Spark Plug

                 The spark plug ignites the fuel-air mixture in the engine cylinder.We have learned that there is no current flow in an open circuit, in most cases this is true. However, if the opening in the circuit is small and a high voltage is present, the high voltage will force the current to jump the small gap, thus completing the circuit. This is the basic principle of a spark plug.

Operation:
                 The spark plug has two conductors called electrodes. One is connected to the high voltage power source
by a high-tension cable and the other is grounded to the engine. Electrodes are separated by a small opening called the gap.

*The high voltage surge from the coil flows thorough the cable to the center spark plug electrode and down the electrode. Then the current jumps the gap to the other electrode and returns to ground
*When current jumps the gap, a spark is created to ignite the fuel-air mixture in the engine’s cylinder.
*Although the spark plug has no moving parts, each part is important.

Outer Sheel:
               Each spark plug has a steel shell. The top of the shell is hexagon shaped so a wrench can fit. The lower part of the shell is threaded so it can screw into the cylinder head. The grounded electrode extends out from the lower threaded part of the shell. The spark plug may have a gasket to help seal it to the engine cylinder head or the seat may be tapered to seal the spark plug. The distance from the flange to the end of the spark plug threads is called the reach. The reach of a spark plug is very important. A spark plug with too long a reach will extend too far into the combustion chamber. The spark plug will run hotter, and may be hit by a piston. A spark plug with too short a reach will run cold and cause misfiring due to fouled electrodes. More on this after.The threaded diameter may vary according to the size of the spark plug hole in the cylinder head.The engine technical manual will give you the exact spark plug specifications for the engine.

Spark Plug Insulator:
          The insulator is mounted in the outer shell. The insulator is usually made of white ceramic or porcelain.
The insulator is held in position and shielded from the outer shell by a gasket and sealing compound. The insulator holds the center electrode and insulates it so current will only flow through the electrode. It must also withstand extreme temperature changes and vibration.

Spark Plug Electrodes:
      The electrodes are made of a metal alloy designed to withstand constant burning and erosion. The center electrode extends through the insulator. One end is connected to a stud screwed into the top of the insulator. The other end extends through the Lower cone of the insulator. The electrode is held in position by sealing compound. The grounded electrode is part of the outer shell. It is bent so the end is directly beneath the center electrode.
*The gap between the two electrodes is a prime factor in a spark plugs operation. This gap must be set to the exact engine specifications in the technical manual.
*If the gap is too small, the spark will be weak and fouling or misfiring may result.
*Too wide a gap may work fine at high speeds, but will misfire at low speeds.
*The surfaces of the two electrodes should be parallel and have squared corners.

Haet Range of Spark Plugs:
          The heat range of a spark plug is as important as the gap setting. In fact, the heat ranges of spark
plugs are used to classify them. The term heat range means the spark plug’s ability to transfer heat at the firing tip to the cooling systems of the engine. Heat transfer is governed by the distance the heat must travel.

Heat at the end of a spark plug with a long insulator cone has farther to travel to get to the cooler engine
cylinder head than heat at the firing tip of a short insulator plug. So the short insulator plug runs cooler than the long insulator plug (Fig. 5.3). Generally, an engine which operates at high speed or under heavy loads will run hotter and require a spark plug with a short insulator cone so the heat will transfer faster. A plug with a long insulator cone is used in an engine that operates at a lower speed. The engine’s technical manual will tell you which spark plug to use.
           The spark plugs’ heat range is an index of its capacity to dissipate thermal energy. The different characteristics of automotive engines regarding operating load, compression, engine speed, cooling, and fuel make it impossible to run all engines with a standard spark plug. The same spark plug may get very hot in one engine type, but may reach only a relatively low temperature in 3 another. In the first case, the air-fuel mixture would ignite on the glowing parts of the spark plug projecting into the combustion chamber (pre-ignition) and, in the second case, the insulator tip would soon become so badly fouled by combustion deposits that misfiring would occur. To ensure that the plug runs between the desired temperatures, plugs with different heat capacities were developed. The so called “heat range”, which is assigned to each spark plug, is used to characterize these heat dissipation capacities. A plug with a low heat range number (e.g., 2-4) indicates a cold plug that quickly dissipates heat to the engine block and cooling system, while a high code (e.g., 7-10) indicates a hot plug that retains heat. By properly selecting the heat range of the plug, it ensures that the plug will operate between the plug’s designed operating range of 500- 900 degrees Celsius. In this range, the spark plug will be self-cleaning, yet will not be hot enough to pre-ignite the air/fuel mixture.

Special Types of Spark Plugs:

There are several kinds of spark plugs that are unique.
Resistor Spark Plugs:
These spark plugs have a resistor between the terminal and center electrode to reduce radio and television interference by the ignition circuit.

Surface Gap Spark Plugs:
Surface gap spark plugs eliminate the problem of grounded electrodes. The spark jumps from the center electrode to the shell. This type of spark plug resists fouling but is used in special high voltage systems.

Insulator Tip Spark Plug:

       Insulation around the tip of an insulator tip spark plug (sometimes called projected tip or turbo action spark plug) extends farther into the combustion chamber than other spark plugs. The tip is cooled by the cool, incoming fuel air-mixture then cleaned by the hot, outgoing exhaust gases. These plugs run hotter at slow speeds because of longer heat transfer distance and colder at fast speeds because incoming charges of cold fuel lower the temperature.

FAQ:
Q. What is the function of a spark plug and why is it a vital engine component?
        A. The function of the spark plug is to introduce the ignition energy into the combustion chamber and to initiate combustion of the compressed air-fuel mixture. It accomplishes this task by generating a spark between its electrodes, which then generates the required heat to ignite a smooth burn of the air/fuel mixture. The spark plug is an important factor in determining optimal performance and reliable functioning of an engine. It must permit reliable cold starting, it must guarantee that there is no misfiring during acceleration, and it must withstand the engine being operated for hours on end at maximum power. These requirements apply throughout the entire service life of the spark plug. For 100 years, Bosch Spark Plugs have been designed for optimal performance, reliable functioning, and to withstand extreme operating conditions. Today, the continuously developing spark plug is a key engine system component. It plays a major role in fuel economy, clean, efficient combustion, and the reliable operation of engines and catalytic converters.

Q. Why do we need to change spark plugs?
                A. During operation, the spark plug is subjected to both wear and to fouling and should be replaced at regular intervals. In the course of its service life, the spark plug undergoes changes that increase the required ignition voltage. When the required voltage reaches a level that can no longer be compensated for by the voltage reserve, the result is misfiring. These changes can be caused by:
• Electrode wear
• Engine wear
• Abnormal operating conditions (pre-ignition, detonation, high oil consumption)
Independent tests have shown that installing premium spark plugs such as Bosch’s Platinum+4 improves fuel mileage by as much as 4.8% over original equipment plugs. Platinum+4 uses racetested technology including multiple electrode paths and large amounts of platinum to generate strong, consistent sparks.

Glossary:
        Multi Ground- Plugs with multiple ground electrodes are called multi ground. These plugs also use surface gap spark patterns and the multiple ground electrodes increase service life.

Surface Gap- Surface gap technology is when the ground electrode is to the side of the center electrode. There can be no ground electrode (racing applications), multiple ground electrodes (PT+4 and PT2), or a single ground electrode. These spark plugs have improved efficiency and a higher probability of igniting a lean air/fuel mixture.

Resistor- Many spark plugs have a special conductive glass seal between the center electrode and the terminal stud. This seal acts as a resistor in the plug which reduces the transmission of pulses of energy to the ignition cables. These pulses can cause Radio Frequency Interference (RFI) with electrical components in the car. For some newer cars, resistor plugs are required for effective communication between the plugs and the electronic ignition.

Radio Frequency Interference (RFI)- When the spark plug creates a spark, a high frequency burst of energy is created. If this energy was to travel through the ignition wires, it could cause interference with other sensitive electronic devices, such as the radio or electronic control units. The resistor in the spark plug reduces this energy before it causes interference with other electrical components.

Fouling- Fouling occurs when a plug becomes contaminated with fuel, oil, or other contaminates that prevent the plug from generating a spark. Most plugs today are designed to reduce fouling and become self-cleaning when they reach 500 degrees Celsius. However, short trips, low speed driving, improper spark plug heat range (too cold), improper timing, or an oil leak in the combustion chamber can cause a spark plug to become fouled.

Bridging- Over time, contaminants can build on the surface of the spark plug if it does not reach self-cleaning temperature often. These buildups can grow between the electrodes until they are actually connected by a bridge of contaminants. This will often cause misfire.

Flashover- Occurs when the spark does not jump between the electrodes within the combustion chamber, but instead jumps between the metal shell and the terminal on top of the plug. This will always cause a misfire since the air/fuel mixture will not be ignited. Bosch spark plugs have a ribbed insulator to prevent flashover.

Quenching- The purpose of a spark plug is to introduce enough heat into the combustion chamber to initiate a smooth burn of the air/fuel mixture. Quenching occurs when that heat generated by the spark is reabsorbed back into the ground electrode, the center electrode, and the ceramic insulation. To reduce quenching and ensure an efficient transfer of heat, Bosch dual and triple electrode ground and Platinum+4 and Platinum2 spark plugs use surface gap technologies.

                            Published by Ravindra.K(Mechanical Engineering)