Spark plugs are one of the most important components in vehicles with a gasoline engine. They are not only responsible for ensuring reliable starting but also play a crucial part in attaining optimum engine performance and dependable operation. This applies particularly to modern engine management systems, where spark plugs and their design are gaining in significance.
The principle of the spark plug is ingenuously simple and simply ingenious. A conductor insulated from the engine passes a high-voltage pulse to an electrode, from where a spark jumps to a ground electrode. The thermal energy from this spark ignites the mixture.
The requirements a spark plug is able to satisfy are governed by the following: The design, the materials used and the manufacturing processes employed.
1. Sharp rib profile on the insulator head
The rib profile acts as a 5-fold creepage-current barrier to prevent sparkover in the head.
Made primarily of aluminum oxide. Its role is to insulate the terminal studs and central electrode from the shell.
3. Terminal studs
The terminal studs are made of steel.
4. Flanged ring
Used to secure and seal the insulator.
Made of steel and nickel-plated to provide protection from corrosion. Used to secure the spark plug in the cylinder head.
6. Conductive glass seal
Electrically and thermally conducting. Connects the terminal stud to the center electrode.
Secures the insulator in place and seals it.
8. Center electrode
Consists of a nickel alloy with copper core.
9. Ground electrode
Consists of a nickel alloy. Assignment, number and geometry have an influence on spark plug technology and service life.
Different advantages are gained from the use of different electrode alloys to suit each particular engine. Depending on the electrode alloy employed, the spark plugs bear the designations Bosch Platinum, Bosch Double-Platinum, Bosch Iridium, Bosch Double-Iridium or Bosch Silver.
Nickel alloy electrodes
On Bosch spark plugs the center electrode is made of a wear resistant nickel alloy with copper core. The copper core has high thermal conductivity to provide protection against thermal overload.
Precious metal electrodes
Modern engines operate at particularly high combustion chamber temperatures. This places strain on the spark plugs, and increases the wear on the electrodes. To compensate for this materials with improved scorch resistance must be used. To achieve this, Bosch relies on precious metal alloys using platinum, iridium or silver.
To ensure reliable ignition for spray-guided direct injection engines, the air/fuel mixture and the ignition spark must be ideally coordinated. This is only possible with precision spark plugs on which the position of the ground electrode is accurately aligned to the combustion chamber situation and the arrangement of the gasoline injector in the course of manufacture.
Cars that are operated by gas are on the increase. The number of newly registered and retrofitted gas vehicles is growing steadily. Natural gas is used as a fuel in various forms: Compressed as CNG (compressed natural gas) or liquefied as LPG (liquefied petroleum gas). As a pioneer of automobile technology, Bosch also assumes responsibility for gas-operated vehicles. Bosch offers you the correct spark plug for any gas-operated vehicle – in original equipment as well as for conversions. As spark plugs are more heavily loaded during gas operation, the change intervals are shorter: around 15,000 km for standard spark plugs and around 30,000 km for platinum spark plugs.
Depending on the engine characteristics Bosch uses varying spark plug air gap designs.
Air gap design:
The ignition spark crosses between the center electrode and the ground electrode by the most direct route, passing through the air/fuel mixture between the electrodes.
Surface gap design:
The ground electrodes are installed in such a way that they can only create a 'surface air gap', producing sparks that are particularly long and powerful.
Surface air gap design:
The surface air gap is designed such that when igniting the spark chooses the best route from the center electrode to the ground electrode. This occurs either via an air gap or a surface air gap. With an air gap the spark crosses via the direct route and in doing so ignites the air/fuel mixture. With a surface air gap the spark passes to the insulator nose tip via the available charge carrier.
Essential for high ignition reliability in supercharged engines with direct gasoline injection: the optimal power transmission of the ignition spark – the strength of the new Bosch pin-to-pin spark plugs.
Bosch pin-to-pin spark plugs
With Bosch pin-to-pin spark plugs, you benefit from the latest in Bosch spark plug technology. The innovative high-performance spark plugs are tailored to the specific requirements profile of supercharged direct gasoline injection engines:
Supercharged engines require a higher ignition voltage. In theory, this can be reduced by spark plugs with a small electrode gap. However, a small electrode gap causes high quenching losses, and therefore poor ignition. To compensate for this, Bosch has developed pin-to-pin spark plugs. In pin-to-pin technology the spark plug has two particularly thin electrodes that cause very low quenching losses, providing for high power transmission and reliable ignition.
When cold, the engine runs with a rich air/fuel mixture and this can produce soot due to incomplete combustion. The soot is then deposited in the combustion chamber and on the spark plugs. These deposits contaminate the insulator nose and create a conductive connection between the central electrode and the spark plug shell. This "shunt circuit" dissipates some of the ignition energy and reduces the energy required for ignition.
The deposition of combustion residues on the insulator nose is highly dependent on temperature and primarily takes place below approx.
At higher temperatures the carbon-containing residues burn off the insulator nose and the spark plug "cleans" itself.
It is important to choose the right heat range for the spark plug and to make sure that the spark plug temperature range remains between 500 and 900 °C. In this temperature range pre-ignition is prevented and the spark plugs are self-cleaning. In addition, oxidation and hot gas corrosion is reduced decreasing electrode wear.
Vehicle manufacturers are continually driving engine development forward, with the aim of making engines more efficient and reliable. As a consequence, modern engines are becoming smaller and more efficient all the time. These smaller engines use less fuel without any loss of performance.
This development means that the compression pressure in the engine is greater, which places particularly demanding requirements on the spark plugs. For example, the high voltage requirement increases. At the same time, in these more compact engines there is less space available for the spark plugs.
The dielectric strength – or the ability of the ceramic to insulate the voltage – is becoming more and more important, since to fulfill the requirements of modern engines the ceramic diameter needs to be smaller and the voltage greater. This requires a commensurate improvement in the insulating properties of the ceramic. Otherwise, if the dielectric strength is not sufficient for the applied voltage, the ceramic is likely to become damaged and misfiring and malfunctions to result.
For this reason, Bosch has developed a special ceramic mixture that provides optimal dielectric strength, perfectly fulfilling the specific requirements of today's generation of engines.
The insulator consists of an aluminum oxide ceramic and insulates the central electrode up to 40,000 V to ground.
In order to make engines more efficient, vehicle manufacturers are reducing the cubic capacity or the number of cylinders (downsizing) and simultaneously increasing the charge air pressure. This necessitates a higher ignition voltage, which in turn requires an insulator with greater flashover resistance. Bosch has specifically developed spark plugs with an extended insulator and cup terminal for this purpose. On these spark plugs the insulator is approx. 8 mm longer. To compensate for this increased length the spark plug now has a stud with a cup to act as the contact connection. The contact pressure spring of the spark plug connector is centered in the cup.
The spark plug specifications are contained in the type designation. It contains all the key features of the spark plug except the electrode gap.