Glow plugs

Heating coil

The heating coil of a self-regulating metal sheathed glow plug is made of a special metal alloy. As soon as a voltage is applied to the glow plug a concentrated area of heat is generated around the end of the metal sheath thereby heating up its surroundings.

By using different configurations of wire diameter and/or length, the heating performance can be adjusted, thus influencing how quickly the plug glows and the temperatures it can achieve.

Control coil

The control or regulating coil is welded in series with the current-carrying centre electrode and the heating coil. As its temperature increases, the electrical resistance of the control coil also increases, thus current flow to the heating coil is reduced. This mechanism provides a means of control dependant on the temperature of the glow plug.

Metal shell

The metal shell of a glow plug represents the electrically negative pole (ground connection).

Insulation powder

The coils inside the glow tube are tightly packed in a special powder: magnesium oxide. Magnesium oxide is both an electrical insulator and a very good heat conductor. The insulation filling fulfils two important functions: It protects the coils from mechanical impact and the severe vibrations created by the combustion process. It also provides an optimal transmission of the produced heat to the sheath and thus the surroundings.

Centre electrode

The battery voltage is applied to the coils through the solid centre electrode.

Connection terminal

The battery voltage is applied at the connection terminal. This may be a threaded post to suit a connector which is secured by a nut or an unthreaded post to suit a push on connector.

Heater tube

The heater tube of a metal sheathed glow plug is made of a heat-resistant alloy such as stainless steel or Inconel. In combination with the packing powder, it ensures that the coils inside are not directly exposed to the combustion gasses or the shockwaves created by the rapidly expanding gasses in the combustion process.

Thread

The thread of a high-quality glow plug is always rolled and never cut. By this production method fast, accurate threads are formed eliminating the possibility of damage to the glow plug bore in the cylinder head.

Insulator

The insulator separates the electrically positive (Connection terminal) from the electrically negative part (metal shell) of the glow plug.

Heating coil

Contrary to a metal glow plug a ceramic glow plug uses a ceramic heating element.

Thread

The thread of a high-quality glow plug is always rolled and never cut. By this production method fast, accurate threads are formed eliminating the possibility of damage to the glow plug bore in the cylinder head.

Insulator

The insulator separates the electrically positive (Connection terminal) from the electrically negative part (metal shell) of the glow plug.

Ceramic casing

The heating coil or heating element of a ceramic glow plug is encased in a high performance ceramic material: silicon nitride. It protects the coil from the high temperatures and vibrations created by the combustion process. It is also an excellent heat conductor allowing the heat energy of the coil to be rapidly released into the combustion chamber.

Metal shell

The metal shell of a glow plug represents the electrically negative pole (ground connection).

Taper seat

The taper seat provides simple but effective gas tight sealing of the combustion chamber without the need for sealing gaskets, etc. Its compact form also allows bore sizes to be kept to a minimum.

The taper faces also provide an excellent electrical ground (earth).

Centre electrode

The battery voltage is applied to the coils through the solid centre electrode.

Connection terminal

The battery voltage is applied at the connection terminal. This may be a threaded post to suit a connector which is secured by a nut or an unthreaded post to suit a push on connector.

Contacting ring

The contacting ring provides the electrical connection at the junction of the centre electrode and the heating element.

Glow plug types from NGK

Improvements in the quality of combustion are met by various glow plug types in a variety of ways. Here you see an overview.

Standard sheathed glow plugs

Standard metal sheathed glow plugs contain only a single heating coil.

The electrical resistance of this coil does not change significantly as the temperature increases therefore the current consumption of this type is constant. Control of this glow plug is usually effected by the application of voltage via an external timer unit or by manual control.

These glow plugs reach an operating temperature of 800°C within 20 - 25 seconds.

Fast heating sheathed glow plugs

Fast heating glow plugs are also equipped with only one heating coil. However, the electrical resistance of the heating coil changes relative to its temperature. Resistance is low when voltage is initially applied and a large current can flow through the heating coil thereby raising its temperature more rapidly than the standard type. This reduces the required pre-heating time. As the temperature rises the electrical resistance increases reducing current flow.

These glow plugs reach an operating temperature of 800°C within 13 - 17 seconds.

QGS metal sheathed glow plugs

QGS stands for Quick Glow System. There are two non-interchangeable types of QGS glow plugs, single and double coil types. The single coil type has a heating coil which is designed to allow an extremely high current flow. The double coil types have a heating coil and a control coil which increases its resistance as the temperature rises. This regulates the current flow to a certain degree and offers some protection against overheating for the heating coil. Both of these types can only be used in conjunction with specifically designed external QGS control units which strictly control the glow plug operation.

For all metal QGS types the following applies: they heat up very quickly and reach temperatures around 900°C within 6 - 10 seconds.

Self-regulating metal sheathed glow plugs

These glow plugs are especially well-suited for engine concepts which require post-glow operation. Self-regulating metal glow plugs (also known as 'SRM glow plugs') contain a heating coil and a control coil which increases the resistance as the temperature increases. Due to the special electrical properties of these coils the SRM glow plug can provide a very fast warm up time and control the final temperature autonomously. Little external control of this pre-heating system is required whilst giving good pre and post heating performance and long service life.

SRM glow plugs typically reach 900°C in 4 seconds.

AQGS sheathed glow plugs

AQGS stands for 'Advanced Quick Glow System'. This major development of the QGS type also has a heating coil and a control coil. The heater tube diameter of the AQGS glow plug at 3.5mm is significantly smaller than that of a conventional metal glow plug which in conjunction with the special coil material technology provides extremely rapid heating. This ensures very stable combustion conditions for the diesel/air mixture. Emissions during the warm-up phase are drastically reduced well before the optimal operating temperature of the engine is reached and long post glow function is also available. Strict pulse width modulation (PWM) control by an electronic control unit (ECU) is essential.<br>AQGS sheathed glow plugs can reach 1,000°C in only 2 seconds.

Self-regulating ceramic glow plugs

The self-regulating ceramic glow plug (also know as an 'SRC glow plug“) has a heating coil and a resistance-regulating coil similar to the SRM metal sheathed type. These coils provide very rapid heating and a high degree of the control of current flow which regulates the final temperature. Due to this level of autonomous regulation this glow plug requires little external control.

SRC glow plugs typically heats up to over 1100°C in 3 seconds and then independently regulates the temperature just under1000°C.

New high-temperature ceramic glow plug

Offering the best in terms of thermal performance and durability, the new high-temperature ceramic (NHTC) glow plug has an electrically conductive ceramic heating element. It reaches an operating temperature of 1000°C in 2 seconds and is capable of post-glow operation at 1350°C for periods in excess of 10 minutes. They offer extremely fast pre-heating times, effectively eliminating any need for a pre-glow period even at very low ambient temperatures. Reduced engine cranking times mean less HC and CO emission even before the engine starts. The higher temperatures achieved in the post glow period result in more stable combustion conditions, better quality of idling and reduced emissions. The “intermittent-glow” operation whereby the glow plugs are energised under certain driving conditions, even when the engine is warm,  assist in the diesel particle filter (DPF) regeneration stage. These features allow modern diesel vehicles to adhere to the especially high specifications of current emissions regulations.

NHTC glow plugs reach 1000°C in 2 seconds and a regulated maximum temperature of 1350°C.

Diagnosis

The appearance of a glow plug can provide clues as to the possible causes of the damage. Here you can see the most common defects and learn what they tell you.

Overheating damage

Breakage as a result of overheating indicates that the spray pattern or volume of the injected fuel is not correct or injection timing is not correct. Worn piston rings and valve guides can lead to excessive quantities of lubricating oil being present in the combustion chamber creating abnormal overheating conditions leading to heater tube damage.

Overvoltage damage/Broken heating coil

Overvoltage damage arises when the wrong glow plug (e.g. 12V instead of 24V) was installed. It is also possible that the control unit is faulty allowing too high a voltage to be applied or for too long a time period. The control unit and alternator should be inspected.

Broken shell or connection terminal

Damage to the metal shell or terminal can occur with the use of the wrong tightening torque or careless use of tools.

Broken shell or connection terminal

Damage to the metal shell or terminal can occur with the use of the wrong tightening torque or careless use of tools.

Broken heater tube

Usually attributable to abnormal combustion conditions caused by incorrect injection timing, the wrong spray pattern or a poorly set flow rate  which results in excessive turbulence, shock waves and abnormal heat characteristics.

Overheating damage/break

Breakage as a result of overheating indicates that the spray pattern or volume of the injected fuel is not correct or injection timing is not correct. Worn piston rings and valve guides can lead to excessive quantities of lubricating oil being present in the combustion chamber creating abnormal overheating conditions leading to heater tube damage.

Overvoltage damage

A swollen or melted heater tube is often the result of using the wrong glow plug (e.g. 12 volts instead of 24 volts). It is also possible that the control unit is faulty allowing too high a voltage to be applied or for too long a time period. The control unit and alternator should be inspected.