Spark plug: Functions, construction, working principle and types

Kunle Shonaike

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A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed aerosol gasoline by means of an electric spark. Spark plugs have an insulated center electrode connected by a heavily insulated wire to an ignition coil or magneto circuit on the outside, forming, with a grounded terminal on the base of the plug, a spark gap inside the cylinder.

The spark plug has two primary functions:

(1) To ignite the air/fuel mixture.

Electrical energy is transmitted through the spark plug, jumping the gap in the plugs firing end if the voltage supplied to the plug is high enough. This electrical spark ignites the gasoline/air mixture in the combustion chamber.

(2) To remove heat from the combustion chamber.

Spark plugs cannot create heat, they can only remove heat. The temperature of the end of the plug\'s firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. The spark plug works as a heat exchanger by pulling unwanted thermal energy from the combustion chamber and transferring heat to the engines cooling system. The heat range of a spark plug is defined as its ability to dissipate heat from the tip.

Construction

(1) Ribs: Insulator ribs provide added protection against secondary voltage or spark flashover and also help to improve the grip of the rubber spark plug boot against the plug body.

The insulator body is molded from aluminum oxide ceramic. In order to manufacture this part of the spark plug, a high-pressure, dry molding system is utilised. After the insulator is molded, it is kiln-fired to a temperature that exceeds the melting point of steel. This process results in a component that features exceptional dielectric strength, high thermal conductivity and excellent resistance to shock.

(2) Insulator:

The insulator body is molded from aluminum oxide ceramic. In order to manufacture this part of the spark plug, a high-pressure, dry molding system is utilised. After the insulator is molded, it is kiln-fired to a temperature that exceeds the melting point of steel. This process results in a component that features exceptional dielectric strength, high thermal conductivity and excellent resistance to shock.

The pointer shows the spark plug insulator. As mentioned above, it is formed from aluminum oxide ceramic. The outer surface is ribbed to provide grip for the spark plug boot and to simultaneously add protection from spark flashover (crossfire).

(3) Hex:

The hexagon provides the contact point for a socket wrench. The hex size is basically uniform in the industry and is generally related to the spark plug thread size.

(4). Shell:

The steel shell is fabricated to exact tolerances using a special cold extrusion process. Certain types of spark plugs make use of a steel billet (bar stock) for shell construction.

(5) Plating:

The shell is almost always plated. This enhances durability and provides for rust and corrosion resistance. The steel shell is fabricated to exact tolerances using a special cold extrusion process or in other specialised cases, machined from steel billet. The hexagon machined onto the shell allows you to use a socket wrench to install or remove the plug.

(6) Gasket:

Certain spark plugs use gaskets while other examples are "gasketless." The gasket used on spark plugs is a folded steel design that provides a smooth surface for sealing purposes. Gasketless spark plugs use a tapered seat shell that seals via a close tolerance incorporated into the spark plug.

(7) Threads:

Spark plug threads are normally rolled not cut. This meets the specifications set forward by the SAE along with the International Standards Association.

(8) Ground electrode:

There are a number of different ground electrode shapes and configurations, but for the most part, they are manufactured from nickel alloy steel. The ground electrode must be resistant to both spark erosion and chemical erosion, both under massive temperature extremes.

(9) Centre electrode:

Centre electrodes must be manufactured from a special alloy that is resistant to both spark erosion and chemical corrosion. Keep in mind that combustion chamber temperatures vary (and sometimes radically). The centre electrode must live under these parameters.

(10). Spark park electrode gap:

The area between the ground electrode and the centre electrode is called the gap. Centre electrodes must be manufactured from a special alloy that is resistant to both spark erosion and chemical corrosion.

(11) Insulator nose:

There are a large number of insulator nose shapes and sizes available, but in essence, the insulator nose must be capable of shedding carbon, oil and fuel deposits at low speeds. At higher engine speeds, the insulator nose is generally cooled so that temperatures and electrode corrosion are reduced.

Working principle

The spark plug is connected to a high voltage source like the magneto or the ignition coil at one end. The other end with the two electrodes is immersed into the combustion chamber. When current passes through the terminal and into the main centre electrode, a potential difference (voltage drop) is created between two electrodes. The gas mixture that occupies the gap between them acts as an insulator and thus the electricity doesn't flow beyond the tip of the centre electrode.

But as the voltage increases, the gases in the gap begin to get energised. Once the voltage increases to the point that crosses the dielectric strength (resistance to conduct electricity) of the gases, they become ionised. Once the gases get ionised, they begin to act as conductors and permit the current to travel through the insulating gap. When the dielectric strength is crossed, the electrons begin to surge through that gap. This sudden movement of electrons rapidly increases the heat in that region due to which they begin to expand rapidly causing a mini explosion which results in the formation of a spark.

Types of spark plug

Spark plugs can be put into two different primary classifications, based on their operating temperatures and construction.

Based on operating temperatures

Once the combustion process is completed in the combustion cycle, the heat generated needs to dissipate. The heat escapes through the exhaust gases, the cylinder wall of the engine and the spark plug surface. Based on the operating temperature and level of heat dissipation, spark plugs can be classified into two types:

(1) Hot spark plug:

A hot spark plug operates in a higher temperature range. It has a lesser ceramic area used to insulate the heat. A hot spark plug dissipates lesser combustion heat and allows the tip and electrode to stay hotter. This ensures that any deposit accumulation is burned off and isn't allowed to stay for long.

(2) Cold spark plug:

For high-performance engines that run hot by default, using a hot spark plug will cause pre-ignition. In extreme cases, it can also lead to the tip melting off. In such cases, a cold spark plug is used. Here the ceramic insulation area is higher and this it will dissipate more heat. But on the flipside, it is prone to greater deposit accumulation. Be sure to follow your instruction manual and use the correct type of plug recommended for your engine for optimum performance.

Based on material used

Spark plugs are further classified based on the material used on the ends of the electrodes.

They are of four types:

(i)Copper- nickel type:

These are the most basic types of spark plugs. Here the centre electrode is made of a copper-nickel alloy as copper on its own is weak and will melt off due to engine heat. Nickel is added to strengthen the plug but even then these are the weakest types available in the market. They are also required to be made with a larger diameter and hence require more voltage for operation.

(ii) Single platinum type:

These plugs have a small platinum disc on the tip of the centre electrode. This platinum tip is exponentially stronger than a copper-nickel coating making this type of plug to last long as well. They are also less prone to debris build up.

(iii) Double platinum type:

These plugs have platinum tips on both the centre electrode and the side electrode. They spark up twice in the combustion cycle, once before the combustion and once during the exhaust stroke. The second spark is wasted and so this spark plug can only be used if your vehicle is equipped with a waste spark ignition type distributor.

(iv)Iridium type:

These are the best spark plugs available in the market. Here the tip of the centre electrode is made of Iridium which is the strongest out of nickel, copper, and platinum. Hence, they are the least prone to deposits and damage. They also have a small sized electrode which requires less voltage for operation as well. Iridium plugs are much more expensive than the other types but then again you pay for what you get.

This is culled from the Internet.

Feedback

I bought a Toyota Venza 2011 model – four cylinder. There's no check light but a technician helped to fix a delayed oil supply to the engine whenever the car is started in the morning or when it has been turned off for at least an hour. The problem was resolved but later the check light came up and the scan result is P0012 (Camshaft position timing over retarded – Bank 1). Please what's problem. Thanks. Michael

Below is the interpretation of the code,

P0012 Toyota camshaft position ‘A’ timing over-retarded bank 1

Meaning

The engine control module controls the oil control valve to regulate the intake camshaft angle. As a result of the angle change, the engine timing is advanced or retarded. Optimising engine timing will help the engine improve torque and fuel economy, and the exhaust emissions decrease under overall driving conditions. The variable valve timing system includes the oil control valve and VVT controller. The ECM detects the actual intake valve timing using signals from the camshaft and crankshaft position sensors, and performs feedback control.

When is the code detected?

After the ECM sends the target duty cycle signal to the OCV, the ECM monitors the OCV current to establish an actual duty cycle. The ECM determines the existence of a malfunction and sets the DTC when the actual duty cycle ratio varies from the target duty cycle ratio.

Possible symptoms

Engine Light ON (or service engine soon warning light)

Possible lack/loss of power

Possible engine rough idle

Possible causes

Damaged ECM

I have asked this question before. Kindly advise me on the type of gear oil to use on my Toyota Previa 1990 model. Will EP 140 be okay for manual engine? Thanks, Henry.

If it is a manual transmission it should be 80W-90

Generic codes

P0683 Glow plug module control To ECM communication circuit fault

Meaning

The ECM monitors the glow plug control module. The ECM sets the OBDII code when it cannot communicate with the glow plug control module.

When is the code detected?

The ECM has detected a communication fault with the glow plug control module circuit

Possible symptoms

Possible causes

Faulty engine control module

P0684: Glow plug control module to ECM communication circuit range/performance

Meaning

The ECM monitors the GPCM. The ECM sets the OBDII code when it cannot communicate with the GPCM.

Possible symptoms

Engine light on (or service engine soon warning light)

P0685: Engine control module power relay control circuit open

Meaning

The powertrain relay is a normally open relay. The relay armature is held in the open position by spring tension. Battery positive voltage is supplied directly to the relay coil and the armature contact at all times. The ECM supplies the ground path to the relay coil control circuit via an internal integrated circuit called an output driver module. The ODM output control is configured to operate as a low side driver for the powertrain relay. The ODM for the powertrain relay also incorporates a fault detection circuit, which is continuously monitored by the ECM. When the ECM commands the powertrain relay ON, ignition 1 voltage is supplied to the ECM, and to several additional circuits.

Tech notes

Check all the engine control module fuses, if the fuses are okay, then replacing the ECM relay should take care of the problem.

Possible causes

ECM power relay circuit poor electrical connection

Blown ECM fuse

Faulty ECM

P0686: Engine control module power relay control circuit low

Meaning

The powertrain relay is a normally open relay. The relay armature is held in the open position by spring tension. Battery positive voltage is supplied directly to the relay coil and the armature contact at all times. The ECM supplies the ground path to the relay coil control circuit via an internal integrated circuit called an ODM. The ODM output control is configured to operate as a low side driver for the powertrain relay. The ODM for the powertrain relay also incorporates a fault detection circuit, which is continuously monitored by the ECM. When the ECM commands the powertrain relay ON, ignition 1 voltage is supplied to the ECM, and to several additional circuits.

Possible causes

P0687: ECM power relay control circuit high

Meaning

The powertrain relay is a normally open relay. The relay armature is held in the open position by spring tension. Battery positive voltage is supplied directly to the relay coil and the armature contact at all times. The ECM supplies the ground path to the relay coil control circuit via an internal integrated circuit called an ODM. The ODM output control is configured to operate as a low side driver for the powertrain relay. The ODM for the powertrain relay also incorporates a fault detection circuit, which is continuously monitored by the ECM. When the ECM commands the powertrain relay ON, ignition 1 voltage is supplied to the ECM, and to several additional circuits.

Possible causes

P0688: ECM power relay sense circuit/open

Meaning

The powertrain relay is a normally open relay. The relay armature is held in the open position by spring tension. Battery positive voltage is supplied directly to the relay coil and the armature contact at all times. The ECM supplies the ground path to the relay coil control circuit via an internal integrated circuit called an ODM. The ODM output control is configured to operate as a low side driver for the powertrain relay. The ODM for the powertrain relay also incorporates a fault detection circuit, which is continuously monitored by the ECM. When the ECM commands the powertrain relay on, ignition 1 voltage is supplied to the ECM, and to several additional circuits.

Possible causes

P0689: ECM power relay sense circuit low

Meaning

The ECM monitors the power inputs. The ECM sets the OBDII code when the ECM remains powered up after the ignition switch has been turned OFF.

Possible causes

P0690: ECM power relay sense circuit high

Meaning

The ECM monitors the power inputs. The ECM sets the OBDII code when the ECM remains powered up after the ignition switch has been turned off.

Possible causes

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All rights reserved. This material, and other digital content on this website, may not be reproduced, published, broadcast, rewritten or redistributed in whole or in part without prior express written permission from PUNCH.

Contact: [email protected]