Spark Plugs vs Glow Plugs: What’s the difference?

In gasoline engines, the fuel is typically introduced to the cylinder at the same time as the air, during the intake stroke. This allows the fuel to vaporize and mix with the air during the compression stroke. Then, the spark plug generates a spark that ignites the fuel for the combustion stroke. 

Diesel engines introduce the fuel at the tail end of the compression stroke, when the air is almost fully compressed in the cylinder. The low flash point, or point at which the fuel explodes, of diesel fuel causes the heat of the compressed air to initiate the explosion of the diesel fuel and power the combustion stroke. 

The combustion stroke in both types of engines begins when the fuel and air mixture explodes in the compressed cylinder. A gasoline engine requires a spark plug to initiate combustion, while a diesel relies on the heat of the compressed air to ignite the diesel fuel. 

What are spark plugs? 

Spark plugs are made of ceramic and metal, with the metal acting as a conductor for electricity and the ceramic acting as an insulator, keeping the electricity from touching things it shouldn’t. 

One end of the spark plug has a metal terminal that connects to your ignition coil, accepting the high voltage charge for every ignition event. The other end of the spark plug has an electrode sticking out past the tip, and an anode that hovers over the electrode to facilitate the spark. 

The spark plug has threads on the electrode end to screw into the top of the cylinder head, with a gasket or crush washer and helps to seal the cylinder. It must have a perfect seal because of the extreme pressures and temperatures that exist in the combustion chamber. 

The electricity flows from the ignition coil, into the terminal of the spark plug and through the metal core of the spark plug, ending at the electrode, where it arcs through the air and returns through the anode. This happens every time the cylinder fires, which will typically be thousands of times per minute. 

The main difference in ranges on spark plugs is in the material that makes up the electrode. Copper was the predominant material for years, but recent developments in spark plug technology have introduced longer-lasting spark plugs made of platinum and iridium. 

The distance between the electrode and the anode on a spark plug is called the gap and every engine has a specification, usually measured in thousands of an inch, for the distance of the gap. It’s important to measure your gaps using a spark plug gap gauge before you install them, as a bad gap can impact the performance of the plugs. 

Depending on the layout of your engine in the engine bay, doing a spark plug replacement can be a fairly easy job. 

What are glow plugs? 

In a diesel engine, combustion is initiated by the heat of the compressed air once the fuel is added. When a diesel engine is very cold it may require some preheating to bring the combustion chamber up to a temperature where combustion can occur and modern glow plug designs can also work with the engine to reduce smoke and other harmful emissions, regardless of the ambient temperature. 

Glow plugs are made of metal and have a threaded section that screws into the cylinder head. Inside the tip of the glow plug, there is a heating coil that, when charged with electricity, becomes hot and glows. 

At the end of the compression stroke, the fuel is introduced through the fuel injectors to the combustion chamber, where it vaporizes and mixes with the air before combusting. The function of the glow plug is to assist in vaporizing the fuel and prompting combustion when ambient temperatures are too low for compression ignition.  

Once the engine is at operating temperature, the glow plug can be idle, but many vehicles use them to fine-tune emissions by maintaining consistent conditions inside the cylinder. 

What is the difference between spark plugs and glow plugs? 

The difference between spark plugs and glow plugs comes down to the combustion cycle of gas versus diesel engines. Gas engines use spark ignition to induce combustion, whereas diesels use compression ignition to prompt combustion.  

• The spark plugs in your gasoline engine emit a spark for every combustion event in the cylinder, meaning that they will cycle many millions of times over their lifespan. 
• The glow plugs in your diesel engine will operate mostly on startup, or during certain conditions to improve emissions.  

Choosing the right plugs for your car

Spark plug and glow plug technology has come a long way in the last 50 years, utilizing more efficient designs and materials that improve the efficiency and longevity of the plugs. 

Spark plugs are now available in several ranges, utilizing exotic materials like iridium and platinum in the electrodes of higher-end models, while less-expensive ranges still use copper in their construction. 

The more expensive iridium or platinum spark plugs can be very close in price to their copper counterparts, so it may be more economical for you to purchase the higher-end plugs for your car, since an iridium spark plug will typically last longer and provide more consistent spark. Many are rated for up to 100,000 miles of use. 

Spark plugs for most vehicles will range in price from less than $10 to around $30 for the higher-end iridium plugs, so the cost to replace a set is not high, when you think of the length of service they provide. It’s a good idea to replace all the spark plugs in your vehicle at the same time, to promote consistency within your engine. 

Glow plugs have approximately the same price range as spark plugs, ranging from around $10 to $30 for most vehicles. They will also typically have a long service life of 100,000 miles or more, so if your diesel-powered vehicle is nearing that mileage, consider doing a glow plug replacement. It can improve emissions and the efficiency of your vehicle. 

When it’s time to do this important service on your vehicle, get glow plugs and spark plugs from AutoZone! Check out these other informative articles on sparkplugs!