Bujias

Precalentadores La comprobación y sustitución de las bujías de incandescencia (calentadores) en los motores Diesel (también llamadas «bujías de precalentamiento») es una operación muy sencilla que podemos realizar nosotros mismos, la única dificultad que podemos encontrar es la que supone la ubicación de las bujías en el motor, que en ocasiones se encuentran en lugares de difícil acceso.

El procedimiento a seguir para la comprobación y sustitución de las bujías de incandescencia es el siguiente:

Desconectar el borne negativo de la batería por seguridad para evitar cortocircuitos.
Desconectar los cables que van a cada uno de los calentadores, para ello aflojar el tornillo pequeño que sujeta el terminal del cable al calentador.
Podemos comprobar que los calentadores funcionan correctamente sin desenroscarlos de la culata, para ello utilizaremos un multimetro.
Una vez quitado los cables limpiar bien alrededor de los calentadores (donde van roscados a la culata) para que no penetre suciedad dentro del motor una vez quitado el calentador. Después echar un poco de aflojatodo (aceite) para que penetre un poco en la rosca y sea mas fácil desenroscar el calentador.
Ahora viene la parte mas delicada, para ello tienes que usar una llave fija, acodada o una llave para bujías que se ajuste a la tuerca labrada en el calentador, desenroscar el calentador como si fuera un tornillo es decir aflojando hacia la izquierda.
Cada vez que quitas un calentador inmediatamente colocas el nuevo para que no entre suciedad dentro del motor.
Por ultimo vuelves a colocar todos los terminales eléctricos (cableado) en los calentadores.

Las bujías de incandescencia o calentadores pueden ir conectados eléctricamente en «serie» o en «paralelo», aunque actualmente y desde hace años se usa mas la conexión paralelo de forma que una bujía averiada no afecta al funcionamiento de las demas.

Método para comprobar que los calentadores funcionan correctamente sin desmontarlos de la culata
Nos puede ocurrir que el vehículo presente dificultades a la hora de arrancarlo (ponerlo en marcha) esto puede ser debido a que «uno» de los calentadores este mal, en este caso el vehículo arrancara con dificultades. Si son «dos» los calentadores que están mal, entonces será casi imposible arrancar el vehículo, dependerá principalmente de la temperatura ambiente. Para comprobar si tenemos un calentador mal, sin desmontarlo de la culata procederemos de la siguiente manera:

Desconectamos el contacto con la llave de arranque del vehículo.
Desconectamos los terminales eléctricos de cada una de las bujías de incandescencia que queremos comprobar.
Con el multimetro seleccionado en la escala de ohmios, colocamos el terminal positivo (cable rojo) en la bujía de incandescencia en su conexión eléctrica. El terminal negativo (cable negro) lo colocamos tocando la culata (sobre metal para hacer masa). El multimetro nos tiene que dar como resultado un valor muy bajo de 1 a 3 ohmios, depende del fabricante (por ejemplo: las bujías de incandescencia que lleva el Renault Megane 1.9D de la marca Bosch, tienen una resistencia interna de 1,3 ohmios). Con este valor de resistencia sabemos que esta se encuentra en buenas condiciones. Una bujía de incandescencia en mal estado nos dará un valor de resistencia muy alto, esto quiere decir que la bobina interna de la bujía de incandescencia esta cortada.}

fuente:aficionados a la mecánica

SPARK PLUG: FUNCTIONS, CONSTRUCTION, WORKING PRINCIPLE AND TYPES

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 which is 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 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 utilized. 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 utilized. 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 specialized 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. Center electrode:
Center 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 center electrode must live under these parameters.

10. Spark park electrode gap: 
The area between the ground electrode and the center electrode is called the gap. Center 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 center 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 center electrode.

But as the voltage increases, the gases in the gap begin to get energized. Once the voltage increases to the point that crosses the dielectric strength (resistance to conduct electricity) of the gases, they become ionized. Once the gases get ionized, 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

Spark Plugs can be put into two different primary classifications, based on their operating temperatures and based on their 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 which is 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 4 types:

1. Copper- Nickel Type: 
These are the most basic types of spark plugs. Here the center electrode is made of a copper-nickel alloy as copper on its own is very 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.

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

3. Double Platinum Type: 
These plugs have platinum tips on both the center 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.

4. Iridium Type: 
These are the best spark plugs available in the market. Here the tip of the center 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.