If you’ve repaired a gasoline engine or a diesel engine, you’ve probably seen two parts with very similar names—spark plug vs glow plug. But they do absolutely nothing the same. So what exactly is the difference between them? Knowing this is beneficial for engine maintenance and troubleshooting problems. Next, let’s take a look.
What Is a Spark Plug?
In gasoline engines, a spark plug is an essential electrical component. Its role is to create the spark that ignites the air-fuel mixture. This starts the combustion process and powers the engine. If no set of spark plugs, a gasoline engine can’t even turn it on.
How Do Spark Plugs Work?
The spark plug connects to the ignition coil, which receives a high-voltage electrical pulse. This charge travels down the plug and creates a powerful spark. The compressed mixture in the cylinder was ignited by a spark, which triggered an explosion that pushed the piston downward. This process will repeat thousands of times per minute as the engine runs. Unlike glow plugs, spark plugs fire continuously during every power stroke. So, they are wearable items that require regular replacement.
What Is a Glow Plug?
The glow plug is a heating element dedicated to diesel engines. Diesel engines do not ignite with sparks, but make the fuel burn on its own, depending on the high temperature of compressed air. But if it’s too cold, the cylinder and air temperature are too low, and won’t heat up enough. It’s time the glow plug plays a role, adding some heat to the combustion chamber, thus making the engine easier to start.
How Do Glow Plugs Work?
As you turn the key, you will see a coil-like warning light on the dashboard. This light shows that the glow plug is working. On startup, the current flows to the glow plug, causing its tip to heat up rapidly and become red. This makes the temperature of the combustion chamber high. When fuel is injected, it is immediately heated, vaporized, and ignited, driving the engine to start. When the engine is running, the heat can maintain combustion, so there is no need to work of glow plug. Unlike spark plugs work continuously while driving, glow plugs are only needed when igniting.
Glow Plug vs Spark Plug: Discover the Differences
Now that we know their basic functions, let’s put them side-by-side to explore the deep-seated differences in their construction and operation. These distinctions explain why they are absolutely not interchangeable.
Material and Design
Each plug is designed to be built for its specific task.
Glow plugs emphasize durability and thermal insulation. It is generally longer and firmer, made of metals and ceramics that can withstand high temperatures, and can withstand strong pressures and extremely high temperatures without damage. At its core is a protected heating coil that is used to heat up quickly before the engine starts.
While the spark plugs are focused on conduction and insulation. It is generally smaller, with a precisely controlled gap at the tip, between the central and ground electrodes. Electrode materials (like nickel, platinum, or iridium) are resistant to corrosion caused by long-term sparks and maintain good electrical conductivity. The layer of white ceramic on the spark plug is not a decoration. It is a key insulating layer used to prevent high voltage from leaking before reaching the electrode.
Operating Temperatures and Conditions
The preheating plug relies on high-temperature heating—the operating temperature can reach about 1000°C (1800°F), which is used to heat the combustion chamber in advance. It only works during the short period of time the engine is started, and automatically stops heating once the engine is running.
The spark plug is different. It does not generate heat itself, but has to be under high temperature and pressure to “survive”. When the gasoline engine burns, the temperature may exceed 2500 °C (4500° F). For a spark plug to conduct thousands of blasts each minute at such a temperature, it must still work stably and continuously.
What Happens When a Glow Plug or Spark Plug Fails?
Both components are wearing parts. Knowing their common fail symptoms can help you spot problems in time before they get bigger. Because the two act differently, the failure performance is also different.
Signs of a Failing Glow Plug
Because glow plugs are for starting, problems will be most obvious then.
- Hard Starting: The engine turned for a long time to start, or would not start at all in cold weather.
- White Smoke from Exhaust: During startup, you may see white smoke, which is due to the cylinder temperature not being high enough to burn sufficiently, causing the unburned diesel get out from the exhaust pipe.
- Rough Idling After Startup: During the first minute or so, the engine may run badly or misfire. Until enough heat has accumulated.
- Glow Plug Light On: The glow plug light on the dashboard may stay on or flash, indicating a fault in the system.
Signs of a Failing Spark Plug
A failing spark plug affects the engine whenever it’s running.
- Engine Misfires: While idling or accelerating, you may feel that the engine is failing sometimes or reacting more slowly.
- Rough Idle: The engine will shake more than usual when parked.
- Poor Acceleration: the accelerator is pressed, but the car cannot release its energy, and its power is obviously not enough.
- Decreased Fuel Economy: the combustion efficiency gets worse, and you may find that the refueling is more frequent than before.
- Check Engine Light On: misfires in spark plugs are one of the common causes of the check engine light coming on.
Spark Plugs vs Glow Plugs: Maintenance
Maintenance schedules for these two parts are also very different.
Spark plugs need routine maintenance. The constant electrical erosion and exposure wear it down. Because the material has different types (traditional copper vs. long-life iridium or platinum), they usually are replaced every 30,000 to 100,000 miles.
Glow plugs, however, are not replaced on a fixed schedule. They are generally only replaced when one or more of them fail. A set of glow plugs can easily last for over 100,000 miles. If you’ve diagnosed a bad glow plug, please replace it. Learning how to replace the glow plug correctly can help ensure reliable starting.
FridayParts Spark Plug vs Glow Plug
Whether you’re doing routine maintenance on a gasoline engine or troubleshooting a diesel engine that’s hard to start, having the right parts is critical. FridayParts is your best choice. We offer various engine auxiliary components for vehicles of various makes and models. Ensure reliable performance and a perfect match. Simply enter your machine details in the FridayParts search box, and we will show you the exact parts you need. Don’t let a failed spark plug or glow plug slow you down.
