A glow plug is a heating device used to aid in the starting of a diesel engine. Unlike a gasoline engine, which uses a spark plug to ignite the air-fuel mixture, a diesel engine relies on compression alone to generate the heat needed for ignition.
However, when the engine is cold, the compression may not generate enough heat to ignite the fuel. The glow plug solves this problem by preheating the combustion chamber, ensuring the engine starts quickly and efficiently, especially in cold weather.
Quick Navigation
The Science Behind a Glow Plug
The fudamental principle behind a glow plug is resistance heating. When an electrical current passes through a resistive material, it generates heat. A glow plug is essentially a small, pencil-shaped heating element with a coiled wire inside. This wire, often made from a material like nichrome, is encased in a protective metal sheath.
When you turn the ignition key, but before you fully start the engine, a current from the car’s battery flows to the glow plugs. The coil inside the plug heats up very quickly, sometimes reaching temperatures over 1000°C (1832°F) in just a few seconds.
This intense heat radiates into the combustion chamber, warming the air inside. When the engine is then cranked, the warmed air combines with the high compression to create the perfect conditions for the atomized diesel fuel to ignite.
This process is what allows a diesel engine to start reliably in chilly conditions.
Anatomy of a Glow Plug
A typical glow plug is a relatively simple component with a few key parts:
- The heating element: This is the core of the plug, a coiled resistive wire that generates the heat.
- The metal sheath: This protects the heating element and is the part that extends into the combustion chamber. It’s designed to withstand extreme temperatures and pressures.
- The electrical terminal: This is where the glow plug receives the current from the vehicle’s electrical system.
- The body: This is the threaded section of the glow plug that screws into the engine’s cylinder head.
Different Types of Glow Plugs
While the basic function remains the same, glow plugs have evolved over time. There are two main types:
- Sheathed-Element Glow Plugs: The traditional type. They have a heating element encased in a ceramic or metallic sheath. They are reliable and widely used.
- Ceramic Glow Plugs: These are a more modern and advanced type. They feature a ceramic heating element, which allows them to heat up much faster and reach higher temperatures than their metal counterparts. This makes them more efficient and reduces the time it takes to start the engine, especially in very cold weather. They are also more durable and last longer.
The Glow Plug’s Role in Modern Diesel Engines
The role of the glow plug isn’t just about starting the engine. In modern diesel engines, especially those with advanced emissions systems, glow plugs can also be used after the engine has started. This is known as “post-glow” or “after-glow.”
The purpose of this function is to help reduce exhaust emissions and smooth out the engine’s idle during the warm-up period.
By keeping the combustion chamber at an optimal temperature, the glow plugs ensure a more complete and efficient burn of the diesel fuel, which in turn leads to cleaner exhaust and a more stable engine performance.
When Glow Plugs Go Bad
Like any other part, glow plugs have a lifespan and can fail.A failing glow plug can cause several noticeable issues:
- Difficult starting: This is the most common symptom, especially in cold weather. The engine might crank for a long time before starting, or it might not start at all.
- Rough idle: The engine might run unevenly or produce a lot of white smoke for a few minutes after starting, as some cylinders aren’t firing properly due to the lack of heat.
- Check engine light: Many modern vehicles will illuminate the check engine light if the engine control module (ECM) detects a fault with one or more glow plugs.
Ignoring a bad glow plug can lead to more significant problems down the line, including damage to the battery and starter from excessive cranking.
