Glow Plug vs Spark Plug [What’s the Difference?]

Last Updated on November 17, 2023 by Jaxon Mike

Glow plugs and spark plugs are both engine ignition devices that are vital for starting internal combustion engines. However, there are some key differences between how glow plugs and spark plugs function and the types of engines they are compatible with.

Understanding the distinctions between these two ignition components can help drivers and mechanics make the right choice when servicing or repairing an engine. This article will compare glow plugs vs spark plugs, explaining how each device works, their pros and cons, and the types of engines they are best suited for.

Here is an in-depth comparison table for glow plugs vs spark plugs:

	Glow Plug	Spark Plug
Purpose	Provides heat to aid in cold starting of diesel engines	Ignites compressed fuel/air mixture in gasoline engines
Design	Metal housing containing a heating element/coil	Ceramic insulator seals and protects center and side electrodes which spark
Location	In the combustion chamber	Threads into cylinder head with electrodes exposed in combustion chamber
Operating Temperature	1000-1400°F	800-3000°F at electrodes
Electrical Requirements	12 or 24 volt power from battery	High voltage (20kV-50kV) ignition coil output
Construction Materials	Housing: steel or cast iron Heating element: steel or ceramic	Insulator: aluminum oxide ceramic electrode: nickel alloy steel Ground electrode: high chromium steel
Lifespan	Up to 20,000 hours	As low as 10,000 miles before replacement needed
Failure Modes	Element oxidation, current leakage, mechanical damage	Fouling, electrode wear/corrosion, thermal shock cracking
Maintenance	Periodic inspection, replacement at tune-ups or failure	Regular replacement at tune-ups, gap adjustment
Cost	CHECK LATEST PRICE	CHECK LATEST PRICE
Advantages	No high voltage ignition required Simple, rugged design	Precision temperature/deposition control at electrodes Higher operating temp for cleaner combustion
Disadvantages	Slow to heat up in very cold temperatures	Sensitive to fouling Higher replacement frequency

What is a Glow Plug?

A glow plug is a heating device used to help initiate combustion in a diesel engine. Diesel engines rely on compression to ignite the air-fuel mixture, unlike gasoline engines which use spark plugs.

The lack of a spark means that diesel engines require very high temperatures in the combustion chamber to get the mixture to ignite. Glow plugs help provide this heat so that the engine can start.

How Does a Glow Plug Work?

A glow plug has a heating element made of a coiled metal wire inside a metal tube or ceramic insulator. When electricity is applied, the heating element quickly heats up to temperatures between 1000-1500°F.

This heat helps warm up the air in the engine’s cylinders so that when the diesel fuel is injected at the top of the compression stroke, the hot compressed air is enough to cause ignition. The fuel will combust and power the engine.

Once the engine warms up to its normal operating temperature, the glow plug is turned off. The heat of compression and combustion is sufficient to keep the engine running without the need for the additional heat from the glow plug.

The glow plug is usually operated automatically when the driver turns the ignition key. It engages for a preset time before and during startup to preheat the cylinders. A light on the dash alerts the driver if the glow plug system is malfunctioning.

Pros and Cons of Glow Plugs

Pros:

• Provide the heat needed for cold diesel engine startup
• Enables diesel engines to have compression-ignition operation
• Simple and robust design
• Long service life if maintained properly

Cons:

• Requires supporting electrical system to control and operate
• Can fail prematurely if subjected to excessive moisture or corrosion
• Not capable of reigniting engine if flameout occurs

What is a Spark Plug?

A spark plug is an ignition device used in gasoline engines to deliver electric sparks to ignite the air-fuel mixture in the combustion chamber. It fits into the cylinder head of each cylinder and connects to the vehicle’s ignition system.

Spark plugs enable gasoline engines to initiate combustion using an electric arc rather than the heat of compression like in a diesel engine. This gives gasoline engines greater flexibility in ignition timing.

How Does a Spark Plug Work?

A spark plug consists of a metal threaded shell housing a ceramic insulator core. At the core’s center is a conductive electrode that is electrically isolated from the metal case.

A gap is created between the electrode tip and the grounded metal shell. When ignition voltage is applied across the gap, it ionizes the air and creates an arc of intense heat and energy, forming a spark.

This spark ignites the compressed air-fuel mixture in the engine cylinder during the power stroke. Well-timed sparks allow ignition to occur at precise intervals, enabling better engine performance.

The heat of the spark is around 4000°F, sufficient to initiate combustion even in cold engine conditions. This means spark plugs allow for easier starting in gasoline engines compared to the pre-heating required for glow plug diesel engines.

Pros and Cons of Spark Plugs

Pros:

• Allow precise ignition timing control
• Work well in cold temperatures
• Produce a hot, focused spark for reliable ignition
• Spark plug design is simple and compact
• Enable greater engine performance optimization

Cons:

• Spark plug gap needs to be adjusted periodically
• Electrode erosion can reduce service life
• Require high voltage ignition system to produce spark
• Vulnerable to oil fouling or damage from wrong spark plug heat range

Key Differences Between Glow Plugs and Spark Plugs

While glow plugs and spark plugs both serve an ignition function in engines, there are several important ways in which they differ:

Ignition Timing

• Glow plugs preheat the engine continuously before and during startup. Their operation is not precisely timed.
• Spark plugs ignite the mixture only at the proper time during the compression stroke for each cylinder. Their timing is precise and optimized for performance.

Fuel Compatibility

• Glow plugs work only with diesel fuel, which auto-ignites under pressure and heat.
• Spark plugs work with gasoline, which requires the electric arc spark to ignite the compressed mixture.

Engine Compatibility

• Glow plugs are exclusively used in diesel engines. The compression ignition operation of diesel engines requires the assistance of glow plug pre-heating.
• Spark plugs are exclusively used in gasoline engines. Gasoline cannot be easily auto-ignited under pressure alone like diesel fuel.

Cost

• Glow plugs are generally cheaper than spark plugs. A basic glow plug can cost around $2-$10 each.
• Spark plugs are more complex in design than glow plugs. They can cost $4-$25 per plug.

Maintenance

• Glow plugs have a long service life if they are not damaged physically. Replacement intervals are around 100,000 miles.
• Spark plugs need more frequent replacement, as often as every 30,000 miles depending on engine and driving conditions. Their electrodes erode over time.

In summary, glow plugs provide simple and robust pre-heating for diesel compression ignition while spark plugs enable precisely timed ignition in gasoline engines. Each technology suits the specific fuel and ignition needs of diesel and gasoline engines respectively.

When to Use Glow Plugs vs Spark Plugs

The type of engine is the primary factor determining whether glow plugs or spark plugs should be used:

Diesel Engines

All diesel engines require the use of glow plugs to preheat the compressed air to initiate combustion during cold starts. The diesel fuel’s auto-ignition properties rely on having the cylinders hot enough prior to injection.

Without glow plugs, diesel engines would be very difficult or impossible to start in cold weather. Diesel engines do not have an ignition system that can produce an electric arc like gasoline engines do.

Gasoline Engines

All gasoline engines are designed for ignition using spark plugs. The spark plugs send timed electric sparks to ignite the mixture.

Gasoline cannot ignite simply from being compressed in the cylinder like diesel. The precision spark event is crucial for controlling the combustion process in a gasoline engine.

Other Factors to Consider

Aside from the engine type, other factors can help determine if glow plugs or spark plugs will be the best choice:

• Operating temperature – glow plugs provide greater assistance for cold startups. Spark plugs can ignite well even at lower temperatures.
• Required ignition timing – spark plug timing can be adjusted for maximum power and efficiency. Glow plugs provide heat but no timing control.
• Altitude – glow plugs work well at higher altitudes where lower air density makes ignition difficult. Spark plugs may require adjustment for higher altitude.
• Engine speed – glow plugs work consistently across engine speeds. Spark plugs can sometimes misfire if engine speed exceeds a certain rpm.
• Fuel composition – glow plugs suit diesel fuel with higher cetane ratings. Spark plugs can work with lower octane gasolines.

In most cases, the built-in engine design will dictate the selection between glow plug or spark plug ignition. But in specialized applications, these added factors can help choose the best technology for the operating conditions.

FAQs

Are glow plugs better than spark plugs?

Neither is inherently better, as they serve different engine types. Glow plugs are optimized to provide robust heating for diesel compression ignition. Spark plugs allow precise spark timing control optimal for gasoline engines. For their intended engines, both technologies work very well.

Can you use glow plugs in a gas engine?

No, glow plugs should only be used in diesel engines. They will not create the timed spark needed to ignite the gasoline-air mix in a gas engine. The engine will fail to start or run properly. Only use spark plugs designed for your specific gas engine.

Do glow plugs wear out?

Glow plugs can eventually wear out, but generally have a long service life. Failure is more likely from corrosion or physical damage rather than electrode wear. Sticking glow plugs are usually from contamination buildup. Proper maintenance and avoiding water ingress will provide the longest glow plug life.

How often should you change glow plugs?

Unless damaged or heavily contaminated, glow plugs usually do not need replacement for around 100,000 miles. Consult your owner’s manual, as some manufacturers recommend changing glow plugs more frequently, such as every 60,000 miles based on engine design and operating conditions.

What happens if glow plugs go bad?

If one or more glow plugs fail, it will make the engine much harder to start, especially in colder weather. The cylinders without working glow plugs will not preheat sufficiently for ignition. This can cause extended cranking times, misfires, rough running, and white smoke until the engine fully warms up. Replace any bad glow plugs promptly.

Conclusion

Glow plugs and spark plugs provide the crucial ignition functions that allow diesel and gasoline engines to operate. Despite their similar role, the two technologies work in very different ways tailored to the specific demands of compression or spark ignition.

Understanding the distinctions in how glow plugs and spark plugs function allows for proper maintenance and repair. Drivers and mechanics must choose glow plugs for diesel engines and spark plugs for gasoline engines for optimal performance and reliability.

While they have some differing benefits, both glow plugs and spark plugs are proven, durable ignition components when installed and maintained correctly in their intended engine application. Knowing their operational differences ensures engines meet starting, power, and efficiency needs.