs.

It is understandable that with mechanical jargon terms can become easily confused. 

We hope that with this article, you now have a better grasp of why diesel engines don’t have spark plugs but rather glow plugs and how they work.

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