Spark Plugs Vs Glow Plugs: What Is the Difference?

Spark Plugs Vs Glow Plugs: What Is the Difference?

Spark plugs and glow plugs sound the same and even look similar. But they are completely different components with different functions. One produces sparks and the other glows (er, that’s obvious). What else then?

What’s the difference between spark plugs and glow plugs? Spark plugs are used in gasoline engines to ignite the fuel mixture. On the other hand, Glow plugs are used in diesel engines to heat the engine.

To highlight the differences between a spark plug and a glow plug, here is a table below:

FeaturesSpark PlugGlow Plug
Type of engineGasoline EngineDiesel Engine
FunctionUsed for fuel ignitionUsed for heating the engine
Source of ignitionSpark produced at the tipGlow produced at the tip
DurationProduce spark continuously to keep the engine runningGlows only once in the beginning to start the engine
TypesCopper, Iridium, and PlatinumIn-cylinder variety and Thermostat variety

Now let us dig deep into it and find out in detail what is a spark plug and a glow plug? What is the structure of these? In addition, how do these work?

What is a Spark Plug?

A spark plug is a device that acts as an ignition source in internal combustion engines that use gasoline as a fuel source. When the piston compresses the air-fuel mixture, the spark plug produces a spark that causes combustion of the air-fuel mixture, increasing pressure inside the chamber and pushing the piston downwards.

The spark plug delivers the spark continuously to keep the engine running. Failure of the spark plug to ignite properly will produce knocking in your engine. In this way, the spark plugs often need to be replaced because of their short life.

As for which engine uses spark plugs, only gasoline engines require spark plugs. Diesel engines do not need spark ignition and hence, don’t use spark plugs.

Structure of a Spark Plug

A spark plug consists of a body and a central electrode that has a terminal end and a tip. A connector connects the terminal end of the electrode with the ignition coil above. The tip of the electrode protrudes into the combustion chamber where it produces a spark to ignite the air-fuel mixture.

Spark Plug

An insulator that is usually a ceramic material covers the central electrode. The insulator provides support to the central electrode and acts as a dielectric barrier so that a high-voltage potential is delivered to the electrode to produce a strong spark.

How does Spark Plug work?

The terminal of the electrode receives high voltage from the ignition coil through the connector. This voltage travels down to the tip where it produces the spark that causes the combustion of the fuel-air mixture.

Combustion builds a high pressure inside the chamber that pushes the piston downward. The piston connects to the crankshaft that in turn rotates the wheels.

Since the gasoline engines have less compression ratio as compared to the diesel engines, the spark plug has to produce a spark continuously to keep the engines running.

To know more, here is a detailed post on spark plug parts and functions.

Why do Gasoline Engines use Spark Plugs?

Gasoline engines have a combustion chamber, which consists of a piston below, and a spark plug fitted above the chamber. Compression of the air-fuel mixture by the piston generates heat. However, gasoline engines have a comparatively lower compression ratio that is insufficient to cause combustion.

Therefore, a spark plug is required, which produces a spark at its tip causing ignition of the compressed air-fuel mixture. In this way, the spark plug has to cause ignition continuously to keep the engine running.

What is a Glow Plug?

A glow plug is a device that acts as an ignition source in the internal combustion engines that use diesel as a fuel source. Diesel engines have an internal combustion mechanism, which means that these engines depend upon the heat of compression to cause combustion.

Glow plugs are required to preheat the air inside the combustion chamber. When the fuel enters the combustion chamber, it will cause combustion of the air-fuel mixture that pushes the piston downwards.

It helps to start the diesel engine in cold weather when the cylinder absorbs the heat of compression that prevents ignition of the air-fuel mixture.

Structure of a Glow Plug

A glow plug is a pencil-shaped device that consists of a central electrode and body. The electrode has an upper terminal and a lower tip that protrudes inside the combustion chamber. The tip contains the heating element that emits light in the visible spectrum when it receives electric current.

The heating element consists of a high-temperature resistant material that is highly temperature independent. A ceramic material covers the electrode that acts as an insulator. It also holds the electrode in place and prevents the glow plug from vibrating.

How does Glow Plug work?

The upper terminal of the glow plug receives electrical voltage from the ignition coil and transfers it to the tip that contains the heating element.

The heating element glows red hot when heated by the electric potential owing to its high resistance. The temperature of the heating element may rise to 1500 degrees centigrade. It helps to heat the compressed air in the combustion chamber so that when the fuel enters the chamber, combustion takes place.

Combustion of the air-fuel mixture increases the pressure in the upper compartment that pushes the piston downwards. It rotates the crankshaft that in turn rotates the wheel.

Why do Diesel Engines use Glow Plugs?

The combustion chamber of diesel engines also consists of a piston below and a glow plug above. However, the compression ratio of the diesel engine is much greater than that of a gasoline engine. It means air compression produces more heat which can cause combustion when the fuel is sprayed into the chamber.

Therefore, the glow plug helps decrease the time required for optimal heat production for combustion to occur. In addition, it is required only once to start the engine, which then runs simultaneously without the aid of a glow plug.

Difference between a spark plug and a glow plug

Sparkplug vs Glowplug


Every engine requires three main factors to work including air, fuel, and heat source. The spark plug and glow plug serve as the ignition source for the combustion of the air-fuel mixture in the combustion chamber, albeit in different ways.

A spark plug acts as a source of ignition in gasoline engines. It works continuously to produce spark owing to the lower compression ratio of gasoline engines. Due to the continuous workload on the spark plug, it has a short life and needs frequent replacement whenever the engine is not working well.

The glow plug assists in the cold start of diesel engines. Diesel engines have a high compression ratio so they do not require glow plugs to work continuously. Therefore, glow plugs have a longer lifespan and are not replaced as frequently as spark plugs.