TekniWiki
Glow plugs
Principles
The diesel engine
The combustion cycles of a diesel engine are similar to those of a petrol engine. However, no spark plugs are required for the ignition of the air-fuel mixture.
The reason: diesel engines are self-igniting. Ambient air is drawn into the cylinder by the downward movement of the piston and then highly compressed by the piston as it returns to top dead centre. The compression ratio in this process can be up to 25:1.
Due to this compression, the temperature of the ambient air rises rapidly and reaches a temperature between 700°C and 900°C. If fuel is now added, it will ignite spontaneously as the heated air will raise the temperature of injected fuel to its auto ignition point. The diesel engine is also known as a compression ignition engine.
Requirements
Pre-heating
For a reliable cold start, especially at low ambient temperatures, diesel engines require the assistance of glow plugs.
The reason: When attempting to start the engine all of the components will have cooled down significantly. As the intake air is compressed the cold cylinder walls, pistons and valves will extract some of the heat energy from the already cold ambient air. Therefore the temperature required for stable spontaneous combustion is no longer reached through compression of the air alone.
This is where the glow plug comes into play. It is installed in the cylinder head so that the heater tube projects into the combustion chamber. The plug heats up as soon as it is supplied with sufficient voltage and depending on the glow plug design, can reach over 1000°C. This raises the temperature in the combustion chamber sufficiently to enable stable spontaneous combustion. This results in good starting performance and the reduction of exhaust smoke and other unwanted emissions. This process before the actual starting of the engine is also called "pre-heating".
Construction
Metal and ceramic glow plugs
Glow plug technology can be divided into two major categories – metal sheathed types and ceramic types.
A heating coil within the protective metal sheath of a metal type plug produces the necessary heat. Ceramic glow plugs, on the other hand, do not require a metal sheath. Instead, their heating element is encased in a special type of ceramic – Silicon Nitride.
Ceramic glow plugs have the ability to heat up more quickly than metal types and in addition can attain higher operating temperatures for extended period of time. They are also more compact. These features are especially advantageous in modern engines.
Metal sheathed glow plug
360° View of a metal sheathed glow plug
The self-regulating metal (SRM) sheathed glow plug is amongst the most frequently used glow plug types.
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.
Ceramic 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.
Special requirements
Special requirements
Over the years, glow plugs have been continually developed in order to meet the demands of both drivers and environmental legislation. This has led to the pre-heating time of modern glow plugs being virtually undetectable; the starting process for the modern diesel is virtually the same as a petrol engine.
Where environmental protection is concerned, the demands placed upon the glow plug have similarly increased. In order to be able to meet current emissions standards (EURO5), glow plugs must now “post-glow" for the first few kilometres of driving until the engine has reached operating temperature.
This strategy ensures an efficiently-running engine and significantly lower levels of harmful emissions. The emission of blue and white smoke is reduced by up to 50 percent.
Another strategy being adopted by vehicle manufacturers is “intermittent-glow”. The glow plugs are energised under certain driving conditions, even when the engine is warm to assist in the diesel particle filter (DPF) regeneration stage.
Glow plug types from NGK SPARK PLUG
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.
Installation
The right torque
A torque wrench is recommended for the professional installation of a glow plug. Even for professionals the estimation of the tightening torque is virtually impossible.
This is because torque is calculated by the multiplication of two factors: the force which is applied and the distance at which that force is applied relative to the centre of rotation.
If a glow plug is insufficiently tightened gas leakage can result and as it is insecure the plug can experience high levels of vibration which can cause damage especially to the ceramic components.
The application of excessive torque can over stress the metal shell which may fail either upon installation or subsequent removal. Less obvious however is the possibility of distortion of the metal shell which reduces the annular gap between the metal tube and the casing. This can lead to over cooling of the regulating coil which allows excessive current to flow through the heating coil, leading to premature failure.
Torque values shown are typical for plugs with an M10 thread. Always refer to manufacturers' installation instruction.
Overview of torques
The tightening torque of a glow plug is dependant on the metal shell thread dimensions and where relevant, the connection terminal thread.
Always refer to vehicle manufacturer installation instructions.
Removing carbon deposits with a reamer
Depending on the design, some diesel engines have a tendency towards carbon build-up around the heater tube. In this case whenever glow plugs are replaced any carbon deposits should be removed from the glow plug bore with a special tool. Failure to remove these deposits can lead to overcooling of the regulating coil which allows too much current to flow to the heating coil and subsequent premature failure.
The use of the correct Hazet reamers - which were developed in partnership with NGK SPARK PLUG - is strongly recommended:
- Identify the suitable reamers for the specific glow plugs part number
- Apply grease to the cutting faces of the reamer
- Screw the reamer into the cylinder head by hand (do not use wrench)
- Unscrew the reamer and clean thoroughly
Video
Installation of a glow plug
Diagnosis
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.
