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20 March 2025 (updated) Glow plug vs spark plug: what's the difference?
Glow plugs and spark plugs are essential for engine ignition, but they serve different purposes. Glow plugs heat diesel engines for combustion, while spark plugs ignite the air-fuel mixture in petrol engines. To maintain the ignition system properly, you need to understand how these parts work and their particularities. Find out more in the guide below.
Are glow plugs and spark plugs the same?
While they serve a similar purpose and look similar, they aren’t the same. Glow plugs can only be used in diesel engines and spark plugs can only be used in petrol-powered vehicles. This is because the method of ignition differs significantly depending on the type of fuel used.
What's a spark plug? Description and meaning
A car spark plug is a small yet essential component in petrol engines, responsible for igniting the air-fuel mixture. It consists of a threaded metal shell, a ceramic insulator, and two electrodes that create an electric spark. Installed in the cylinder head, it withstands high temperatures and pressures while ensuring smooth combustion.
Spark plugs come in various types, including copper, platinum, and iridium, each designed for durability and performance. Regular maintenance prevents misfires, poor fuel efficiency, and engine power loss.
What does a spark plug look like?
Spark plugs are typically shorter and bulkier with a central electrode and ground electrode at the tip, designed to generate a spark. They have a threaded metal shell and an insulator around the center electrode. It has a white ceramic insulator surrounding a central electrode, which extends from the top to the bottom. At the tip, a ground electrode bends over the central electrode, creating a small gap where the spark jumps.
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What do spark plugs do?
A spark plug generates a high-voltage spark to ignite the compressed air-fuel mixture inside a petrol engine’s cylinder. This controlled explosion drives the piston down, producing power to propel the vehicle. It plays a critical role in engine performance, fuel efficiency, and emissions control.
A faulty spark plug can cause misfires, rough idling, or starting issues. Regular inspection and timely replacement help maintain optimal combustion, ensuring smooth acceleration, better fuel economy, and a longer engine lifespan.
How many spark plugs in a petrol engine?
As a rule, there is one spark plug installed in each cylinder. So, if you have a V6 engine, you’ll have 6 spark plugs to replace, and if you have a V8 engine, you’ll have 8. You should take this into consideration when weighing up the maintenance costs of different vehicle models.
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What is a glow plug? Description and meaning
A car glow plug is a heating device used in diesel engines to assist with cold starting. It consists of a metal body with a heating element at the tip, which quickly reaches high temperatures when electrified. Unlike spark plugs, glow plugs do not create sparks but preheat the air in the combustion chamber to ensure efficient ignition.
They are crucial in cold weather, reducing engine strain, emissions, and the startup time. The main types available include metal and ceramic glow plugs, offering varying heat rates and durability.
What does a glow plug look like?
Glow plugs are longer and thinner than spark plugs, resembling a small metal probe. The tip houses a heating element, often sheathed in metal, which glows red-hot to aid ignition. The upper end has an electrical terminal for power connection. Unlike spark plugs, glow plugs lack an electrode gap, instead relying on heat to ignite the air-fuel mixture in cold diesel engines.
What do glow plugs do?
Glow plugs preheat the air in a diesel engine’s cylinders, making it easier for fuel to ignite during startup, especially in cold conditions. They remain active for a short time after ignition to ensure smooth combustion. Without functioning glow plugs, starting a diesel engine can be difficult, leading to misfires, excessive smoke, and rough idling. Their role is crucial for reducing emissions and improving fuel efficiency.
How many glow plugs in a diesel engine?
A diesel engine typically has one glow plug per cylinder. For example, a four-cylinder engine has four glow plugs, while a six-cylinder engine has six. The larger the engine, the more a replacement will therefore cost.
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