It doesn’t matter if you’re someone who follows the automotive industry or not, chances are that most of you might’ve heard the term spark plugs. And while spark plugs are used to create a spark that ignites the air-fuel mixture in the engine’s cylinder, glow plugs are entirely different.
Before we move onto what glow plugs are, one of the major differences between these two is that spark plugs are used in petrol cars whereas glow plugs are majorly used in diesel cars.
In this article, we’ll not only talk about the function of glow plugs and how they differ from spark plugs but also explain how they work along with when should you replace them.
What Are Glow Plugs?
To understand what are glow plugs and how they work, we first need to understand the working of diesel engines.
Unlike petrol engines that rely on a spark for ignition, diesel engines are compression-ignition (CI) engines. This means that compression of the air-fuel mixture in the cylinder is used to generate enough heat that causes the mixture to combust.
While this might not sound very different from a petrol engine, the problem with a diesel engine is that it takes a lot of time to reach the operating temperature. This is because the air-fuel mixture in a diesel engine is denser than in a petrol engine, which means it takes more time to heat up and reach the point of combustion. This also means that diesel engines have a higher compression ratio compared to gasoline engines.
This is where glow plugs come into the picture. They are used to preheat the air-fuel mixture so that it can reach the point of combustion faster. They have a cylindrical or rod-shaped design with a heating element at one end and a connection for an electrical power source at the other end. The heating element is typically made of a metallic material that is resistant to high temperatures, such as nickel or copper.
How Do Glow Plugs Work?
We now know that a glow plug is a heating device used to aid in the starting of internal combustion engines. These plugs are generally used in diesel engines to preheat the air in the cylinders so that the fuel will ignite more easily. This is especially important in cold weather or at high altitudes where the air is thinner and the fuel will not ignite as easily.
When the engine is started, the glow plugs are automatically turned on. The electricity flows through the heating element in the plug, causing it to heat up. The heat from the plug is then conducted through the air in the cylinder, raising its temperature and increasing the likelihood of successful ignition when the fuel is injected.
Glow plugs can achieve temperatures of around 800°F – 1000°F within seconds. They tend to stay on for a few seconds after the engine is running to ensure that the engine runs smoothly and efficiently.
Also, these plugs are typically located in the cylinder head of the engine, with one plug for each cylinder. They are controlled by the engine’s electronic control module (ECM) and are activated when the engine is cold, such as when it is starting up or when it has been sitting idle for an extended period of time.
What Are the Three Phases in a Glow Plug?
The glow plug goes through three phases after it has been turned on. These include preheat, heat up, and keep warm
Preheat
Older diesel cars did not start as soon as the ignition was turned on, instead, the glow plugs would first require some time to heat up so that the engine can successfully start. This happens in the preheat phase, it could start as soon as the user unlocks the car or opens the doors of the car. In this phase, the plug is first activated and the temperature builds up to 800°F-1000°F. At this point, the diesel fuel is able to start to combust.
Heat Up
The heat up phase occurs next and the temperature continues to rise to 1400-1600°F. This is when the combustion process becomes fully self-sustaining.
Keep Warm/Post Glow
The keep warm phase follows and the glow plug maintains a temperature of 800°F-1000°F. The post glow, as it is also called, is when the glow plug remains active even after the engine has started. This is particularly useful in the city or in stop-and-go traffic conditions.
What Is the Structure of a Glow Plug?
The structure of a glow plug consists of a heating element, a metal sleeve, and a metal shell. The heating element is an electric wire embedded in ceramic or metal. This element is connected to a positive terminal and a negative terminal, which when connected to a power source, causes the element to heat up. The metal sleeve and metal shell provide protection for the element and help to evenly distribute the heat.
When To Replace Your Glow Plugs?
Over time, glow plugs can wear out and need to be replaced. Here are a few signs that it may be time to replace your glow plugs:
- The engine is taking longer to start
If it’s taking longer than usual for your engine to start, it could be a sign that the glow plugs are wearing out. When the plugs are new, they heat up the air in the combustion chamber quickly, making it easier for the engine to start. But as they wear out, they take longer to heat up the air, making it harder for the engine to start.
- The engine is misfiring
Another sign that the glow plugs may be wearing out is if the engine is misfiring. This can happen because the plugs are not heating up the air in the combustion chamber evenly, causing the air/fuel mixture to ignite unevenly. This can lead to a misfire.
- The engine is running rough
If the engine is running rough, it could be because the glow plugs are not providing enough heat. This can cause the engine to run lean, which can lead to a loss of power and a decrease in fuel economy.
- The check engine light is on
If the check engine light is on, it could be because the glow plugs are not working correctly. The light may come on because the engine is misfiring or running rough.
- There is black smoke coming from the exhaust
If you notice black smoke coming from the exhaust, it could be because the glow plugs are not providing enough heat. This can cause the engine to run rich, which can lead to black smoke coming from the exhaust.
If you notice any of these signs, it’s important to have the glow plugs checked and replaced if necessary. Waiting too long to replace them can lead to more serious problems, such as damage to the engine.
To Conclude
Glow plugs are an important part of diesel engines and play a vital role in starting and maintaining the engine’s performance. Knowing when it’s time to replace a glow plug, and how to properly do so, is essential for ensuring your engine’s longevity. Make sure that you consult a professional before replacing the glow plug.
