How Do Glow Plugs Work?

Last Updated on December 3, 2023 by Jaxon Mike

Glow plugs are small heating devices found in diesel engines that help aid cold starting. They preheat the air in the combustion chamber to ensure the air-fuel mixture ignites properly when cold. Without glow plugs, diesel engines would be very difficult to start in cold weather.

The glow plug has a heating element that can heat up to over 1000°F when energized. This heat is transferred into the combustion chamber, warming the air to improve ignition. They allow diesel engines to start quickly even in very cold ambient temperatures.

Glow plugs are controlled by the engine’s computer, which energizes them for a predetermined time when starting the engine. The warm air created by the glow plugs allows the engine to turn over and start smoothly. Once the engine warms up, the glow plugs are turned off until needed again at the next cold start.

How Glow Plugs Work?

Glow plugs consist of a metal housing that threads into the cylinder head. Inside is a heating element, typically made from a nickel-chromium alloy. The heating element has a tip that protrudes into the combustion chamber and gets red hot when energized.

Construction and Components

• Housing – Made of steel and threads into the cylinder head. Provides mounting and heat transfer.
• Heating Element – Made of Nichrome alloy. Heats up when current flows through it.
• Tip – Protrudes into the combustion chamber. Made of metallic ceramic. Transfers heat into the chamber.
• Insulator – Electrically isolates heating element from housing. Usually magnesium oxide powder.
• Lead Wire – Provides electrical connection to the power source.

The housing threads into the top of the cylinder head with the tip exposed in the chamber. The insulator prevents the heating element from shorting to the housing. The lead wire connects the heating element to the vehicle’s electrical system.

Heating Element and Tip Design

The heating element is designed to quickly reach over 1000°F. It uses a coiled wire design and Nichrome alloy to maximize heat output and minimize electrical resistance.

The tip is made from a metallic ceramic material able to withstand high temperatures. Its shape promotes heat transfer into the air. The design varies between surface ignition and sheathed types.

Electrical Operation

Glow plugs receive current from the vehicle’s battery when energized. A glow plug relay controls the battery voltage to the plugs. The engine computer determines when to activate the relay based on coolant temperature, engine speed, etc.

At startup in cold weather, the computer energizes the glow plug relay, sending current through the heating elements. The glow plugs heat up and warm the combustion chambers. After 10-60 seconds, the computer deactivates the relay, cutting off the current.

Stages of Operation

Glow plugs function in two stages:

• Pre-glow – Plugs activated before cranking to preheat chamber. Lasts 10-60 seconds depending on temperature.
• Post-glow – Plugs stay on for 1-5 seconds after starting. Further aids ignition.

The pre-glow warms the air before starting. Post-glow helps ensure ignition as the engine 

cranks. The colder it is, the longer the pre-glow time needed.

Function During Cold Start

When cold starts, the glow plugs have two main functions:

• Warming the combustion chamber – Heating the air improves ignition by making the fuel/air mix more combustible. Prevents misfires.
• Aiding fuel ignition – The hot tip acts as an ignition source to help ignite the fuel. Provides extra assurance of ignition.

The heated air and hot surface cause the fuel to ignite as soon as the engine starts cranking. This allows quick, reliable cold starting that would otherwise be very difficult with diesel’s high compression and poor cold fuel vaporization.

So in summary, the glow plugs heat the air in the cylinders, making ignition much easier when cold. The engine computer controls their activation based on ambient temperature and engine condition. They preheat before cranking and stay on briefly after to aid cold starting.

Types of Glow Plugs

There are a few different designs of glow plugs used in diesel engines:

Fast Light-Off Glow Plugs

These have a thin, exposed heating element that heats up very quickly. The thin wire and maximized surface area lead to fast light-off times under 10 seconds. The exposed element requires a protective cap when not in use.

Surface Igniter Glow Plugs

Surface igniter glow plugs use a thicker heating element with a round, flattened tip protruding into the chamber. The large surface area of the tip promotes heat transfer. These take slightly longer to heat up than fast light-off plugs.

Sheathed Element Glow Plugs

Sheathed element glow plugs embed the heating element in metal prongs for protection. The prongs transfer heat from the element to the tip. This protected design is more durable but slower to light off.

Coiled Element Glow Plugs

These use a coil-shaped heating element to maximize resistance and heat output. They have good heating power but slower light off than surface igniter plugs. The embedded coil design provides protection.

Glow Plug Controls

The vehicle’s engine computer controls when the glow plugs are activated using several key inputs and strategies.

Control Module

The engine control module (ECM) dictates glow plug operation based on:

• Coolant temperature sensor – Measures engine temp
• Ambient air temperature sensor – Measures outside air temp
• Engine speed sensor – Monitors rpm
• Timer – Tracks pre-glow duration

Using these inputs, it energizes the glow plug relay accordingly to activate the glow plugs.

Pre-Glow Time

The ECM determines the required pre-glow time based on coolant and air temp. The colder it is, the longer the plugs need to heat the chambers before cranking.

Typical pre-glow times by temperature:

• Above 50°F – No pre-glow needed
• 40-50°F – 3-5 seconds
• 30-40°F – 5-10 seconds
• 20-30°F – 10-20 seconds
• 10-20°F – 20-30 seconds
• 0-10°F – 30-60 seconds

The ECM uses a timer to control the pre-glow duration needed for the temperature.

Post-Glow Time

Post-glow keeps the plugs energized after starting for a brief period. This aids ignition and smooths out initial combustion.

Post-glow times are much shorter than pre-glow – usually 1-5 seconds. Colder temperatures will utilize longer post-glow.

Wait-to-Start Time

Some engines use a wait-to-start light on the dash. This indicates when the optimal pre-glow heating has been completed before cranking.

The light illuminates after energizing the glow plugs. Once the programmed wait time is reached, the light goes out, signaling cranking can begin.

Altitude Compensation

At higher altitudes, lower air density reduces combustion efficiency. To compensate, the ECM increases pre-glow time at altitude to provide additional heating.

Voltage Management

When energized, glow plugs cause a voltage drop which can disrupt ECM operation. Strategies to manage voltage drop include:

• Staged plug activation – halves power draw
• Delayed activation – staggers turn-on times
• Voltage boosting – regulates system voltage

Proper voltage control allows the substantial power draw of the glow plugs during pre-glow.

Glow Plug Maintenance

Like any engine component, glow plugs require periodic inspection and maintenance to ensure proper operation.

Checking for Proper Operation

Glow plugs should be checked before winter to confirm functionality:

• Visual inspection – Check for corrosion, damage
• Continuity test – Use a multimeter to check the element circuit
• Activation test – Energize plugs and check that the tip heats up

Catching any issues at the start of winter prevents cold start problems down the road.

Replacement Interval

Most manufacturers recommend replacing glow plugs at 60,000-100,000 mile intervals. The heating element can wear over time, reducing heating performance.

Plug life can be shortened by frequent cold starts, short trip driving, and excessive idling. Check plugs earlier if cold start issues arise.

Installation Tips

When installing new glow plugs:

• Apply anti-seize to threads – Prevents seizing in cylinder head
• Tighten to spec – Overtightening can damage housing
• Ensure lead wire clearance – Prevent chafing and electrical issues
• Reprogram control module – Initializes new plug characteristics

Proper installation ensures maximum plug life and performance.

Failure Modes

Some common reasons for glow plug failure include:

• Tip deposits – Carbon buildup insulates the tip and reduces heat transfer
• Seizing – Rust and corrosion cause plugs to seize in the cylinder head
• Wire damage – Chafing, voltage spikes, corrosion compromises wiring
• Element deterioration – Metal fatigue, and erosion degrade heating element
• Cracking – Thermal cycles and vibration cracks ceramic tip over time.

Catching minor issues early prevents more significant failures down the road.

Diesel Cold Starting Aids

In addition to glow plugs, some other common methods used to aid diesel cold starting include:

Block Heaters

Block heaters warm the engine coolant when plugged into an electrical outlet. This allows starting the already-warm engine at ambient temperatures down to -20°F.

Battery Heaters

Battery heaters warm the battery to maximize cranking power in extreme cold. This improves cold cranking amps when batteries are sluggish.

Oil Pan Heaters

Oil pan heaters heat the oil before starting to promote quicker oil flow and circulation when first started. This reduces engine wear.

Intake Air Heaters

These heat the air going into the engine to improve cold air density and combustibility. Some systems use exhaust heat while others are electrically powered.

Fuel Heaters

Fuel heaters use electric elements to warm the diesel fuel going to the injectors. This aids cold fuel atomization and vaporization.

So in summary, glow plugs are the primary cold start aid, while supplemental systems can provide additional heating of other engine components and fluids.

Frequently Asked Questions

What are some trouble signs of failing glow plugs?

Difficulty starting, longer cranking times, rough cold idle, and increased white smoke on startup indicate glow plugs may be failing and unable to properly preheat the combustion chambers.

How can you test the glow plug operation?

Use a multimeter to check for continuity through the heating element. Also, check for proper voltage at the glow plug connector during pre-glow mode.

What happens if a glow plug seizes in the cylinder head?

A seized glow plug can lead to the tip breaking off in the combustion chamber when trying to remove it. This requires the removal of the cylinder head to extract the broken tip.

Should you pre-glow before every startup?

Pre-glow is only needed when the coolant temperature is below 50°F. Once warmed up, the engine should start without pre-glow. Excessive pre-glowing can shorten the glow plug life.

Why do diesel engines have more difficulty starting when cold?

The low volatility and poor vaporization of cold diesel fuel makes it harder to properly mix and ignite when cold. Glow plugs help offset these cold starting challenges.

Conclusion

Glow plugs provide an invaluable function in diesel engines by preheating the combustion chambers for easy cold starting. Their simple yet highly engineered design allows reliable operation in even the coldest weather.

Given proper maintenance and operation within design limits, glow plugs will provide many years of cold weather starting assistance over the life of a diesel vehicle. Advanced designs continue to improve their efficiency and expedite engine warmup.

So the next time you turn the key on a cold winter morning, you’ll know it’s the quick-heating glow plugs that allow your diesel engine to fire up seamlessly and get you on your way.